Manufacturing Process

RWTH edition RWTH Aachen Fritz Klocke Manufacturing Processes 1 Cutting Translated by Aaron Kuchle 123 Professor Dr.-Ing. Dr.-Ing. E.h. Dr. h.c. Dr. h.c. Fritz Klocke Laboratory for Machine Tools and Production Engineering RWTH Aachen University Steinbachstr. 19 52074 Aachen Germany [email protected] Translated by Aaron Kuchle Yeungnam University Korean Language Institute 214-1 Dae-dong Gyeongsan Gyeongbuk 712-749 Korea [email protected] ISSN 1865-0899 e-ISSN 1865-0902 ISBN 978-3-642-11978-1 e-ISBN 978-3-642-11979-8 DOI 10.1007/978-3-642-11979-8 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2011925556 © Springer-Verlag Berlin Heidelberg 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: deblik, Berlin, Germany Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Preamble About Compendium “Manufacturing Processes” Key factors for quality and economic efficiency of industrial production are the choice of the manufacturing processes and their design. Manufacturing Technology is an elemental part of the fundamental knowledge of machining engineers. Also design engineers have to gain knowledge in this field, since they have high responsibility for the manufacturing costs. However, the students as well as practising experts who are willing to enhance their knowledge have the problem to collect information. To the current day there is no extensive, but still clear description of manufacturing processes focussing on the technology itself. In order to counter this necessity the compendium at hand is supposed to present an overall picture of the most common machining and non-machining manufacturing processes. Additional to the description of the techniques these volumes are desired to deliver an insight in the underlying physical principles whenever it is necessary for the understanding of the processes. The apportionment of the compendium “Manufacturing Processes” into Volume 1: Cutting Volume 2: Grinding, Honing, Lapping Volume 3: Electrical Erosion and Hybride Processes Volume 4: Forming Volume 5: Casting, Sintering, Rapid Prototyping groups techniques with similar active principles together. In front of the first volume is placed a technique-spanning section to the tolerances and questions of the workpiece measuring techniques used in manufacturing. Within the individual volumes was tried to avoid an encyclopaedic listing of the techniques. The book series are primarily intended for junior scientists in the fields of manufacturing technology and construction. In addition, the practitioner will be able to refurbish or extend his knowledge. The variety of manufacturing problems v vi Preamble About Compendium “Manufacturing Processes” is as large as the multiplicity of the products, and alone with text book wise sayings manufacturing questions are not to be solved. We wish that this book offers starting points and ways to its readers, on which they can come up with successful solutions by engineering thinking. Aachen, Germany September 2010 Fritz Klocke Preamble for Volume 1 “Cutting” The available volume treats machining with geometrically defined cutting edges. It approaches both the practical engineer and the student of engineering sciences. This book is based on the lecture “Manufacturing Technologies I and II” and the pertinent exercises, which are held at RWTH Aachen University. The arrangement of the book results from the experiences, which were gained when lecturing in the chronology of the course. The structure of the book is oriented to a large extent at didactical criteria. The first section deals with the metrology and the workpiece quality, followed by a chapter about the fundamentals of cutting. Based on the common technological principle of the cutting processes and their variants, the second section deals with the mechanisms occurring at the cutting edge and the resulting loads on the tools. The next section deals with the demands on the cutting materials as well as their manufacturing processes and application. The text presents topics like the use of the finite element method for cutting processes as well as cutting fluids. A detailed chapter about the machinability of the most important work piece materials, such as steel and cast iron, light metal alloy and high temperature resistant materials imparts a broad knowledge of the alternating coactions of work piece material, the cutting material and the process parameters. The necessity for process control, used sensor systems and application possibilities are regarded. Due to the increasing relevance and application in industrial manufacturing plants, monitoring systems are discussed in their own chapter. Finally, the different cutting processes and their kinematic characteristics are presented in detail. For their cooperation with the compilation of the available edition I would like to thank my co-workers, Dr.-Ing. M. Abouridouane, Dr.-Ing. K. Gerschwiler, Dipl.-Ing. M. Arft, Dipl.-Ing. C. Essig, Dipl.-Ing. P. Frank, Dipl.-Ing. A. Krämer, Dipl.-Ing. D. Lung, Dipl.-Ing. H. Sangermann, Dipl.-Ing. R. Schlosser, Dipl.-Ing. vii viii Preamble for Volume 1 “Cutting” P. Vogtel, as well as Dipl.-Ing. S. E. Cordes, who was additionally responsible for the coordination of the work on this book. Further I would like to thank the many former co-workers, who participated in the past German editions and now have leading positions in industry and research companies. Aachen, Germany September 2010 Fritz Klocke Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Metrology and Workpiece Quality . . . . . . . . . . . . . . 2.1 Manufacturing Disturbances and Manufacturing History 2.2 Measuring and Testing . . . . . . . . . . . . . . . . . . 2.2.1 Measurement Errors . . . . . . . . . . . . . . 2.2.2 Macro- and Microgeometry of Components . . 2.3 Length Testing Devices . . . . . . . . . . . . . . . . . . 2.3.1 Material Measures . . . . . . . . . . . . . . . 2.3.2 Gauges . . . . . . . . . . . . . . . . . . . . . 2.3.3 Indicating Measuring Instruments . . . . . . . 2.4 Surface Inspection . . . . . . . . . . . . . . . . . . . . 2.4.1 Surface Parameters . . . . . . . . . . . . . . . 2.4.2 Subjective Surface Inspection . . . . . . . . . 2.4.3 Surface Measurement . . . . . . . . . . . . . . 2.5 Inspection of the Workpiece Rim . . . . . . . . . . . . 2.5.1 Surface Layers . . . . . . . . . . . . . . . . . 2.5.2 Inspection of the Surface Rim Zone . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 8 9 10 13 13 13 14 21 22 26 27 30 31 33 3 Fundamentals of Cutting . . . . . . . . . . . . . . . . . . . . . 3.1 The Cutting Part – Concepts and Terms . . . . . . . . . . 3.2 Reference Systems . . . . . . . . . . . . . . . . . . . . . 3.3 Basic Process Variants . . . . . . . . . . . . . . . . . . . 3.4 Chip Formation . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 The Cutting Process . . . . . . . . . . . . . . . 3.4.2 Different Types of Chip Formation . . . . . . . . 3.5 Kinematic Surface Roughness . . . . . . . . . . . . . . . 3.6 Mechanical and Thermal Strain on the Cutting Section . . 3.6.1 Influence of the Geometry of the Cutting Section 3.7 Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.1 Wear Mechanisms . . . . . . . . . . . . . . . . 3.7.2 Causes of Wear . . . . . . . . . . . . . . . . . . 3.7.3 Forms and Dimensions of Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 39 42 45 47 47 49 54 55 68 71 71 74 85 ix x Contents 3.8 Cutting Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8.1 Shear Plane Theory . . . . . . . . . . . . . . . . . . . 3.8.2 Application of Plasticity Theory to Cutting . . . . . . 4 Cutting Tool Materials and Tools . . . . . . . . . . . . . . . 4.1 Overview of Cutting Tool Materials . . . . . . . . . . . 4.1.1 Classification of Hard Cutting Tool Materials . 4.2 Tool Steels . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Cold Work Steels . . . . . . . . . . . . . . . . 4.2.2 High Speed Steels . . . . . . . . . . . . . . . 4.3 Cemented Carbides . . . . . . . . . . . . . . . . . . . . 4.3.1 Historical Development . . . . . . . . . . . . 4.3.2 Cemented Carbide Production . . . . . . . . . 4.3.3 Components of Cemented Carbides and Their Properties . . . . . . . . . . . . . . . . . . . . 4.3.4 Microstructure . . . . . . . . . . . . . . . . . 4.3.5 Classification of Cemented Carbides . . . . . . 4.3.6 Uncoated Cemented Carbides and Cermets . . 4.3.7 Function Gradient Cemented Carbides . . . . . 4.3.8 Coated Cemented Carbides and Cermets . . . . 4.4 Coatings . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Coating Methods . . . . . . . . . . . . . . . . 4.4.2 Specific Properties of Hard Material Coatings . 4.4.3 Substrate Pretreatment . . . . . . . . . . . . . 4.5 Ceramic Cutting Tool Materials . . . . . . . . . . . . . 4.5.1 Cutting Ceramics . . . . . . . . . . . . . . . . 4.5.2 Superhard Non-metallic Cutting Tool Materials 4.6 Tool Designs . . . . . . . . . . . . . . . . . . . . . . . 4.6.1 Solid Tools . . . . . . . . . . . . . . . . . . . 4.6.2 Tools with Inserts . . . . . . . . . . . . . . . . 4.7 Tool Preparation . . . . . . . . . . . . . . . . . . . . . 87 87 92 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 95 97 99 100 101 110 111 112 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 116 117 123 123 126 127 128 149 161 161 162 173 184 185 187 193 5 Finite Element Method (FEM) . . . . . . . . . . . . . . . . . 5.1 Basic Concepts of FEM . . . . . . . . . . . . . . . . . . 5.2 Lagrangian and Eulerian Considerations of the Continuum 5.3 Explicit and Implicit Methods of Solution . . . . . . . . . 5.4 Combined Thermal and Mechanical FEA . . . . . . . . . 5.5 Nonlinearities . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Material Laws . . . . . . . . . . . . . . . . . . . . . . . 5.7 Software . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Phases of a Finite Element Analysis (FEA) . . . . . . . . 5.10 The Use of FEM in Cutting Technology . . . . . . . . . . 5.10.1 Continuous Chip Simulation . . . . . . . . . . . 5.10.2 Segmented Chip Simulation . . . . . . . . . . . 5.10.3 Simulation of the Cutting Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 197 199 199 200 200 201 204 205 205 206 207 209 212 . . . . . . . . . 7. . . 7. .4 Chip Form .3 Surface Quality . . . . . . .5 Machinability of Cast Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. . . . . . . . .2 Carbon Content . . . . . . . . . . . . . . .2. . . . 6. . . . . . . .1 Determining Tool Life Parameters . 7. 7. . . . .7. . .4 Nitrided Steels .5 Selection of Cutting Fluids . . . . . . . . . 7. . . . . . . . . . . . . . . . . . . .6 Reducing or Avoiding the Use of Cutting Fluids . . . . . . . . . . . . . . . . . . .2. 7. . . . .4. . . . . . . . . . . . . . . . . . 6. . . . . . . . . . . 7. 7. . 237 239 239 240 243 245 248 248 249 250 252 254 256 263 263 266 267 268 269 271 274 280 280 283 290 290 293 296 311 314 327 327 331 . . . . 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. . . .2. . . . . . . . . .5 Cutting Speed . . . .1 Reducing Cutting Fluids .6. . . .1 Tool Life . . . . . . .6. . . . . . . . . .3. . . . . . . . . . . . . . . . . . . . . . .2 Water-Based Cutting Fluids . . . . . . .2 Fibre-Reinforced Plastics . . . .3 Alloying Elements . . . . . . . . . . . . . . . . . . . . .6 Machinability of Non-ferrous Metals . . . . . . . . . . . . . . . 7. . 6. . . .6. . .6. . . . . . . . . . 7. .7 Non-rusting Steels . . . . .3 Titanium Alloys . . . . . .5. . . 7. . . . . . . . .6. 6.6. .4. 7. . .3 Heat-Treatable Steels . . . . . .6. . . . .4 Machinability of Various Steel Materials 7. . . . . . . . . . . . . . . . .1 Non Water-Miscible Cutting Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3. . . . . . .1 Metallographic Constituents . . . . . . . . . . . . . . . . . . . . 7. . . . . . . . . . . . . . .4. . . . . . . . . . . . . . . . .2 Resultant Force . . . . . . . . . . 6. . 7. . . 7. . . . . . . . . . . .4. 7. . . . . . . . . . . . . . . . . . . . . . .2 Types of Cutting Fluids . . . . . . . . . . . . . . . . . 7. . . . . . . . . . . .6 Hardened Steels . . . 6. .Contents xi 6 Cutting Fluids . . 7. . . . . . . . . . . . . . . .1 The Functions of Cutting Fluids . . . 219 219 219 220 221 222 223 226 227 228 229 233 7 Tool Life Behaviour . . . .5 Tool Steels . . 7. .4. . . . . . .4 Copper Alloys .7 Machinability of Non-metals . . . .4 Effects of the Cutting Fluid on the Machining Process . . .1 Machining Steels . . . . . . .5. 7. . . 7. . . . .2. 6. . . . . . . . . . .2 Minimum Quantity Cooling Lubrication (MQCL) 6. . . . . . . . . . . . . . . . . . . . . . . . 7. . . . . 7. . . .1 Graphite . . . . . . . . .4. . . . . . . . . . . . . 7. . . . . .2 Machinability . . .2. 7. . . .2 Grey Cast Iron . . . . . . . . .2. . . .5 Nickel Alloys .2. . . 7.6. . . . . 7. . . . . . . .7. . . . . . . . . . . . . 7. . . . . . . . . 7. . . . .3. . . .4. . .3. . . . . . . . . . . . 6.3 The Machinability of Steel Materials .3 Guidelines on the Use of Coolant Emulsions . . . . . . . . . . . . . . . . . . . . .2 Magnesium Alloys . . .1 White Cast Iron . . . . . . . . . . . . 6. . . . . .4 Types of Heat Treatment . . . . . . . . . . . . . .2 Case-Hardened Steels . . . . . . . .3 Avoiding Cutting Fluids . . . . . . . . . . . . . . . . . . . . .1 Aluminium Alloys . . . . .3. .2 Signal Processing and Monitoring Strategies . . 10. . .2.2 Profile Broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . 339 339 339 342 350 351 352 354 371 9 Processes with Rotational Primary Movement . . . . . . . . . 9. . .3. . . . . . . 8. . 9. . . . . . . . . . . . . . .2 Cylindrical Turning . . . . . 9. . . . .1 Profile Counterboring . . . . . . . . . . .1. . .5 Form Turning . . . . . . .1. .2 Optimizing the Cutting Parameters . . . . . . . . . . . 10. . .2. . . . . . . . . . . . . .xii Contents 8 Process Design and Process Monitoring .6 Further Process Variants . . . . . . . . . . .2 Process Monitoring . 9. . . . . . . . . . . . . . . . . . . . . . . . . . 8. . . 10. . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. . . . . . . . . . . . . . . . . . . . . . . 9. . 8. . .1. . . . . . . . . . . . . . . . . . . . . . . . . 8. . . . . .1 Bandsawing . . . . . . . . . . . . . 8. 469 Subject Index . . . . . .1 Finding Economical Cutting Parameters . . . . . . . . . . . . . .3 Circular Sawing . . . . . . . . . .3 Planing and Shaping . . .1 Process Variants. . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Hacksawing . . . . .2. .1.2 Rotary Drilling . . . . . . . . . . . 9. . . . . .1. .1 Broaching . 383 383 385 386 387 388 389 390 390 395 416 416 418 439 439 442 442 . 9. . .1 Face Turning .4 Planning Methods and Tools . . . . . .1 Face Shaping and Face Planing . . . . 9. . . 9. . . . . . 8. . .3 Drilling . . . . .1. . . . 8. . . . . . . . . . . . . . . . . . . . . .1. . . . . 8. . . . . .4 Sawing . . 9. . . . . . . . 10. . . . . . . . . . . 9. . .1 Sensors for Process Monitoring . . . . . 10. . .1 Face and Circular Broaching . . . . . . . . . . . . . . . . .3. . . . . . . . . . . . . . . . . . . .3. . . . . . . . . . . . . . 9. . . . . . . . . . . . . . . . . . . . . . . 9. . . . . . . . . . . . . . . .1. . . . . . . . . . . . . . . . .1.1. . . . . . . . . . . . . . .4. . . .3 Form Broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Helical Turning . . . 445 445 446 449 451 454 457 458 459 466 References . . . . .1. . . . . . . . . .3 Calculating the Machine Hour-Rate .2 Shaving .2 Gear Shaping . . . . . . . . . . . . . . . . . . . . . . .4 Profile Turning . . . . . . . . . . . .3 Gear Planing . . . . . . .1. 10. . . . 9. . . 497 10 Processes with Translatory Primary Movement 10. . . . . . . . . . . .4. . . . . Specific Characteristics and Tools 9. 9. . . . . . . .1 Turning . . .1. . . . . . . .3. . . . . . . . . . . . .2 Milling . . . . . . . . . . .1 Cutting Parameter Limits . . . . . . . . . . . . . vc = 1 m/min Cubic boron nitride Carbon fibre reinforced plastics (CFRP) Computerised Numerical Control Chemical Vapour Deposition Continuous wavelet transform Tool diameter Direct current Diamond-like carbon Strain gauge Polycrystalline diamond Discrete wavelet transform Young’s modulus Electron beam-PVD xiii .Symbols and Abbreviations Capital Letters A A Ac1 Ac3 Aα Aα Aγ A AFK AIP AV BAZ Cf mm2 % Cv min CBN CFK CNC CVD CWT D DC DLC DMS DP DWT E EB-PVD mm2 mm2 mm2 mm2 mm min mm Cross section Stretch ratio Transformation temperature (Start of austenite formation) Transformation temperature (End of austenite formation) Flank face Minor flank face Rake face Shear face Aramid Faser reinforced plastics Arc Ion Plating Offset Machining center Axis intercept of the expanded TAYLOR equation. f = 1 mm Axis intercept of the expanded TAYLOR equation. xiv ECM EDX F Fa Fc FD Ff Fp F FFT FVK GFK H HB HC HF HF-CVD HM HRC HSC HSS HT-CVD HV KA KE KI KF.min KMH KML KR KZ KB KM KSS KT L LARC MQCL MQC MQS MQL MSIP MT-CVD NC Symbols and Abbreviations N N N N N N N mm C C C C/St. C/h C C C mm mm mm C/h Electro chemical machining Energy-dispersive X-ray spectroscopy Resultant force Active force Cutting force Thrust force Feed force Passive force Shear force Fast Fourier transform Fibre-reinforced plastic (FRP) Glass-reinforced plastic (GRP) Stroke length Brinell hardness Diamond coated cemented carbide Fine grain cemented carbide Hot-Filament-Process-CVD Cemented carbide Rockwell hardness High Speed Cutting High Speed Steel High temperature-CVD Vicker hardness Imputed amortizations Energy costs Maintenance costs Minimal manufacturing costs per workpiece Machine hour-rate Machine and labour cost hourly rate Room costs Imputed interest Crater width Crater center distance Cutting fluid Crater depth Hourly wage Lateral Rotating Arc Quantity Cooling Lubrication Minimum quantity cooling Minimum quantity lubrication Minimum quantity lubrication Magnetron Sputter Ion Plating Moderate Temperature-CVD Numerical control . 2%–yield strength (Depth of) Surface roughness Averaged surface roughness Radio Frequency Major cutting edge Minor cutting edge Rounded cutting edge Short-Time Fourier Analysis Displacement of the cutting edge in the direction of the flank face Displacement of the cutting edge in the direction of the rake face Tool life Temperature Melting temperature Room temperature Temperature on the chip upper side Voltage Feeding voltage Width of flank wear land .Symbols and Abbreviations NE-Metalle PA-CVD Pc Pe Pf Pf Pn Pne Po Pp Pr Ps PW PKD PM PM-HSS PVD Q R Ra Re REM Rkin Rm Rp0.2 Rt Rz RF S S SKV STFT SVα W W W W C  μm N/mm2 μm N/mm2 N/mm2 μm μm μm mm SVγ mm T T Tm Tr Tsp U U0 VB min ◦C ◦C ◦C ◦C V V mm xv Non-ferrous metals Plasmaactivated-CVD Cutting power Effective power Assumed working plane Feed power Tool cutting edge normal plane Effective cutting edge normal plane Tool orthogonal plane Tool back plane Tool reference plane Cutting edge plane Effective power Polycrystalline diamond Powder metallurgy Powder Metallurgy High Speed Steel Physical Vapour Deposition Charge Electrical resistance Average roughness Elastic limit Scanning electron microscope (SEM) Surface quality Tensile strength 0. b = h = 1 mm Specific feed force. b = h = 1 mm . b = h = 1 mm Specific resultant force Specific passive force.-% B bfα C cp cpk dij dW/dt f fa fok fmax fmin fr fz f0 H hch hcu.1 ki kp1.1 mm mm mm mm mm mm m/min pC/N m/min mm mm mm mm mm mm mm 1/s mm mm μm mm N/mm2 N/mm2 N/mm2 N/mm2 Width of cut Depth of cut Maximum depth of cut Optimal-cost depth of cut Hydrogenfree amorphous carbon film Hydrogenated amorphous carbon film Atomic ratio Width of undeformed chip Width of chamfer Speed of sound Process capability index Rectified process capability index Piezoelectric coefficient Growth of wear Feed Axial feed Optimal-cost feed Maximal feed Minimal feed Radial feed Feed per tooth/cutting edge Eigenfrequency Undeformed chip thickness Chip thickness Maximal chip thickness Minimal chip thickness Strain sensitivity Specific cutting force.max hmin K kc1.xvi VBmax VHM Wc We Wf WZ-D WSP Z ZTU Symbols and Abbreviations mm J J J mm Maximal width of flank wear land Cemented Carbide Cutting work Effective work Feed work Tool diameter Insert Atomic number Time Temperature Transformation (TTT) Small Letters ae ap apmax apok a-C a-C:H at.1 kf1. Symbols and Abbreviations lc lf mc mn pKSS rε rß sd T T te.min tg tv te ter th tn tr ta-C vc vch vf vfax ve x z m m mm bar mm mm mm s min min min min min min min min m/min m/min m/min mm/min m/min mm xvii Cutting path Feed path Exponent of the specific cutting force Normal module Cutting fluid pressure Corner radius Rounded cutting edge radius Coating thickness Pitch Time Minimal time per unit Basic time Additional time Time per unit Recovery time Main process time Auxiliary process time Setting-up time Tetrahedral hydrogenated amorphous carbon film Cutting velocity Chip velocity Feed velocity Axiale feed velocity Effective velocity Profile offset Number of teeth Greek Letters α α eff αf αF αo α oe β βo β oe γo γn γ ne γ oe γ eff ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ Tool clearance angle Effective tool clearance angle First tool orthogonal clearance (Broaching) Clearance angle at the pitch diameter (Gear shaping) Tool clearance angle Working clearance angle Wedge angle Wedge angle Working wedge angle Tool orthogonal rake angle Tool normal rake angle Working rake angle Wirk-Orthogonalspanwinkel Effective rake angle . xviii ε ε¯ ˙¯ε ε˙ εk εr η ϑ κr κ re λs λse μ φ φ φf ϕ ρ σ σn ω Symbols and Abbreviations s–1 1/s ◦ ◦ ◦C ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ kg/m3 N/mm2 N/mm2 rad/s Strain Shear strain Shear strain speed Strain rate Critical strain Tool included angle Effective cutting speed angle Temperature Tool cutting edge angle Working cutting edge angle Tool cutting edge inclination Working cutting edge inclination angle Coefficient of friction Phase Shear angle Slide angle Feed motion angle Density Stress Normal stress Angular speed . human beings could already adapt stone tools with intentionally produced edges to specific machining tasks by varying the geometry of the cutting edge.1007/978-3-642-11979-8_1. RWTH edition. C Springer-Verlag Berlin Heidelberg 2011 DOI 10.000 to 50.  1 . The discovery of how to extract metals such as copper.” [Schl11]. but the speed of these cutting edges is a function of the machine moving them. At the beginning of the 17th century. 1. leading to the development of cemented carbide in 1923 by SCHRÖTER [Schr23.. as is shown in early tool findings from the Palaeolithic Age (Fig. all the influencing F. DRP25] at the SOCIETY FOR ELECTRIC LIGHTING as well as its use in machining by FRIEDR. so as wages increase. SCHLESINGER said: “The dividends may lie on the cutting edge of steel. the cutting machine is a trump card. at the turn of the century.Chapter 1 Introduction From about 12. TAYLOR. there was increased exploration into manufacturing technology at the beginning of the 19th century. constant improvements in iron smelting led to the preferred use of iron and steel as construction materials instead of other metals known then and in place of wood. tin and iron was a huge milestone in the history of material and manufacturing technology. Klocke. Krupp AG was a major contribution to improving productivity and economy [Sche88]. leading by the second half of the 19th century to the first systematic investigations into cutting methods and initiating a completely new research area. At the end of the 19th century further research led to the discovery of new cutting tool materials and. Following this was the invention of oxide-ceramic cutting tool materials and their application in cutting after 1938 by OSENBERG as well as WENTORF’s development and synthesis of the superhard cutting tool material cBN (cubic-crystalline boron nitride) in 1956 [Went57].1). Subsequent research followed in this direction. KRUPP AG as WIDIA cemented carbide. In order to fulfill the constantly increasing requirements made on workpiece quality and to make machining processes more economical. a significant contribution to the history of manufacturing technology [Tayl07]. In light of these developments. which had predominated until then. to the development of high speed steel by F. Since about 700 B.W. tools were almost exclusively made of iron. Inspired by the growth of the textile industry and the discovery of the steam engine.000 years ago. The refinement of cutting tools with wear-resistant cemented carbide coats starting in 1968 by the companies Sandvik Coromant and Friedr. Manufacturing Processes 1.C. 2 1 Introduction Drill Saw End scraper Gravettian point Double tines Round scraper Side scraper Notched point Stemmed-base hafted scraper Fig. . In this pursuit. 1. acc. As an introduction to the subject matter. process kinematics. cutting tool materials and their coatings.1 Flint tools. the compendium will be preceded by a general chapter providing a survey of the most important attributes of workpieces and their test methods. to KERND’L [Kern72] parameters relevant for productivity and workpiece quality must be considered: cutting edge geometry. materials. wear behaviour as well as auxiliary materials. cutting parameters. the following quotation by SCHLESINGER can act as a guideline: “The machine tool must be designed with a view to the place it is used”. 1 Manufacturing Disturbances and Manufacturing History The basic task of manufacturing is to provide workpieces with specified quality characteristics in the required quantity in the most time. such as roughness. 2. If a characteristic value lies outside of the permissible tolerance. Thus important functional characteristics must be tested either already during manufacturing or at the end of manufacturing. they cause manufacturing trends. RWTH edition. This group also includes disturbances resulting from the engagement kinematics between the tool and the workpiece. disturbances caused by thermal influences. form and position of geometrical elements. the accuracy of dimension. C Springer-Verlag Berlin Heidelberg 2011 DOI 10. These may include hardness values. such as deflections effected by the workpiece weight or clamping errors. for example. Klocke. residual stresses or certain structural properties. For this reason. Manufacturing Processes 1.and cost-efficient way possible. disturbances caused by dynamic forces which lead to either self-starting or forced oscillations. such as generated cut deviations occurring during hobbing or diffraction effects on optical surfaces caused by systematically applied tool marks. Microgeometrical parameters include. Systematically acting disturbances cause deviations in characteristics. the functional integrity of the process requires that certain properties are tolerated although they lie below the surface in the peripheral surface zone.1). Important disturbance factors which must be taken into consideration as possible causes of defects are: disturbances caused by static forces.1007/978-3-642-11979-8_2. they can F. The fluctuations of characteristics occurring in every manufacturing process can basically be derived from the influence of either systematically or randomly occurring disturbances. However.  3 . and disturbances caused by tool wear (Fig. If the causes of systematic errors are known. Systematic errors are caused by the system. Macro-geometrical properties include. Every manufacturing process is affected by variable disturbances. which can be both external and internal (occurring within the process itself). They are reproducible under identical boundary conditions and change the characteristic in a single direction. it is defective. for example. Functionally determinative quality characteristics can be defined through macrogeometrical properties.Chapter 2 Metrology and Workpiece Quality 2. characteristic surface values. such as process heat or internal sources of heat in the machine tool. the functionally determinative properties of the components are provided with tolerances. mean roughness value and polishing depth. the work result becomes unstable. In practice. Changes of the cutting force caused by sudden cutting depth fluctuations and the workpiece deflections associated with them may also be of systematic nature (Fig.33. The process capability index is always taken as the dominant process . These process capability indices can only be applied if the variation of the characteristics are normally distributed.4 2 Metrology and Workpiece Quality Statical disturbances Thermal disturbances • Clamping fault • Internal heat sources • Deflection • Strain • • Process heat ••• • Environmental effect • ••• Prozess Dynamic disturbances Tool wear • Self-starting oscillation • Displacement of cutting edge • Forced oscillation • Flank wear e. be statistically recorded. Within real processes. Examples of systematic errors are geometric errors of machine guiding elements and the development of cutting edge offset through the continuous wear of turning tools used in turning processes. 2. the effects of which cannot be predicted in a strictly deterministic way. 2. 2. it is frequently required that cp ≥ 1.e. triggered over basement • ••• • Flank roughness • Grinding wheel wear • ••• Fig.2). random temperature fluctuations or sudden self-starting oscillations (rattling oscillations).3). The influence of random disturbances on the work result can. Randomly occurring disturbances lead to changes to characteristics.g. on the variation of a quality characteristic) is process capability or the process capability index (Fig. They influence the result in both a positive and a negative direction. Examples of this include fluctuations in the structure of a material. Thereby. An important parameter describing the effect of random disturbances on a quality characteristic (i. it is not always easy to clearly decide between systematic and random influences. however.1 Variable disturbances in manufacturing be corrected and compensated. when it can not be guaranteed that all active elements work securely during the process being at its performance limits.Manufacturing Disturbances and Manufacturing History 5 Dimensional error Passive force FP / N / µm Nominal diameter 2. 2. 2. Manufacturing progresses result from a manufacturing chain or a manufacturing sequence. . significant on the process capability.1 Feed direction 1000 800 600 400 200 Cutting path 10 5 0 Workpiece material Cutting tool material Cutting speed Feed Cutting path Ck55 N HW P10 vc = 160 m/min f = 0. In general. There can also be a significant interaction between the current step in the process and previous changes. The output properties of the part following a certain manufacturing step in turn become the input factors for the processing steps which follow. If. cutting edges are randomly failed during a cutting process because of cutting edge fractures or layer failures. several manufacturing steps are necessary to manufacture a part. for example. Thus a manufacturing history develops (Fig. this can have an immediate. Another example of critical process conditions is when the quantities of heat entering the workpiece at high process performances cause thermal strain to induce an increased variation of geometrical characteristics. peripheral surface zone) are left behind on the part. surface.25 mm Cutting part geometry: γ0 = 6° κr = 70° α0 = 5° εr = 90° rε = 0. characteristic changes (geometry.8 mm lS = 0° Fig. With every manufacturing step. These examples reveal that measuring and testing procedures in manufacturing are necessary both to guarantee the quality of the manufactured part and to start and develop processes.2 Influence of passive force on dimensional accuracy design criterion when large numbers of pieces must be manufactured with the highest possible level of productivity. Variation in manufacturing batches occur.4). 67 = 1.33 cp cpk = 5.6 2 Metrology and Workpiece Quality UTL 20 _ 3s^ s _ 15 NV LTL _ _ _ x x . 2. c Damping and stiffness Fig.3 Process capability indices Lu Ll Lu Enveloping cut deviation Anisotropism Ll Forging.00 = 5.00 Process capability indices cp = cpk = Tolerance band = Process spread UTL .NV|min 3 σ^ UTL: Upper tolerance limit LTL: Lower tolerance limit NV: Nominal Value Fig. cold extrusion k1a Turning milling c1a k2b Machining center Lu.LTL 10 cp cpk cp cpk = 0.LTL 2 ( 3·σ^ ) Minimum proximity to process limit = = Half process spread _ x : Mean value of the random sample ^ : Estimation of the spread of the totality σ _ s : Mean deviation of the random sample T _ 6s _ |x .4 Manufacturing history (Source: Getrag Ford) Ll Hard machining Hardening c2b Lu Texture knx cnX Grinding machine tool Geometry Assembling .71 = 0.71 x _ x x = 2.00 cp cpk = 1. 2.l Upper and lower limit Lu Ll Machining center k.50 = 1. 6 Stability bounds when milling a BLISK stabil instabil stability bound Speed / min–1 .2. 2. The hole in a transmission part shown in Fig. chucking 0. machining Manufacturing dispersion: Affected by residual stress. 2.5 Representative case study – box part The hole in a transmission part shown in Fig. 2. 2. long and slander tools Work piece: Blisk (Blade integrated Disk) made of TiAl6V4 Tool: Depth of cut/mm End milling cutter (D = 8 mm) Source: MTU Fig.008 Requested cpk = 1.5 could not Manufacturing process: Forming. There can also be a significant interaction between the current step in the process and previous changes.5 could not be manufactured in a procedurally secure way with the required process capability index and specified. 5-axis-milling Manufacturing dispersion: Process instability.01 0. chatter marks.67 Box part: Automobile-gearbox made of Mg-alloy AZ 91hp Standard tolerance IT 4 Fig.1 Manufacturing Disturbances and Manufacturing History 7 Manufacturing process: Primary shaping. The milling process is only stable for certain combinations for cutting speed and depth of cut. This is due to released residual stresses which arised in the previous die-casting process. A 100% characteristic check is prescribed for many safety-related parts.8 2 Metrology and Workpiece Quality Manufacturing process: Forming. 2. broaching Manufacturing dispersion: Process anomaly. However.6 shows a part manufactured by milling with long. drilling.2 Measuring and Testing Quality characteristics must be tested during the manufacturing due to the boundary manufacturing conditions mentioned above. but also impermissible changes in the peripheral surface zone. / MPa Smeared material 1000 Low thermal influencing High thermal influencing 10000 100000 1000000 Work piece: Turbine disk made of Inconel718 Tool: VHM-drill (D = 8 mm) Numbers of change of load Fig.7 Surface damages when drilling with a worn drill be manufactured in a procedurally secure way with the required process capability index and the demanded productivity. Figure 2. 2. not only impermissible macro-geometrically or micro-geometrically recognizable changes may limit the process. These characteristic stability areas must be known. . tool life decrease of the part Source: Rolls-Royce 50 µm Without anomaly Ktxσappl. turning. These stresses damaged the peripheral surface zone in a way that significantly diminished the fatigue strength of the part. Though. Figure 2. When these areas are exceeded. otherwise the process cannot be carried out at the performance limit. a 100% test does not assure freedom of errors. slender end mills. rattling oscillations occur.7 shows a turbine disk which was exposed to high mechanical and thermal stresses caused by machining the drill holes. This can be done by means either of samples or of a 100% test. optical and electronic nature. distances between surfaces or angular positions of surfaces.g. Mechanical material measures may be. Measurement is the comparison of a characteristic with a measurement standard.2 Measuring and Testing 9 Testing means determining whether the test sample exhibits the required characteristics. visual and tactile tests. In contrast to this. Measuring instruments and gauges are distinguished by material measures corresponding to the measurand. Material measures can be of mechanical. e. The result is information regarding whether the specified requirements were satisfied or not (go or not-go). every measurement result is subject to error. thus the distinction between subjective testing and objective testing. The causes of measurement errors may lie in the measuring method. gauges and aids. Table 2. Possible reasons for measurement errors are shown in the ISHIKAWA diagram in Fig. This determination can be made in different ways. Gauging is the comparison of the test object with a dimension or a form.1 Survey of manufacturing qualities Standard tolerance IT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Primary shaping Die forming (warm) Extrusion (cold) Deep drawing Turning Drilling Reaming Milling Grinding Erosion 2. Subjective testing is performed by means of the sensory perception of the tester. divisions or counters. In objective testing. Indicating measuring instruments have movable markers. the measured value reading can be either direct analogue or digital. form and surface quality. the tester is supported by testing equipment. 2. measuring and gauging. the measuring . electric. When using these devices. Intermediate values cannot be defined. The result is a measured value.2. for example. such as dimension. Table 2. There are two further methods which fall under this category. Examples of this are the evaluation of burr formations on the edges of parts and of shadings on polished surfaces.2.1 Measurement Errors As a matter of principle.8. Testing equipment is subdivided into measuring instruments.1 provides a survey of the attainable accuracy values when using different manufacturing processes. the gauge body corresponds merely to the dimension or the dimension and form of the required characteristic. 2 Macro. It may differ from the real surface. In order to achieve a high measurement accuracy. or at least to estimate them in terms of their magnitude. It can principally be distinguished between systematic and random disturbances in measurement procedures.8 Causes for measuring errors by the measuring procedure instrument. see DIN EN ISO 4287) is the surface actually present on the part. The real surface (primary surface. DIN 4760 offers a general system for organizing structural deviations (Fig. 2.and Microgeometry of Components 2. 2. where applicable. 2. the measured object.2.2. as well [DIN1319a]. Macro-geometrical parameters refer to deviations of dimension. since every measuring method can only approximate the real surface.10 2 Metrology and Workpiece Quality Measuring method Preparation Unconsidered disturbances Aptitude Abbé principle error Preliminary arrangements Cleanliness Protocol Calibration Environment Temperature Air pressure Humidity Vibration Extraneous light Interference field Measurement error Aim of the measurement Testing deviation Measured object Measuring forces Evaluation Aptitude Routine Assembling Visual acuity Measuring uncertainty Aptitude Attention Mistake Measuring device Measuring person Fig. the person measuring. the distinction is often made between macrogeometrical parameters and the surface quality. A geometrically ideal surface is assumed in designs . Therefore it is necessary to define the term “surfaces” at the beginning. The transitions between these categories are not always clearly definable. The surface quality is defined by roughness parameters.2. the preparation involved and the environment.9). The actual surface is the metrologically registered surface. to minimize them. form and position.1 Structural Deviations With regards to components. compensate them. it is necessary to take the abovementioned influences into consideration and. 2.2 Measuring and Testing 11 Structural deviations (in superelevated representation) 1st Order: form deviations 2nd Order: waviness 3rd Order: grooves (roughness) 4th Order: scores, scales (roughness) Not easily representable 5th Order: textural structure 6th Order: lattice structures of the material Superposition of structural deviations 1st up to 4th Order Fig. 2.9 Structural deviations, acc. to DIN 4760 and forms the basis of tolerances. In Fig. 2.9 six orders of structural deviations are defined on the basis of these observations. Structural deviations of the 1st order (see also the following sections) are frequently the result of systematic errors. With regards to waviness, i.e. the structural deviations of the 2nd order, one cannot clearly define whether they are caused by systematic or random influences. The unbalance of a rotating tool and any periodical oscillations caused by it are forced, while sudden rattling oscillations are self-starting. In general, fundamentally different actions must be implemented in order to exclude any systematic or random causes of error. Structural deviations of the 3rd order also occur regularly. They are to be attributed to the penetration between tool and workpiece and are often determined by means of penetration calculations. Examples of these are kinematic roughness associated with turning, surface marks created in peripheral milling and generated cut deviations created in hobbing. In such cases, the structural deviations can be influenced in a targeted way by means of generation kinematics and tool design. The higher orders of structural deviation are primarily random in their occurrence. Examples of structural deviations of the 4th order include chip formation processes and removal processes. Roughness of the 5th order is rendered visible by structural properties on the surface. This can play a significant role in the high-precision machining of metallic optical mirrors. Thus in high-precision turning of multicrystalline metals, grain boundaries may become visible because the individual crystals exhibit varying orientations and therefore varying stiffnesses. In this case, anisotropism of the grains becomes visible on the surface. 12 2 Metrology and Workpiece Quality In general, all the structural deviations on a real surface are superposed. Filters are employed to separate roughness and waviness in a measurement process [DIN EN ISO 4287]. The following sections will treat macrogeometrical deviations of structure, form and position, as well as the corresponding measurement technology. Section 2.4 and the following sections describe the metrological recording of higherorder structural deviations. 2.2.2.2 Form Deviations Form deviations refer to deviations from a specified ideal geometrical property, such as straightness, evenness, roundness or cylindric form [DIN EN ISO1101, VDI2601]. The following will introduce some examples of form errors and their causes (Fig. 2.10). • A cause for deviations from the cylindric form of a workpiece can be the incorrect alignment of the workpiece on the machine tool with respect to the tool. • Another cause for form deviations is the continuous alteration over time of the tool geometry caused by wear. • Thermal displacements of the workpieces, tools and machine tools can also lead to form deviations. • Deviations from cylinder form can also arise when the workpiece deflects because of a radial strain. This can happen, for example, e.g. when turning long, slender parts when the workpiece is not supported. • Roundness errors can develop through the incorrect clamping of the workpieces on the machine tool. 2.2.2.3 Position Deviations Position deviations are deviations of the position of a geometrical element with respect to a reference, such as an edge, circulation line or axis of the predetermined position. In general, the position of two surfaces or axes in relation to each other is indicated by simple length or angle specifications. Fig. 2.10 Deviations from circularity and cylinder form 2.3 Length Testing Devices 13 Fig. 2.11 Deviations from concentricity, straightness and parallelism Position deviations as shown in Fig. 2.11 may arise, for example, through incorrect clamping or defective clamping devices. 2.3 Length Testing Devices 2.3.1 Material Measures A material measure in length measurement technology represents lengths or angles by means of fixed distances or angles between surfaces or lines. Setting standards are material measures. The material measures used most frequently are gauges. The most important and most precise material measures in length measurement technology are parallel gauge blocks. They consist of two plane-parallel measuring surfaces made of hardened steel or cemented carbide. Parallel gauge blocks are used in prismatic and cylindric forms. Gauge blocks are composed of grades. A standard set contains 5 grades each containing 9 blocks, from which almost any dimensions can be assembled (see Fig. 2.12). The measuring surfaces are superfinished and even enough to allow, after careful cleaning of the measuring surfaces, individual gauge blocks to be joined together so that they adhere. Fig. 2.12 Gauge blocks (Source: Mahr) 2.3.2 Gauges A gauge measures dimensions or forms generally according to limit dimensions [DIN2257]. It can be distinguished between inspection gauges, limit gauges and form gauges (Fig. 2.13). Go 2 Metrology and Workpiece Quality Gu 14 21,5 Fig. 2.13 Types of gauges: inspection gauges, form gauges, limit gauges Not go side Not go side Not go side Not go side Go side Go side Go side Go side Drill hole Shaft Square profile Multitooth profile Fig. 2.14 Different types of limit gauges Inspection gauges are parts of a gauge set for which the measure is assembled from the combination of gauges. Examples are the combination of parallel gauge blocks and the setting of feeler gauges. Form gauges allow for the testing of profiles using the light-slit method. They include angles, radius gauges and thread gauges. Limit gauges function according to Taylor’s principle: The go gauge must be designed in such a way that the dimension and form of the workpiece can be inspected when combined with the gauge. Not-go gauges are only used to inspect single dimensions. Limit gauges used most frequently are calliper gauges, limit plug gauges, gauging rings and thread gauges (Fig. 2.14). Gauges are the simplest testing equipment used in industrial manufacturing. 2.3.3 Indicating Measuring Instruments Indicating measuring instruments consist of a material measure and a display mechanism. The indicating range is the range of measured values that can be displayed on a measuring instrument. The measuring range is the part of the indicating range for which display deviations lie within specified or agreed limits. The suppression range of a measuring instrument is the range which must be exceeded in order that 2.3 Length Testing Devices 15 the instrument can begin to display values. The hysteresis of a measuring instrument equals the difference of the display values determined by measuring from different directions. The hysteresis of a measuring instrument is often not constant, which means that only a certain lower limit is indicated. The reversal error is principally a systematic error. It can be compensated. The sensitivity of a measuring instrument is the ratio of an observable change on the measuring instrument display to the causative parameter. Using length measuring instruments, the sensitivity is defined as the ratio of the range of the indicating element (e.g. the indicator) and the range of the measuring element (e.g. the spindle or measuring arm). Conversion between the measurement range and the display may be based on different physical principles. The most frequently used conversion elements are mechanical, electrical, pneumatic, optical and electronic conversion elements. 2.3.3.1 Instruments with Mechanical Converters Instruments with mechanical converters utilize mechanical or optical indicating elements. In length measurement technology, the most often used measuring instruments function on the basis of mechanical converters, e.g. threads, racks, gearwheels and blade segments (Fig. 2.15). With mechanical callipers with sliding glideways as the conversion element, the material measure is mounted on the bar on which the display, i.e. the slider, is moved. Mechanical callipers have two different scales which can be read off against each other by the observer. The scale on the slider is called a vernier (Fig. 2.15). In the case of micrometers, highly precise threads are used as converters and as a material measure. The system comprises a measuring spindle running in a sleeve, whose forward face is designed as the measuring surface and whose back end carries a ground thread as material measure. Typical thread pitches are 0.5 and 1 mm. As seen with callipers, two different scales are read off against each other. One scale is mounted on the sleeve, the other on the scale drum. In order to avoid influencing Fixed measuring jaw Measuring ambos Moveable measuring jaw Vernier Scale sleeve Thimble Clamp Bar Linear scale Fig. 2.15 Setup of callipers and micrometers Measuring spindle Coupler 16 2 Metrology and Workpiece Quality Prestressed spring Rack Arm with blade segment Pinion Plug gauge Fig. 2.16 Setup of dial gauges and dial comparator the measurements through large fluctuations in the measuring force, the latter is limited to 5–10 N by means of a coupler (ratchet coupling). Micrometers are designed both as outside micrometers for external dimensions and as inside micrometers for internal dimensions (Fig. 2.15). Dial gauges are length measuring instruments equipped with racks and gearwheels as conversion elements which allow a larger view of the path of the plug gauge. The material measure lies in the gear mechanism, the conversion element which leads to measurement reversal errors due to anisotropic friction and potential tolerance. Needle positions greater than 360◦ are possible (Fig. 2.16). Dial comparators are the most precise mechanical length measuring instruments. They are equipped, unlike dial gauges, with a gear mechanism as lever arm system, gear segment and pinion as conversion elements. Thereby, the movement of the plug gauge is transmitted to the needle. The design only allows needle positions smaller than 360◦ (Fig. 2.16). In principle, all the instruments with mechanical conversion elements described here can be equipped with frictionless optical indicator elements in order to improve precision. Given constant conversion behaviour, the more precise indications the measuring instrument should display, the longer the indicator must be. However, since there is only a limited amount of space in the instruments and, above all, since the inertia of the indicator can lead to distorted measurement results when the measurement range is small, optical indicator elements are used. The principle is basically this: a beam of light is directed to the material measure and, according to the position of the material measure, this beam is reflected in a different direction. The optical indicator then displays the measurement range of the material measure without friction and inertia. Further potential for increasing precision can be exploited by means of a fine graduation of the scale which can no longer be observed with the naked eye, thus necessitating optical magnifying aids in the form of microscopes. By means of the microscope, a mechanism consisting of an objective and an ocular, a magnification of a real image by the objective can be viewed with the ocular. In this way, a remagnified virtual image of the object is created. The total magnification through 2.3 Length Testing Devices 17 a microscope is the product of the objective and ocular magnifications. The scale graduation can be applied in the ocular directly or by means of an ocular plate. Microscopes including measuring oculars are called measuring microscopes. 2.3.3.2 Instruments with Electrical Converters These instruments are used to determine changes in the measurand caused by alterations of electrical properties, such as resistance, inductance and capacity. The turns of a coil, the distance between plates of a condensator and the resistance of an electrical conductor can all serve as material measure. Inductance, for example, is altered by changing the immersion depth of an iron core; the capacity of a plate capacitor is a function of the plate distance (Fig. 2.17). Position encoder Sensor Output to measuring amplifier Position encoder L1 R4 L2 R3 s UA UB = Bridge-output voltage = Bridge-supply voltage = Capacity = Measurement range ε o = Electric constant ε r = Dielectric constant A = Surface area d = Distance UA UB C s Fig. 2.17 Capacitive and inductive measuring principles C= ε o˙ εr˙ A d UA =C ˙ s UB 18 2 Metrology and Workpiece Quality Inductive position encoder Fig. 2.18 Inductive working sensor in real execution An alteration of the ohmic resistance can be effected by changing the conductor length and/or by a cross-sectional variation [Hoff04, Grot05]. Figure 2.18 shows a real execution for multi-point measuring with inductive working sensors. 2.3.3.3 Instruments with Optical Converters These length measuring instruments exploit the wave property of light. The wavelength of light is used as the material measure. Figure 2.19 shows the functional principle of a laser interferometer. Reference path Wedge prism of the reference beam Measuring path Displacement Laser Mirror Polarizing beam splitter Interference of measuring und reference beam Detector Fig. 2.19 Longimetry with laser interferometer Wedge prism of the measuring beam Length Testing Devices λ1 λ3 λ2 ϕ3 Time Ampl. ϕ2 ϕ3 19 Ampl. Ampl. 2.3 λ Time Time ϕ2 l Light mixture of different wavelengths Monochromatic and coherent light Monochromatic light λ ≠ const. ϕ ≠ 0 λ = const. ϕ = 0 λ = const. ϕ ≠ 0 Ampl. Ampl. λ Time Time Interference of two coherent light waves with the same phase length (Addition of illuminance) Interference of two coherent light waves out of phase (destructive interference) λ: Wavelength ϕ : Phase difference Fig. 2.20 Optical measuring principle, interference A light source emits coherent, monochromatic light whose beam is split into a reference beam and a measuring beam at a polarizing beam splitter. These beams are reflected on a respective wedge prism and combined again through superposition. Finally, the combined beam is detected and evaluated. If the wedge prism of the measuring beam is shifted, interferences occur because of the difference of the optical paths of the two partial beams after combining them. These are fluctuations in light intensity via eliminating and amplifying light. Integral multiples of the wavelengths lead to an amplification, phase differences of 180 degrees to an elimination of light (Fig. 2.20). The number of interference lines is directly related to the path being measured. Since the wavelength of the light is a function of temperature and also air pressure and humidity, these influences are compensated within the device. The advantages of such systems lie in their large measuring range and especially in their contactless measurements. 2.3.3.4 Instruments with Pneumatic Converters These instruments use pressure and flow rates as material measures (Figs. 2.21 and 2.22). This measuring principle can be used for both tactile and contactless measurement (Fig. 2.22). There are two types of such measuring instruments: high-pressure and lowpressure instruments. High-pressure instruments work with an operating pressure greater than 0.5 bar, low-pressure instruments with an operating pressure lower than 20 2 Metrology and Workpiece Quality Volume measuring method 1. Air filter 2. Pressure control 3. Flowmeter 4. Measuring nozzle Velocity measuring method 1. Air filter 2. Pressure control 3. Venturi-nozzle 4. Flow of valve 5. Measuring nozzle 6. Differential pressure manometer 1 3 2 4 s 1 2 3 4 h 6 5 s Fig. 2.21 Measuring principle of pneumatic distance measurement Air inlet Valve Guide body Measuring nozzles Follower Drill hole Work piece Tactile measurement Contactless measurement Fig. 2.22 Pneumatic sensor 0.1 bar. The pressure range between 0.1 and 0.5 bar is outside the operating range of the instrument [DIN2271a]. Figure 2.23 shows the functional principle of contactless pneumatic measurement. The left side shows an external measurement, the right side an internal measurement. If the measuring rod is designed in a way that both measuring nozzles 3.4 Surface Inspection The task of characterizing a technical surface consists in attaining a complete topological record of its three-dimensional geometry and describing it. dial gauges and dial comparators. The measurement of freeform surfaces and tooth-flank topographies on spur gears and bevel gears with coordinate measuring devices is currently state of the art (Fig. deviations from hole roundness and diameter can be determined by turning the rod or the workpiece. then an exact centring in the hole is unnecessary. Therefore. This path measurement system is also executed with callipers. especially if a 100% test is required.5 Electronic Measuring Instruments These devices work with photoelectric sensors. in order to simplify the measuring process.2. only parameters for surface roughness along a single measured length are registered (one-dimensional parameters). 2. It is also realized in coordinate measuring devices and machine tools. The advantages of contactless pneumatic measurement lie in the self-cleansing effect of the measuring device (i. The measuring rods are often specially adjusted to the measurement of a specific workpiece. an exact control system and effective software are required which can influence the quality of the measuring result and the applicability of the data [Pfei01]. The material measure is realised through the light-dark fields. Since the type of hole measurement described is a two-point measurement. To be precise. 2. the workpieces do not generally have to be cleansed of oil. it is a mechanical guidance system whose positions are detected and indicated optoelectronically. Often. by which light-dark fields of a gauge are converted into electric signals.24). 2.23 Functional principles and examples for pneumatic measurement work within the linear range of the characteristic diagram. since the air currents of the two nozzles are added and the sum represent the measure for the total gap width. pneumatic measurement is used predominately in serial production. This . 2. dirt or micro-chips) and in the quickness of the measuring process. Impulse gauges and code gauges are preferably used as path measurement systems.e. for economical reasons.4 Surface Inspection 21 ps ∅dm pa Annulus Measuring nozzle pa Test sample h Work piece surface Measuring nozzle Fig. To control measuring axes and to evaluate data.3. 25 Registration of structural deviations by surface cuts Cross section parallel to the tangential cross section .g. 2. The surface can be detected by means of surface sections or the bearing surface (Fig. a profile section will be used to illustrate some of the basic terms of surface inspection technology (Fig. multi-dimensional surface descriptions must be used. 2. First. For certain applications. These sections must be statistically representative for the entire surface [DIN2257].24 3-D coordinate measuring machines procedure is frequently followed in practice and is sufficient in many cases.4. With respect to a profile.25) [DIN4760]. tangentially or at other angles. in optics or for characterizing grinding wheel surfaces. the distinction is made between the test length Lt of the surface section being metrologically detected and Profile Tangential cross section Profile α Vertical cross sections Diagonal cross section Fig.1 Surface Parameters In most of the procedures used in industrial surface inspection technology. e. Profile sections are sections which are oriented to the surface normally. 2.26). 2. the following will exclusively treat one-dimensional parameters. more comprehensive. However. only structural deviations of the second or higher orders are analyzed and measured on surface sections. 2.22 2 Metrology and Workpiece Quality Fig. 26). 2. The detected profile (actual profile) of a surface depends on the measuring procedure and the filter used.4 Ra Zv2 Zv1 Zt1 x Measured length L Fig.75 mm (Rauheit) R-Profil(roughness) R-Profile 250 µm 250 µm Fig. 2.Surface Inspection L1 L2 L3 L4 L6 L5 Zp1 Z (x) 23 Zp6 2. different profiles can be determined [DIN EN ISO 4287]. These are the P-Profile (primary profile). This is oriented in such a way that the surface areas above and below the middle profile line are equally large (Fig. 2.27). which is used for evaluation (L < Lt ) [Grot05]. The reference profile shifted within the measured length perpendicularly to the geometrically ideal profile is defined as the middle profile.26 Fundamental terms of surface inspection technology the measured length L. Manufacturing technology: Short-stroke honing Average roughness: Ra = 0.27 Separation of waviness and roughness by wave filter . thus it only represents an approximate image of the actual surface. 2.75 mm Grenzwellenlänge: Cutoff wavelength: λλ== 0. By using different measuring procedures and depending on which filter is used. the R-Profile (roughness profile) and the W-Profile (waviness profile) (Fig.75 Scanning system: HAT 25/6 Scanning path: 10 mm P-Profile (waviness and roughness) µm 1 1µm Without wave filter 250 µm 250 µm W-Profil (Welligkeit) W-Profile (waviness) µm 11µm Low-pass filter 250 µm High-pass filter µm 1 1µm 250 µm 0. the bearing ratio curve – also called the ABBOTT-FIRESTONE curve – can be determined (Fig. The bearing ratio of the roughness profile Rm r(c) is calculated as follows:  Xi L Rm r(c) = (2.28). According to DIN EN ISO 4287.1) 0 Horizontal parameters (distance parameters) are also designated as bearing lengths. to DIN EN ISO 4287 0 20 60 100 Rmr (c ) / % . 2. Forming a ratio of of the summed single bearing lengths and dividing by the measured length yields the relative bearing length (material ratio) in a specified section depth c.2) c Z(x) x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x Measuring length L Fig.28 Bearing ratio curve of the profile. By creating sections at different depths c.24 2 Metrology and Workpiece Quality The examples given below refer to the R-Profile (roughness) (Fig. • The total height of the profile Rt : the sum of the highest profile point and the depth of the deepest profile valley within the measured length L. acc. 1 Ra = L L |Z(x)| dx (2.26). • The greatest height of the profile Rz : the sum of the height of the highest profile point Rp and the depth of the deepest profile valley Rv within a sampling length Li . • The depth of the deepest profile valley Rv : value of the y-coordinate Z(x) of the deepest point of the profile of the middle profile line within the sampling length Li . • The mean roughness value Ra : the arithmetic mean of the values of the y-coordinates Z(x) within a sampling length Li . They are determined by means of tangential sections. the following roughness parameters can be distinguished: • The height of the highest profile point Rp : value of the y-coordinate Z(x) of the highest profile point of the middle profile line within the sampling length Li . 2. 2. one must consider the fact that it is not necessarily possible to deduce the manufacturing process when using one-dimensional parameters for describing the surface [Abou76].1 0. Table 2.4 0. which manufacturing process is to be used to create that parameter.04 Achievable average roughness Rz / µm .25 0.4 Surface Inspection 25 The bearing ratio can be calculated on the basis of the primary profile (Pm ). Due to the characteristic engagement conditions between the workpiece and the tool one would also have to specify.5 1 0. Table 2. This problem is somewhat alleviated if multiple one-dimensional surface parameters are used instead of just one when describing the manufactured surface or if surface reference standards are available (Figs. for each surface parameter. With respect to general specifications on achievable surface values. the roughness profile (Rm ) and the waviness profile (Wm ). 2. 2. Using the average roughness.30).29 Surface quality for several manufacturing operations 630 1000 400 300 250 25 160 16 10 4 2.2.29 and 2.2 Achievable average roughness Primary shaping Die forming Extrusion Turning Drilling Reaming Milling Grinding Erosion Rough and finish turned Honed Planed Grinded Fig.2 provides an overview of the surface roughness values which can be achieved with different manufacturing processes. Face milling Cylindrical turning 2.5 0.4 3.2 1.6 0.8 Roughness Rt (µm) 12.5 6.3 Horizontal milling Arithm. average 50 roughness Ra (µm) 32 16 8.0 4.0 N6 N5 ISO R 1302–1971 N3 N2 N8 N4 Reaming Flat grinding N7 N5 N6 N10 N9 N7 Flat lapping Roughness markl Surface mark DIN 140 0.05 0.4 0.2 0.1 Roughness Rt (µm) 10 6,0 Arithm. average 1.6 0.8 roughness Ra (µm) 0.55 2 Metrology and Workpiece Quality 3.0 1.6 1.0 26 Fig. 2.30 Surface reference standard (Source: Rupert) 2.4.2 Subjective Surface Inspection In some cases, a visual inspection can determine whether the processed surface meets the specified requirements. A testing device frequently used in the workshop is the straight edge, which is used to check the evenness of a surface using the light-slit method. In workshop operations, spotting straight edges or spotting plates are used to detect form deviations or contact patterns on the workpiece surface (Fig. 2.31). Impression methods using lacquers have been developed for surfaces which are difficult to access (e.g. drill holes, internal surfaces). The lacquer is applied onto the Spotting medium Black areas Light areas (spotting medium) (lowest points) Work piece Spotting plate Spotting straight edge Fig. 2.31 Surface inspection by spotting Grey gleaming areas (bearing points) 2.4 Surface Inspection 27 test surface and pulled off after drying. The contact patterns attained on the lacquer film thus attained can then be evaluated. A frequent use of spotting is detecting contact patterns in quality testing and in the assembly of gear teeth (especially bevel gear teeth). Applying spotting paste and then rolling the teeth under light strain causes the bearing points on the tooth flanks to become visible. These bearing points can then be compared to the reference contact pattern. Nowadays in individual cases machine-tool guiding elements also become scraped, therefore, spotting is used to judge the surface. 2.4.3 Surface Measurement All the physical principles mentioned in the context of length measurement technology are also fundamentally applicable to surface measurements. Since mechanical and optical measuring methods have proven to be the most effective in this field, the following discussion will limit itself to these methods. 2.4.3.1 Mechanical Measuring Methods Devices for surface measurement which function on the basis of a mechanical functional principle are generally referred to as stylus instruments. Mechanical measuring methods are classified as either scanning methods or sensing methods. In the scanning method, a contact stylus descends upon the surface to be tested at a specified frequency. The surface is guided under the needle with a constant feed rate. The path of the contact stylus can be visualized mechanically, optically, electrically or electronically. The contact stylus can either be raised to a fixed level (the WOXEN principle) or raised by a fixed amount from the respective point of impact to the surface (differential tactile procedure) (Fig. 2.32). In the differential tactile procedure, the impact energy of the contact stylus is lower in comparison to the WOXEN principle and exhibits marginal dispersion. As a result, the penetration depth of the needle remains constant, the measuring accuracy being thus higher than in the case of the WOXEN principle. In the case of instruments functioning according to the sensing method, the contact stylus is guided continuously over the surface. The needle rises and falls in line with the profile pattern Scanning method Woxen principle Differential tactile procedure Fig. 2.32 Mechanical measuring methods Sensing method Differential tactile procedure 28 2 Metrology and Workpiece Quality Plane of reference system Oscillating scanning system Caliper slide Semi-rigid System Scanning arm Caliper slide Caliper Caliper Table Skid Caliper Skid Fig. 2.33 Different surface scanning systems (Fig. 2.32). The lifting motion is indicated relative to a reference point defined in the device or to a reference level. Here too, mechanical, electric, optical and electronic converters are used. Sensing profile methods are the most widespread in practice. Independently of the design type of the contact stylus instruments used, three system designs are distinguished (Fig. 2.33). Plane of Reference System In this system, the scanning unit is guided on a reference surface (plane, cylinder) which corresponds to the ideally geometrical surface of the test sample and is oriented along the surface to be measured (Fig. 2.33). Aside from errors arising due to the calliper geometry, this scanning system provides a faithful transmission of the roughness and waviness values of the test sample. When measuring small or very large surfaces, however, the handling of this scanning system can frequently become unwieldy. An alternative is the reference surface contact system. Here, the workpiece is conveyed on very precisely guided slides in a horizontal direction beneath the firmly anchored scanning system. Besides roughness, the macrostructure of a surface can also be detected in certain areas. Semi-Rigid System In semi-rigid systems, the scanning unit, which contains the contact stylus, is guided on the skid gliding on the surface to be measured (Fig. 2.33). This system has the advantage that it requires little space and is thus suitable for measuring small or hard-to-access surfaces. A disadvantage is that the system requires an orientation to the surface to be measured which can cause parts of the profile to be transmitted in a distorted manner. Oscillating Scanning System Two glide skids guide the scanning unit of the oscillating scanning system (Fig. 2.33). The latter is oriented towards the surface to be measured and is thus 2.4 Surface Inspection 29 comfortable to operate. However, it requires more space than the single skid system and can therefore not be used for small or hard-to-access surfaces. The distortions caused by long-wave profile sections are smaller than those in half-rigid systems, because the skids are flat and farther apart. Nevertheless the waviness must be filtered out in many cases [Henz68]. Usually, the magnification gauges for x- and y-coordinates are selected in a highly varying way in profile records. This is necessary because the measuring lengths (abscissa) lie in the mm region and roughness parameters are represented on the ordinate which lie in the μm region. The optical impression of the profile record is thus strongly distorted in comparison to reality. The contact styluses used in contact stylus instruments often have an apex radius of 2 μm. They function with a bearing strength of 0.5 N, which can result in considerable surface pressures (up to 6000 N/mm2 ), which possibly cause alteration of the test surface. On the other hand, excessively large contact stylus radii distort the result; they act like mechanical filters. Thus the optimal conditions must be determined on the basis of the material of the test sample and documented in their entirety in the measurement report. 2.4.3.2 Optical Measuring Methods White-Light Interferometer This technology differs from length measuring technology both in that it employs white light, i.e. light with the entire wavelength spectrum, and in that it does not just use one beam, as with laser interferometers, but rather an entire bundle. A reflectedlight microscope is used to display an image of a section of the test object on a detector (e.g. CCD camera). By using different interference lenses, a beam splitter can be used to superimpose a highly accurate reference surface with the image of the test object on the same scale. The interferences which are then created can be detected and evaluated (Fig. 2.34). The topography of the test object creates a spatial modulation of the light intensity in the interference image. Depending on the surface to be measured, two different measuring modes are used: one to characterize very flat surfaces with an average roughness value Ra < 1 nm and one for all other surfaces. Steps and roughnesses of up to several millimetres high can be displayed using this technology. Fringe Projection The fringe projection method functions according to the triangulation procedure, in which equidistant stripe patterns are observed and evaluated at a certain angle, i.e. the triangulation angle. The stripe patterns on the test object are detected and evaluated from a certain position, with the projected stripes following the arbitrary form of the test surface. These stripes appear from the observer’s standpoint to be “interferences”, though they only appear this way because the location of the projection 2 Metrology and Workpiece Quality x Displacement Reference mirror Test object Displacement 30 Light Lenses z Interference of measuring and reference beam Detector Fig. 2.34 Principle of a white-light interferometer (two-beam interferometer) surface points varies in relation to that of the projector. The stripe patterns then are evaluated interferometrically on the basis of fluctuations in light intensity. The most important geometrical values on which this method is based are: • the real distance of the projected stripes • the stripe distance registered from the position of observation • the triangulation angle Fringe projection is used, for example, to judge surfaces bent over a large area, such as those found in deep-drawing tools and on deep-drawn parts. The advantages of this method are a high measurement speed and a spatial or laminar scanning. A disadvantage is that the test objects may not be transparent or reflective. 2.5 Inspection of the Workpiece Rim The functional behaviour and applicability of a component depend not only on its macrogeometry and surface roughness, but also from the physical properties of the material both on the interior and near the surface. While the inspection of material properties properly belongs to the field of materials science and materials testing and thus cannot be treated here in further detail, the following will discuss some of the properties of technical surfaces and of layers near the surface (i.e. the rim zone) and methods used to measure rim zone properties. Technical surfaces can be categorized according to types of load into the following groups: 2.5 Inspection of the Workpiece Rim 31 • external surfaces of technical products of all types, i.e. visible surfaces, covering surfaces, indicating surfaces, etc. They are generally mechanically unstrained, but are exposed to climatic or environmental stresses. • surfaces subjected to heat, radiation or electrical currents, such as insulating surfaces, electrical contacts, or the like. Such surfaces are referred to as thermally stressed, radiation stressed or electrically stressed surfaces. • surfaces which come into contact with fluids or gases. On the one hand, a corrosive stress may be predominant, or a flow stress with cavitation and erosion processes may occur in the case of flowing media. Also, surface boundary currents may be influenced via microstructures (ribblets). • surfaces in mechanical contact with moved counter bodies. This strain is referred to as tribological stress. These stresses are found, for example, in typical machine elements, such as bearings, couplings, brakes, gear-teeth, etc. With this type of stress, different kinds of wear may occur. • optical surfaces used to form and conduct electromagnetic waves. Optical surfaces are produced with mirrors and transparent components. The basic beam-conducting and forming principles are reflection, refraction and diffraction. • biologically stressed surfaces exposed to the effect of microorganisms. 2.5.1 Surface Layers The properties of technical surfaces relevant to component behaviour are determined through the entirety of the physical and chemical properties of the surface layer. These properties include textural structure, hardness, strength and residual stresses in the rim zone near the surface. The surface rim zones of technical bodies are created through machining processes. A part of the energy used to create the surface always flows into the workpiece and is either stored or causes change processes in the base material. Thus every machining process also causes a change of the surface rim zone vis-à-vis the base material. Whether these changes affect the functionality of the workpiece must be tested and confirmed in individual cases. The distinction is frequently made between the external and the internal boundary layer. Figure 2.35 shows the structure of both boundary layers. The layers referred to here cannot be clearly defined; there are no fixed boundaries between them. For further reference, please refer to the following sources: [Schm36, Schl51, Czic03]. 2.5.1.1 The External Boundary Layer The external boundary layer is located between the surrounding atmosphere and the atoms of the base material embedded in the crystal lattice. It is generated via the reaction between the material and the atmosphere during and after machining. The external boundary layer encompasses the following individual layers: 32 2 Metrology and Workpiece Quality External boundary layer Grease-/ oil film 10–100 µm Adsorption layer 0.3–0.5 nm Oxide film 1–10 nm Internal boundary layer > 5 μm Plastic deformed and hardened Elastic und plastic deformed Elastic deformed Matrix Fig. 2.35 Layer structure of metallic surfaces, acc. to SCHMALTZ [Schm36] Reaction Layers/Oxide Film On the surface of metals which are either freshly machined or created through breakage can form thin reaction layers/oxide films. These layers may only have a thickness of 10–20 molecular layers. They may, for example, influence wetting capability or adhesive behaviour. Adsorption Layer After a short duration of time of being exposed to the air, the oxide layer is covered with an adsorbed layer of water and gas. This adsorption layer becomes considerably important, for example, in the context of electrical resistance when metallic contacts are present. Grease/Oil Film Coolants in particular cause a grease and oil film to deposit itself after machining on the adsorption layer. This film ranges from 10 μm to several 100 μm in thickness. A grease and oil film can still be detected even after cleansing. These films can exert considerable influence interface formation in further galvanic, adhesion, or PVD/CVD processing. 2.5.1.2 The Internal Boundary Layer The internal boundary layer is the layer of the machined workpiece bordering on the base material and thus possesses practically the same chemical composition as the 2.5 Inspection of the Workpiece Rim 33 Sharp-edged tool Worn tool Cutting tool material: HC Material: C15 vc = 200 m/min f = 0.1 mm 50 µm 50 µm Fig. 2.36 Plastic deformation of the workpiece rim by drilling base material. The physical structure and the expansion of this layer in the direction of the workpiece interior depend both on the material used and the manufacturing operation used. The transition from the affected rim structure to the unaffected base structure is continuous. Figure 2.36 shows a model of the plastic deformation of the structure of case-hardened steel C15 after a drilling operation. Larger mechanical and thermal stresses caused by worn tools lead to clear modifications of the textural structure. In steel material processing, for example, given high enough temperatures and high cooling rates, a hardening zone may develop in the outermost surface layer. In the layers below this, annealing processes are possible due to the lower temperatures and cooling rates in these layers (Fig. 2.37). As shown in Fig. 2.37, a hardening zone has developed which has an extremely negative effect on the flank bearing strength. 2.5.2 Inspection of the Surface Rim Zone Since, in addition to surface geometry, the physical properties of the areas near the surface play an essential role, these properties must be tested, too. Therefore methods familiar in the field of materials testing are applied: • • • • • material analysis structure and texture investigation hardness testing fracture mechanical testing residual stress measurement Structural changes caused by increased thermal stresses of the rim zones near the surface can be determined in structure and texture investigations which are standardly used in materials science. This is done by metallographic preparations. Crack detection in rim zones of workpieces can be either destructive or nondestructive. A crack is a locally limited detachment of the material structure of small width but often considerable length and depth. The crack can arise through internal 34 2 Metrology and Workpiece Quality Pitting at the tip of a tooth RF 200 µm Grinding burn with new hardening zone left flank LF 200 µm Vickers hardness / HV0,05 1000 900 Right flank 800 RF 700 LF 600 500 df dw Left flank Left flank 400 300 0 2 4 6 8 Distance from edge d / mm 10 Fig. 2.37 Grinding burn during gear tooth grinding tensions or through external force effects. For further reference, please refer to the specified literature [Schu04]. 2.5.2.1 Residual Stresses Residual stresses are characteristic for the internal tension of a load-free workpiece. This tension results from inhomogeneous elastic and elastic-plastic deformations which are present without the influence of external forces and torques. For every component, there is a balance of all the internal forces and torques created by residual stress. Residual stresses are subdivided into macro-residual stresses (type 1) and micro-residual stresses (types 2 and 3). Residual stresses caused by mechanical and thermal influences is treated in the chapters devoted to the relevant manufacturing operations. The basic formation mechanisms of thermally and mechanically induced residual stress are shown in Fig. 2.38. Residual stresses become significant in the context of dynamic stresses. The attempt is therefore often made to use manufacturing processes in finishing which create compressive residual stresses in the surface rim zone in order to minimize the probability of crack formation and crack growth. Such operations include, for Compressive residual stress (mechanical caused) Plastic deformation Yielding point Compression(–) sE– Tensile residual stress (thermal caused) High-temperature limit of elasticity sE+ ain Str 35 Tensile (+) Inspection of the Workpiece Rim e s lea Re Deformation path Yielding point Compression (–) Stress Stress Tensile(+) 2.5 Co oli n He g Temperature ati ng High-temperature limit of elasticity Plastic deformation Fig. 2.38 Formation of residual stress example, finish and surface rolling as well as shot peening. In contrast, tensile residual stress is probable in electrical discharge machining, because of its thermal operating principle; in grinding and all manufacturing operations which use geometrically defined cutting edges, both mechanical and thermal load spectrums occur during processing, a prediction of the stress state is not easily possible. Methods of Analysis For all processes, the metrological definition of residual stresses occur indirectly through the measurement of strains, electromagnetic parameters or speed of sound. Below, the basic methods used in manufacturing technology will be explained. For further information on this topic, please refer to the specified sources [Peit92, Hauk87]. Mechanical Methods The mechanical methods of analysis include the borehole method and the toroidal core method. These are destructive methods which release residual stresses in the rim zone near the surface through the insertion of very small holes or ring grooves in this zone, which causes an altered stress state and thus strains, which can be measured with wire strain gauges, for example. The average residual surface tension can then be calculated from the strains in consideration of the material behaviour. By removing layers of the surface rim zone at different surface positions, distributions of residual stresses can be defined up to the depth corresponding to the drill’s diameter. Since drilling or milling processes only cause reproducible strain changes at a depth of several hundredths of millimetres, this method cannot be used to determine residual surface stress with strong rim zone gradients. 2. The BRAGG condition applies here: n · λ = 2 · a · sin  (2. λ Source of radiation Detector ϑ ϑ ϑ a Fig.3) Altered lattice distances a of the material phases can be calculated very accurately from the displacement of the diffraction lines. 2. the elastic properties of the material phases are used.40 Goniometer X-ray tube .36 2 Metrology and Workpiece Quality X-ray and Neutron Diffraction In research and practice methods are used which utilize the diffraction of monochromatic X. In the case of many materials. The chosen measuring direction defines the stress components to be detected.40 shows the customary device for measuring residual stresses (goniometer). In order to calculate residual stresses from strain measurements. their anisotropic behaviour can even be taken into consideration when measuring at the respective lattice levels. Figure 2.or neutron radiation on the crystalline lattice structure of the material phases (Fig. This allows the definition of the entire stress tensor by measuring in multiple directions. 2.39).39 Principle of X-ray diffraction on crystalline lattice Swivel table Specimen Rotating table Detector Fig. which in turn allow to draw conclusions about residual stresses. neutron sources can also be accessed at selected locations. The applicational potential of these methods are limited above all by the material state. Such methods are based on the assumption that the sound velocity is proportional to the lattice strain. X-ray diffraction is used in practice to define residual stresses. superposition permeability. The X-ray method has a depth of penetration which is limited to an extent depending on the absorption properties of the material and the wavelength used for the X-radiation. Depending on the material and the range of frequencies analyzed. dynamic magnetostriction and magnetic and acoustic BARKHAUSEN noise. Magnetic Methods Magnetic analysis methods use the influence of mechanical stresses on resetting in ferromagnetic materials. Magnetic methods are characterized by compact and easily manageable test set-ups. Acoustic Methods Residual stresses are generated through lattice strains of the material which result in an altered propagation of structure-borne sound. These changes can be measured by means of an externally applied alternating magnetic field. with which it is possible to detect depth structures up to some millimeters without removing surface layers. Disadvantegous is that these methods they are limited to ferromagnetic materials and have to be calibrated to known material states. Residual stress depth profiles with great rim depths can be produced through the electrochemical removal of layers of the surface rim zone. This acoustic-elastic effect can be used to analyze stresses by means of ultrasonic measurements.5 Inspection of the Workpiece Rim 37 Diffraction methods demand high requirements on measurement technology and on occupational safety (radiation protection). The simultaneous analysis of several magnetic parameters improves the evaluation of the quantitative measuring data. Given the usual measuring parameters. significantly greater depths of penetration are possible for light metals. In research. the average penetration depths vary between approximately 1 μm for tungsten carbide and 5 μm for steel.2. . surface rim zones can be gauged ranging from a few micrometers to over 1 mm. This is because microstructural errors and textures exert as great an influence on sound velocity as structural gradients. The measurands are coercive field strength. C Springer-Verlag Berlin Heidelberg 2011 DOI 10. in which layers of material are mechanically separated from a workpiece in the form of chips by means of a cutting tool.  39 . cutting edge S. Klocke.2 (machining with geometrically defined cutting edges) and 3.1 shows an idealized cutting wedge [DIN6581]. Several concepts and terms are necessary to describe cutting part. designations and terms used to describe the geometry of the cutting part are set down in DIN 6581. Figure 3. In the standard. geometry and position to the workpiece are determined.1 The Cutting Part – Concepts and Terms The concepts. Manufacturing Processes 1. RWTH edition. The idealized cutting wedge is made up of two faces: A rake face and a flank face. Machining is defined as follows: cutting. which cut in a line. The angle between these two faces is designated as the wedge angle β. of which the cutting edge number. Deformation is achieved by means of a relative motion between the tool and the workpiece that brings about a transfer of energy [DIN8580]. What all processes in the group of geometrically defined cutting edges have in common is that they use a tool.Chapter 3 Fundamentals of Cutting DIN 8580 defines machining as all process variants of the third main group “Cutting”. in which form is altered by means of reducing material cohesion. The rake face Aγ is the face of the cutting edge where the chip runs off. These terms make it clear that the cutting wedge F. According to DIN 8580. this basic classification is further refined in order to categorise machining procedures [DIN8589]. which is turned towards the new workpiece surface (the cut surface). The first part of this compendium on manufacturing processes deals exclusively with process variants of Group 3. the term “machining” will only be used in the sense of cutting with geometrically defined cutting edges. For this reason. The cutting part is the active part of the tool where the cutting wedges are located with the cutting edges. machining comprises Groups 3.3 (machining with geometrically undefined cutting edges) in the manufacturing classification system.2. 3. which will be described in the following.1007/978-3-642-11979-8_3. The flank face Aα is the face on the cutting wedge. 3. in the simplest case of one major cutting wedge and one minor cutting face (Fig. the selected cutting point. . 3.1 Description of the idealized cutting wedge (tool) should always be regarded in connection with the workpiece. To position the components of effective cutting speed clearly. two angles are defined: • The effective cutting speed angle η as the angle between the effective cutting direction and the direction of primary motion (see Fig. Generally. This means that considerable importance should be attached to process kinematics. the workpiece is assumed to be fixed. 3. Usually. more complex tools are used.1) • The feed motion angle ϕ as the angle between the feed direction and the direction of primary motion (see Fig. The resulting velocity vector is designated as the effective cutting speed ve . It can be divided into two components. composed of several cutting wedges.2). One model concept is often used for the sake of a simplified description of process kinematics.40 3 Fundamentals of Cutting Trace of plane of rake face Aγ Chip Tool Trace of plane of flank face Aα β Cutting speed vc η ϕ Workpiece Effective cutting speed ve Feed velocity vf Trace of plane of cutsurface Fig. At the selected cutting point. This curvature is described by the cutting edge radius rß .1) Since there are no ideally sharp tools in practice. In almost all cases the transition between flank and rake face is curved. 3. This model simplifies the actual kinematics by summarizing the spatial velocity fields in one point. cutting edge rounding is taken into consideration (Fig. the velocity fields can be represented in a summarizing fashion by means of vectors. the cutting velocity vc in the cutting direction and the feed velocity vf in the feed direction. that of the selected cutting point. Up to this point.3). 3. the concepts have been explained using a simple cutting wedge formed by two faces. all motions are carried out by the tool. These vectors can be summarized in turn by vector addition in one total vector. acc.2 Cutting edge with cutting edge radius Direction of primary motion Tool shank Rake face Aγ Direction of feed Major cutting edge S Major flank face Aα Minor cutting edge S′ Cutting edge Minor flank face A'α Fig. the minor cutting edge S towards the machined face [DIN6580]. If the selected cutting point is on the minor cutting edge. 3. to DIN 6581 Correspondingly. 3. 3. we speak of major and minor cutting edges.1 The Cutting Part – Concepts and Terms 41 Trace of plane of rake face Aγ Chip Tool Trace of plane of flank face Aα β Cutting edge radius vc rβ η ve ϕ vf Workpiece Fig. The major cutting edge S is always turned towards the cut surface.3. .6). The flat areas of the major and minor cutting edges are joined by the corner radius r (Fig. the concepts defined in the former are named accordingly and furnished with an apostrophe ( ).3 Cutting edges and faces of the wedge. right). the reference plane of the tool-in-use system is oriented orthogonally to the effective direction. The reference systems are in agreement when the cutting direction corresponds to the effective direction. In contrast. left. to DIN 6581 Pre . In actual cutting processes however. In the tool-in-hand system. position and direction of motion of a cutting wedge. all planes contain the selected cutting point.2 Reference Systems In order to describe the location. reference systems are utilized in which characteristic planes are defined that are valid for all process variants.5): • The basic plane.42 3 Fundamentals of Cutting 3. First. The tool-in-hand system was developed for tool design as well as for the production and testing of cutting tools.4 Tool frame of reference (left) and tool-in-use system. the tool angle is measured without considering process kinematics. Assumed direction of primary motion Effective cutting direction Pfe Pp Pf Direction of primary motion ve vc Direction of feed Assumed direction of feed vf Ppe Pr Fig.4.4. 3.4). the reference plane of which is oriented orthogonally to the assumed cutting direction. 3. acc. the effective angle deviates from the nominal tool angle under certain circumstances due to process kinematics (exception: the wedge angle). 3. In both systems. It contains the rotation axis (if present) and lies perpendicularly to the assumed cutting direction. The tool-in-hand system is a system. 3. and 3. the tool-in-hand system will be considered (Figs. The two standardized reference systems are the tool-in-hand system and the toolin-use system (Fig. is the tool reference plane Pr . For this reason. a tool-in-use system is also defined (Fig. upon which all other planes are based. the same signs are used as in tool-inhand system. The tool-in-use system is rotated towards the tool-in-hand system by the effective cutting speed angle η. • The tool back plane Pp stands perpendicularly on the tool reference plane Pr and perpendicularly on the assumed working plane Pf .3.6 Position of tool cutting edge angle κ r • The cutting edge plane Ps runs tangentially to the cutting edge S and perpendicularly to the tool reference plane Pr .5 Orientation in tool frame of reference Trace of tool back plane Pp Trace of tool cutting edge plane Ps Assumed direction of feed re κr Trace of assumed working plane Pf Tool Tool reference plane Pr Fig. The tool cutting edge normal plane Pn is identical to the effective cutting edge normal plane Pne . which stands for “effective”. since it is not oriented to the tool reference plane but rather to the major cutting edge. .2 Reference Systems 43 Pn Po Assumed direction of primary motion Assumed direction of feed Ps Pr Fig. In tool-in-use system. 3. • The tool cutting edge normal plane Pn is perpendicular to the cutting edge S. 3. • The tool orthogonal plane Po is perpendicular to the tool cutting edge plane Ps . they are followed however by an e. • The assumed working plane Pf is perpendicular to the tool reference plane Pr and parallel to the assumed feed direction. which are measured in the tool cutting edge normal plane Pn . is defined as the angle that determines the position of the major cutting edge S in the tool reference plane Pr .7). The tool normal rake angle γ n is customarily used as the orientation angle. In the following. • the orientation angle around a rotation axis that stands orthogonally on the tool cutting edge normal plane Pn : This rotation is executed by one of the tool angles α n or γ n . as it has a large effect on the cutting process (Fig. is defined as the angle that determines the position of the major cutting edge S in the tool cutting edge plane Ps (Fig. For an optimal cutting process. Trace of assumed working plane Pf Major cutting edge S λs Tool Trace of Tool reference plane Pr Assumed direction of primary motion Tool-cutting edge plane Ps Fig. • the orientation angle around a rotation axis that stands orthogonally on the tool cutting edge plane Ps : The tool cutting edge inclination λs between the major cutting edge S and the tool reference plane Pr . 3. It is measured mathematically positively on the basis of the assumed working plane Pf (Fig.6). the tool orthogonal wedge angle β o is measured in the tool orthogonal plane Po . measured in the tool cutting edge plane Ps . measured in the tool reference plane Pr . the same index is assigned to them that signifies the plane in which these angles are measured. certain orientations are responsible for three rotatory axes in space. these angles will be explained exemplarily in the tool-in-hand system: • the orientation angle around a rotation axis that stands orthogonally on the tool reference plane Pr : The tool cutting edge angle κ r between the tool cutting edge plane Ps and the assumed working plane Pf .44 3 Fundamentals of Cutting In order to designate all tool and effective angles clearly. 3. 3.7 Position of cutting edge inclination λs . 3. Information about three angles around the three rotatory axes in space is required in order to determine the orientation of the cutting edges or the positions in space of the rake and flank faces. For example.8). or the effective side rake angle γ fe is measured in the working plane Pfe . they can still all be subdivided into three main categories: • free. The angle between the flank face Aα and the tool cutting edge plane Ps . The angle between the rake face Aγ and the flank face Aα is called the tool orthogonal angle β o . 3.8 Position of wedge in tool frame of reference It is measured mathematically positively between the tool rake face Aγ and the tool reference plane Pr in the tool cutting edge normal plane Pn . measured in the tool orthogonal plane Po . • free. 3. The tool normal orthogonal clearance angle α n lies between the tool flank face Aα and the tool cutting edge plane Ps .3. the assumed working plane Pf and the tool cutting edge normal plane Pn – must in total make a 90◦ angle. The kinematic factors of tool cutting edge angle κ r .3 Basic Process Variants Despite the large number of cutting process variants.3 Basic Process Variants 45 Cutting line of planes Pn and Pr Trace of plane of rake face Ag γn Tool Trace of plane of flank face Aa βn Assumed direction of primary motion αn Assumed direction of feed Trace of tool cutting edge plane Ps Tool cutting edge normal plane Pn Fig. The tool normal wedge angle β n is always fixed on the tool and cannot be changed by tool rotation. orthogonal cuts. wedge angle and rake angle – defined in the tool orthogonal plane Po . the clearance angle. For further information. tool cutting edge inclination λs and tool normal rake angle γ n are important influencing parameters on the cutting process. see DIN 6582. measured in the tool cutting edge normal plane Pn . diagonal cuts and • bound. diagonal cuts. As rule. is designated as the tool orthogonal clearance angle α o . . The angle between the rake face Aγ and the tool reference plane Pr is defined as the tool orthogonal rake angle γ o . The general case. 3. The following marginal conditions must be realized in the case of the free. orthogonal cut. diagonal cut is more general and contains the free. • Tool cutting edge inclination λs is equal to 0◦ (orthogonal). orthogonal and free. In the case of the uninterrupted cut. • The tool cutting edge angle κ r can take on values that are not equal to 90◦ (diagonal). contains all abovementioned special cases and. 3. Both conditions don’t have to be met simultaneously. also permits the engagement of the minor cutting edge (bound). 9.9 Free. • Arbitrary tool cutting edge inclinations λs are permissible (diagonal). it is a free.10). this cut can be executed. for example. diagonal cut. by means of longitudinal face turning or cross cylindrical turning (see Sect. temporal interruption is infinitely small (the cut is continuous). As soon as one of the last two marginal conditions is fulfilled. 3. diagonal cut: • Only the major cutting edge is engaged (free). The free. diagonal cut (Fig.9). Practically speaking. • The tool cutting edge angle κ r is 90◦ (orthogonal). in which the cut takes place only intermittently.46 3 Fundamentals of Cutting vch Tool Tool vch Chi p Chip λs vc vc Workpiece Workpiece Fig. The interrupted cut is the general variant among them. which can be brought about by the following marginal conditions: • Only the major cutting edge is being engaged (free).1) allowing for the above marginal conditions (Fig. orthogonal cut is a special case. All the basic process variants can be further extended by adding the following categories: • The uninterrupted cut and • the interrupted cut. in addition to the marginal conditions of the free. although this can occur. diagonal cut. diagonal cut The free. Both process possibilities will be described . the bound. 3. the direction of cutting section stress is determined by defining the tool normal rake angle γ n .4 Chip Formation At the beginning of the chip formation process. . the cutting speed vc and the depth of cut ap . 3. To put it another way. either tough or brittle material behaviour can be realized [Karm11. the deformed material forms a chip. causing it to deform elastically and plastically.4 Chip Formation 47 Fig.3. Böke14]. This has a large effect on chip formation. The amount of the particular stress is influenced by the cutting parameters cutting speed. feed velocity and depth of cut. The property of plastic deformability is not solely related to the material. During cutting. After the maximum permissible material-dependent shear stress is exceeded. As far as process kinematics is concerned. there are therefore six basic process variants.10 Bound diagonal cut vch Chip Too l νc ce Workpie in great detail in later chapters (see Chaps. the cutting section penetrates the material. it can also be brought about in a targeted way by altering the stress. In total.1 The Cutting Process By varying the direction and amount of stress. Soko55. Djat52.4. The amount of stresses is influenced by the feed velocity vf . Contingent on a given cutting section geometry. the direction of a particular stress can be set by the process parameters tool normal rake angle. the same material can be tough or brittle depending on the direction and amount of stress. tool cutting edge angle and tool cutting edge inclination. In order to ensure chip formation. the material begins to flow. 9 and 10). 3. Bram60]). which runs off the rake face of the cutting section. a minimum chip thickness and depth of cut must be exceeded (see Chap. Bram61. the tool cutting edge angle κ r and tool cutting edge inclination λs . 7 [Moll39. the chip formation process can be made visible. Leop80.48 3 Fundamentals of Cutting Fig. to Leopold [Leop00] independently of how it behaves at room temperature and under single-axis tensile loads (tension test). Chil71. since it allows for a temporal and local definition of chip formation on the superimposed grid [Leop80]. The particular stress must therefore be optimally adjusted to the given material properties with process kinematics in both direction and amount. In this representation. To clarify plastomechanical processes during chip formation. With this method. chip formation models can be made. There is brittle fracture behaviour for these materials even at high temperatures due to their material structures. we can see a continuous plastic deformation that can be subdivided into four zones. acc. 3. Figure 3. If however the material has higher deformability. The transition from the workpiece structure (a) to the chip structure (b) is made by simple shearing (shear zone).11 Model representation of deformation. When cutting brittle materials. This is not the case when machining cast iron.11) so that material deformation during cutting can be observed (the visioplasticiy method) [Hast67. The tensile load under simultaneous perpendicularly . glass and ceramics. detachment first occurs in front of the cutting edge in zone (e). and with the help of this. When cutting steel.12 is a schematic representation of the chip formation process as sketched with the help of a photograph of chip initiation (right). minor deformation on the shear plane can already lead to material detachment. Special marginal conditions exist during chip formation: both high rake face temperatures (about 770–1700◦ C) as well as high deformation speeds (of an order of magnitude of 104 /s). a grid is superimposed on the workpiece (Fig. Finite element analysis is another possibility for modelling chip formation. The visioplasticity method can help to verify FE modellings. 3. Leop00]. one must bear in mind that brittle behaviour only occurs at very low temperatures. 12 Location of chip initiation active pressure leads.4 Chip Formation 49 Structure of the workpiece Shear plane vc Structure of the chip a Shear zone 0. Cutting conditions under which cemented carbide tools operate result in deformation and material heating durations in the order of magnitude of milliseconds. The chip resulting from the described chip formation process is designated as a continuous chip.3. it can occur in different ways. Sliding over the tool surfaces causes further plastic deformations to arise in the boundary layers. Figure 3. to strong deformations on the peripheries of the rake face (c) and cut surface (d). ERNST [Erns38. Fig. These stresses – in conjunction with temperatures prevalent in the contact zone. segmented chips and discontinuous chips. Continuous chip formation is characterized by an evenly deformed material . 3.13 provides corresponding figures from the tension test. Erns41] has made a phenomenological classification of chip formation types.4. 3. which can amount to over 1000◦ C in the continuous chip formation zone – lead to deformations with sheer strains ε¯ between 0.2 Different Types of Chip Formation When chip formation has been ensured. the deformation texture of which forms parallel to the rake face. 3. together with the high temperatures prevalent here. gives the impression of a viscous flow process with an extremely high degree of deformation. Other chip types include lamellar chips. the heating velocities are theoretically around 106◦ C/s [Opit70].1 mm e b d c vch Flank face rake face Cut surface Tool Tool Cut surface Fig. For the sake of comparison. The “flow zone” (the non-etched white zone on the bottom of the chip).0 and sheer strain speeds ε˙¯ of up to 106 /s.8 and 4.13 shows a quantitative profile of normal and tangential stresses resulting from the resultant force components acting on the rake face. 3. the cause of which is explained by temporally highly altered friction conditions between the chip and the tool (stick-slip) or by dynamic stress transfer [Scha64]. acc. the cause of which is assumed to be in the temporally highly uniform friction conditions between the chip and the tool. vc = 160 m/min Cutting temperature: Chip flow velocity: Heating velocity: Average normal stress: Average shear stress: Shear strain Shear strain rate σn T1 ~ 1030 °C vch = 67 m/min 106 °C/s σ−n = 350 N/mm2 − τ = 250 N/mm2 Tension test: ε ~ 0. can be recognized in the chip structure.50 3 Fundamentals of Cutting Workpiece Material: C45E. Tool Material: HW-P20. ap = 2 mm. f = 0. The high level of local structural deformation is . The frequencies of the vibrations caused by stick-slip are in the range of kilohertz and have small amplitudes. shear bands.8 < ε < 4. that are characteristic of this type of chip formation. Lamellar chip formation is characterized by an unevenly deformed material structure between the chip and the tool. to KÖNIG [Köni67] chip structure. Locally enhanced structural deformations.2 Chipping: 0.25 mm.0 Tension test : ε ~10–3 s–1 Chipping: ε ~ 104 s–1 Distribution τ of stress w σ vch τ d T3 vc T4 Flow area T5 Tool vch Detail A Fig.13 Conditions of the cutting process. there is a detachment of material areas. • Discontinuous chips most form when cutting brittle materials with uneven microstructures such as certain types of cast iron and stone. the microstructure is uniform in the cutting area. The frequencies of these effects are much lower.15 gives a tool-oriented overview of the dynamic system of spheres of influence. Shear bands appear when machining high-strength materials with high levels of deformability.4 Chip Formation 51 explained by the fact that the thermally caused material softening is more dominant than its mechanical hardening. but are torn off the surface. these vibrations do not so much influence chip formation in the sense of material deformation as much as chip formation from outside. If the stress condition in the shear zone exceeds the deformability of the material (shear strength). cast iron. which then fuse with each other again. The chips are not detached. whereby this is not only the case for brittle materials like cast iron but also can come about if deformation causes embrittlement in the microstructure. tool.g. • Lamellar chips form when εB < ε0 < εF or the microstructure is not uniform or vibrations lead to variations in chip thickness. which brings about . but rather fracture takes place without plastic deformation. This leads to the formation of segmented chips. fibre-reinforced plastic or titanium alumnides. The form when εF < ε0 . This can be conceived of as a special case of lamellar chip formation in which highly localized deformations (shear bands) arise as well.14 summarizes the principal chip types [Vier70]. • Segmented chips consist of chip segments that are separated in the shear plane and fuse together again. the entire system – consisting of the machine tool. stone. This process basically distinguishes itself from the other processes of chip formation by the fact that no plastic deformation occurs before fracture. In addition to vibrations. Research into these phenomena relies on the work of ZENER and HOLLOMON. SCHWERD has made microcinematographic experiments and suggested the concept of “discontinuous chip” [Schw36]. Discontinuous chips can be observed in the case of materials with very brittle properties. Segmented chips can also be formed at extremely low cutting speeds (vc = 1 − 3 m/ min). in the area of a thousand hertz.3. often causing damage due to small breakings from the workpiece surface. • Continuous chips form when the material has sufficient deformability (εB > ε0 ). Lamellar chips can form with high feeds as well as with high cutting speeds. deformation does not cause embrittlement and chip formation is not impaired by vibrations. workpiece and fixtures – is influenced by further dynamic effects such as the regenenerative effect or directional coupling [Weck77]. since they are the cause of dynamic alterations in the nominal feed. Figure 3. Figure 3. who have described them and conceived the term “shear band”. e. They are also especially frequent when machining with high cutting speeds [Hopp03]. all complexly overlapping and mutually retroactive. In contrast to the higherfrequency chip formation dynamics. 15 Influence on chip formation the various types of chip formation. Opit70.and continous discontinous chip chip 4 Discontious chip Elastic region Plastic region Yield range 1 2 3 4 Shear strength τ Shear strength τ 1 Continous chip ε 0: E B F Strain ε E: B: F: Degree of deformation in the shear plane Elastic limit Breaking limit Fraction point ε0 Degree of deformation ε Fig. 3.52 3 Fundamentals of Cutting 2 Lamellar chip 3 Segmented chip Range of lamellar-. 3. the materials to be machined. Alti00]. . This section will not deal with low-frequency dynamic effects on the part of the machine tool.14 Types of chip depending on material properties. Range of segmented. Weck77. See the relevant literature for further information [Kron54. acc. tool geometry and process kinematics. to VIEREGGE [Vier70] Range of temperature Material properties Tool wear Friction and Deformation Range of velocity Process parameter Tool geometry Mechanism of chip formation: • Continous chip formation • Lamellar chip formation • Segmented chip formation • Discontionous chip formation Fig. This system is heavily influenced by the velocity fields. investigations into chip roots are also possible with higher cutting speeds. 3. During the cutting process. In the case of the equipment used today.17).17). Klop26] (see Fig.18) [Buda68]. the tool is suddenly accelerated out of the cut.16). Fig. Merc45. With such cameras. chip roots are held “in statu nascendi”.16 Microcinematograpy One method with which chip formation is directly observable. 3. Klop23. Gente has recently developed a method in which the delay time and the masses moved can be reduced in comparison with the “explosion method” [Gent02] (see also Fig. In this way. the first application of which on the cutting process was already carried out by KURREIN and then later by KLOPSTOCK [Kurr05. 3. In this process. Warn74]. . The development of this process has been advanced by several researchers in the course of time [Schw36. chip formation is observed through a quartz glass pane while photos are taken or films are recorded successively in short intervals.000 images per second. The acceleration is brought about by a predetermined breaking point on the swivel device that breaks during the cutting process due to an impulse caused by an explosion [Lola49] (Fig.4 Chip Formation 53 Feed motion Tool Primary motion Buffed and etched Workpiece Quarz glass pane Hollow spindle Tool reference plane Pr Fig. Today there are high-speed cameras available that can capture 150. Nevertheless. is microcinematography. various methods have been developed for cut interruption. One disadvantage of this procedure is the limitation to very low cutting speeds (vc = 1 cm/min). In order to be able to study chip formation more effectively.3. The advantage is that that chip root is detached from the workpiece so that the effective area can be spared during the following metallographic preparation. Other methods have been developed in which a chip root is produced in the workpiece by means of a predetermined breaking point (brittle fracture. providing information about chip formation after subsequent metallographic preparation. Oxle59. 3. the process has helped to explicate basic phenomena. the phenomena can be made visible at the lower threshold at higher speeds or especially during micro-cutting. Spaa71. 3. The following shows by way of example the generation of surface quality for a simple external turning process. .18 Interruption of cutting process by predetermined breaking point. 3. to LOLADSE [Lola49] (left) and GENTE [Gent02] (right) Tool Predetermined breaking point Fig. acc. 3.19). including the associated undeformed chip cross-sections in the case of changing engagement parameters (see also gear manufacture) or micro-geometrical surface parameters. the penetration of the workpiece by the tool is geometrically evaluated taking into consideration the kinematics in the tool reference plane Pr (Fig.54 3 Fundamentals of Cutting Gun Moment 1 Moment 1 Sample Moment 2 Tool Moment 2 Direction of primary motion Tool Baffle Sam Buffed sheet steel with high Young’s modulus ple Fig. Since chip formation processes are ignored. we speak in this case of the generation of kinematic surface roughness. For this. 3. to BUDA [Buda68] 3.17 Interruption of cutting process. acc.5 Kinematic Surface Roughness Calculations of interpenetration between the tool and the workpiece can provide key information about the cutting process. feed force Ff and passive force Fp (Fig.19 Geometric ratio of engagement in the cutting process 3. these resultant force components are usually detected metrologically with the help of piezoelectric force sensors. represented here in the turning process as an example. to DIN 6584 .20 Resultant force and its components in the cutting process. acc. 3.6 Mechanical and Thermal Strain on the Cutting Section The resultant force F.20). can be decomposed into the components cutting force Fc . 3. 3.6 Mechanical and Thermal Strain on the Cutting Section 55 z x rε f Rt rε –Rt M1 κr f/2 f M2 Rt = rε − rε2 − f2 4 or Rt = f2 8 ⋅ rε Area of validity: f < 2 · rε cos(κ r + ε r – 90°) and κ r + ε r < 180° Fig. Currently.3. ve vc Primary motion (Workpiece) Ff vf Fp Fc Fa Fz FD Direction of feed (Tool) F : Resultant force Fc : Cutting force Ff : Feed force Fp : Passive force Fa : Active force FD : Thrust force vc : Cutting speed vf : Feed velocity ve : Effective cutting speed Fig. 3. Merc45a] is the basis for determining the forces acting on the tool cutting edge (Fig. 3. or FΦT and FΦN FκN FκN Fκ T A Fγ N Ffp Fp κre FκT Fγ T λ se = 0° FΦ N Sectional view A-A vc FΦ T Shear plane h Φ Fκ N βoe FΦT Fγ T Fcγ N Fc Workpiece hch vch γoe Fγ N FΦN αoe Tool FcγN h Undeformed chip thickness hch Chip thickness vc Cutting speed vch Chip speed α oe β oe γ oe κ re λ se Φ Tool orthogonal clearance Wedge angle Tool orthogonal rake angle Working cutting edge angle Working cutting edge inclination Shear angle Fig. The following applies as long as the inclination is λs = 0◦ and the influence of the minor cutting edge is slight: Fγ N = Fc cos γo − (Ff sin κr + Fp cos κr ) sin γo (3.2) .56 3 Fundamentals of Cutting A h vf Ff Fc Cutting force Ff Feed force Fp Passive force Ffp Resultant of Ff and Fp Normal force of major cutting edge Tangential force of major cutting edge Normal force of rake face Tangential force of rake face Normal force of shear plane Tangential force of shear plane Resultant of FγT and FγN.21 Components of resultant force in working plane of reference (top) and working plane (below) The analysis of forces derived for the orthogonal cut by Merchant [Merc45. Assuming an ideally sharp cutting edge and neglecting flank face wear.21). the tangential force Fγ T and the normal force Fγ N acting on the tool can be calculated from the force components.1) Fγ T = Fc sin γo + (Ff sin κr + Fp cos κr ) cos γo (3. a neutral tool orthogonal rake angle (γo = 0◦ ) was selected.22 In the cutting process the mechanical strain of the cutting edge results from normal and tangential forces and their components acting on rake and flank face. In contrast to Merchant’s original work. 3. the cutting force Fc and the feed force Ff act perpendicularly/tangentially to the rake face. to MERCHANT [Merc45. 3.21). The friction coefficient. As research has shown. Because of the (in most cases) small difference between the angles in the tool-in-hand system and the tool-in-use system.22 clarifies for the simplest case of an orthogonal cross section with a neutral tool orthogonal rake angle. Designation of the acting forces and their signs is derived from the standards ISO 3002/4 and DIN 6584.3. VB > 0 Cutting edge . and the forces were shown with the effective cutting direction on the tool cutting edge.6 Mechanical and Thermal Strain on the Cutting Section 57 where the bracket term in Eqs. calculations are made within the tool-in-hand system for the sake of simplicity. (3. the normal force of the flank face amounts to about 40% of the feed force and thus to h Φ F ΦN Fa Chip F Fz ΦT = Fc Ff vf Tool Fγ N vc Fγ T acc to: Merchant Fig. For the case γo = 0◦ . which can be calculated from the cut and feed forces.2) corresponds to the normal force of the major cutting edge FκN (Fig. As Fig. κ r = 90°. the cutting and feed forces on the worn tool are composed not only of the normal force of the rake face Fγ N and the tangential force of the rake face Fγ T but also of the tangential force of the flank face FαT and the normal force of the flank face FαN . thus contains not only rake angle friction but also forces acting on the flank face caused for example by elastic deformation of the workpiece surface and by tool wear [Klau65]. 3.1) and (3. Merc45a] Fα N VB Fα T |Fc | = |Fγ N |+ |Fα T | |Ff | = |Fγ T |+ |Fα N | True for: γ o = 0°. acc. the undeformed chip thickness h increases proportionally to the reduction of the width of undeformed chip b. Figure 3.13. cutting velocity. 3.58 3 Fundamentals of Cutting Feed force Ff Passive force Fp Fig. Their profile is qualitatively reproduced in Fig. and depth of cut (qualitative) Cutting force components Feed f Fc Ff Fp Tool cutting edge angle κr Cutting force components Cutting force Fc Cutting force components Cutting force components about 66% of the rake face component in the feed direction [Spaa67. The profile of feed force Ff and passive force Fp over the tool cutting edge angle κ r results from the geometric position of the cutting edge with respect to the workpiece axis. Yet this is only valid if the depth of cut is larger than the corner radius of the tool. The reduction of forces with increasing cutting speed is caused by the reduction of material strength at higher temperatures. A percentage of approximately 10% of the cutting force Fc was ascertained for the tangential force of the flank face.23 shows the dependence of the static components of the resultant cutting force Fc . since with a larger cutting edge angle the resultant force component aimed in the feed direction increases. The components of the resultant force increase proportionally over the depth of cut ap .23 Components of resultant force depending on feed. The size and direction of the resultant force are strongly influenced by the cutting parameters and cutting section geometries used. Since cutting Fc Ff Fp Cutting speed vc Fc Ff Fp Depth of cut ap . The extremes in the profiles of the resultant force components over cutting speed can be ascribed to growth of built up-edge. depth of cut ap and the tool cutting edge angle κ r qualitatively in a linear coordinate system. tool cutting edge angle. If the tool cutting edge angle is increased. Lutz68]. 3. Materials that are difficult to cut result in values of 1100 N/mm2 . and its maximum is reached at κr = 90◦ . The average normal and tangential stresses originating from the resultant force components acting on the rake face are between 350 and 400 N/mm2 or between 250 and 350 N/mm2 when machining construction steel [Köni72]. Ff and Fp on the feed f. cutting speed vc . 0 % 4. 3.24 provides some standard values for how the components of the resultant force change if the tool orthogonal rake angle or the cutting edge inclination is varied.0 % Tool cutting edge inclination 1.5 % 1. about 100% for the passive force Fp and about 20% for the cutting force Fc . This information can vary greatly however and should only be seen as reference values.5 % 10.5 mm: About 90% for the feed force Ff . resulting in a larger positive tool orthogonal rake angle.0 % Increasing Cutting force Fc Tool orthogonal rake angle 1.6 Mechanical and Thermal Strain on the Cutting Section Influencing variables 59 Change of cutting force components per degree angle variation passive force Fp Decreasing Feed force Ff Tool orthogonal rake angle 1. generally leads of a reduction of cutting forces.0 % 4.5 % 5.24 Influence of cutting edge and rake angle on the components of the resultant force force Fc is proportional over depth of cut ap (= ˆ width of undeformed chip b) but increases degressively over feed (= ˆ undeformed chip thickness h). linearly dependent magnitudes in . Changing the corner radius also does not have a significant effect on the forces as long as the condition 2r ≤ ap is met.7) is another variable that influences resultant forces.5 % 5.0 % Tool cutting edge inclination 1. Several specific characteristic values can be distinguished for calculating the components of the resultant force.0 % Fig. these forces increase when flank face wear is dominant since the friction surface between the workpiece and the flank face becomes larger. Crater wear. Figure 3. a light reduction of Fc with increasing κ r is the outcome of both changes. Varying effects on the components of the resultant force can be observed depending on the type of wear. To identify empirical laws. Changing the tool orthogonal clearance angle in the range of 3 ≤ αo ≤ 12◦ has no mentionable effects on the resultant force components. it is usually advisable to relate the measurement to already known.5 % 10. Quantitative determinations concerning force increase with increasing tool wear can only be approximated due to the large number of influencing parameters.3.5 % 1. Tool wear (see Sect. On the other hand. The following reference values can be assumed approximately for force increase up to a width of flank wear land of VB = 0. These component forces are directly proportional to the width of undeformed chip b. 3. the relevant cutting forces are measured with constant cutting speed.1 and (1–mc ) order to minimize the number of variables going into the physical law. To determine kc1. 3.1 is determined by . cutting experiments are carried out for the combination of workpiece material and cutting tool material under investigation.8 1.25). In this case.0 h / mm Fig. 3. If we now plot the values thus found over undeformed chip thickness h in a double logarithmic plot.1 0.1 ) + (1 − mc ) · log h (3. depth of cut and cutting section geometry and plotted in accordance with Fig. 3.1 = 1740 N/mm2 mc 1000 1 800 1 –mc = 0.4 0. we form the quotient Fc  from the cutting force Fc and the width of undeformed chip b.7265 600 45° 400 1 Fc' = kc1.1 ⋅h1–mc 200 0.6 0. In these experiments. The corresponding linear equation log(Fc /b) = log(kc1.25 Graphical determination of characteristic values kc1.1 and (1–mc ).25.3) can be converted into the KIENZLE Equation Fc  = kc1.1 is the cutting force required to detach a chip of undeformed chip width b = 1 mm and undeformed chip thickness h = 1 mm.2 0. the measurement points arrange themselves in a straight line (Fig.60 3 Fundamentals of Cutting Cutting force Fc = Fc’ / (N/mm) Width of undeformed chip b 2000 kc1. The exponent (1–mc ) designates the gradient of the straight line Fc  = f (h) in the double logarithmic system.4) The specific cutting force kc1. The required specific cutting force characteristic parameter kc1.1 · h(1−mc ) (3.0 Undeformed chip thickness 2. 1 mm. we can assume We ≈ Wc and Pe ≈ Pc for most cases.6) The values determined in this way are however only valid for undeformed chip thicknesses of h > 0. Since the heat centres are identical with the deformation centres. Figure 3. Values for kc and (1–m) can be found in [Köni82].5) Fp  = kp1. Corresponding equations and characteristic values can be defined for resultant force components Ff and Fp : Ff’ = kf1.8) Feed energy Wf and feed power Pf : Wf = lf · Ff (3.26 provides an overview of the segmentation of the total active energy into shear. The mechanical work used in cutting is almost completely converted into thermal energy.12) Due to the relatively low feed speeds and feed paths. whereby shear energy has the largest share for most undeformed chip thicknesses.9) Pf = vf · Ff (3.03–3% of the corresponding cutting energy or cutting power.1 · h(1−mp ) (3.3. Cutting energy Wc and cutting power Pc : Wc = lc · Fc (3. According to DIN 6584. The tangent of the angle between the straight line and the x-axis is the desired gradient value (1–mc ).10) Effective energy We and effective power Pe are understood as the sum of all corresponding cut and feed amounts: We = Wc + Wf (3.1 · h(1−mf ) (3. the energy for cutting is the result of the product of the paths to be travelled or paths travelled and the components of the resultant force acting in their direction. For this reason. cutting and friction energy as a function of the undeformed chip thickness [Vier70]. The illustration shows that the amounts of the different types of energy depend on the undeformed chip thickness.11) Pe = Pc + Pf (3.7) Pc = vc · Fc (3. Correspondingly the powers arising during the cutting process result from the product of the speed components and the components of the result force acting in their direction. the shear . feed energy/power during turning amounts to only about 0.6 Mechanical and Thermal Strain on the Cutting Section 61 extrapolating the undeformed chip thickness to h = 1 mm. acc.8 Tool orthogonal rake angle: γο = 10° 1. 3. radiation and convection to the environment. the degree of deformation on the bottom of the chip is much higher in the flow zone than in the shear zone. The illustration in Fig. The heat arising in the individual development locations is dissipated by thermal conduction. corresponding temperature fields form in the workpiece and tool that change until equilibrium between added and removed heat is achieved.13 shows.4 0.62 3 Fundamentals of Cutting Shear energy Effective energy We Deformation energy Cutting energy Friction of flank face We = Fe ·Ie Latent energy and heat Friction energy Friction of rake face 700 Material: / (m·daN/m) 600 Tensile strength: Rm = 800 N/mm2 Total energy Cutting speed: 500 Shear energy 300 b = 4.0 Width of undeformed chip: h / mm Fig. 3.26 Segmentation of effective energy in cutting process depending on chip thickness. 3. because the energy required to overcome friction between the chip . Most of the mechanical energy (in this case 75% and generally more than 50%) is converted in the shear zone. its temperautre will be at least equal to that of a particle in the shear zone. to VIEREGGE [Vier59.25 mm Tool orthogonal clearance: αο = 5° 200 Effective energy We vc = 100 m/min Width of undeformed chip: 400 Path length due to primary motion 55NiCrMoV6 Friction of rakeface 100 Friction of flank face and cutting energy 0 0 0. so one can expect the highest temperatures between the chip and the tool. Since the thickness of the flow zone is very small in comparison with the shear zone however. the material is intensively heated on the chip bottom side and the tool on the rake face. Most of the heat is dissipated by the chip.27 left gives information about heat that is absorbed/dissipated by the workpiece. If we consider one material particle in the cutting zone. As a result of this heat balance. As Fig.6 0. these higher temperatures should not also be equated with a high energy conversion. Vier70] zone and the friction zones on the tool come into consideration as heat sources. chip and tool. The right side of the figure shows such a temperature field.2 0. As it glides further into the contact zone. Axer55. 3. chip. Beye72. about 200–300◦ C .3. Axer55].29) [Denk90. The level of the temperatures acting on the cutting section depends on the machined material. to KRONENBERG [Kron54] and VIEREGGE [Vier59] and the rake face is almost completely converted into heat. Laus88. In comparison to the rake face. Vier55. KÜSTERS demonstrated temperatures of over 1000◦ C on the rake face when cutting steel with cemented carbide [Küst56]. Dama90]. tool wear and the cooling medium. Küst54. The reason for this is the the growth of built up-edge arising in this cutting speed range (see Sect. acc. temperatures on the flank face are generally low (Fig. Denk90. Dama90.2). 3. Beye72.32 mm Tool orthogonal rake angle: γo = 10o Fig. but rather at a certain distance from it on the rake face depending on the cutting conditions.7. The maximum temperature does not appear directly on the cutting edge. Küst56. Already in 1956.28 provides an impression of the order of magnitude of the mean cutting temperatures to be expected on the rake face as a function of cutting speed for various cutting tool materials. Determining the temperatures arising during cutting has long been the subject of intensive research [Gott25. In the vc = 20–50 m/min range. Figure 3. 3. Laus88. Ehme70a. Ehme70a. Since this process only takes place in boundary layers of the chip and cutting tool material it heats up the rake and chip bottom side all the more strongly the less time is available for heat dissipation due to the higher cutting speeds. the cutting tool material. the temperature profile is non-linear in the double logarithmic coordinate system.27 Distribution of heat and temperature in workpiece. and tool in the process of steel cutting. Lenz66. LoCa94]. Lang49.6 Mechanical and Thermal Strain on the Cutting Section 63 Workpiece 300 310 Chip o 5% 75% 2% 18% 380 C 130 80 500 30 600 700 650 600 For steel 400 450 500 Tool Material: Steel Flow stress: kf = 850 N/mm2 Tool material: HW-P20 Cutting speed: vc = 60 m/min Width of undeformed chip: h = 0. Schm53. The size of the temperature difference between the rake face and the flank face. which interferes with direct heat conduction. the selected cutting conditions. Later investigations have qualitatively confirmed these results again and again [Cass94. 3. since the cutting process duration (and thus the achievement of a thermally stationary state) is too short for other methods .8 mm rε Cross section of undeformed chip: ap · f = 3 x 0. which vary according to technique. Vier53. Schm53. According to the information provided by the literature [Ehme70a.25 mm2 Cutting time: tc = 15 s 350 300 10 16 25 40 63 100 250 Cutting speed vc / (m/min) Fig. especially during finishing operations with high cutting speeds and small feeds. temperatures of over 800◦ C can be found on the flank face depending on the selected marginal conditions when cutting with cemented carbide tools.28 Mean rake face temperature when cutting with cemented carbide tools according to [Ehme70a].64 3 Fundamentals of Cutting 1150 1100 1050 1000 950 HW-P1 x 900 Temperature of rake face ϑ / °C 850 800 750 x x End of growth of– build–up edge HW-P30 x x 700 x 650 HS12–1–4–5 600 550 Tool material: HW–P10. Beye72].30) [Lowa67]. The closer the rake face moves towards the cutting edge. Axer55. Vier55. P30 High speed steel HS12–1–4–5 Material: C53E Geometry of cutting section: 500 450 400 κr εr αo γo λs 6° 6° 0° 70° 84° 0. 3. This means that temperatures are very high on the flank face as well. Küst56. experimental setup and measurement position. There are various methods and devices available for temperature measurement during the cutting process (Fig. The only metrological techniques that come into question for measuring temperature during the cutting process are time-resolution measurement techniques. the more the temperatures prevalent on the rake and flank faces conform. Küst54. is strongly codetermined by the position of the contact zone on the rake face. 3. Radiation pyrometer 4.5 mm 4° 0° 45° 90° 45° to cutting edge 2 κγ εr rε ap·f = 3 ·0. Two-chiselmethod 4. Coat of paint at the inner surface of a splitted tool 4.1.2. acc.29 Measurement of temperature with embedded thermocouple on cemented carbide. Thermo-electric measurement 2. Radiationmeasurement 3.2. Caloric heat measurement 2.4.6 Mechanical and Thermal Strain on the Cutting Section 65 Weld 1000°C 1200°C 3 2 1 0 4 800°C 1 600°C 500°C Compensating resistance Material: Cutting tool material: Cutting speed: Cross section of undeformed chip: 2 Isolation of silicon varnish Ni-tube ∅ 0. Assembly of a complete thermocouple Fig.3. One-chisel method 3.3. Measurement of infrared radiation with PDS-cell 3. Assembly of an isolated wire as themocouple . 3.20 mV 400 °C Example for the distribution of temperature inside the cutting edge CrNi-Wire ∅ 0.74 mm Fig.3.1.2.13−0. to Küsters [Küst56] Temperature measurment during machining 1. Measurement with thermo-colours 3.30 Temperature measurement in cutting processes 4.1. Infrared photothermometry 3. Coat of paint at an uninvolved surface of the tool 2.07 mm 30Mn4 HW-P10 vc = 180 m/min Cutting wedge geometry γo αo λs 0° Bevel width: 0. 3. this can lead to much lower measurements. since the thermoelectric voltage arising in the contact zone between the cutting insert and the chip would otherwise be dismantled through the machine in a short circuit. there is also the distance between the thermocouple surface and the internal measurement point. These techniques generally have low temporal resolution. In the case of sheathed thermocouples with isolated measuring points.29). Abra97] or in the workpiece [Osul02]. in cooperation with REICHEL. The workpiece and the tool must be clamped in isolation.30. temperature measurement with thermocouples. This causes a difference in temperature between the measurement surface and the thermocouple. 3. The thermocouples included are classified as encapsulated thermocouples.66 3 Fundamentals of Cutting (e. Both tool and workpiece form the heat soldering joints. Both methods require electrically conductive materials and cutting tool materials. pyrometers and photo thermometry are of technical interest. Pyrometry is the contact-free determination of absolute temperature by measuring the inherent radiation of a body without spatial scanning of the object field. The most important techniques in radiation measurement. Contact between the passing chip and the workpiece outside of the contact zone must therefore also be avoided. cutting tool materials with varying thermoelectric properties must be used. the one and two-chisel methods. Thermography provides a pictorial representation of temperature distribution. which determines temperature by measuring the heat radiation emitted from a surface.g. Temperature measurement with thermocouples is among the most common techniques today. Qure66. In the case of the two-chisel method. Temporal resolution is influenced by the response time of the thermocouple and heat transfer between the thermocouple and the device under test. Thermal compounds are used to improve heat transfer between the thermocouple and the surface. sheathed thermocouples and single filament thermocouples. This process can detect temperatures of up to 1200◦ C. Vier70]. GOTTWEIN developed the one-chisel method as the first direct method for measuring temperature and then. Of the processes introduced in Fig. Due to the extremely high temperature gradients with short test times characteristic of cutting processes. Herb26. coating with temperature-sensitive colours or powders with constant melting points). Both methods are based upon the principle of a thermocouple. Installing a thermocouple into the tool or workpiece allows for a point-wise determination of the temperature field (Fig. Thermocouples can be applied in blind holes in the tool [Küst56. are pyrometry and thermography. The maximum values dominant in the friction zones cannot be measured in this way. Another disadvantage is that direct contact between the thermocouple and the test object is necessary and that the holes used to position the thermocouples can significantly affect the distribution of temperature. Also problematic is the contact heat transfer resistance between the surface under test and the thermocouple due to the roughness of the bore. the two-chisel method [Gott25. Radiation techniques have decisive advantages compared with thermoelectric . The greatest disadvantage of the single-chisel method is the protracted and elaborate process needed to calibrate the thermocouple which must be carried out again for every cutting tool material and workpiece material combination. while the tool clamp is the cold soldering joint. Commercial cameras usually work with long wavelengths and large broad-bands. narrow-band partial radiation. Thermography with infrared cameras can be used as an alternative to pyrometrical measurements. The precision of total radiation and broadband partial radiation pyrometers are especially influenced by factors that alter the spectral grade of emission of the surface. ελ1 ∼ ελ2 . wavelength. Some high-speed infrared cameras Workpiece Chip Lens Dichroic beam-splitter Point of measurement IR-filter Insert Glass fiber (∅ = 0.31) has the advantage that the spectral grades of emission ε1 and ε2 of the surface need not be known.7 µm. An error in measurement will only result if both wavelengths λ1 and λ2 differ greatly. Since the two selected wavelengths lie directly next to each other. angular position.3. acc. calibrating the measurement device for a particular surface is very difficult. and they are also contact-free. due to dust for example.42 mm) Photodiode Amplifier Characteristics • Measurement of two discrete wavelength bands λ 1 = 1. 3. which makes it possible to measure lower temperatures but which also has a negative effect on the attainable accuracy. 2 µs) • Independence from grade of emission (no calibration required) • Large range of temperature measurement (250–1200 °C) • Contact free measurement Fig. Further advantages of this principle are that the measured temperature is independent of signal dampening.31 Build-up of a two-colour pyrometer in principle. to MÜLLER [Müll85] . and λ 2 = 2 µm which are set into ratio • High temporal resolution (approx. The two-colour pyrometer (Fig. Moreover. In cutting. Since the emission grade is the function of many factors like temperature. so long as both signals are dampened equally. effects such as surface roughness and oxidation influence the grade of emission of different surfaces greatly. material and surface condition. The advantage to this is the pictoral representation of temperature information.6 Mechanical and Thermal Strain on the Cutting Section 67 methods: the time resolution is much higher (whereby pyrometers are principally faster than infrared cameras). two-colour and multi-colour pyrometers have been developed. To limit the influence of the grade of emission on measured temperatures. One significant problem when measuring for an exact absolute temperature with a radiation method is the dependence of the radiation emitted on the grade of emission of the surface. Scanning cameras that work with a single detector are too slow for fast processes. the temperature of objects that are smaller than the optical field of vision can be measured without error [Müll85]. 3. 6. we can select from a diverse array of cutting section geometries. Töns95. There are numerous parameters that may have an effect on temperature: the type of cutting process. cutting parameters and tool geometry. A direct comparison of the temperatures measured by various authors is only rarely possible. Figure 3.1 Influence of the Geometry of the Cutting Section Depending on the cutting task at hand. Determining the tool angle always involves a compromise that can only do approximate justice to various requirements. possible coatings of the cutting tool materials.32. since experimental conditions generally deviate from each other. most measurements indicate an apparent rise in temperature at low to average cutting speeds. Heat radiation emitted from the chip bottom side passes through the window via a mirror integrated into the chisel holder and reaches an infrared camera [Müll96]. Typical tool angles in steel cutting are given in Fig. while at high speeds there is an approximately constant temperature level. Cutting speed clearly has the greatest influence on the maximum temperatures that arise in the contact zone between the chip and the tool.68 3 Fundamentals of Cutting offer integration times in the range of microseconds. Moreover. the measurement position is also crucial as well as metrological parameters like local or temporal resolution. 3. The duration of the cutting process and the measurement time must also be taken into consideration. One major problem in the metrological determination of cutting temperatures. the use of cutting fluids and cutting parameters such as feed. materials. Another highly promising method for determining temperatures in the boundary layers between the material and the cutting tool material is the implantation of a window made of diamond in the tool contact zone. besides poor accessibility. is the limited local resolution of the available measurement techniques as well as the small range (only a few micrometers) within which the maximum temperatures arise. Local temperatures in the boundary layer between the cutting tool material and the sliding chip also cannot be captured with existing measurement methods. material. In the case of the basic process. This technology is however not yet suitable for cutting processes with high wear. .33 shows how a change in cutting section geometry influences cutting parameters. 3. depth of cut and cutting section geometry. Geometry selection depends on • • • • the cutting tool material. cutting speed. which are applied to tool contact zones and determine temperature distribution within the contact zones during the machining process [Zieb95. Kloc97]. Promising attempts to determine thermal tool load more accurately than previously have been made with sensors in thin-film technology. making it possible to capture fast processes without motion blur. cutting tool materials. 4 to 2 mm Decreasing chatter.33 Influence of cutting edge geometry on characteristic machining values 3. If α o → 0◦ . surface . Decreasing cutting force Decreasing wear Increasing stability of cutting section Increasing of passive forces. If it is large.1. Increasing stability of cutting section – + λ s = –6° to +6° rε = 0.1 Tool Orthogonal Clearance α o Wear on the flank face (denoted by the width of flank wear land VB) is mostly determined by the size of the tool orthogonal clearance. if the wedge angle is too small. Decreasing cutting force Increasing surface quality Controlled chip flow Fig.6 Mechanical and Thermal Strain on the Cutting Section Geometry of cutting section Tool Cutting Cutting Tool edge orthogonal orthogonal edge rake angle clearance inclination angle Cutting tool material γο αο High speed steel (HSS) –6° to +20° 6° to 8° Cemented carbide 69 Tool included angle Corner radius λs κr εr rε –6° 10° 60° 0.4 to to to to +6° 100° 120° 2 mm –6° to +15° 6° to 12° Fig. 3.3. this increases the danger of breaking on the cutting edge.32 Tool angle for machining of steel Lower minimum chip thickness Increasing stability of cutting section – γ o = –10° to +20° + Improved chip formation Improved surface Reduced cutting force Decreasing wear α o = 6° to 12° Increased stability of cutting section Chamfer of the rake face κ r = 10° to 100° Decreasing wear Decreasing chatter. also.6. the cutting section is weakened in two respects: a heat build-up can ensue in the tool that can possibly lead to impaired or loss of hot hardness. 3. It is responsible for the detachment of the material to be cut. which can lead to a decreased tool service life. The thereby increased deformation of the passing chip and the large cutting forces result in a high temperature load on the cutting section. highly positive tool orthogonal rake angles can thus lead to tool breakage as a result of cutting section weakening. 3. a chip flow that is supported by a positive tool orthogonal rake angle often has insufficient chip breakage (tendency towards the formation of continuous chips). be either positive or negative. in contrast to α o .6. 3.5 Cutting Edge Inclination λs A negative cutting edge inclination can stabilize the cutting process to a large extent since the lead of the tool does not take place on the cutting edge but rather towards the middle of the cutting edge.1. Small tool included angles are required especially for copier and NC machining.1.1. the tool orthogonal rake angle γo and the tool orthogonal clearance form a right angle α o (Fig. On the other hand. increasing the danger of clattering vibrations due to growing instability of the cutting process. 3. This causes the specific cutting edge wear to sink.6.6. The possible range is limited by specifying the position of the major cutting edge and by making sure that the angle between the minor cutting edge and the feed direction is at least 2◦ in order to avoid subsequent shaving of the minor cutting edge on the workpiece. Together with the wedge angle β o .3 Tool Included Angle εr Due to the tool stability that is desired under extreme cutting conditions. Negative tool orthogonal rake angles increase cutting edge stability (used for example for planing and machining workpieces with apertures. The primary advantages of a positive tool orthogonal rake angle are the low cutting and feed forces as well as a usually improved workpiece surface quality.4 Cutting Edge Angle κ r At constant feed and depth of cut. the tool included angle εr should be as large as possible. This results in an improved load profile. The size of the tool orthogonal rake angle γo affects the stability of the wedge. 3.8).6. rolling skin or cast skin). so that the danger of cutting edge breakage due to local overloading is reduced. the passive force Fp rises with decreasing κ r . so small cutting edge angles are used especially to machine high strength materials in order to keep tool load and wear low. Wedge Angle β o The tool orthogonal rake angle γo can.1. the width of undeformed chip b increases with decreasing κ r . Increased crater wear appears on the rake face.2 Tool Orthogonal Rake Angle γ o . First cuts that have minimal loads are of particular importance especially in the case of interrupted . 3.70 3 Fundamentals of Cutting wear increases because pressure weldings appear on the contact areas of the friction partners to a greater extent. However. 3. 3. 7. whereby the following relation is approximately true: Rt = f2 8rε (3. 3.6. which must be absorbed by the machine tools (stiffness perpendicular to the main spindle!).7.7 Wear During the cutting process.7.2. brought about by mechanical causes. 3. The cutting edge inclination also influences the direction of chip flow.7 Wear 71 cross sections (e. decreasing the surface quality. high cutting speeds and high temperatures. Together with the selected feed. abrasion. A negative cutting edge inclination can result in the chip getting diverted to the workpiece surface.3). deformation.34).1 Wear Mechanisms The main mechanisms that cause wear are adhesion. producing scores and .1. cutting tools reach the end of their service life because of continuously increasing wear on both rake and flank faces. Negative cutting edge inclinations induce large passive forces.g.e.1 Abrasion Abrasion occurs when rough areas of the counter body or particles present as intermediate material or also as counter bodies penetrate into the surface of the cutting tool material and simultaneously make a tangential motion. 3.3. contact and relative motion of a solid. in milling or planing) as well as in the machining of cast iron and forged parts (workpieces with transverse drill holes. tribochemical reactions and surface disruption (Fig. shrinkage cavities). liquid or gaseous counter body. Small corner radii have the advantage of a smaller clattering tendency due to smaller passive forces. The cutting tool materials used are subject to an extremely complex load collective characterized by high compressive stresses.13) Large corner radii improve the surface quality and cutting stability. i. it influences the attainable workpiece surface quality to a great extent (see Sect. separation and friction processes take place in the area of the cutting edge. Using cutting parameters common in practice. This is explained as the progressive loss of material from the surface of a solid body.6 Corner Radius rε The corner radius to be selected depends on the feed f and the depth of cut ap .1. acc. The various forms of abrasion are designated as groove.35 Types of material damages caused by abrasive parts. hard areas of roughness or hard particles penetrate into the surface of the stressed material. Micro-plowing and micro-cutting are the predominant wear processes in the case of ductile materials.35). and the material finally fails due to fatigue. Material particles are hereby generated by means of cracking and crack growth in the stressed surface. Habi80]. Often. This can however occur if the material is repeatedly pushed to the groove edges due to the simultaneous effect of many abrasive particles or due to multiple effect of a single particle. fluid erosion.72 3 Fundamentals of Cutting Adhesion Abrasion Tribochemical reaction Surface damage Fig. the material is plastically deformed within the wear groove and pushed towards the groove edges. . to ZUM GAHR [ZumG87] micro-cutting. compact counter body (counter body grooving) or by loose particles in the case of sliding load (particle grooving). microcracks also arise. 3. microcutting and microcracks (Fig. Ideal micro-cutting leads to material removal in the form of a chip. to ZUM GAHR [ZumG92] Micro-plowing Micro-cutting Microcracks Fig. ZumG92. acc. In the case of microplowing. The volume of the wear particle created in micro-cracking is much larger than that of the continuous wear groove [ZumG87. creating scores or grooves by a sliding movement. Wear can be caused by a rough. the volume of which is equal to the volume of the wear groove created. notch or blast wear. In the case of groove wear. mill. 3.34 Schematic illustration of the four main mechanisms of wear. The process of material damage can be subdivided into micro-plowing. the term “groove wear” is used synonymously with the term “abrasive wear” [Habi80. In the ideal case. 3. In the case of brittle materials. no material is removed. Czic06]. 2 Adhesion Adhesion is defined as the formation of bonds between certain molecules [Erin90.1. it is sometimes referred to as adhesive wear. new material surfaces are generated that. As a wear mechanism this process is understood as one in which atomic bonds (e. weak or no adhesion bonds at all [Habi80]. The propensity of solid bodies to form adhesion bonds can be evaluated with the help of their surface energy. especially that of the coating system) should have as little polar energy as possible in order to minimize that propensity [Bobz00. in the form of micro-welds) are formed in the contact zone between the material and the cutting tool material.3. Neis94. the surface energy of cutting tools (i. particles can be transferred from one wear partner to the other. Whether a material has the propensity to adhesion can only be answered with reference to the material of the counter body. Tribooxidation changes the properties . reinforced and sheared off during a tangential motion of the friction partners.7.e. come in a chemically highly active state into contact with the tool surface under high pressures and temperatures. If this transfer is the primary cause of wear. electron exchange or electric polarisation. with the cutting tool material [Tell93]. Andr59.3 Tribooxidation The term “tribooxidation” refers to chemical reactions of the cutting material and the material with components of the intermediate material or the surrounding medium as a result of activation caused by friction.7.7 Wear 73 3. [Bobz00] describes a measurement process that makes it possible to compare the surface energies of coating systems with respect to their polar and dispersive parts. the formation of micro-welds between two friction partners) can come about by atomic interaction (chemical adhesion) between the partners. Adhesion is a pairing property that depends on the characteristics of the base and counter bodies. The cutting process thus provides very good opportunity for the appearance of adhesively caused interactions [Bömc89. It can also result from mechanical snagging (mechanical adhesion) of the workpiece material. Siko63].1. Erin90]. ZumG87]. 3. Kloc05].e. Contact angle measurement is one method for ascertaining the surface energy of solid bodies. The process of adhesion (i. approaching that surface on the atomic level. The strength of an adhesion bond is described by the adhesion coefficient. without adsorption or reaction boundary layers in statu nascendi. By means of adhesion. such as thermally induced diffusion processes. These bonds are then deformed. During chip formation. which has plastically become extremely deformable under high temperatures.g. Since it is the polar energy share that is primarily responsible for the propensity of a solid body to adhesion. Plastic deformation processes lead to the formation of large real contact areas. although other wear mechanisms are usually also involved in the formation of loose wear particles [Habi80]. This is defined as the quotient of the normal force FN with which two solid bodies in relative motion are pressed against each other and the opposing force FA that must be applied to undo the bond formed by adhesion [Habi80. with which it may either form strong. 5 Surface Damage Surface damage occurs as a result of tribological alternating stresses. 3. Erin90. Together with extraordinarily high mechanical and thermal stresses. there is a difference in concentration due to the differing compositions of the material and the cutting tool material which also is preserved in the material because of the constantly added chip [Neis94.4 Diffusion Diffusion is the thermally activated change of position of individual atoms [Horn67]. in which wear particles can be formed by a single stress process. Opit70b]. 3. alternating mechanical stresses lead to structural changes. Diffusion processes appear especially when high cutting speeds are used. Habi80].2 Causes of Wear Friction processes in tool contact zones are comparable to those of dry friction in a vacuum. which is usually very small but can still certainly be measured as a quantity of wear. The diffusion of atoms from the tribologically loaded areas of the tribopartners leads on the one hand to a direct loss of material. Reaction products can be formed that are either removed with the chip or remain stuck to the cutting tool material as a coating.74 3 Fundamentals of Cutting of the external boundary layer. thereby leading to high temperatures in the contact zone. In the area of the contact zone. Whether tribooxidation increases the amount of wear essentially depends on the hardness of the reaction products formed in comparison with the hardness of the cutting material. In this period. 3. surface damage is usually preceded by a longer incubation period during which no measurable wear occurs. This involves a temperature-dependent physicochemical process in which the wear resistance of the cutting tool material can be reduced by foreign substances diffusing with it or its own components diffusing away from it. the formation of wear particles by means of structural changes as well as cranking and crack growth is prepared [Habi80]. Diffusion of essential alloying elements can lead to decreased hardness and thus to reduced resistance of the cutting tool material to abrasion [Neis94. Wear can be increased or reduced by this. much more serious is the potential reduction of the wear resistance of cutting tools by the diffusion of certain alloying elements. . cracking. The high pressures and temperatures in the contact zones characteristic of the cutting process provide an ideal setting for diffusion processes between the material and cutting tool material. fatigue. Erin90].1. Furthermore. A decrease is especially possible when the reaction layers prevent direct metallic contact between the base and counter bodies. the tool is generally worn out quickly [Rabi65. crack growth and even to separation of wear particles [Habi80]. As opposed to abrasion.7. Krag71.7. limiting the effect of adhesion [Erin90. Opit70a. however.1. Habi80. In the stresses surface areas.7. Tell93]. the cutting tool material and the material approach each other on the atomic level. scaling. In the case of counter body grooving.36): • • • • • mechanical wear (abrasion). diffusion.2. the following separate causes can be given for the collective term “wear” (Fig. we must presume that the loose particles generated are pressed into the softer counter bodies flowing by them. Groove wear in the form of counter body or particle grooving can be seen as the dominant form of wear in this category.36 Causes of wear in cutting processes According to current information. These particles can originate from the workpiece material (oxides. feed et al. Köni65. nitrides) or be transferred by adhesion from the cutting tool material to the workpiece or chip bottom side. Ehme70c]. . the shearing off of adhered material damage to the cutting edge due to mechanical and thermal overstress. tribological stress on the cutting tool material is based on the abrasive effect of hard particles fixed on the contact surface of the workpiece or the chip. These can arise directly from abrasion (micro-cutting. oxidation Cutting temperature (cutting velocity. 3. Ehme70b. contributing to further wear by counter body grooving. 3.Wear 75 Diffusive wear Total wear 3. microcracks) or from surface disruption.) Fig. these may also be adhesion particles or products of tribooxidation removed by abrasion or surface fatigue. These processes overlap to a large extent and are only partially separable from each other with respect both to their cause and to their effect on wear [Opit67. Due to the high pressures and temperatures that predominate in the contact zones on both rake and flank faces. carbides. In the case of particle grooving. 3.1 Mechanical Wear (Abrasion) Mechanical wear or abrasive wear occurs at both low and high cutting speeds. However.7 Abrasion Adhesion High temp. loose particles are the cause of wear.7. This makes it possible for it to take over the function of a chip-removing tool. The types of wear resulting from this strong adhesive tendency can range from material bonding on the rake and flank faces to de-coating of coated tools in the area of the contact zone. Wear first reaches a maximum at the cutting speed at which the built-up edges reach their largest dimensions. cutting tool material particles that have broken away from the cutting tool material flow over the rake or flank face under high pressure.5 kHz. which is favourable for adhesive processes. The term adhesive wear is used as soon as material is separated in the cutting tool material. This can cause further wear by micro-cutting or microcracks.7. Figure 3. in the case of austenitic steel materials on their face-centred cubic crystal lattice. Adhesion is also responsible for the formation and growth of built-up edges. but has at least two distinct extremes [Opit64]. within one or within both bodies.2. these particles lead to increased flank face wear and considerably deteriorate the surface quality of the workpiece (Fig. Tungsten carbide. the basis of hardness and wear resistance in conventional WCCo cemented carbides. This is made possible by the property of certain materials to harden during plastic deformation. The material adhering to the cutting edge is deformed by chip pressure. the crystal lattice of the binding metal cobalt is face-centred cubic above 690 K. despite higher cutting speeds. material separation can occur in the boundary layer. resulting in a strong tendency to adhesion when machining austenitic steels. The titanium-based coating systems commonly deposited on cemented carbides also have face-centred cubic structures. Depending on the cutting conditions. The high ductility of ferritic materials is based above all on their relatively low strength. in which material is transferred from the material to be cut to the cutting tool [Erin90. also called “self-wear” [Ehme70a]. This process. According to it. Since the chip is diverted with the built-up edge and not the rake face. Ferritic and austenitic steel materials have a high propensity to adhesion with the cutting tool material. In the case of high hardness and removal frequencies up to about 1.2 Shearing-Off of Adhered Material Particles (Adhesion) In the case of micro-welds being sheared off. This can be ascribed to the fact that reinforcement of the built-up edge . Habi80. ZumG87]. 3. has a hexagonal crystal structure. The reason for this is above all the high plastic deformability of these materials. Built-up edges are highly reinforced layers of the machined material that take over the function of the tool cutting edge as bondings on the tool. making it very hard. A wear minimum appears at the cutting speed at which no more built-up edges form.76 3 Fundamentals of Cutting Due to abrasion or surface disruption on the cutting edge or in the contact zones on the rake and flank faces. built-up edge particles slip periodically between the flank face and the cutting surface.37). flank face wear does not increase with cutting speed continuously. On the other hand.38 shows a wear-cutting speed function (VB-vc graph). 3. Flank face wear decreases after exceeding the maximum. is of particular importance especially with respect to the formation and development of flank face wear. crater wear is usually negligible. 315 mm2 0. It is shifted to lower cutting speeds by any kind of measures taken . The position of the maxima and minima of the VB-vc graph is contingent on temperature. but as a whole across the rake face.04 0 1 2 3 5 10 20 30 50 100 vc = 30 m/min Cutting speed vc / (m/min) Geometry of cutting wedge: αo γo λs κr εr re 8° 10° –4° 90° 60° 1mm Cutting time tc = 30 min Fig.08 0. 3.16 0. 3. It becomes unstable and no longer migrates partially between the cut surface and the flank face.38 Wear on flank face and growth of build-up edge begins to decrease as a result of recrystallization processes.7 Wear 77 1 2 3 4 Width of flank wear land VB / mm Fig.: HW-P30 Cross section of undeformed chip: ap· f = 2 · 0.24 Material: C53E Cutting mat.37 Scheme of the periodic growth of build-up edge 0.20 0.3.12 vc = 20 m/min 0. Breakage of the cutting edge can occur both on the top and bottom sides of the insert. especially when tough materials are being cut. cooling) shift the extremes accordingly to higher cutting speeds [Opit69. Small amounts of breaking occur when the workpieces contain hard non-metallic inclusions that originate during deoxidation of the steel [Opit64a.2. Beck69]. This dynamic pressure threshold load can lead to fatigue failure.g. Opit62]. reaming or shaving). In the case of such breaking.g.78 3 Fundamentals of Cutting to increase the cutting temperature (e. especially in the case of manufacturing processes with relatively small cross-sections of undeformed chip (e. Parallel Cracks In the case of interrupted cuts (e. higher feed. on the material. such as breaking. Cutting Edge Chipping Large cutting forces lead easily to breaking on the cutting edge (cutting edge chipping) or corner if the wedge angle or tool included angles of the tool are too small or the cutting tool material used is too brittle.g. Cut interruptions can also lead to breaking. the chips of which tend to stick.39). cutting .g. It cools down after the tool exits the workpiece. During tool engagement. the cutting tool material is subject to strong mechanical alternating stress. parallel cracks. The quickly alternating stress in the formation of lamellar chips can also lead to parallel crack formation if a critical stress cycle is exceeded [Domk74.g. Measures to reduce the cutting temperature (e. among other things. to parallel cracks (Fig.3 Mechanical and Thermal Overstress Damage to the cutting edge. smaller tool orthogonal rake angle. Opit66. is the consequence of mechanical or thermal overstress. The difference between the highest and the lowest temperature depends. the profile of the fracture surface is determined by the direction of the cutting force [Köni75]. Sintered oxides and more wear-resistant types of cemented carbide are sensitive to this type of local overstress. Such stresses originate mainly while working with interrupted cuts. Comb Cracks Comb cracks are a form of damage to the cutting edge as a result of thermal alternating stresses (Fig. Peke74]. Distinct breaking of cutting tool material on the major and/or minor cutting edge can also be caused by chips striking against the cutting edge or during the rotation of shaft sections because of chip jamming between the cutting insert and the workpiece. 3. 3. e.39). comb cracks or plastic deformation. especially in the case of milling with cemented carbide tools. the cutting edge quickly heats up to high temperatures. when cutting titanium materials. higher material strength).7. milling). Quick consecutive cutting force changes lead. 3. while temperature change cracks form only gradually in the cutting part in the course of the cutting edge engagement. In the case of interrupted cuts. These should be distinguished from thermoshock cracks such as develop during turning or milling while using cutting fluid. the use of cutting fluids is particularly important with respect to the size of the temperature difference because it has a much larger quenching effect than air. The comb cracks arising in the cutting part during milling or also in short-cycle turning are generally temperature change cracks. 3. . Cooling is advantageous to the formation of comb cracks in the case of cemented carbides and ceramic cutting tool materials. Thermoshock cracks are formed by a nonrecurring steep temperature change. to VIEREGGE [Vier59] conditions and the ratio of the paths covered in the material and in the air.39 Comb and parallel crack formation in milling. The profile of the comb cracks is aligned with that of the isotherms of the temperature field in the cutting part. the stresses leading to the formation of temperature change cracks are below the strength of the cutting tool material. The development of temperature change cracks thus requires several periodic temperature changes as a rule.3. While in the case of thermoshock the thermally caused tensile strain arising in the contact zone exceeds the strength of the cutting tool material and leads directly to cracking. acc.7 Di Wear 79 cti on of co m b cr ac ks Direction of primary motion re Comb and parallel cracks ce e fa Rak KM vc = 200 m/min GJS70 l lle f para tion o Direc cracks Comb cracks VB e k fac Flan 42CrMo4+QT Temperature Stress Tensile + 0 – Compressive vc = 275 m/min Stress Temperature Tensile + 0 – Compressive –y Heating during cutting process Cooling Fig. • diffusion of Co into the steel. the geometry of the insert and. its high temperature properties like hardness. Plastic Deformation Plastic deformation of the cutting edge arises when the thermomechanical stress effecting the tool cutting edge exceeds the deformation resistance of the cutting tool material. the cutting parameters. 3. usually cobalt. on the part of the cutting tool material. they can relatively quickly achieve a critical size leading to fracture. Plastic deformation of the cutting edge results in a considerable increase in wear and can lead to sudden disruption of the tool by cracking and shearing off of the cutting edge. With a corresponding output length. they can also become the source of further cracks [Gers98].g. thus leading to considerable weakening of the cutting edge [Gers98]. It thus limits above all the cross-section of undeformed chip and cutting speeds applicable in rough turning. Tool steel and high speed steel already become soft at temperatures at which diffusion can hardly manifest itself (e. Cutting edges made of tool steel or high speed steel deform the stronger the smaller the difference is between the temperature of the cutting edge and the annealing temperature of the cutting tool material. Depending on the bending stress which the cutting edge is subject to during the cutting process and which are superimposed over thermally induced stresses. Temperature change cracks can be observed in both wet and dry cuts. the cutting cycle and the number of cutting cycles play a decisive role in their formation. diffusion wear must be expected at high cutting speeds and mutual solubility of the partners. Cemented carbides deform more the more binder phase there is. . The use of cutting fluids leads to a strong reduction of the amount of cutting cycles required for cracking [Gers98]. Plastic deformations also appear in the case of cemented carbides and cermet. When cutting with uncoated cemented carbide substrates.40): • diffusion of Fe into the binder phase Co. 3. the cracks become larger with increasing numbers of cutting cycles. As a result of the notching effect coming from the crack.80 3 Fundamentals of Cutting Due to the high energy level thermoshock cracks propagate with greater speed. they can acquire very large depths in the cutting tool material (> 1 mm). the following reactions can occur (Fig.2. whereby Fe and Co form a gapless series of mixed crystals. Crack growth is largely transcrystalline. about 600◦ C for high speed steel).7. The influencing parameters are the strength of the material to be cut. however only at higher temperatures (cutting speeds) and under higher forces as is the case for tool or high speed steels. compressive strength and creep behaviour. Besides the amount of thermal load.4 Diffusion In the case of heat wear resistant cemented carbide tools. Depending on the stress level. 40 Presentation of the diffusion phenomena in cemented carbide tools • dissolution of tungsten carbide to form mixed and double carbides in the form of Fe3 W3 C. into the steel. In the presence of Fe the solubility is increased to 1. i. W) C Co-W-C-mixedcrystal Fe WC: Dissolution into: Fe3W3C. The maximum solubility of carbon in cobalt is about 0. Ni 30 Fe 20 Co 10 0 µm 5 0 5 Fig. both of which accelerate dissolution.7% at 1200◦ C. The diffusion of carbon passes into the cobalt phase. (FeW)6 C and (FeW)23 C6 . To this purpose. Cr. which is in turn the prerequisite for the dissolution of tungsten carbide. Then the samples that were fused together on the contact surface were separated on an abrasive cut-off machine perpendicularly to the contact level.5–2%. (FeW)23C6 C Co Tool Chip Cutting material Cemented carbide WC-Co W. cemented carbide-material pairs were annealed at a temperature of 1100◦ C. and . Co 50 Mass percent % Material X6CrNiMoTi17−12−2 W 40 Fe.7 Wear 81 (Ti. Ta. (FeW)6C. The carbon released during the dissolution of tungsten carbide migrates in the direction of lesser concentration. 3.6 MPa and a holding time of 2 h in a protective gas atmosphere of argon.3. It lends itself to the formation of iron mixed carbides and increases the reception of carbon in cobalt. a pressure of 12.e. The diffusing iron thus introduces two reactions. The diffusion processes that are possible between cemented carbide and steel materials can be shown very effectively with the help of annealing tests. Nb. In contrast. This prevents direct contact between the substrate and the material. The aluminium oxide layer thus functions as a diffusion barrier. Since diffusion phenomena depend on temperature.42). caused by the diffusion of iron into the cemented carbide leads to a weakening of the structure and lowers the resistance of the cemented carbide substrate against abrasion.32 mm. In the opposite direction (i. there is a diffusion of iron. thus increasing the resistance of the cutting tool material to crater wear and increasing its heat wear resistance quite substantially. Mixed carbides on the basis of titanium and tantalum/niobium have. As the line scans of annealing tests in Fig.41). The most effective way to reduce crater wear is coating the cemented carbide substrate. 3. no more diffusion takes place in the case of the coated cemented carbide with Al2 O3 as an intermediate layer.42 show.40 show. Mixed carbides reduce the diffusion of iron into the cobalt binder phase. tc = 40s Coated cemented carbide. nickel and chrome are found in the ZrN surface layer.82 3 Fundamentals of Cutting after these sample halves were grinded and buffed. the cutting speed influences on an elementary level the speed with which crater wear develops. Uncoated cemented carbides can thus only be used at relatively low cutting speeds (vc < 100 m/ min). The consequence of this is the formation of distinct crater wear (Fig. diffusion phenomena take place under the selected annealing parameters between the uncoated cemented carbide and the austenitic steel material here selected. f = 0. The dissolution of tungsten carbide. in comparison to WC. ap = 2 mm vc = 200 m/min. f = 0. 3. 3. diffusion processes between the cemented carbide substrate and the material can by all means still occur in the case of TiN or TiCN layers. tc = 20 min vc = 120 m/min. Cemented carbide layers seal the substrate surface. The elements iron. As the quantitative line scans in Fig. 3. The cemented carbide substrate also has a serious effect on crater formation.41 Formation of crater wear on an uncoated and a coated cemented carbide. the distribution of elements was analysed with a microprobe perpendicularly to the contact surface cutting tool material/material.2 mm. thus reducing or suppressing diffusion phenomena. chrome and nickel into the cutting tool material. the bearer of hardness and wear resistance. while tungsten and cobalt are in the TiN boundary layer (Fig. ap = 2 mm Material: X6CrNiMoTi17–12–2 Fig. 3.e. In the aluminium oxide intermediate layer Uncoated cemented carbide. from the cutting material into the material) tungsten and cobalt diffuse. to GERSCHWILER [Gers04] . On the side of the steel material. acc. a much higher chemical stability towards iron. electrically isolated oxide layers function as diffusion barriers on cemented carbides as opposed to semiconducting nitride or carbon nitride layers can be explained as follows. The result of this is that “component diffusion” becomes impossible. For reasons of electron neutrality. As opposed to uncoated cemented carbides. While diffusion of iron through the cemented carbide layer into the cemented carbide substrate is indeed possible in the case of cemented carbides coated with TiN and TiCN.6.5Co6.8MK W Co TiN Ti (C. Conversely. The diffusion coefficients are indeed still small. chrome. In electrically isolating oxides like Al2 O3 or HfO2 . The observation that.5Co. Diffusive transport is already possible at the temperatures prevalent on the tool during machining of > 800◦ C. excluding air or water.3.N) Al2O3 ZrN X6CrNiMoTi 17-12-2 Fe Cr Ni Fig. electrons can migrate through the layer together with the ions. no migration of electrons or holes.8MK TiN Ti (C.) In semiconductive or conductive layers however.42 Verification of the diffusion phenomena between coated cemented carbide and steel dependant on the coating system on the basis of annealing tests the traceable amount of these elements is reduced to 0 mass-%.N) TiN X6CrNiMoTi17-12-2 W Co Fe Cr Ni WC-7. but the necessary diffusion paths are very short. tungsten or cobalt have diffused through the Al2 O3 intermediate layer on the opposite side of that layer.7 Wear 83 WC-7. 3. The consequence is that these oxide layers act as barriers as long as there is no way for electrons to get around it. There is above all no indication that iron. this diffusion is not as significant as is the case for uncoated substrates. and the transport of oxygen ions is also slow enough (in oxides this transport is empirically slower than cation transport by several orders of magnitude. the iron atom cannot “release” its electrons outside the layer and then migrate through the layer as an ion. only ion diffusion is possible. which theoretically can be split into an ion and an electron. “Component” refers to an atom or iron. leading finally to the iron element migrating through the layer to alloy with the substrate. the coat provides the substrate . conveyance of Co or W into the chip is also possible. 84 3 Fundamentals of Cutting with additional protection from the abrasive action of the flowing chip. 3. But as soon as the hard material layer in the contact zone of the rake face has been rubbed off and the substrate exposed. together with the abrasive effect of the chip on the substrate. Scaling varies in importance depending on the cutting tool material alloy and cutting temperature (Fig. Annealing time: 3 h exp. to an acceleration of crater wear. 3. whereby Increase in mass / % 20 15 HW-K10 Atmosphere: air Temperature: 1000 °C HW-P25 Annealing temperature: 500 °C Annealing time: 3h exp.43).2. diffusion processes now once again taking place between the substrate and the chip contribute. 3.43 Gain of weight during the annealing process in air dependant on the used cutting tool material . 3.7. we can recognize several annealing colours near the contact zones. indicating scaling (oxidation) of the cutting tool material. cemented carbide layers help to reduce abrasive wear to a significant extent.41). to air HW-K10 HW-P25 HW-P10 HT-P10 Fig. Cemented carbide layers thereby ensure the high performance level of coated tools and allow for much higher cutting speeds when cutting steel materials in comparison with uncoated cemented carbides (Fig. to air 10 HW-P10 5 HT-P05 HT-P10 HT-P20 0 1 2 3 4 HW-K10 5 Annealing time / h Annealing temperature: 1000 °C. Especially because they are much harder than the substrate. Cemented carbides already begin scaling at 700–800◦ C.5 Scaling If we examine a tool after cutting. 46).44). 3. The cutting part is worn on the rake face (crater wear) as well as on the major and minor flank faces (flank face wear) (Fig. Even under the usual cutting conditions. an oxide film is formed on WC tools in the area of the cutting edge under the effect of the cutting temperatures and atmospheric oxygen.44 Oxidation zones at a cemented carbide turning tool those made of pure tungsten carbide and cobalt oxidize more than those alloyed with titanium carbide or other carbides [Kief65].3. Scaling is of no practical importance for tool steels and high speed steels since their heat resistance is exceeded before their surfaces are oxidized more strongly. flank face wear tends to be strongest on the edge of the contact zone on the major and minor cutting edge of the tool. the ends of the contact zones on the flank face.3 Forms and Dimensions of Wear Wear phenomena that are generated on the cutting part during cutting vary greatly depending on the type and duration of stress. i. The destructive effect of oxidation on the structure of cemented carbide can be observed especially clearly on the minor flank face. minor flank face and rake face (Fig. Such “notch wear” is caused by the abrupt transitions of mechanical and thermal stress prevalent on the end of the contact area. 3. develops in a wart-like fashion and can lead to corner breaking [Köni75].25 mm Feed: Depth of cut: ap = 3 mm Rake face Oxidation zones Flank face Fig. 3.e.45 shows wear forms that occur primarily on turning tools. Figure 3. A complex tungstencobalt-iron oxide is formed that. the abrasive character of the sharp .7. 3. This film covers the areas to which atmospheric oxygen has free access.7 Wear 85 Oxidation Material: C53E Cutting tool material: HW-P30 Cutting velocity: vc = 125 m/min Cutting time: tc = 20 min f = 0. which continues on the rake face. Depending on the cutting parameters and the combination of workpiece and cutting tool material. as a result of its larger molar volume compared with cemented carbide. 3 mm Insert geometry: VBMW160412 Notch wear on: Minor cutting edge Major cutting edge Cutting tool material: Ceramic Material: Nickel-based alloy Cutting parameters: vc = 220 m/min f = 0. flank face wear and notch wear dependent on the used cutting tool material and the machined material .16 mm tc = 1.3 mm Insert geometry: RCGX090700T00515 Fig.16 mm tc = 2.45 Characteristic wear forms at the cutting part during the turning process Cutting tool material: Cermet Material: 42CrMo4+QT Cutting parameters: vc = 400 m/min f = 0. partly overlapping the cutting edge in a palisade-like manner Spalling of cutting edge along major cutting edge Notch wear on minor cutting edge Notch wear on rake face Plastic deformation of corner of cutting edge to the point of breaking Spalling of cutting edge due to chip impact or clamped chips Layer flaking due to plastic deformation of cutting edge Layer flaking beneath width of flank wear land Notch wear on flank face Wear on flank and minor flank face Delamination on cutting edge beyond contact zone Fig.6 min ap = 0. 3.86 3 Fundamentals of Cutting Cratering on rake face Smeared material. 3.3 min ap = 0.1 mm tc = 12 min ap = 0.46 Examples for the formation of crater wear.5 mm Insert geometry: SPGN120308 Cutting tool material: PCBN Material: Nickel-based alloy Cutting parameters: vc = 300 m/min f = 0. In particular.46). Figure 3.1 Shear Plane Theory The model representation of the shear plane is based on ideal cutting conditions. 3. to DIN ISO 3685 chip edge or the feed comb and the tool edge as well as direct contact with the atmosphere (Fig.47 is a schematic representation of the dimensions of wear. SVα KB: Crater width KM: Distance betw. we distinguish the width of flank wear land VB. crater depth KT and the crater centre distance KM from which the crater ratio K = KT/KM is formed.8 Cutting Theory 87 SVα: Displacement of cutting edge toward flank face SVγ: Displacement of cutting edge toward rake face rε C VBN b/4 B A N Wear notches on major cutting edge b Sectional view A-A KT KB VBB VBB max. the displacement of cutting edge toward flank face SVα and rake face SVγ . acc. . the representation of mechanical loads as substitute forces and ideal plastic material behaviour. crater centers KT: Crater depth VBC Width of flank wear land VB Wear notches on minor cutting edge A KM crater SV A γ Plane Ps SV α Fig.3.8 Cutting Theory 3. cutting edges that are not subject to wear. 3. Required are: • • • • ideally sharp cutting edges. 3.47 Wear forms and measured quantities at the cutting part.8. 3. orthogonal cut. Moreover.48). This is a plane which is inclined toward the cutting edge plane Ps by the shear angle F.48 Model representation of the shear plane The chip formation theory derived from this model (shear plane theory) assumes that deformation takes place on only one plane. If the effects of the corner radius and minor cutting edge are negligible in comparison with that of the major cutting edge. the shear plane. the shear angle establishes the relation between undeformed chip thickness h and chip thickness hch . the shear plane passes through the selected point of cutting edge just like the tool-inhand and tool-in-use reference system (Fig. With the shear angle we can macroscopically consider material and cutting tool material properties as well as thermal loads and friction conditions which predominate during contact of the chip with the rake face. the theory of the shear plane can also be used for the bound cut. In the following the shear plane P will be considered in the tool-in-hand reference system. hch = cos ( − γn ) ·h sin (3. By definition. The requirements made on process kinematics are fulfilled when: • tool cutting edge angle κr = 90◦ • tool inclination λs = 0◦ We are thus assuming a free.14) .88 3 Fundamentals of Cutting Trace of tool reference plane Pr γn Trace of shear plane PΦ hch h Tool Φ Trace of cutting edge plane Ps Workpiece Selected point of cutting edge z Po ≡ Pf ≡ Pn y Fig. The shear plane model has the advantage of making complicated cutting processes highly comprehensible. 3. MERCHANT calculated the chip speed vch with the help of the shear angle F (Fig. the term degree of compression is also used.8. three classical applications of shear plane theory will be described. In conclusion.49 Velocity plan.1. to MERCHANT [Merc45] Po ≡ Pf ≡ Pn .3. This concerns volume consistency. 3. | vch | = sin · | vc | cos (γn − ) (3. the term chip compression λh is used analogously [Krys39]. to show that the theory of the shear plane makes it possible to calculate various quantities with complicated relations. cos ( − γn ) hch = (3. In cutting theory.49) [Merc45]. 3.8 Cutting Theory 89 By means of the shear angle we can now calculate the surface area of the shear area A in the shear plane: h A = b · (3.17) Trace of tool refernce plane Pr γn Trace of shear plane PΦ hch h Tool Φ Trace of cutting edge plane Ps vch Workpiece z vΦ vc Selected point of cutting edge y Fig. even if the shear surface and slip surface are nearly identical under certain marginal conditions.15) sin The ratio of undeformed to deformed magnitudes is designated in forming technology as deformation.1 Calculation of Chip Speed From simple trigonometric relations (law of sine) in the velocity plan.16) λh = h sin The shear angle  is a purely theoretical quantity and should not be confused with the actual slip direction of the material. If the material is compressed and only the change in height is described. 3. acc. acc. The idea behind this is that a mass flow flows through a control surface. ⇒ | vc | · A = | vch | · Ach (3.8.1. in this case the shear surface. can be represented by the resulting resultant force Fz .20) The component of the resultant force in the shear plane.19) 3. the shear force F . With the help of the angle relations in the circle of THALES we obtain according to Fig.50 Circle of THALES for the force diagram. In the first place we have: τ = | |F A (3.21) Trace of tool reference plane Pr γn Trace of shear plane PΦ Tool Φ Trace of cutting edge plane Ps FΦ Fc Ff z ρ − γn FΦn Fz Fγn Fγ Po ≡ Pf ≡ Pn y Fig. which will be shown in the following steps. 3. 3. to MERCHANT . Assuming that the density of the material is not altered by mechanical and thermal load.50: z | · cos ( + ρ − γn )  | = |F |F (3.18) From which follows: | vch | = h 1 · | vc | = · | vc | hch λh (3.2 Calculation of Resultant Force It is assumed that the shear angle is a function of the shear stress in the shear plane.90 3 Fundamentals of Cutting The continuity equation can also be used to calculate chip speed vch . we obtain: m ˙ = const. 3. the position of the main axial system is assumed to be approximately in the direction of the resulting contact stress (Fig. • The position of the shear plane is determined by the minimum of cutting energy. the principle stress is not represented by the stress tensor but simply by vector addition.21): z| = |F τ ·b·h sin · cos ( + ρ − γn ) (3.24). the tool orthogonal rake angle and the friction angle can be derived from Fig. The value of cutting force can be read off the circle of THALES in Fig.26) 3.8.50).51.27)  = − (ρ − γn ) 4 MERCHANT has suggested a possibility of calculating the shear angle that makes the following assumptions [Merc45.50: .8 Cutting Theory 91 The amount of resultant force can now be calculated from Eqs. maximum shear stresses occur at an angle of 45◦ . 3. For purposes of simplicity. The function relation between the shear angle. He made the following assumptions: • The theory of the shear plane is valid. 3. 3. (3. For calculations of the cutting process.25) (3.1.51). • The maximum shear stress leads to material collapse. The following is valid for the shear angle: π (3.23) f | = |F z | · sin (ρ − γn ) |F (3.3 Calculation of the Shear Angle KRYSTOF developed a model for the simple calculation of the shear angle [Krys39].24) If we now insert Eq.22) The functional relations of the values of the resultant force components cutting force and feed force can be taken from the circle of Thales (Fig. (3. 3.22) into Eqs. Merc45a]: • The theory of the shear plane is valid. (3.20) and (3. c | = |F z | · cos (ρ − γn ) |F (3. the friction angle approximates the orientation of the main axial system. the following is valid for the values of the resultant force components: cos (ρ − γn ) · τ · b · h sin · cos ( + ρ − γn ) sin (ρ − γn ) · τ f| = ·b ·h |F sin · cos ( + ρ − γn ) c| = |F (3. In this case. In the main axial system.23) and (3. 92 3 Fundamentals of Cutting Trace of tool reference plane Pr 1 γn Trace of shear plane PΦ Tool Φ τΦ Trace of cutting edge plane Ps − π 4 Fz ρ Fγ 3 Fγn Po ≡ Pf ≡ Pn Fig. 3.: ∂ 2 Ec . to KRYSTOF [Krys39] c| = |F b·h cos (ρ − γn ) · τ · cos ( + ρ − γn ) sin (3. acc.: ∂Ec =0 ∂ suff.51 Principle of the maximum shear stress.28) The necessary and the sufficient conditions for calculating the position of the energy minimum are nec. : c| lc · ∂|F =0 ∂ suff. and the cutting path is not a function of the shear angle. we can simplify to: nec.: c| ∂ 2 |F .29) Since the cutting energy is calculated as the product of the cutting force and the cutting path.= 0 ∂ (3. this theory makes it possible to take a material-specific slip angle into consideration instead of an idealized 45◦ . It should .2 Application of Plasticity Theory to Cutting HUCKS and OPITZ developed a shear angle equation by applying MOHR’s slip theory to the orthogonal cutting process [Huck51.30) From which follows for the shear angle: = 1 π − · (ρ − γn ) 4 2 (3. Mohr06].31) 3. Opit53. As opposed to the models of KRYSTOF and MERCHANT.= 0 ∂ (3.8. 3: Principle normal stress directions cti on Trace of cutting edge plane Ps ≡ Pp zFE ire yMT ire Fig. The friction coefficient is dependent on the cutting speed and consequently alters the principle stress direction. which will be neglected in the following. acc. The slip direction on the other hand is inclined towards the principle stress directions by a materially dependent angle ϕ.8 Cutting Theory 93 Trace of tool reference plane Pr 1 Trace of shear plane PΦ Sli γn pd zMT yFE ω cti on ϕ Φ1 σy τzy τyz Tool τyz σρ y y τzy 3 Φ3 Po ≡ Pf ≡ Pn Sli pd 1. This relation makes it clear.3. why different cutting speeds yield different shear angles. to HUCKS [Huck51] again be pointed out that the previously known shear angle need not necessarily be equal to the angle given by the actual position of the slip surface.52). In this case the directional angle of the slip surface towards the tool cutting edge plane is designated as the shear angle . This angle can be . If we now determine the principle normal stress with the help of MOHR’s stress circle. 3. Due to the unhindered flow of the chip over the face there is a very small amount of compressive stress in this direction. There are therefore two compressive stresses and four shear stresses on the surface element under consideration. 3.52 Surface element at the cutting part. for example.32) The angle between the rake face and the principle stress direction is thus determined by the friction coefficient between the rake face and the chip. The shear stresses also appear on all remaining faces of the quadratic element as a result of the law of equality of shear stress. since the direction of the shear plane was previously only purely geometrically determined by ideal considerations. First a square surface element is considered on the rake face that experiences both shear and compressive stresses from the tool (Fig. we establish the validity of the following equation: ω= 1 arctan(2 · μ) 2 (3. With the help of the angles γ . For steel. HUCKS’s work made the fundamentally new discovery that the stress field in the chip on the cutting edge depends. This approximation was proven to be in accordance with experiments made on materials with envelope lines that are inclined and parallel to the σ -axis. resulting in a normal and tangential force in the shear plane.52 relative to the rake face.34) 1 = ϕ − arctan(2μ) + γn 2 Slip angle 3 is in the still undeformed area.94 3 Fundamentals of Cutting calculated from the compressive flow stress σ D and the shear flow stress τ F with the help of flow tests and MOHR’s stress circle:   1 σD − 2τF ϕ = 45 − arcsin 2 σD (3. With the help of the shear angle. ϕ.36) To sum up. which is why only slip angle 1 (which is identical with the shear angle) is significant. To do this. the stresses that are active in the shear plane are multiplied with shear plane surface. HUCKS did not experiment with materials with bent envelope curve [Huck51]. The shear angle calculated here presumes an extrapolation of the stress conditions via the shear plane and the width of flank wear land as well as the formation of a continuous chip. the slip angle 1 can now be calculated: 1 (3. 3. We will in this context proceed without the derivation of HUCKS’s formulae. since there the shear stress is at a maximum [Huck51].33) For an ideal plastic body which is approximately realized by steel at the yielding point. the flow plane is under 45◦ in relation to the principle stress direction. we obtain the principle stress and slip directions shown in Fig. and ω. . the resultant forces can now also be calculated. which has an envelope line that is parallel to the σ -axis. With this. the following formulae for cutting and feed force result with width of undeformed chip b and chip thickness h according to HUCKS: ⎡ ⎤ 1 + cot g(φ)⎦ (3.35) Fc = τ0 · b · h · ⎣  2 μ2 + 14 ⎡ ⎤ cot g(φ) Ff = τ0 · b · h · ⎣  − 1⎦ 2 μ2 + 14 (3. only on the friction coefficient and the normal direction of the envelope line [Huck51]. besides its intensity. we can say that the highly numerous experimental relations in chip formation depend only on a few numbers of materials and the friction coefficient. Klocke.  95 . for example. reproducible wear behaviour. Manufacturing Processes 1.1007/978-3-642-11979-8_4. Should we consider all these characteristics simultaneously. bending strength and toughness. Development in the cutting tool material sector is therefore far from finished. are affected by wear. 4. F. However. RWTH edition. Cutting tool materials should have the following properties in order to do justice to the stresses placed on them: • • • • • • • • • hardness and pressure resistance. Developments in the cutting tool material sector are concentrating on optimizing and modifying chemical composition. inner bonding strength. 4. machine and labour costs. we are then faced with the requirement for the “ideal” cutting tool material (Fig. and with them both manufacturing times and tool. high temperature strength. edge strength.Chapter 4 Cutting Tool Materials and Tools Tool change times. geometry and coating in order to broaden the areas of application of cutting tool materials and tools in accordance with the requirements of modern production. 4. there is no one cutting tool material that unifies all the required properties. is the physical opposition of hardness and toughness. small propensity to diffusion and adhesion.1 Overview of Cutting Tool Materials The cutting tool materials used in cutting can be summarized as (Fig. manufacturing methods. but is constantly aiming both to improve cutting tool materials that are already established as well as to discover new materials for use in the manufacture of cutting tools. Wear is affected in turn by the properties of the cutting tool materials.2): • tool steels. oxidation resistance. abrasion resistance. • cemented carbides. One reason for this.1). C Springer-Verlag Berlin Heidelberg 2011 DOI 10. 4.Ni Cutting .Nb)C-Co TiC/TiN .Ta.4 Cutting Tool Materials and Tools to ol m 1 BN ut ti ng DP 2 Id ea lc Cutting speed Wear-resistance and heat-resistance at er ia l 96 Al2O3 ceramic Coated cermet Al2O3 + TiC Si3N4 ceramic Cermet Coated cementedcarbide Cemented-carbide on tungsten-carbide-basis Fine grain and submicron grain cemented carbide Coated HSS DP: 1 Considering the hardness 2 Considering the hardness and the temperature HSS Durability and bending-strength Feed Fig.Co. 4.ceramics Oxide-ceramic Mixed-ceramic Silicon-nitride-ceramic Super-hard cutting tool materials Boron-nitride Diamond Fig.2 Classification of cutting tool materials for machining .1 Schematic classification of several cutting tool materials Cutting tool materials for chipping with geometrical defined cutting edge Cold work steels Tool steels High speed steels Cemented carbide WC-Co WC-(Ti. The goal of DIN ISO 513. 4. diamond or boron nitride. DIN ISO 513 provides six main application groups and thus six classes of workpiece-materials. The hardness and wear resistance of the cutting tool materials increases in this order. for which they are most suitable for cutting. N. K. On the other hand. An application group thus contains comparable cutting tool materials of different manufacturers. Examples are HW-P10.3 are valid for other hard cutting tool materials based on ceramic. “Cermets” – cemented carbides based on titanium boron nitride – are notated with HT. depending on wear resistance and toughness. ceramic. Tool steels include unalloyed and alloyed cold work tool steels as well as high speed steel. Corresponding abbreviations from Fig. Every main application group is subdivided into particular application groups (Fig. these now new major application groups were provided with the labels N for nonferrous metals. diamond and boron nitride are classified in accordance with Fig. 4. These are notated with a code letter for the main application group they belong to and with an index.1. . M. It is also possible for a cutting tool material from one manufacturer to be categorized under multiple application groups if suitable to them. 4. which are classified with the code letters P.4. although they may differ in wear behaviour and performance. and CA-K10. and cast iron (K). Manufacturers of cutting tool materials should assign their cutting tool materials to the appropriate application group.3. the notation and application of hard cutting tool materials of cemented carbide. The indices are merely reference numbers indicating a certain sequence. their bending strength decreases in this order. Expanding earlier norms. According to it. coated cemented carbides and coated cermets with HC. stainless steel (M). M and K but now only designate with them the main application groups that comprise the material groups steel (P). is above all their assignment to materials. mixed and non-oxide ceramic. They provide no information about the extent of the wear resistance or the toughness of a cutting tool material. DIN ISO 513 thus maintains the established code letters P. uncoated cemented carbides based on tungsten carbide with a WC grain size of ≥ 1 μm take the abbreviation HW and with grain sizes of < 1 μm the notation HF. The major machining group K included according to the old standard not only cast iron materials but also a number of other materials.1 Overview of Cutting Tool Materials 97 • cutting ceramics and • super-hard cutting tool materials made of boron nitride and diamond. 4. besides that of identifying cutting tool materials. The term cemented carbide refers to conventional cemented carbides based on tungsten carbide as well as “cermets” based on titanium boron nitride.4).1 Classification of Hard Cutting Tool Materials According to DIN ISO 513. 4. S for special alloys and H for hard materials. The higher it is within each application group. the lower is the material’s wear resistance and the higher its toughness. In DIN ISO 513. The index refers to the toughness and wear resistance of the cutting tool material. HC-K20.4). Ceramics is the generic term for cutting tool materials made of oxide. S and H as well as by colour (Fig. but also with other components than oxides CN Silicon-nitride-ceramic. predominantly made of Aluminium-oxide (Al2O3). but coated Fig.98 4 Cutting Tool Materials and Tools Cemented carbide Code-letter Group of materials HW Uncoated cemented carbide. 4. reinforced CC Cutting-ceramic like above. acc. but coated 1) These cemented carbides are also called "Cermet" Ceramic Code-letter CA Oxide-ceramic. to DIN ISO 513 .3 Notation of hard cutting tool materials for machining. predominantly made from tungsten-carbide (WC) with a grain size < 1 µm HT 1) HC Uncoated cemented carbide. but coated Diamond Code-letter Diamond category DP Polycrystalline diamond DM Monocrystalline diamond Group of materials Boron-nitride Code-letter Boron-nitride category BL Cubiccrystalline boron-nitride with a low content of boron-nitride BH Cubiccrystalline boron-nitride with a high content of boron-nitride BC Cubiccrystalline boron-nitride like above. predominantly made from tungsten-carbide (WC) with a grain size = 1 µm HF Uncoated cemented carbide. predominantly made from silicon-nitride (Si3N4) CR Cutting-ceramic. based on aluminium-oxide (Al2O3). predominantly made from aluminium-oxide (Al2O3) CM Mixed-ceramic. predominant titan-carbide (TiC) or titan-nitride (TiN) or both Cemented carbide like above. but coated Diamond Code-letter Group of materials DP Polycrystalline diamond DM Monocrystalline diamond Boron-nitride Code-letter Group of materials BL Cubiccrystalline boron-nitride with a low content of boron-nitride BH Cubiccrystalline boron-nitride with a high content of boron-nitride BC Cubiccrystalline boron-nitride like above. We distinguish between cold work steels.2 Tool Steels DIN EN ISO 4957 defines tool steels as high-grade steels suited to machining and processing materials as well as handling and measuring workpieces. acc. nickel and cobalt. increasing wear-resistance of the cutting tool materials Increasing feed.2 Tool Steels 99 Main application group Code-letter P Code-color Workpiece-material blue Steel: All kinds of steel and cast-steel. cast-iron with ductile graphite. nonmetal materials N01 N10 N20 N30 N05 N15 N25 S brown Specializations and titanium: Highly heat resisting special-alloys based on iron. titanium and titanium based alloys S01 S10 S20 S30 S05 S15 S25 H grey Hard materials: Hardened steel. increasing durability of the cutting tool materials Fig. 4.exept stainless steel witch austenitic structure Application group Hard cutting tool materials P01 P10 P20 P30 P40 P50 M01 M10 M20 M30 M40 P05 P15 P25 P35 P45 yellow Stainless steel: Stainless austenitic and austenitic-ferritic steel and cast-steel red Cast-iron: Cast-iron with flake– graphite. hot work steels and high speed steels.4 Application and classification of hard cutting tool materials. castiron for chill-casting H01 H10 H20 H30 H05 H15 H25 M K N M05 M15 M25 M35 Increasing cutting speed. annealed cast-iron K01 K10 K20 K30 K40 K05 K15 K25 K35 green Nonferrous metals: Aluminium and other nonferrous metals. to DIN ISO 513 4. Cold and hot work steels are suited to purposes in which the surface temperature during engagement .4. hardened cast-iron-materials. hand saws. pressure casting dies or tools for forging machines and extrusion presses.9 115CrV3 1. quenching in an oil or water bath (a high cooling speed is necessary.07 – 0. shear-blades for cutting of steel.35 Files. Tools for both cutting and forming are made out of cold work steels and high speed steels.40 0.42 0.35 0.6 – 0.25 – 0.10 – – – 1.50 – – – 0. martensite hardness) and tempering (with the goal of a partial loss of hardness in order to increase toughness).0 1. Unalloyed tool steels (carbon steels) contain up to 1.12 – – – – Files.20 0. ductors. 4.5 Examples for alloyed and unalloyed cold work steels Cutting-tools.15 0. bits C85W 1. chisels. 4.2.0 0.5) [DIN EN ISO 4957. chisels.90 0. 4.5 – 0.8 X210CrW12 1. scissors.20 – – – 1.25 0. Hot work steels are used primarily for forming tools such as forging dies.25 0.10 0.1830 0.2210 1.30 0.20 0.30 0.1563 1. axes.40 0.15 0.7 – 1.2328 0.15 11. broaches.40 0.40 0. hatchets.80 Hammers.60 – – – 0. ductors. High speed steels on the other hand can be applied at temperatures of up to 600◦ C.1730 0.10 0.50 0. Wegs95].25 0.85 – – – 0. paper knifes Alloyed cold work steels 45CrMoV7 1.30 1.25% C Unalloyed cold work steels Material grade Material number C Si Composition Mn Cr Mo V W Examples C45W 1.70 Wood saws.80 0.30 0.47 0.100 4 Cutting Tool Materials and Tools is generally under 200◦ C.2436 2. steel blades for segmentbuzz saws C125W 1.40 0. consisting of heating to austenitizing temperature. Cold work steels can be classified as unalloyed and alloyed steels (Fig. woodworking-tools .0 Chisel of all kinds 0.1 Cold Work Steels Cold work steels obtain their wear and toughness properties from heat treatment.0 – – – – 2.05 – – 0. Paper knifes 0.8 Fig. screwdrivers.45 12.10 0. milling cutters and turning tools for grooving and parting-off operations as well as for finishing. which limits the cutting speeds with which they can be used. In contrast to cold work steels. retention of hardness and high temperature strength (chrome.2. tungsten. which. cobalt and chrome as their main alloy elements.5). and they have higher hardness. molybdenum.2 Tool Steels 101 and small amounts of Si and Mn. The hardness and wear resistance of unalloyed tool steels depend on their martensitic structure.5% Cr. twist drills. Hardness and wear resistance are increased by the martensite tempered in the matrix and the embedded . partially precipitated as stable special carbides. Alloyed tool steels have about 1.25% C as well as up to 1. Moreover. Their hardness of about 60–67 HRC is preserved up to 600◦ C in temperature. yet toughness decreases simultaneously and thus the material’s sensitivity during heat treatment and tool use becomes greater. High speed steels are high-alloyed steels containing tungsten. They can be used at cutting temperatures of up to 200◦ C. The advantages of alloyed tool steels in contrast to unalloyed are the increase in wear resistance (addition of carbide-forming elements). This new group of cutting tool materials was called “high speed steels” (HSS). 0.2 High Speed Steels A new cutting tool material was introduced for the first time in the year 1900 at the world’s fair in Paris. 4. gouges. the alloy elements dissolved in the matrix W. thread die cutting) and to manufacture tools for repair work. threadcutting tools.4. They have relatively high bending fracture strength and thus have favourable toughness properties. are responsible for retention of hardness. they continue to have a broad range of application.2% W. V and Co. allowing for improved hardenability. 4. their matrix is characterized by improved retention of hardness. All unalloyed tool steels are shell hardeners. since their cost is lower than high speed steel (HSS) tools because they have fewer alloy elements. 1. Mo.2%V. The productivity of machining processes could thus be increased considerably. Their area of application extends mainly to hand tools such as files. the critical cooling speed is lower. cold work steels are used only rarely for metalworking on machine tools. reamers or on saw blades for woodworking (Fig. While the hardness of high speed steels are affected by the hardness of the base material (martensite) and the number and distribution of the carbides. with which TAYLOR could realize significantly high cutting speeds.5% Mo and 1. i. molybdenum. Wear resistance increases with hardness and carbon content. Due to both this and their machinability. they do not harden all the way through along the entire cross-section but only on the surface of the workpiece. vanadium. reamers. especially for tools with sharp cutting edges and small wedge angles such as broaching tools. vanadium alloys) and in their higher hardness (carbon in solution). They are used above all in steel machining with low cutting parameters (reaming.e. Due to their low hot hardness. 5 – – (+) + HS10-4 ...5 – – + – HS12-1. High speed steels are classified according to their W and Mo content into four alloy and performance groups (Fig.102 4 Cutting Tool Materials and Tools carbides (especially Mo-W double carbides.10 – – (+) + HS6-5-2 – + – – – – (+) + HS6-5-2. This alloy is used in the production of turning tools and profile steels of all kinds. 4.2.1 Classification of High Speed Steels High speed steels are denoted with the letters “HS” and the percents of alloy elements in the order W-Mo-V-Co (e. for finishing tools..2.3 Fig. Its Co content also gives it high hot hardness and retention of hardness. cast irons that are difficult to machine. Composition W .6 Alloy and performance groups of high speed steels . Cr and V carbides).average load . nonferrous metals and non-metallic materials.6) [DIN EN 10027a]... .4 . The type containing Co HS18-1-2-5 has good retention of hardness.Mo . Carbide formation and hardening is promoted by additional alloying of chrome.Co I II III IV For conditioning of steel under.g. Group I includes the steels that contain high amounts of tungsten (18% W).5 – – + – HS2-9-1 + – – – HS2-9-2 – + – – HS2-10-1-8 – – + – 18% W 12% W 6% W + 5% Mo 2% W + 9% Mo HS6-5. Tools made of this alloy are used for roughing high strength steels. 4..highest load Symbol < 850 N/mm2 > 850 N/mm2 Roughing Finishing HS18-0-1 + – – – HS18-1-2.V . 4. They are suitable for machining highly heat-treated Cr-Ni steels and non-ferrous metals. The material HS12-1-4-5 of the group with 12% W has excellent wear resistance due to its high V content. HS10-4-3-10).3 . high performance milling cutters and automatic tool machines. the universal or standard type HS6-5-2 is used [Habe88]. improves hardening and toughness. a higher amount of alloy elements means higher tool costs. Steels containing molybdenum are especially tough. 4. tappers. • Carbon: the source of hardness in the base material. • Vanadium: exists as the primary carbide VC and increases wear resistance (finishing). They are characterized by high toughness. • Molybdenum: forms carbides.7 provides a list of the main applications of high speed steels [DIN EN ISO 4957. increases hot hardness. milling. participates in carbide formation. which has negative consequences particularly when manufacturing complicated moulding tools.2 Areas of Application Figure 4. retention of hardness and wear resistance. machinability becomes more difficult. It is suitable for manufacturing machining tools for roughing or finishing such as twist drills.2. temperature resistance and retention of hardness. As a result of its balanced alloy structure. This use of high-alloyed high speed steels lends itself especially to the solution of machining problems in which an increase in high temperature strength or . • Cobalt: shifts the limit of overheating sensitivity to higher temperatures. At the same time however. Generally. especially for highly stressed milling tools. its high toughness and good wear resistance. Steels of both of these groups that have low amounts of or no cobalt are used to manufacture tools of all kinds.4. hobs). Molybdenum replaces tungsten from metallurgical points of view and is more effective at equal masses because it has an approximately twice as large volume percentage due to its lower density.2. increases hot hardness. turning. making it possible to achieve higher hardness temperatures. Types containing cobalt are used for machining with simple tools when robust stresses are expected (drills. As the amount of alloy elements increases. this alloy is very diverse. They are universally applicable for roughing and finishing tasks. so does the effectiveness of these cutting tool materials with respect to improved wear resistance and lifetime. • Chrome: improves hardenability. Most carbides are dissolved and the hot hardness increases. retention of hardness and wear resistance. broaches etc. reamers. Increases wear resistance because it forms carbides. Usually. The most important group are high speed steels with 6% tungsten and 5% molybdenum. planing and broaching tools.2 Tool Steels 103 The last two groups are represented primarily by steels containing tungsten and molybdenum. milling cutters of all types. The basic effect of alloy elements in high speed steels is summarized once more in abbreviated form in the following: • Tungsten: forms carbides. molybdenum can replace tungsten (has half its density!). The cost effectiveness of a manufacturing process is thus also determined by the choice of high speed steel. Wegs95]. reamers.g. counterboring drills. 4. HS6-5-2-5. highly loaded twist drills and taps.2. to DIN EN ISO 4957 toughness would have a great effect. roughing-tools with high durability HS10-4-3-10 1.3243 High speed milling cutters. 4.and cold working-tools. such as the qualities HS12-1-4-5 and HS10-4-3-10. the cast-metallurgical fabrication of high speed steels remains the most economically important. High speed steels alloyed with cobalt (e.3344 Highly loaded taps and reamers. reamers.3348 Milling cutter. broaching-tools HS2-9-1-8 1. cutting and shaving wheels HS6-5-2-5 1. turning. . taps Fig. twist drills. DIN EN ISO 4957 Main use HS6-5-2 1. are characterized by improved wear properties at increased temperatures stresses and are suited to machining tasks that place the highest requirements on the wear resistance of the tools. Despite attempts to develop alternative fabrication techniques.3247 End-milling cutter. broaching-tools.and highly loaded milling cutters. automated works. pull broaches. turning-tools for automated works. A comparison of both fabrication methods is provided in Fig.8. woodworking.and turning-tools of all kinds. high speed milling cutters. broaches.3 High Speed Steel Fabrication High speed steels can be fabricated using cast-metallurgical [Habe88] or powdermetallurgical methods [Duda86. buzz saws HS6-5-3 1. twist drills. acc.7 Main applications of the most important high speed steels. planing.2.3207 All-purpose usage for roughing.and finishing-works.3343 Standard high speed steel for all cutting tools for rough turning or finish turning.104 4 Cutting Tool Materials and Tools Steel grade Material name according no. planing-tools. taps and twist drills. HS18-1-2-5) are ideal for machining tasks that place increased requirements on the high temperature strength of the tools. woodworking-tools HS2-9-2 1. threading dies. milling cutters of all kinds. Beis82]. 4. Steels that contain vanadium in addition to cobalt. 4. 4. only a small part of the block is fluid during remelting.2 Tool Steels Casting 105 Recast (ESU) HotMachining deformation Cast-metallurgical fabrication Melting Powder-metallurgical fabrication Gas spray aerated Housing powder Water spray aerated powder Hot isostatic pressing Heattreatment Grinding Hot Machining deformation Cold Sintering HotMachining isostatic ρ < 100 % deformation pressing Softening Cavitypressing Sintering ρ = 100 % Hot isostatic repressing Fig. In order to avoid cracks or warpage during hardening. Fig. not the whole block. This is followed by a forging process (to break up the ledeburite and carbides) with a possible intermediate heating of the workpiece. especially in the case of tools that are irregular or difficult to mould. as well as by rolling (1200◦ C). Conventionally produced high speed steels thus tend to segregate (demix) during the hardening phase. electro slag remelting (ESR) can further improve the degree of purity and toughness of high speed steels.9). High speed steels require a highly complex hardening sequence due to the high amounts of different alloy elements. 4. Independently of previously applied measures such as improved melting process or seeding as well as improved design of the block format. Such phenomena are usually causes of lifetime deviations in later tool use. The disadvantage is the higher fabrication cost. it is then subjected to block tempering (900◦ C) (homogenization. it is advisable to anneal the tools with a minimal amount of stress in order to reduce machining stresses before the last mechanical processing. . In this way. The softened condition is in most cases the post practical both for cutting and cold forming and offers the most favourable output structure for hardening.8 Production-sequence of the powder. macroscopic demixings such as carbide agglomerations can be reduced and slag inclusions removed.and cast-metallurgical fabrication of high speed steels Cast-Metallurgical Fabrication After melting and casting (1550◦ C) the steel in moulds. In this process. which is especially pronounced in the case of large cast blocks and high-alloyed steels. Softening improves machinability. 9 Cast-metallurgical fabrication and heat treatment of high speed steels . each 0. An ne al in al in g g al in air 2. 400 °C (in salt bath) C 2nd preheating stage 850 °C D E F G 3rd preheating stage 1050 °C Hardening temperature ~1200 °C Hot bath 500 bis 600 °C 50–80 °C Fig.106 4 Cutting Tool Materials and Tools HS10-4-3-10 HS12-1-4-5 HS12-1-4 HS6-5-2 1280 1240 1240 1240 1230 3× 560 °C 2 × 570 °C 1 × 550 °C 2× 570 °C 2× 560 °C 2× 540 °C Code = Grade HS18-1-2-5 Hardening temp. 4.5–1 h 1600 1000 800 600 400 200 Block turning 1200 Melting and casting Temperature T / °C 1400 Block tempering Forging Rolling Softening 0 Slow oven cool-down F B If heated in a vacuum furnace Air Air g ne An 3. A ne oil/air C Annealing duration each approx 1–2 h An D Annealing 1. E Balancing-temperature Finishing works Preparatory work Temperature T / °C St re s tre s re at lie m f en he t at W ar m in g up Au st en iti sa tio n Q ue nc hi ng Time t /h Air G Time t / h A 600 to 650 °C B 1st preheating stage aprrox. [°C] Annealing. Figure 4. Due to their structural composition. After soft deformation. or the standard designation of the corresponding cast-metallurgically fabricated high speed steel is applied. On the other hand. Gas spray aerated powder is housed and undergoes hot isostatic pressing. PM steels are characterized by a homogeneous structure (no carbide segregation) with an even distribution of fine carbides. the aim is to reach the full theoretical density. the American nomenclature is often used. Numerous cutting experiments have shown that they are at least equal to conventional high speed steels with the same nominal composition. Powder-metallurgically obtained high speed steels have become increasingly important in recent years. Water spray aerated powder undergoes a cold isostatic pressing after softening. such as nickel-based and titanium alloys. Final pressing takes place later in a forging process downstream. Sintering is not performed until the full theoretical density is reached. They are designated according to the company that manufactures them.2. PM steels have better grindability and higher toughness.4 Heat Treatment of High Speed Steels HSS tools are subjected to heat treatment in order to give them their required hardness [Habe88].4. especially when cutting tool materials that are difficult to machine. the semi-finished products are conventionally machined into tools.8). With respect to their efficiency as cutting tools.9 shows a time-temperature chart for the heat treatment of high speed steel HS6-5-2. The practical consequences of this are reduced warpage in both transverse and longitudinal directions during hardening and annealing due to the more homogeneous microstructure – an especially important aspect in the case of tools that are long or are premachined with a small stock allowance – as well as improved toughness resulting from the higher degree of purity and the generally more homogeneous structure. PM steels become advantageous under high mechanical stress resulting from greater feeds.2 Tool Steels 107 The advantage of the ESR process is that the end structure is more even. powder-metallurgically fabricated high speed steels have no material indexes of their own. . 4. The sintering process can sometimes be followed by a hot isostatic repressing. As opposed to cast-metallurgically fabricated high speed steels. First they are hardened (heating and maintaining an austenitisation temperature followed by high-speed cooling) and then annealed several times. and then the semi-finished products are sintered in a vacuum furnace.2. mouldings such as indexable inserts are hardened in matrix presses. Powder-Metallurgical Fabrication The starting materials for powder-metallurgical fabrication of high speed steels is powder obtained by means of gas or water spray aeration of the molten mass (Fig. they generally have a somewhat higher alloy content. powder-metallurgically fabricated high speed steels vary to some degree. During sintering. 4. To simplify their classification. has a small amount of segregations and a high degree of purity. Until now. 10 is a schematic depiction of the processes that occur during the annealing of high speed steels. high speed steels must be heated very slowly in order to avoid heating stresses and cracks. this second increase in hardness leads to hardness values that are clearly higher than those existing after quenching. the microstructure of high speed steels consists of martensite. residual austenite and carbides. coarseness must also be avoided. Quenching intensity. nitrogen or other gasses. Warming up thus generally takes place in two or three preheating stages at 400. which in the case of gasses is much lower than with liquid media. a temperature of 1150–1250◦ C is held for hardening. The cutting tool material hardness resulting M ~ 66 M2 S A2 Hardness/HRC M1 A1 A Annealing temperatureT /°C ~ 600 Fig. 4. quenching can take place in different media. 850 and 1050◦ C. High speed steels are annealed at temperatures of 540–580◦ C. hot baths (salt or metal molten baths) or gas.10 Schematic illustration of cutting tool material hardness resulting from the superposition of various effects during the annealing of high speed steel . Depending on the critical quenching speed and on the shape and size of the workpieces. oil.108 4 Cutting Tool Materials and Tools Due to their poor heat conductivity. including water. The goal of austenitisation is to dissolve as many carbides as possible. The tools must therefore undergo annealing after hardening. is not dimensionally stable during use and exhibits an increased fracture tendency. due to the high amount of residual austenite and the instability of the structure. it has insufficient wear resistance. After quenching. This is defined as the increase in hardness at higher annealing temperatures after an initial decline in hardness from annealing at low temperatures. the temperature range of secondary hardness. The hardness of this structure is unsuitable for cutting tools. Under normal hardness and annealing conditions. Gaseous media include air that is still or in motion. Figure 4. On the other hand. can be increased by raising the flow speed and pressure. After reaching the level of austenitisation. whereby the dissolution of the secondary carbides increase with a rising hardening temperature and extended holding times. carbide precipitation from the residual austenite and the conversion of residual austenite into secondary martensite – lead to the increase in hardness known as the secondary hardness effect. highly wear-resistant hard material coats such as titanium nitride (TiN) or titanium aluminium nitride (Ti. if austenitisation occurs correctly. Their advantages are optimal heat transfer and the possibility of executing partial heat and surface treatments. The cause of this is precipitation of the carbon trapped in solution and the associated reduction of crystal lattice tension.5 Surface Treatment The wear properties of HSS tools can be further improved by treating the surface by nitrating (enriching the surface layer with nitrogen by thermochemical treatment at temperatures of 500–580◦ C). chrome-plating (deposit of a hard chrome layer with a thickness of 5–50 μm at temperatures of 50–70◦ C) or coating (PVD coating with thin. As the annealing temperature increases. All three processes – carbide precipitation from the martensite. The last few years have seen increasing amounts of heat treatments taking place in vacuum furnaces. Extremely finely distributed carbides precipitate from the martensite (M) above 350◦ C and from the residual austenite (A) above around 450◦ C. the matrix becomes low in carbon and alloy elements. The advantages of this method are above all its environmental friendliness as well as the clean and shiny workpiece surfaces it yields.2. .2. Until now.2. which leads to the decline in hardness observable during annealing at up to about 350◦ C. The maximum of the hardness profile is shifted to higher temperatures (560◦ C). causing another increase in hardness (M2/A1) due to precipitation hardening.Al)N. hardness can be higher after annealing than in the hardened state [Habe88].4. The precipitation of the carbides acts against this process. Quenching the workpieces from hardening temperature is achieved by flooding them with nitrogen at a pressure of 10 bar.2 Tool Steels 109 from the superimposition of various effects is plotted as the cumulative curve S as a function of the annealing temperature. The gas is circulated such that it flows at high speed through the batch and then through a heat exchanger for recooling. Components are quenched in this manner to 50◦ C. and.6 HSS Indexable Inserts Indexable inserts made of high speed steel can be manufactured by precision casting. steam annealing (formation of a thin iron oxide layer on the tool surface in a steam atmosphere at about 500◦ C). the hardness of martensite (M) decreases continuously (M1). 4. by machining pre-products or directly using powder-metallurgical methods. heat treatment usually took place in salt baths. 4. The important disadvantages are the high costs involved in detoxifying the salts and the necessity of cleansing the workpieces of salt residue after treatment. As a result of carbide formation.2. so that part of the residual austenite is converted to secondary martensite (A2) during cooling from annealing temperature. but they assigned to the non-oxide ceramics as “metallic hard materials” [Horn06.g.4 Cutting Tool Materials and Tools X5CrNi13–4 n = 65 min–1 v f = 35 mm/min Cutting speed vc 70 m/min Material: rpm: Feed velocity. this technology allows us to fit tools with various fit-for purpose cutting tool materials (Fig.3 Cemented Carbides Cemented carbides are composite materials.P30 Fig. electrical conductivity). 4. cermets contain titanium nitride. which are embedded in a soft metallic binder phase made of cobalt and/or nickel. the most important advantage of sintered HSS indexable inserts is their increased flexibility in tool use due to the use of indexable insert technology. In the case of manufacturing processes such as milling or drilling. The binder phase is responsible for binder the brittle carbides and nitrides to a relatively solid body. Wähl86]. Salm83]. .: Profile milling cutter for rotors Requirements for the cutting tool material High Sufficiant temperature wear ductility resistance at low vc HSS 98 m/min 110 Cemented carbide 345 480 Diameter D/mm Indexable inserts HS10-4-3-10 P20 . Additionally. Hard materials are the bearers of hardness and wear resistance. 4. Indexable inserts made of precision casting have so far gained no importance for steel machining [Bong91.11). powder-metallurgical fabrication of HSS indexable inserts for example exploit the cost-intensive alloy elements much more fully.11 Fit-for-purpose design of indexable inserts composed of high speed steel and cemented carbide (Source: Fette) While pre-products still require further machining. The carbides are on the boundary between metals and ceramics. Besides lower manufacturing costs and more efficient utilization of material. 4. They still have metallic properties (e. They are composed of the carbides of transition metals (fourth to sixth subgroup of the periodic system). pressure resistance and hightemperature wear resistance. labeling WIDIA 1928 Carboloy (General Electric) 1931 WC-TiC-Co cemented carbide 1934 TiC-Mo2C-Ni (1st generation of Cermets) producer Plansee.3 Cemented Carbides 111 The advantages of cemented carbides include good structural uniformity due to their powder-metallurgical fabrication. cemented carbides were first introduced as new high performance cutting tool materials at the Leipzig Trade Fair under the name WIDIA (Fig. K. Cemented carbides made for patent reasons without tungsten based on TiC and Mo2 C with Nickel as the binder proved in many cases to be 1923 Patent for the fabrication of WC-Co-HM. 4.12 Milestones in the development of cemented carbides . Initially. Böhlerit (Böhler) 1942 Coromant (Sandvik) 68/69 Implemention of coated cemented carbide 1970 Fine grain cemented carbide 1973 since 73/74 Spinodal .12). 4. could be machined easily with the new cutting tool material. labeling Titanit S 1937 Titanit (Plansee). In contrast to the high speed steel tools used hitherto. WC-Co cemented carbides were used exclusively to machine cast iron materials. This period also saw the development of the first cermet. producer Krupp. Schröter 1927 Leipzig Trade Fair: 1st WC-Co-HM. This changed with the introduction of titanium carbide as an alloy component. 4.1 Historical Development In 1927. Due to the high amount of crater wear. cemented carbides have the same hardness as high speed steel at room temperature. At 1000◦ C. With cemented carbides containing TiC.cemented carbide (1st Cermet with TiN) 1993 Submicron grain and nano crystalline cemented carbide Accelerated developing of Cermets Fig. these cemented carbides were not suited to machining long-chipping steel materials. Materials such as chilled cast iron. similar sensational cutting speed increases were made possible in steel machining as previously was the case when machining cast iron with WC-Co cemented carbides.4. Kola92]. cemented carbides allowed the use of twice or three times the cutting speed with the same cross-section of undeformed chip. opening up completely new dimensions in cutting technology. The capacities of the machine tools of the time were far beneath the potential of the new cutting tools. It is also possible to manufacture types of cemented carbides with various properties by means of intentionally changing the amounts of hard material and binder [Sche88.3. high hardness. which had been very difficult to cut with HSS tools. This was a revolutionary development at the time. Spri95]. Kola92]. 4. The combination of tough cemented carbide substrates with highly wear-resistant hard material coats led to an enormous increase in possible cutting speeds and tool standing times. both hardness and bending strength could be increased with the same amount of binder [Sche88.e. Häus90. With the development in 1973 of “spinodal” cemented carbides. The individual components of the cemented carbide are weighed out as powder to a batch and homogenized in mixers. Multi-layer coats on substrates with binder-rich rim zones or gradient structures are new developments [Sche88. The fabrication method used depends mainly on the geometry and quantity of the product to be manufactured [Sche88. Kola93. The CVD and PVD methods are the most important coating process variants today. very different forming methods can be used. Due to the great variety of shapes that cemented carbide components can assume.112 4 Cutting Tool Materials and Tools excessively fragile for the machinery of the time [Sche88. hexan) should protect the powder from oxidation during . drilling. which contained titanium nitride as a further hard material component. the basic form of today’s highly efficient cermet was created. combined fabrication and special methods (injection moulding). ÖP31. making these cutting tool materials especially interesting for cutting operations with high thermal stress on the cutting edge. the grinding fluid (alcohols. Kola92. By reducing WC crystallite size to under 1 μm. using additional machining steps such as cutting. production and cutting performance. i. whereby cemented carbides can be increased in hardness only with a reduction in toughness. Kola92]. The starting material is a cemented carbide in a pre-sintered or cold-isostatically pressed condition.2 Cemented Carbide Production Cemented carbides are fabricated using various powder-metallurgical means (Fig. the consistency of which is still chalky [Sche88. Kola93. This is due to their high chemical stability and high-temperature wear resistance. Complicated moulding in small quantities. Further development of cemented carbides in the following years led to continuous improvement of their composition. The influence of carbide grain size on the properties of cemented carbides was recognized early on. screws. Gill95]. rolling rings or matrices are made indirectly. turning or milling. such as pistons. acetone. About two thirds of all cemented carbide products are manufactured by direct fabrication. The relation. 4. Cermets are today among the high performance cutting tool materials that meet the demands of modern cutting technology excellently by allowing for the use of high cutting speeds with moderate feeds and realizing long standing times with a high level of reliability. Kola92. primarily indexable inserts. The most general distinction is between direct and indirect fabrication. could be overcome with the development of fine grain cemented carbide.13).3. The introduction of coated cemented carbides at the beginning of the 1970s was another great innovation. In the wet grinding following this. DRP23. the first Cermet. Spri95]. The granulate is produced with the help of spray drying or granulation processes. loosely bonded powder material (pressed part. wet grinding. Daub95].4. separating Coating Finished products Grinded mouldings for wear protection Thick beams Profiles Drills.13 Fabrication sequences for the production of workpieces made of cemented carbide (Source: Widia) grinding and guarantee optimal dispersal of all the components in the suspension. green compact) is increasingly compressed due to diffusion-controlled place-shifting processes (surface diffusion. After the grinding process is finished. wet-sieving Pressing additives Vacuum drying. VC. Shaping the indexable inserts is achieved by compressing the granulate in matrix presses. milling cutter Indexable inserts Mining tools Small bearing parts Short beams Beams. profiles Extrusion Vacuum drying dispersing HM-injection moulding material Injection moulding Debindering Sintering HIP Grinding. In principle. To produce indexable inserts.Nb)C. Within the fabrication sequence. grain . Cr3C2 Blending.3 Cemented Carbides 113 Tungstencarbide WC Auxiliary metal Co. the powder mix is prepared in accordance with the subsequent shaping method. the powder is transformed into granulate in order to assure good flow properties and a suitable granulate size for compression into matrices. 4. since it is here that the component obtains the mechanical and technological properties that are essential for its functional capability. screening Spray drying HM-powder Granulation HM . Mo2C. Ni Alloying elements TiC. sintering process is probably the most important operation. profiles d < 22 mm Drills with integrated coolant bore Small mass products with complex geometry Fig.Granulate Cold isostatic compression Cold isostatic compression Machining Debindering Sintering Plasticising Vacuum drying dispersing HM-modelling material Powder compaction Debindering Sintering/ HIP Semi-finished products beams. (Ta. sintering is defined as a thermally activated material transport in which the compressed. The matrix press method allows for short cycle times and is thus especially suited to producing large quantities [Kola92. a further thermally activated material transport of tungsten and carbon takes place in accordance with the solubility of WC in Co. The applicable cutting speed is limited at higher temperatures due to its tendency to dissolution and diffusion. the entire binder phase. In addition. Up to a temperature of 700◦ C. Nb)C in cemented carbides. tungsten carbide precipitates again from the liquid binder phase. is molten. Molten baths first appear at about 1300◦ C. • Co: Cobalt is still unrivalled as a binder metal for cemented carbides based on tungsten carbide. but rather (as it is called in professional jargon) sintered with liquid phase [Sche88. • NbC: NbC has a similar effect as TaC. Cermets containing nitrogen have as a rule a very fine-grained structure. The consequence of this is that TiC cemented carbides have considerable high-temperature wear resistance but little bonding and edge strength. Kola92]. tantalum carbide has a grain-refining effect. Their use is preferred when cutting steel materials with high cutting speeds. WC is soluble in Co. At a sintering temperature of about 1400◦ C.3 Components of Cemented Carbides and Their Properties • WC: Mono tungsten carbide is the most important hard material phase in technical sintered cemented carbides. Both carbides appear as a mixed crystal (Ta. linear shrinkage can reach 20%. Atoms of the hard material phase still dissolved after solidification in the binder phase stabilize the cobalt-rich binder phase in the cubic crystalline crystal lattice structure until room temperature. thus improving toughness and edge strength. It is decisive for fabricating cemented carbides that not the entire alloy system is converted to a molten state. • TiN: Titanium nitride is the property-determining component in all modern cermets. WC is also even more wear-resistant than TiC and TaC. grain growth is inhibited. penetrates into all the pores and causes the hard material particles to glide together under the influence of surface tension into the smallest space. the inner bonding strength does not decrease as sharply as is the case for TiC. This is due to the high level of solubility of WC in cobalt and . • TaC: In small amounts. Tungsten carbide dissolves increasingly in the molten bath. In a solid state. The physical properties of cermets are based on those of titanium carbon nitride. composed of cobalt and tungsten carbide. otherwise the cobalt converts into hexagonal form below 417◦ C [Sche88.3. • TiC: Titanium carbide has a low tendency to diffusion. This particle rearrangement and re-precipitation process leads to tight packing with minimal surface energy. resulting in the high inner bonding and edge strength of WC-Co cemented carbides. TiN in steel is even less soluble and is thus more resistant to diffusion than titanium carbide. Kola92]. Depending on the green density and sintering parameters. 4. volume diffusion). TiC can join with WC to make a composite carbide. Cemented carbides high in TiC are therefore brittle and fragile.114 4 Cutting Tool Materials and Tools boundary diffusion. The liquid phase wets the carbide. TiC and TiN are completely mixable. Nitrogen causes an increase in wear resistance. As the mass cools from sintering temperatures. Cemented carbides should have high hot hardness and pressure resistance for the sake of increasing the resistance of the cutting edge to plastic deformation. 4. nickel is used as a binder for cermets.14). They should however also have high bending strength and therefore sufficiently high toughness and resistance to cracking.Ta)C TaC Wear resistance TiC Co-Concentration WC-Grain size Composite carbide concentration Cutting tool material attributes High wear resistance High ductility Co-Concentration: Co-Concentration: WC-Grain size: WC-Grain size: Composite carbide concentration: Composite carbide concentration: Fig. cobalt is also added with nickel as a binder in cermets today in order to improve their high temperature properties. crumbling and fracture. 4. fracture toughness is reduced. tough cemented carbides are low in hardness and pressure resistance. But since nickel is more easy to deform than cobalt. Tantalus carbide has a favourable effect on temperature change strength. TiC TaC Bending strength (Ti. As the concentration of composite carbides goes up.14 Influencing factors for wear resistance of cemented carbides . while hardness and pressure resistance are decreased (Fig. These opposing cutting tool material properties are influenced decisively by the microstructure of the base material. In general. Hardness Pressure resistance The properties of the cemented carbide substrate are of key importance with respect to the wear resistance and efficiency of uncoated and coated cemented carbides.4. • Ni: due to the improved wettability of hard materials.3 Cemented Carbides 115 to the good wettability of tungsten carbide crystals by the molten WC-Co binder phase [Sche88]. The tendency is that fracture toughness increases with the cobalt content and average grain size. Mo) (C. Kola93a]. By selective solution. The carbide skeleton is filled with the binder phase. Ta. mainly however the dissolution and re-precipitation processes taking place during liquid phase sintering. this crystal form is less well developed in types that are fine-grained and low in cobalt. 4. Ta. Köni90].15). the Ni-Co molten bath is enriched with carbide components. Cubic mixed crystals exist both in both cubical (with rounded edges) and nearly spherical shape (Fig. Kola89a. the tungsten carbide is usually in the form of prisms with triangular bases.3. W. The core/shell structure of the hard materials is characteristic of their microstructure. Leop87. Kola89. As opposed to cemented carbides containing larger amounts of cobalt. Composite carbides precipitate as a rim zone around the remaining hard material grains when cooling down from sintering temperature [Mosk66. Mo) Ni3 (Ti. Ni (Ti. N) (Ti.116 4 Cutting Tool Materials and Tools This is exploited in many cases and. Al) Main component Schematic Schematic Fig. W) C Binder phase: Co-W-CMixed crystall Binder phase: Co. The causes of this are demixing phenomena in the hard materials (spinodal demixing). Ta. Rudy73. W. The high material costs put a tight limit on the use TaC alloys however. W. Cermet HT-P15 Cemented carbide HW-P10 Chart of reflected electrons Chart of reflected electrons Hard material particles: Hard material particles: TiN (Ti. Nb. in which crystal growth is less hindered. N) WC (Ti. Ta.4 Microstructure In conventional cemented carbides based on WC. N) (Ti. The structure of cermet contains only rounded composite carbon nitrides.15 Microstructure of conventional cemented carbides and cermets . Mo) (C. W) (C. 4. 4. Schi89. in the case of substrates for milling. Kief71. the ratio TiC/TaC is modified in favour of tantalum carbide [Sche88. The back scatter electron image in Fig.2–0.N) in the centre. surrounded by a titanium and nitrogen-rich shell.Nb)C-Co and • TiC/TiN-Co. • WC-(Ti.Ta.N) shell consists primarily of a titanium-tungsten-molybdenum composite carbide containing a relatively large amount of tungsten and/or molybdenum. These are cemented carbides based on: • WC-Co. The lighter (Ti.5 μm). .3.15 shows this varying core/shell structure very clearly. they are not suitable for machining soft steel materials. Deviating from this.5 Classification of Cemented Carbides Cemented carbide cutting tool materials can be subdivided into three groups.5–0. If the initial powder consists of unalloyed binary hard material components. The initial powder consists in this case of single carbides. They can contain up to 0.3. usually titanium nitride or titanium carbide are in the cores (dark cores). which is surrounded by a relatively titanium-rich composite carbon nitride (Ti.3 μm). They are mostly used for short-chipping materials. The hard materials can exhibit widely varied textural structures depending on composition and grain size.Ni. composite carbides and carbon nitrides. 4.Ni.Ta. When prealloyed powders are used consisting of composite carbides or quaternary carbon nitrides.4). 4. 4.W)(C. non-ferrous metals and non-metals.Mo)(C. high temperature materials as well as in stone and wood processing (Fig.8 μm) and ultrafine (0.Ta.N). the standard DIN ISO 513 distinguishes only between cemented carbides with grain sizes ≥ 1 μm (HW) and those with grain sizes of < 1 μm (HF). WC-Co cemented carbides were subdivided in accordance with the average WC grain size in the sintered microstructure into fine (0.4. cast iron materials.3 Cemented Carbides 117 The microstructure of the hard material particles is determined by the components contained in the original powder.W.5. Due to the strong diffusion tendency of tungsten carbide. According to previous terminology. The term “cermet” is now used for cemented carbides based on TiC/TiN-Co.16). The hard material particles with the dark cores have Ti(C. higher concentrations of molybdenum and/or tungsten are also found in the cores (light cores). 4.1 WC-Co Cemented Carbides The cemented carbides of this group consist almost exclusively of hexagonal tungsten monocarbide and the binder phase cobalt. submicron (0.Nb)C as doping additives to control structural fineness and consistency (Fig. WC-Co cemented carbides are characterized by high abrasion resistance. The hard material particles in the light core contain this composite carbide inside. 4.8–1.8 mass % VC and/or Cr3 C2 and/or up to 2 mass % (Ta. 7 Poisson-constant 0. finishing. Moreover. Köni93. Gill01].2 1730 1580 1420 1290 5700 5400 5000 4500 1600 2000 2350 2450 650 630 590 580 Fracture toughness (N⋅m1/2/mm2) 6.2–0.22 0.5 5.3 μm (acc. they have a small inclination to adhesion and to wear by diffusion.6 12.6 5.21 0.22 Heat conductivity (W⋅m–1⋅K–1) 80 80 70 65 Coefficient of thermal expansion (293–1073 K) (10–6⋅K–1) 5.8–1.16 Composition and attributes of conventional WC-Co cemented carbides.8 and 0.22 0.0 5. Daub95.6 14.5 μm in the sintered microstructure. edge strength and toughness. 4. Submicron and Ultrafine Cemented Carbides Submicron and ultrafine cemented carbides have until now been the terms used in literature and practice for WC-Co cemented carbides with an average WC grain diameter of 0.118 4 Cutting Tool Materials and Tools Cemented carbide Application group (DIN ISO 513) HW-K05 HW-K10 HW-K25 HW-K40 WC-4Co WC-6Co WC-9Co WC-12Co 15. to DIN ISO 513 grain size ≥ 1 μm) are still widely used in cutting technology applications where high demands are placed on cutting edge sharpness and toughness.3 12. Valuable submicron and ultrafine cemented carbides are superior to conventional fine-grain cemented carbides in hardness. The small grain diameter gives there cemented carbides a special combination of attributes: reducing the WC crystallite size below 1 μm with equal binder content leads both to increased hardness and bending strength (Fig. These attributes . e.1 14. Drey01. in steel milling. This property makes submicron and ultrafine cemented carbides applicable to a wide variety of tasks [Kola92.9 14. grooving and parting-off or in the manufacture of threads [Kola92].17). Köni90a. 4.5–0. acc.9 9.g.9 Grade Density (g/cm3) Hardness HV 30 Pressure resistance (cylinder test) (N/mm2) (ISO 3369) (ISO 3878) (ISO 4506) Bending strength (N/mm2) (ISO 3327) Young’s modulus (103⋅N/mm2) (ISO 3312) Fig. to Kolaska [Kola92] Conventional Fine-Grain Cemented Carbides (HW) Conventional uncoated WC-Co fine-grain cemented carbides (fine cemented carbides) with an average grain diameter of 0. Group A: Mixed carbide content >10–11 mass %. for broaching. the cemented carbides of this group can be classified according to their use into two subgroups.Nb)C-Co Cemented Carbides The cemented carbides of this group contain.2 WC-(Ti. e. oxidation resistance and diffusion resistance to iron materials. Their main area of application is in machining long-chipping steels (Fig.Tool life T / min (B): Bending strength N/mm2 (H): Hardness HV30 14 12 Fine grain WC-6Co-Cemented carbide Submicron grain Turning of white iron (80 Shore) vc = 16 m/min 4300(B) 3000 2000 Standard 2000(H) 119 1780 Cemented Carbides 1580 4.5.4). tantalum. the cemented carbides of this group are characterized by a high level of .g.18). Due to their mixed carbide content. Drey01.0 mm Indexable insert: SPGN 120308 Sharp-edged 6 4 2 κ r = 75° 0 Standard Fine Submicron grain grain Fig. mixed carbides (MC) made of titanium.1 mm 10 8 ap = 1. 4. Submicron and ultrafine cemented carbides are used whenever high toughness. 4. milling and shaping of heat treated and hardened steels. Compared with WC-Co cemented carbides. they are characterized by improved high-temperature properties. Due to the mixed carbide content. This is especially the case for hot hardness and high temperature strength. Köni93. Köni90a.17 Microstructure and attributes of fine and submicron grain cemented carbides in comparison to standard K-class cemented carbides (Source: Widia) are necessary when hardened materials must be machined with finishing quality and the smallest machining allowances. 4.3 f = 0.3. niobium and/or zirconium carbide (Fig. Daub95. besides tungsten carbide.Ta. in cast iron machining and for machining fibre-reinforced plastics and non-ferrous metals [Kola92. Gill01]. high wear resistance and the highest edge strength is required of the cutting edge. 4. 0 11.5 6.6 11.0 10. Ta.0 82.5. They are utilized above all in steel and cast steel cutting.7 12. acid and heat-resistant steels with austenitic structures as well as for alloyed or hard austenitic/ferritic cast iron materials.5 Density (g/cm3) 10. with the exception of rust and acid-resistant steels with austenitic microstructures.5 Poisson constant 0.7 17. 4.22 0. tantalum.1 9. 4. Group B: Mixed carbide content < 10–11 mass %. a low tendency to diffusion with iron materials and low abrasion.5 10. They are especially suitable for cutting rust.9 6.0 9. Nb)C-Co cemented carbides.18 Composition and attributes of WC-(Ti.0 6. to KOLASKA [Kola92] high-temperature wear resistance.1 13. niobium.22 0. .0 31.5 84.0 10.22 0.3 (ISO 3369) Hardness HV 30 (ISO 3878) 1560 1500 1490 1380 1700 1550 Pressure resistance (cylinder test) (N/mm2) 4500 5200 4600 4450 5950 5500 (ISO 4506) Bending strength (N/mm2) (ISO 3327) 1700 2000 2200 2250 1750 1900 Young’s modulus (103⋅N/mm2) (ISO 3312) 520 500 550 560 580 570 Fracture toughness (N⋅m1/2/mm2) 8.4 6.23 0. Present-day cermets are complex multi-material systems that can contain a number of additional elements such as tungsten.120 4 Cutting Tool Materials and Tools HW HW HW HW HW - - - - - - P10 P15 P25 P30 M10 M15 Composition (mass percentage) WC (Ti.9 9. Ta. molybdenum or complex carbides.2 7.Ni Cemented Carbides (Cermets) Cemented carbides based on titanium carbide and titanium nitride with a Ni.3 TiC/TiN-Co.22 Heat conductivity (W⋅m–1⋅K–1) 25 20 45 60 83 90 Coefficient of thermal expansion (293 K–1073 K) (10–6⋅K–1) 7. The cemented carbides assigned to this group have relatively good high-temperature wear resistance and abrasion resistance.5 25.6 13. acc.5 9.5 10.0 78.3 10.0 6.7 6.0 13.3.5 11.0 72. from which intermetallic phases are formed during sintering.Co binder phase are designated as cermets (formed from ceramic + metal).5 10.0 64. Nb)C Co 60.0 Cemented carbide Application group (DIN ISO 513) HW Fig.23 0. Köni93.3 1.21 Heat conductivity (W⋅m−1⋅K−1) 9.5 82.7 0. Some essential differences to WCbased cemented carbides are their significantly smaller thermal conductivity coupled with their greater thermal expansion. cermets are characterized by their lower density.3 Cemented Carbides 121 Cermet Application group (DIN ISO 513) HT . cermets are especially good for planing steels.8 11.19 [Kola92]. They thus have a strong tendency to form comb cracks.22 0. 4.1 Composition (mass percentage) Carbon nitrides Additive nitrides Co/Ni Density (g/cm−3) (ISO 3369) Hardness HV 30 (ISO 3878) Pressure resistance (cylinder test) (N/mm2) (ISO 4506) Bending strength (N/mm2) (ISO 3327) Young’s modulus (103⋅N/mm2) (ISO 3312) Fig. cermets have been predominantly utilized to machine .6 10. Until now.0 0. high resistance against abrasive wear and low adhesion.1 7. Kloc96].P05 HT .21 0.4 85. resulting in a lower total heating of the cutting insert.4 9.19 Composition and attributes of cermets.0 7. have a low tendency to diffusion and adhesion as well as a high level of high-temperature wear resistance. Near the contact zones however.P20 89. to KOLASKA [Kola92] The composition and some characteristic attributes of three typical cermet types is given in Fig.7 Coefficient of thermal expansion (293 K–1073 K) (10−6⋅K−1) 9.0 Poisson constant 0.9 10.8 13. the temperature gradient increases inside the cutting tool material.7 6. Compared to conventional cemented carbides.P10 HT . 4.5 9.0 15.0 16. The result of this is that cermets react much more sensitively to temperature change than conventional cemented carbides. Due to their lower heat conductivity. this leads to high tensile and pressure stresses in the cutting tool material. Due to their high edge strength. acc.4. especially in interrupted-cut machining [Köni90a.0 1650 1600 1450 5000 4700 4600 2000 2300 2500 460 450 440 Fracture toughness (N⋅m1/2/mm2) 7. a larger portion of the cutting heat is dissipated with the chip. Cermets are very hard. In association with the large thermal expansion coefficient.2 7. Because of their clear tendency to .Nb)C-Co. they allow for higher cutting speeds in comparison with conventional cemented carbides. are used successfully in average rough turning operations and in milling (Fig.3 0. ap = 1 mm Width of flank wear land VB/mm Material: Cutting conditions: 3 20 10 5 Cutting time tc /min 30 vc = 220 m/min f = 0–25 mm ap = 1–1.20). At the same time.05 Coated cemented carbide Cutting speed vc /(m/min) 400 300 C45+N. generally leads to better surface qualities than coated cemented carbides are capable of producing in planing and finishing. The high wear resistance of the cutting edges.Ta. f = 0.1 mm.3 mm ap = 1–1.30 0. But they are also suitable for cutting speeds below 100 m/min.122 4 Cutting Tool Materials and Tools 500 Fine finishing ap < 0. 4. CeramTec) steels with high cutting speeds and small cross-sections of undeformed chip. due to these properties.5 mm Fig. Basic areas of application include both turning and milling.21). Rm = 700 N/mm2 vc = 400 m/min.4 Cermet C45E Tool life quantity: 240 vc = 400 m/min f = 0. 4. The development of tougher cermet types has led to the expansion of their field of application to include more average roughing conditions (Fig.5 mm Finishing ap < 2 mm 200 Rough finishing ap < 3 mm 100 0. in association with the low diffusion tendency and high oxidation resistance.5 mm Tool life quantity: 150 0 0 0. 4.50 0.10 Cermet 0. Tougher cermet types. Cermets also are suitable for grooving and thread turning.20 Field of application of cermets during turning steel (Source: Kennametal Widia.2 Feed f /mm 0. corresponding to the range P15 to P25 of conventional cemented carbides based on WC-(Ti.40 Cemented carbide HW−P10 0.20 0.1 0. 4. 4. finishing. Kloc96]. .: 50CrV4. D = 16 mm. Köni93. D = 8 mm. (Cemented Carbide. e. ae = 1 mm Dry cutting Fig. 250 – 300 HB End milling cutter. Köni90a.21 Milling with cermets (Source: VW-Kassel.6 Uncoated Cemented Carbides and Cermets Uncoated conventional cemented carbides are still used consistently in cutting technology whenever there are high demands placed in cutting edge sharpness and toughness properties. Kennametal) form comb-type fractures. Cermet-inserts) Coolant: Emulsion 100 Material: Tool: Cutting cond. According to DIN ISO 513. in steel milling.3. whose composition and/or microstructure differ in the rim zone and kernel area.3 Cemented Carbides 123 Chamfering of gears 600 150 Cemented Carbide -TiN PVD 50 HSS -TiN Cutting speed (m/min) 300 25 % 70 % 200 Tool life quantity (pieces) Tool life travel path lf / m 260* Cermet Peripheral down-cut milling *Limited by the nmax of the maschine Cermet Cemented carbide K25 0 100 % 200 300 Cutting speed vc / (m/min) Tool costs per workpiece Material: 16MnCr5 End milling cutter.g. The transition from the rim zone to the kernel area is continuous [Berg03].4.3. cermets should always be dry-milled [Drey97. uncoated cemented carbides based on tungsten carbide take the abbreviation HW and uncoated cermets the prefix HT (Fig. 4. z = 3 fz = 0.3).7 Function Gradient Cemented Carbides Function gradient cemented carbides are defined as materials.025 mm ap = 8 mm. (HSS -TiN) Tools: and D = 18 mm. They are however in direct competition with coated cemented carbides and cermets. 4. grooving and parting off operations or in thread manufacture. which are replacing them in an increasing amount of applications. Other method variants include nitrogen pressure treatment. the strength of cemented carbides is generally completely sufficient. e.g.g. In planing operations. The mixed carbides near the surface gradually dissolve. three types of function gradient cemented carbides will be introduced. 4. Such measures are. An edge effect is characteristic of these processes. Moreover. cobalt penetrates from inside towards the outside. and still is. zirconium carbides) can also be contained in these rim zones. via cobalt enrichening and mixed carbide depletion in a approx. Such modifications. Köni90.124 4 Cutting Tool Materials and Tools The goal in the development of function gradient cemented carbides was. The cobalt content in the rim zone is higher than within. which are less hard but more wear-resistant than cubic carbides or carbides of other transition metals (e. in which the cemented carbide contains a defined excess of carbon and a certain sintering regime is adhered to during cooling [Berg97. nitrides. 4. Nb)C mixed crystals (Fig. in which the cemented carbide is subjected to a nitrogen treatment under increased pressure after sintering. the rim zone of which is free of hard and brittle (Ti. nitrogen is introduced into the initial cemented carbide mixture as TiN or Ti(C. The fabrication of rim zones free of mixed carbides can take place in various ways. to improve even further the wear resistance of coated indexable inserts (especially in the case of dynamic stress) by modifying the rim zone. On the other hand.22) and consists practically of nothing but tungsten carbide and cobalt up to a depth of about 50 μm. To reduce impact sensibility. In the most commonly used method. and the creation of zones free of mixed carbides without nitrogen. Yohe93]. The growth rate follows a parabolic temporal law. In the following. Ta.3. In this case. only practical when applied to indexable inserts intended for larger cross-sections of undeformed chip and interrupted cuts.7. however.N). In this way. the end effect of which is that the zone free of mixed carbides is thinner on sharp edges than on even surfaces [Berg97]. In this way. cemented carbide substrates with high hardness but very tough rim zones of lesser hardness and increased resistance against cracking can be produced. a rim zone that takes over or supports the wear-reducing effect of the coating (mixed carbide enrichening) is advantageous [Berg97]. The result of this is the formation of a titanium activity gradient. Titanium diffuses into the interior and accumulates there on existing cubic mixed crystals. a nitrogen concentration gradient arises in the cemented carbide plate from the inside outwards. The nitrogen dissolving in the liquid phase diffuses from the cutting tool material into the surrounding atmosphere. It increases with a rising nitrogen content and decreases with a rising titanium content. The same is valid for tantalum and niobium. 50 μm thick rim zone. .1 Cemented Carbides with Rim Zone Free of Mixed Carbides Gradient cemented carbides are understood as cemented carbides. The titanium nitride decomposes during the sintering process. both cracking and fracture sensitivity of the insert are reduced [Berg97]. The basic principle behind it is the formation of a nitrogen gradient in the rim zone of the cemented carbides [Berg97]. a coarse WC grain is advantageous. In the case of turning operations with tools of the application groups P20–P40.22 Influence of the rim zone on the tool life of CVD tools (Source: Sumitomo) The thickness of mixed-carbide-free rim zones produced in this fashion is up to 50 μm. .3 mm ap = 0.4. indexable inserts with rim zones free of mixed carbides are widely used [Berg97]. tools coated according to this design achieve significantly longer standing times. such toughness-increasing measures are not used. At best a minor cutting edge rounding can still be undertaken. thus designed cemented carbides have significant advantages in their wear behaviour compared with coated ones. Therefore.5 mm Multilayer coating 360 % gradiented rim zone Multilayer coating on gradiented rim zone 100% Tool life quantity P25 Hardness gradient 10 µm 1000 1200 1400 1600 Hardness HV Fig. As has been shown in practice.3 Cemented Carbides 125 Material = C25 vc = 200 m/min f = 0. Especially when turning with interrupted cut. The methods described can therefore only be used for indexable inserts that are already in their final shape after the sintering process. although they would be highly desirable. 4.15–0. in the case of highly accurate milling inserts that are ground.5–1. 126 4 Cutting Tool Materials and Tools 4.7. especially when one succeeds in designing the coating and the rim zone such that neither are clearly delineable from each other after the coating process [Leng06.g.N)-rich rim zones showed clearly improved wear behaviour compared with uncoated cemented carbides. They can however also be conventionally coated. a transition of the WC-Co zone to the base cemented carbide develops. the composition of which becomes more rich in carbon with increasing depth. This guarantees that the coating adheres better.3. The thickness of the surface carbon nitride layer can be varied by changing the treatment time. first a rim zone free of mixed carbides is produced. i. Cutting experiments with gradient cemented carbides with Ti(C. TiN).g. minimal. coated cemented carbides and cermets have the prefix HC (e. however. a zone enriched with Ti(C.g. carbon nitrides (titanium carbon nitride.g. Although uncoated cermets are already relatively high in wear resistance. Ti(C. This process of enriching the rim zone with Ti(C. .7.N) a few micrometers beneath the surface in the interior of the substrate. P20. aluminium oxide. K20) on which is applied a 5–20 μm thick hard material coating made of carbides (e.e. 4. inserts with internal Ti(C. hard material coating also can further improve their wear performance and efficiency just as with the conventional cemented carbides. Then nitrogen pressure is altered at a relatively high temperature so that nitrogen diffuses into the interior through the mixed-carbide-free WC-Co rim zone. Leng04].N)) and/or oxides (e. titanium carbide. The substrates are first tightly sintered and then impinged with nitrogen. By means of this one-sided diffusion of nitrogen. 4.2 Gradient Cemented Carbides with External Ti(C. In milling experiments.N) enrichment exhibited excellent wear behaviour in comparison with TiAlN-coated standard inserts [Leng06.3 Gradient Cemented Carbides with Internal Ti(C.3. Leng04]. The thus “coated” cemented carbides are efficient alternatives to uncoated and conventionally coated tools.g. Al2 O3 ). The rim zone functions as a membrane that permits the diffusion within of nitrogen but prevents the diffusion without of titanium. Diffusion of Ti through the WC-Co layer is. TiC).N) Enrichment The hallmark of these cemented carbides is an enrichment of nitrides and carbon nitrides on the surface.N) is called coating the substrate during the sintering process. HC-K20). Diffusion of nitrogen into the rim zone leads to the formation and enrichment of carbon nitrides high in nitrogen on the surface. nitrides (e.8 Coated Cemented Carbides and Cermets Coating cemented carbides with thin.3. titanium nitride.N) Enrichment In this substrate variant. Coated cemented carbides consist of a relative tough base body (e. highly wear-resistant hard material coatings is one of the milestones in their continued development. According to DIN ISO 513. In turning.4. These processes are superimposed to a large extent and are only partially separable in their causes and in their effects on wear. 4. diffusion and oxidation phenomena (Fig. the use of coated cemented carbides and high speed steels for machining various materials is state-of-the-art. the percentage is about 95% and in milling about 60%. The toughness attributes of a substrate are of decisive importance for applications with interrupted cut. abrasion. While the hard material layer improves wear resistance. But in general. The high efficiency of coated cemented carbides is made even more clear by the fact that in the meantime more than 80% of all cemented carbides used in cutting have a highly wear-resistant hard material coating. flank face and notch wear at both low and high cutting speeds. cemented carbides have been coated with hard materials in order to improve their wear resistance and thus also their performance capability. lessens adhesion between the tool and the workpiece and acts as a diffusion barrier. several hard materials possess the required prerequisites for improving resistance effectively against crater. Today. The primary function of the hard material layer is to inhibit contact between the material. at high temperatures. the task of the substrate is to act as an effective support for the hard material layer and to provide the composite body (consisting of substrate and coating) with sufficient hot hardness and. in particular. diffusion and oxidation that determine wear on the tool.4 Coatings Since about 1968.23 Wear phenomena at coated cutting tools Volume effects . which is caused by adhesion. With their high hardness and chemical stability. the toughness of the cutting edge is much more responsible Adhesion Delamination Abrasion Oxidation Diffusion Surface effects Strains Crack formation Crumbling Fractures Fig. thereby reducing tool wear. Due to input and output impacts and associated mechanical and thermal stress variations.23). it is above all adhesion and.4 Coatings 127 4. besides abrasion at low cutting speeds. toughness. 4. N) 5 10 Al2O3 105 TiN Ti(C. layer deposition lies within a specific pressure/temperature range for each method and includes some characteristic coating systems.N) (Ti. the composite material “coated cemented carbide” must first exhibit sufficient resistance to cracking.Zr)(C. Only when these demands are met can.23). (Ti.128 4 Cutting Tool Materials and Tools for its dimensional accuracy than wear resistance. 4.24 shows an overview of technically established coating methods for the production of wear-resistant hard material layers. abrasion. wear mechanisms like adhesion.Zr)(C.N) Al2O3 PVD 600 800 1000 Coating temperature T /°C 1200 Fig.N) (Ti. Method variants include • chemical vapour deposition (CVD) and • physical vapour deposition (PVD).Al)N (Ti. For the interrupted cut. delaminations and – as long as the contact zone temperature and contact time are sufficient – oxidation and diffusion be effectively hindered. • The coating process may not reduce the inner bonding strength. analogously to the interrupted cut. which are given in bold in the illustration [Berg03].N) Hf(C.24 Comparision of pressure and temperature conditions of coating methods with characteristic coating systems (Source: Kennametal Widia) .Hf)(C.4. Figure 4. 4. especially the toughness of the substrate.1 Coating Methods Coating cutting tools can be done chemically and physically.N) MoS2 WC/C Pressure p /Pa 104 MT-CVD 103 CVD 102 101 PA-CVD 100 10–1 10–2 400 Al2O3 / ZrO2 / TiOx TiC TiN Ti(C.e. From this we can derive two requirements for coatings [Köni92]: • The hard material layer has to drastically reduce the effect of all (if possible) wear mechanisms involved in wear process.N) (Ti.N) Zr(C. 4.N) Ti(C.Al)(O. i. crumbling and fracture (Fig.N) TiN Ti(C. As can be seen. Figure 4.7 3070 2950 3420 2980 7.7 7. 700–900◦ C) • PA-CVD (Plasma activated CVD. such as TiC.1 Grey white Melting point (°C) 570 White 451 Uncolored Young’s modulus 590 (103⋅N /mm2) Fig.25 provides a comparison of some essential physical characteristics with the properties of tungsten carbide.44 6.4 Coatings 129 4.05 1580 3100 2300 2800 1700 HV30 2600 1850 2300 1200 1100 352 530 380 200 3700 3380 2050 2700 8.0 7.25 Characteristics of hard materials for coatings in comparison to a cemented carbide of the application group HW-K10 (Source: Kennametal Widia) .4 6.1 CVD Processes CVD processes are chemical reactions that take place in gaseous phase under rough vacuum conditions (103 –105 Pa) and with the addition of thermal or radiant energy.94 3.9 7.1. There are three types of CVD coating procedures: • HT-CVD (high temperature CVD.35 12. the main component of classical cemented carbides.6 (at 1000 °C) 1000 °C) 5. TiN. Ti(C.K–1) 348 6 (at 0.99 5.6 < 0.25 provides an overview of the most common hard materials manufactured with the CVD process.4.7 9.93 5.3 6. The chemical reaction is determined by the laws of thermodynamics and thus by the partial pressure of the gaseous components and temperature.6 6. forming technically useful solids (hard materials) as well as volatile products.7 9.30 13. WC6Co Crystal system TiC TiN ZrC ZrN HfC HfN α-Al2O3 ZrO2 HfO2 fcc fcc fcc fcc fcc fcc rhombohedral monoclinic monoclinic 4.5 Metallic grey Thermal conductivity (W⋅cm–1⋅K–1) 495 Grey Coefficient of thermal expansion (10–6.N). heteropolar (ionic) bonds such as Al2 O3 .4.46 Density (g/cm3) 14.9 80 33 29 38 21 29 22 Gold yellow Metallic grey Light yellow Metallic grey Green yellow Colour 5. The most common coating materials are based on hard materials with primarily metallic bonds. Figure 4. 900–1100◦ C) • MT-CVD (medium temperature CVD. 450–650◦ C) A broad palette of hard material coating systems can be synthesized with CVD methods. 4.7 Micro hardness HV0. but also covalent bonds such as diamond [Damm04]. is vaporized and transferred with methane (CH4 ) to a reaction vessel. In this way.26 Schematic illustration of a CVD coating unit (schematic) Gas exhaust . which can contain several thousand indexable inserts (Fig. Among the most technically advanced methods is the deposition of coating systems with nanostructures. thermally activated HT and MT-CVD methods can only be used to produce equilibrium phases. The HT-CVD Process The high temperature CVD process (HT-CVD) is the classic CVD method for coating cemented carbides. 4. Titanium carbide is formed at temperatures of 900–1100◦ C and a pressure below atmospheric pressure in a hydrogenous environment (H2 ) in a chemical reaction following this equation: Coating furnace CH4 H2 Gas inlet H2 H2+TiCl4 Fluid TiCl4 Evaporator Fig. In order to produce a TiC coating. the low temperature PA-CVD process is also capable of producing metastable phases such as TiAlN. With it. titanium tetrachloride (TiCl4 ).130 4 Cutting Tool Materials and Tools Because the process is near equilibrium. which is produced by an easily reproducible material transfer between the surface of the substrate and the reactive periphery [Ever94].26). 4. On the other hand. HM tools can be coated at temperatures of 900–1100◦ C. complexly doped hard material phases made of Al2 O3 or superlattice structures made of TiN/TiB2 can be produced with a single coating layer thickness of 5 nm [Damm04]. High temperature coatings are characterized by a high level of adhesive strength on the substrate. for example. From this chart. which offer us many starting points for the improvement of toughness in coated cemented carbides. processes taking place during coating in the rim zone (formation of eta phases). They include on the one hand the properties of the substrate to be coated (chemical composition. making the material vulnerable to unacceptable warpage. AlON and others as single coats or in different combinations as multilayer coats. (Ti. grain size. The reaction products rinse the substrates so that no shading effects arise. pressure conditions. Cemented carbides of the first generation exhibited an additional approx. The coating material is formed by chemical reaction from the gaseous phase directly on the surface of the parts to be coated. In this case.23% carbon. It shows typical coating temperatures and times for the HT-CVD. MT-CVD. In contrast. We will now consider more closely processes in the rim zone (eta phase formation).N). Parts with complex geometries can thereby be coated thoroughly and consistently without difficulty.Hf)(C. W3 Co3 C). texture. bending and pressure strength. residual stresses present in the substrate. PA-CVD and PVD methods. thermal expansion. Ti(Cx Ny ). . The causes for this loss of toughness in HT-CVD coating are extraordinarily complicated. the “temperature/time cycle” in the coating process and coating thickness based on all the above factors.1) This process allows us to deposit hard materials like TiC. TiN.4. Figure 4. A further disadvantage to the HT-CVD coating process is that the toughness of the coated cemented carbide body is reduced in comparison to the uncoated substrate. The high process temperature is problematic in the case of coating tool and heattreated steels. crystalline structure. 3–5 μm thick brittle zone with an “eta phase” (W6 Co6 C. Al2 O3 . temperature/time cycle). there is a danger of forming brittle phases in the interface.N). (Zr/C. Cemented carbides that are coated with the classic high temperature CVD process are characterized by high wear resistance due to their relatively thick hard material coatings (up to 20 μm for turning and up to 6 μm in the case of milling). thickness and adhesion of the applied coating are all mutually responsible for toughness reduction in HT-CVD coating. coating must be followed by rehardening. pre-treatment) and process control on the other hand (gas atmosphere. Furthermore. caused by decarburization processes in the rim zone [Schi89].4 Coatings 131 1000◦ C TiCl4 + CH4 + n H2 −−−−−−−−→ TiC + 4HCl + n H2 10 − 150 mbar (4. no changes in constitution are to be expected in the case of low temperature coating methods. During the high temperature CVD coating process. transition zone (interface) and hard material layer as well as the microstructure. Embrittlement from the eta phase has a negative influence on the toughness of coated cemented carbides. as in the PA-CVD or PVD processes [Köni90]. we can see that in the classic HT-CVD and medium temperature CVD process the area of eta phase precipitation is passed through/tangent.27 shows the time/temperature transformation chart for a cobalt alloy with 5% tungsten and 0. The logical consequence of these discoveries is that more and more coating methods have gained significance for coating cemented carbides in the past few years. which can be used at lower coating temperatures in comparison with the classic HT-CVD process.132 4 Cutting Tool Materials and Tools 1200 Co-5W-0. With increasing coating thickness and coating temperature.27 TTT-chart of a cobalt alloy with typical temperature sequence of several coating processes (Source: Widia) The comparative examination of the bending strength of thin. toughness loss is relatively low. bending strength is reduced. The MT-CVD Process By using acetonitrile (CH3 -CN) instead of a methane (CH4 )/ nitrogen (N2 ) gas mixture. In the case of milling therefore. in which the tool is subject to dynamic stresses. it is possible to lower the coating temperature in comparison with the HT-CVD method by up to 200◦ C [Chat86].1 1.23 C 1000 HT-CVD Temperature T /°C WC 800 Eta' Eta MT-CVD Co3W 600 PA-CVD PVD 400 200 0. carbon .0 10 Time t /h 100 1000 Fig.01 0. 4. The effect is especially negative in cutting operations with interrupted cut. cemented carbides with thinner hard material coatings (4–6 μm) than in turning are used. Reduction of wear protection due to the reduced coating thickness must be expected. 4. With these coating thicknesses. In the 700–900◦ C temperature range.28). variously coated WC-Co cemented carbides shows what effect coating temperature and coating thickness have on these toughness-relevant parameters (Fig. 28 Influence of coating method and coating thickness on the bending strength of coated cemented carbides (Source: Widia) nitride coatings of the elements Ti.N) Changes of bending strength / % 0 PA-CVD –10 400 °C 550 °C PVD –20 700 °C 550 °C –30 CVD 700 °C –40 F –50 CVD 2. 4. 4. Both MT-CVD coated and PVD coated cemented carbides are clearly superior to uncoated cemented carbides in performance. • The coatings applied with the MT-CVD method exhibit lower tensile residual stresses compared with HT-CVD coatings.5 –60 0 2 4 6 Coating thickness / µm 8 10 Fig. In higher cutting speed ranges.4. the performance of the 3 μm thick .6 14. VDI3324]. Trial results from the longitudinal turning test show the same qualitative dependence on cutting speed for MT-CVD coated cemented carbides as for the uncoated substrate (Fig. Zr and Hf can be applied with high deposition rates. • The danger of decarburization and thus of embrittling eta phases is lower for cemented carbide substrates. As opposed to the conventional HT-CVD process.4 Coatings 133 10 TiN TiN/Ti(C. the MT-CVD method has the following advantages: • Thermal stress on the cutting tool material is lower due to the reduced coating temperature at simultaneously higher deposition rates.29) [Köni92. 2 mm ap = 2.N)-TiN coating do not differ. the cutting edges succumb to flank face wear and no longer to cracking and crumbling. Transverse cracks and cutting tool material crumbling occur later. The PVD coating tends to show advantages at low cutting speeds. It extends the range of applicable cutting speeds clearly to lower cutting speeds to a range that is also covered by coated HSS. At low cutting speeds. There are significant performance advantages compared with conventional coatings.29 Influence of coating methods and cutting speed on the toughness behaviour of carbides in an interrupted longitudinal turning test Arc-PVD-TiN coating and the 6 μm thick MT-CVD-TiN-Ti(C. The causes for the improved toughness are the unaffected properties of the substrate and the more favourable compressive residual stresses resulting from PVD coating of the cemented carbides [Köni90]. resulting in longer tool standing times and higher machining quality. 4.134 4 Cutting Tool Materials and Tools Material: 42CrMo4+QT Rm = 980 N/mm2 Substrate: P30/40 SPUN120312 f = 0. Fewer comb cracks form and crack development is slower than it is the case with HT-CVD coatings. . but in a reproducible way by a continuous wearing of the flank face. MT-CVD coated cemented carbides are used above all in milling. Their excellent toughness properties make these cemented carbides also ideal for milling with cutting fluids.5 mm 100000 Number of impacts n 50000 Arc-PVD 10000 5000 MT-CVD 1000 Uncoated 500 100 60 80 100 200 300 400 Cutting speed vc / (m/min) Fig. The end of their standing times is no longer brought about by cutting edge crumbling. however. One example is the coating sequence TiN-Ti(C. have the advantage that crack development is hindered in the hard material coating. The outside coating composed of Al2 O3 has the smallest thermal expansion coefficient. 4. In addition. for example. Coatings with compressive residual stresses. Considerable larger compressive residual stress is measured with the same coating sequence if it is supplemented externally with a further Zr(C.N) or Hf(C. stress-free annealing in the substrate cemented carbide always occurs at the coating temperatures typical for the CVD process. the Al2 O3 coating exhibits a small amount of compressive residual stress. there are also CVD systems with compressive residual stresses available. After cooling from coating to room temperature.CVD) Internal stress (N/mm2) Pressure –1000 1000 Strain TiAlN Al2O3 Ti(C.N) (MT . in the case of multilayers. coatings deposited by means of the HT or MT-CVD process exhibit tensile residual stresses. Compressive residual stresses can also be produced when using the CVD method by a corresponding choice and arrangement of individual coating systems.CVD) Cemented carbide substrate Cemented carbide substrate Fig. between the individual coatings. generally familiar in the case of PVD coatings. 4. the inside TiN coating the largest. no longer exclusively true.4. Tensile residual stresses originate when cooling from coating temperature to room temperature and result primarily from the difference of the thermal expansion coefficients between the substrate and the coating or.30 Influence of coating methods and coating structure on residual stresses in hard coating systems .N) coating (ratio C:N = 1:1) (Fig.N) (MT . In the meantime. during grinding [West00]. This is. Compressive residual stresses are therefore the most beneficial to performance in interrupted turning and in milling in general.30). and the fracture strength of the coating/substrate composite is largely maintained.N)-Al2 O3 . leading (as opposed to the low temperature PVD process) to a reduction of the (compressive) residual stresses introduced. PVD-Coating system CVD-Coating system Internal stress (N/mm2) 17 µm –4000 –2000 –3000 0 –1000 –3000 –2000 0 Pressure 4 µm Hf(C.4 Coatings 135 Residual Stresses in CVD Coating Systems As a rule. At these temperatures. nitrides or oxides of the coating. the thermal energy alone is insufficient for introducing the chemical reaction needed to form the hard materials from the gaseous phase. coatings made of titanium nitride. additional energy is added to the process by means of the pulsed plasma of a low pressure glow discharge. In this way. followed by one or more titanium carbon nitride intermediate layers. As opposed to the uncoated substrate. Berg05]. titanium carbide. The TiN surface layer functions as a diffusion barrier during coating and prevents brittle eta phases forming in the interface [Köni90]. Due to the non-equilibrium conditions during deposition. titanium carbon nitride and aluminium oxide can be deposited individually in crystalline structure by changing the gas composition with individually or in an arbitrary sequence. the classic high temperature CVD process is combined with the MT-CVD process. “plasma enhanced” – precipitation of fine-grained hard material coatings takes place at the. For this reason. . West00. Modifying the Layer Structure The layer structure of many multilayer systems usually begins with a thin titanium nitride layer coating the substrate.136 4 Cutting Tool Materials and Tools To produce these multilayer systems. Berg03. The application example “side milling” proves this (Fig. The Ti(C. They act as a kind of “buffer” between the relatively soft and tough substrate and the much harder and more brittle carbides. there is also a possibility of synthesizing metastable systems like (Ti. As in the HT-CVD process. The sensitivity of the composite to comb cracking and failure due to crumbling is much lower than it is the case for HT-CVD coated cutting tool materials. whereby the precipitation of the carbon nitride coatings takes place with the help of the medium temperature CVD process [Drey97. PA-CVD coated cemented carbides measure about 30% higher bending strengths than HT-CVD coated ones. much lower coating temperatures of 450–650◦ C. The residual stresses are in the compressive range after coating.N) layers building upon the TiN surface layer can consist of one layer or several layers differing in stoichiometry. in contrast to HT and MT-CVD methods. The attributes of the cemented carbide remain largely unaffected by the lowering of temperature during coating. are only possible at much higher temperatures [Tabe89.31). while it is over 300% larger after PA-CVD coating. Köni89]. 4. The Plasma Activated CVD Process In the PA-CVD process (“plasma assisted” or “plasma activated”) – also known in the literature as PE. chemical reactions are made possible that. The properties of PA-CVD coated cemented carbides have a positive effect on their performance when cutting strong steel materials in interrupted cut. at thermodynamic equilibrium.Al)N using PA-CVD [Lemm03]. the tool life travel path is slightly diminished after HT-CVD coating due to thermal influence on the substrate as well as tensile residual stresses in the hard material coating. 2 μm. not in micrometers (Fig. The toughness of the coating/substrate composite is affected not only by the thickness of the entire coating. With a constant total coating thickness of 10 μm.08 mm ae = 42 mm ap = 6 mm 4 0 Cemented carbide P25 / TiN. they are often designed in the form of fine lamellar. the wear resistance of the CVD-coated cemented carbides increases. Due to its high chemical stability as a coating material.32).N) with Al2 O3 or. 4. 220 HB Cutting tool material: HW-P25 8 vc = 80 m/min fz = 0.33. These are multilayer coatings with individual layers whose thickness is given in nanometres. Total coating thicknesses. fine grained multilayer coatings (Fig. but it only has a small amount of thermal shock resistance and toughness. 4. 4. PA-CVD-coated Uncoated HT-CVD PA-CVD Coating method Fig. the failure rate of the coated cutting tool material due to fracture clearly increases with the thickness of the Al2 O3 layer [Naka88].32 illustrates this using the example of a coating made of TiC and Al2 O3 . In order to reduce the brittleness of coating systems containing aluminium oxide. 4. as in Fig.32. sixty 80 nm thick layers and thirty-six 50 nm thick layers are deposited on a top layer composed of titanium nitride. 4. In the case of coating systems 1 and 2 shown in Fig.31 Cutting edge durability of a PA-CVD-coated cemented carbide in side milling (Source: Widia) With increasing coating thickness. with aluminium oxynitride (AlON).4 Coatings 137 16 Tool life travel path Lf / m Side milling cutter 12 Material: 60WCrV8. but their bending strength and toughness is reduced. whereby each layer can be thinner than 0. are around 5 μm in the case of milling and between 12–20 μm for turning. Such multilayer coatings consist usually of a combination of TiN or Ti(C. aluminium oxide does indeed have excellent resistance against crater wear.33). but also to a large extent by the coating material and the thickness of the individual coating components. Using PA-CVD coating technology. we are now also capable of depositing nanolayer coatings. Figure 4. In .4. depending on the intended use. Coatings such as these can also be composed of a large number of individual layers. 05 when the coating thickness is below 10 nm. 4. N2 and Ar. For example. whereby the latter – so-called composite coatings – are finely distributed in the Al2 O3 base matrix of a further oxide. on the fact that coating materials below a certain thickness take on completely new properties. 4. among other things. 4. As can be seen in Fig. is condensed from a gas mixture comprising AlCl3 . both zirconium oxide ZrO2 and hafnium oxide HfO2 have very low thermal conductivity. however it has a hardness of over 3000 HV0. demands on thermal resistance of the tool cutting edge are increased.32 Fine lamellar multilayer coating (Source: Widia. CO2 .05. TiCl4 . 4.34. they are combined with Al2 O3 [West00]. Lowering the thickness of the coating increases hardness. oxide coatings are especially suitable to meet these demands. relatively soft in comparison to hard materials like TiC.4 Cutting Tool Materials and Tools Wear resistance 138 PVD CVD TiN AlON Coating thickness Bending strength PVD PA-CVD TiN AlON CVD TiN Coating thickness Single layer thickness Al2O3 / TiC CVD − coating Al2O3 / TiC Total coating thickness for all tools 10 µm AlON TiN AlON 1 µm / 9 µm 4 µm / 6 µm Transistion layers 6 µm / 4 µm Substrate 3 µm 0 10 20 30 40 50 60 70 80 90 100 Occurance of failure / (%) (cutting edge breakage) Fig. H2 . Since both oxides are insufficiently hard by themselves. with a hardness of about 1200 HV0. Due to their low thermal conductivity. in addition to aluminium oxide. e. AlN is. there are as much as 2000 single layers of titanium and aluminium nitride of 9 nm thickness that are applied alternately. consisting of three components.34). the oxide layer. At high cutting speeds or during dry machining. HfO2 oder ZrO2 (Fig. This can be accomplished in the form of multilayer coating systems with single or multi-phase layers. In the case of the Al2 O3 /ZrO2 /TiOx composite coatings shown in Fig.25.g. ZrCl4 . Al2 O3 then exists as a κ-phase and ZrO2 as a phase mixture in tetragonal/monoclinical (coating temperature > 990◦ C) . toughness and wear resistance. This effect is based. Sumitomo) the case of the coating system TiN-AlN. Doping brings about higher deposition rates. Due to their nanodispersive distribution.33 Nanolayer created by a PA-CVD coating process (Source: Kennametal Widia) or monoclinical form (coating temperature < 990◦ C) depending on the coating temperature. titanium is added to the gas mixture as TiCl4 and boron as BCl3 . In the fracture structure image.4. 4. smoothing the surface of CVD-coated tool bodies by means of brushing. For example. 80 nm 139 Coating system: TiN-18x(Al2O3-TiN) Coating thickness: 2 µm Single layer thickness: approx. are effectively dissolved in the Al2 O3 lattice. Kloc01.05 Fig. 4. the ZrO2 particles can be recognized as finely distributed in the Al2 O3 matrix.4 Coatings Coating system: TiN-30x(TiCN-TiN) Coating thickness: 5. makes it possible to reduce deposition temperature and clearly improves wear resistance at high thermal cutting edge loads (Fig. they have a positive effect on coating growth and the grain size of the hard materials forming the coating system [West01. Doping layers made of Al2 O3 with titanium or boron (or TiN layers with boron) leads to the formation of very fine-grained coating structures. 9 nm Hardness of coating: 3200HV0. The highly fine-grained coating structure is originated by means of a small addition of titanium as doping agent.2 µm Single layer thickness: approx.35) [Kath03]. The doping products TiOx und Bx Oy . In the CVD process. Aftertreatment Aftertreatment helps to improve further the performance characteristics of coatings. 50 nm Coating system: 1000x(TiN-AlN) Coating thickness: 9 µm Single layer thickness: approx. Berg03]. especially in the case of interrupted cut. well detectable in the EDX (energy dispersive X-ray) analysis but still not clearly identifiable as a phase. jet treatment or polishing is . 34 Fracture structure of CVD-coated cemented carbides with modern coating-design (Source: Kennametal Widia) TiN A TiN B Ti(C.140 4 Cutting Tool Materials and Tools HfO2 1 µm 1.N) (MT-CVD) TiN (12–13 µm) 9. 4.N) MK reduced rim zone A Single layers of Al2O3 and TiN endowed with titan and boron B Single layers of TiN and Ti(C. 4.N) endowed with boron Fig.5 µm 6 µm Ti(C.35 Formation of microcrystalline layer structures via doping with titanium and boron (Source: Ceratizit) .5 µm TiCN (MT-CVD) TiN (16–18 µm) Al2O3 /ZrO2 /TiOx Composite 7 µm TiCN (MT-CVD) 5 µm TiN (12–17 µm) Fig.5 µm Al2O3 /HfO2 /TiOx 0.5 µm Al2O3 with dispersed HfO2 9. Attempts to improve the toughness and resistance of cemented carbides substrates to crack development without reducing their hot hardness led to the development of substrates with graded rim zone structures [Sche88. Ta. sputtering). This would also have positive effects on the following coating process. On the other hand. The coating and application of such cemented carbides is presently state of the art. In addition. Nb)C mixed crystals and consist practically of only tungsten carbide and cobalt up to a depth of approx. Köni90. which spread from the hard tensile stressed material coating into the cemented carbide substrate.35). 50 μm (mixed carbidefree or reduced rimzone. For example. Since brushing only covers the cutting edge area. The reason for this is assumed to be cracks. CVD coatings lengthen standing times enormously. but also reduces the range within which the wear and standing time of coated hard material indexable inserts are scattered [Berg03]. in addition smoothing. thereby also reducing their capacity to bear high stress peaks during cut interruptions [Köni90. Further developments in this area are aimed at establishing the basic properties of a coated cemented carbide body – hard outside and tough inside – already during sintering [Leng04].5. Fig. all-round smoothing with jet treatment is being given increasingly more preference. The initial benefit of these methods was the possibility of coating HSS tools with complex geometries. Jet treatment not only improves the residual stress condition. oxidation layers and corrosion layers can also be produced. . Arc-PVD. decorative layers.500 N/mm2 ) in place of tensile residual stresses (1000 N/mm2 ). they also lower the toughness of indexable inserts. also causes an alteration of the tensile residual stresses as a function of the treatment parameters.4 Coatings 141 the common practice today for removing potential cracking sources. the rim zones of which are largely lacking hard and brittle (Ti. Schw89]. 4. after jet treatment. The fabrication of function-graded cemented carbides with rim zones lacking mixed carbides and with external or internal Ti(C.4. the Al2 O3 layer in the Al2 O3 /ZrO2 /TiOx coating system has. In the meantime. The goal was to modify the rim zone of the cemented carbide such that crack development towards the centre of the indexable insert was hindered or suppressed.2 PVD Coating The early 1980s saw the succession of the CVD process by the PVD process in various process variants (vacuum evaporation. Modifying the Substrate Rim Zone In comparison to uncoated cemented carbides. 4.1.N) enrichment is described in more detail in Sect. It is of particular importance that the jet treatment. compressive residual stresses (–3. Köni92]. 4.3. these processes are also being used for cemented carbide and cermet coating. One possible solution are cemented carbides.4. The vapour atoms exhibit average free path lengths of up to several meters at these pressures. Therefore. Rödh87.142 4 Cutting Tool Materials and Tools The fundamental differences between PVD coating and the classic HT-CVD process can be summarized as follows [Sche88.36). Gühr89. Residual compressive stresses reduce the risk of fracture formation under shock lead and thermal cycling. CVD coatings have better adhesion due to the effects of evaporation and diffusion. A columnar fine crystalline layer is formed [Möhw96]. • Because of shadowing effects. the substrate material must be moved in the coating chamber in order to avoid shadowing effects and inconsistent coating thicknesses. • The bending strength of the substrate remains largely unaffected by the low coating temperature. so that temperature-sensitive substrate materials can also be coated. Schi89. Due to the straight path of the particles. • PVD coatings exhibit residual compressive stresses. VDI3198. • High number of potential coating systems and substrate materials. consistent layer thicknesses can only be partially realized by means of very costly devices for rotating the parts to be coated. Knot89. Since the latter is considerably colder than the vapour. We can distinguish several different methods of PVD coating: • methods with thermal evaporation (vacuum evaporation) • methods with arc evaporation (arc-PVD) • methods with cathode evaporation (sputtering) Thermal Evaporation (Vacuum Evaporation) In vacuum evaporation. since the layer thickness is diminished with increasing depth. Rass96]. Vacuum evaporation allows for high coating rates (coating thickness increase per unit time) and is used above all for optical applications. in mass production for large-scale metallization of films. which currently limit the possible layer thicknesses to 3–6 μm. Köni90. plastic parts and paper as well as for coating turbine blades (EB-PVD) [Sche88. VDI3824]: • The process temperature of 160–600◦ C means relatively low temperature stress on the substrate materials. they generally do not interact with each other and proceed straight to the substrate. • In the PVD process.N). multi-component metastable phases can also be deposited under the non-equilibrium conditions of physical vapour deposition. the particles condense on the substrate in accordance with the principle of water vapour. Inner contours are as a rule only coatable up to a depth/diameter ratio of 1. workpieces that are to be coated require a very careful pretreatment of the surface and process control in order to guarantee sufficient adhesion of the coating. usually the coating material is evaporated in a crucible with a resistance heater or an electron beam gun (EB-PVD) in high vacuum at a pressure of 10–3 –10–6 Pa (Fig. These phases often exhibit structures that do not exist under equilibrium conditions. • Besides stable coating systems such as TiN and Ti(C. . An example of this is the coating system TiAlN. 4. nitrides or oxides. . Due to the high energy density in the focal point of the arc. The liquid particles that are carried away are found again as dropshaped defects (“droplets”) in the layer [Bran05]. the target material is vapour-deposited and ionised by means of an arc in high vacuum.. ). to Möhwald [Möhw96] In order to form hard material coatings composed of carbides. For example.4 Coatings 143 Gas inlet Target Watercooled recipient High vacuum coating plant evaporation by: . The arc travels either randomly (random arc) or in a controlled fashion (steered arc) by means of the vaporizing source. acc. bonding strength is relatively low due to the low particle energy of less than 2 eV. but molten particles are also torn out at the same time. The appearance of droplets is . [Haef87]. usually connected as a cathode (Fig. This method is distinguished by its high degree of ionisation of up to 90% and thus makes it possible to produce high quality coatings with thick structures and good adhesive properties.4. the substrate is reactively vapour-deposited with the help of a reaction gas (N2 .36). These are undesirable because they adhere to the substrate insufficiently [Möhw96].electron beam Electron beam Evaporator Vacuum pump Energy supply Fig. 4. Arc Evaporation (Arc-PVD) In the arc-PVD method. 4. . CH2.36 PVD-Process – Vacuum evaporation. In the case of simple reactive evaporation. the material is explosively evaporated and propelled.resistance heating or by . from 2Ti + C2 H2 the hard material layer 2TiC and the gas H2 are formed. One process variant is the LARC method (Lateral Rotating Arc Cathodes). Gas inlet Target Sputtering plant .without additional magnetic field: Diodes-Sputter-Source . an inert gas (e.37 PVD-Process – Cathode evaporation (sputtering) . fewer droplets are produced.37).g. no oxide coatings can be deposited with this method. 4. atomic groups and molecules of the coating material via impulse exchange [Rass96] (Fig..144 4 Cutting Tool Materials and Tools the main disadvantage of arc evaporation. Since evaporation by means of cathode arcs requires that the coating material is electrically conductive. this method is not suited to depositing corrosion-resistant coatings [Möhw96]. The basis of this method are rotating. The magnetic field is created by permanent magnets and coils that are shifted and steered vertically and radially. Droplet emission can be partially reduced by optimizing the process parameters. In contrast to conventional arc technology. resulting in smoother coatings [Csel03]..with additional magnetic field: Magnetron-Sputter-Source Electrode as additional ion source Plasma-room Evaporator (Target) Additional magnetic system Vacuum pump + -1.5 kV Fig. Cathode Evaporation (Sputtering) In a low-pressure plasma. argon) is ionized by applying high voltage. and since the droplets can lead to pores in the coating. 4. water-cooled arc cathodes [Damm04]. The positively charged inert gas ions are propelled onto the target switched as a cathode (coating material) and knock out atoms. since the material is evaporated not thermally but by means of impulse transfer. O2 . in which there is a high-frequency AC field that is used for non-conductive materials. .37). The plasma consists of ions.1–1 Pa pressure.g. magnetron sputtering involves the arrangement of permanent magnets behind the target (Fig. . . electrons and uncharged atoms [Möhw96].e. or oxides. Due to the overlapping of magnetic and electric fields. nitrides. this can be achieved with reactive sputtering. If not only metals or metal alloys are to be deposited. thus making it the anode. The disadvantages of RF sputtering can be surmounted by using pulsed DC sputtering (MF: medium frequency sputtering. CH4 .e. Al2 O3 . a reaction gas (e. In this case. TiN. The plasma burns between the target which acts as an anode and the substrate connected to the ground or the recipient wall. . The use of direct current (DC) plasma. N2. TaC. in which the sputtering material is electrically conductive. . not only metallic.4 Coatings 145 As opposed to vacuum evaporation. Almost all substances [Haef87] and composites of very diverse melting substances can be evaporated. the magnetic field lines of which vertically penetrate the cathode plate and the target affixed onto it. The coatings produced with sputtering at low vapour-deposition rates have good adhesive properties. 4. From this reaction gas are obtained the non-metallic components of the coating (e. Magnetron Sputtering In comparison to pure diode sputtering. it is possible to freeze metastable composite phases due to their rapid condensation from the gaseous phase. thus synthesizing a completely new material type that is often superior to stable boundary systems [Leye04]. This evaporation method is universally applicable.g. H2 . coatings that do not exist in equilibrium). this technology is of lesser importance in the creation of wear-protection coatings due to the appreciably higher capital costs in comparison with DC sputtering and the significantly lower deposition rates [Lemm03]. sputtering also involves the possibility of depositing metastable coatings (i. The average free path length of the particles at this pressure amounts to a few millimetres. After evacuating the recipient. Because the process conditions are far from equilibrium conditions. We differentiate between DC sputtering. a glow discharge plasma is created. i. As in the arc method. up to 50 kHz). the electrons in the plasma are . high-frequency (RF) plasma and the combined use thereof as well as a nearly unlimited number of possible coating materials making sputtering enormously flexible with respect to the development of application-specific component coatings [Rass96]. sputtering requires a process gas (usually argon) as well as a high voltage source. and RF sputtering.) is injected in addition to the process gas. . At a target voltage of –1 to –5 kV.4. but also isolating layers as well as materials with high melting points can be deposited.). but also carbides. the process gas is admitted at up to 0. Although RF sputtering has important advantages when atomizing insolating materials. Erke05].PTM ). 4. asymmetrical bipolar and bipolar two-cathode magnetron sputtering [Lemm03. Fig.I. Figure 4.38 Fracture pattern of (Ti.146 4 Cutting Tool Materials and Tools directed on extended spiral-shaped paths. but completely new coating systems can also be synthesized due to the extremely enhanced ion currents [Lemm03. not only coating systems usually reserved for RF technology can be deposited. With this technology. It was the invention of magnetron sputtering that has first made possible an economic utilization of the sputtering effect for coating deposition and thus the commercial deposition of wear-protective layers by means of sputtering technology [Lemm03. Pulse technology makes it possible to deposit conductive and electrically isolating layers in almost arbitrary stoichiometry as well as nano-structured single and multi-phased layers with new property combinations. Bobz00]. deposited with conventional DC-technique (left) and using bipolar-pulse-technique (right) (Source: CemeCon) . Pulse technology thus also creates the possibility of depositing crystalline γ-Al2 O3 at substrate temperatures of < 500◦ C or producing (Ti. The coating process with pulsed plasmas is also known as high ionisation pulsing (H. We distinguish between three process variants: unipolar. In this way.38 shows a (Ti. Leye04].Al)N layers with a AlN content of up to 66 mol% (Fig.39).Al)N-layers on cemented carbide substrates. One can clearly recognize the alteration of the coating micromorphology towards finer crystalline structure made possible with the increased plasma ionisation of pulse technology [Lemm03]. optimized morphology have led to the development of pulsed DC plasmas. the probability of ionisation is increased as well as the deposition rate as the discharge voltage is reduced. High-Ionisation Pulsing Rising interest in isolating.Al)N coating created with conventional DC technology in comparison to one made with the help of pulse technology. mostly oxidic wear-protection coatings and the necessity of increasing ion density in the area of the substrate in order to deposit layers of dense. 4. 40).39 Metastable phase diagram of the system TiN-AlN. Ion plating thus involves surrounding the substrate with gas ions.Al)N+ (Al. here too a reactive gas is fed into the chamber.4 Coatings 147 900 Fig. The metal vapour is ionized by means of electrodes arranged in a gas space and electromagnetic fields. Part of the ionised particles are accelerated towards the substrate (Fig. to Cremer [Crem98] 800 (Ti. coating properties such as density and residual stress are modified and adhesion to the substrate is improved. The resultant coatings are very adhesive and tight. arc ion plating (AIP) and ion plating with low-voltage arc discharge are among the process variants that have become the most significant in tool coating. 4. 4.Ti)N 700 T / °C 600 500 400 (Ti. . Ion plating can also be utilized for purifying and activating the component surface prior to coating (also called etching) or for activation during coating [Rass96].2 0. ion plating is defined as a variant of PVD coating.4. The prerequisite is a sufficiently high number of ionised steam and process gas atoms.Ti)N (wurtzite structure) 100 TiN 0. in which the condensing layer is altered by means of an additional electric field applied to the substrate. In ion plating. nitrides or oxides for coating purposes. the substrate is coated with a negative voltage or “bias voltage”.6 0. This method is called reactive ion plating [Möhw96. By bombarding the growing layer with ion. acc. This method variant is independent of the type of process and physical principle involved in creating the metal vapour. For tool coating. A variety of ion plating plant designs have emerged as practicable coating variants.Al)N (NaCl structure) 300 200 (Al.8 AlN xAlN Ion Plating With respect to process control.4 0. All PVD methods in which a bias voltage is used to modify the coating are termed “ion plating” [Bobz05]. In order to produce carbides. Haef87]. so that plastics and materials with low annealing temperatures can also be successfully coated [Bobz00]. . which can result in high coating temperatures (approx.. Nearly all materials are depositable with this method. the substrates are heated to a much smaller degree than in the AIP process. Their energy is partially converted into heat. Magnetron Sputter Ion Plating (MSIP) MSIP guarantees a stable plasma and high atomization rates [Bobz00].5000 V Rotating arc plasma channel Vaporising via vacuum arc vaporiser: Contact ignition device Magnets for stabilisation rather leading of the arc Vacuum pump Fig. Since the ionisation degree of the atomized particles is under 5%.40 PVD-Process–Ion plating. which impact the substrate at high speed.. This method is characterized by the high degree of ionization of up to 90% and can thus be used to produce high quality coatings with dense structures and good adhesion. the target material is vaporized and ionised by means of an arc in high vacuum.148 4 Cutting Tool Materials and Tools - + 10. to MÖHWALD [Möhw96] Arc Ion Plating (AIP) In the AIP process. The pre-voltage applied to the substrate accelerates the ionised particles. Coatings deposited with MSIP exhibit very good mechanical and optical properties. 4. 550◦ C) in the substrates. acc. 41).4 Coatings 149 Low-Voltage Arc Coating Ion plating with low voltage arc discharge is characterized by a low acceleration voltage. . Besides these coating systems.N).g.N). No droplets are formed when ion plating with low-voltage arc discharge is used for coating. Due to the unsurpassed hardness of diamond. directed towards the coating target. The vapour made of the coating material arising from the electron bombardment is about 50% ionised and. 4. built up edge formation is almost completely suppressed due to lowered adhesion between the flowing chip and the cutting tool material in the entire cutting speed range under investigation of vc = 10–63 m/min (Fig.Al)N-WC/C). Ti(C. to improve their wear-protective effect. This method is ideal for depositing nitride or carbide layers by introducing the corresponding reactive vapours [Haef87]. This jet is directed towards the target (anode) in a water-cooled crucible.4. Different coating systems can be deposited as monolayers (e. TiC. combined as multilayers with single layer thicknesses in the micrometers (e. since the substrate is applied with a voltage of –200 V. Built up edge formation is characteristic of machining relatively soft or tough steels at low cutting speeds. those that are characterized by a low friction coefficient such as DLC (diamond-like carbon) and MoS2 -coatings are successfully used for wearprotection. with which an electron jet is created from a low-voltage arc discharge.N).N)Al2 O3 -TiN.Cr)N) or. Ti(Al. TiN-Ti(C. These lead to dimensional and shape inaccuracies as well as to reduced workpiece surface quality. (Ti. the use of coolants – a constant ecological problem whose maintenance and disposal are ever costlier – is superfluous. (Ti.g. One measure for reducing built up edge formation among others is the use of broaching oils.N).HF. Characteristic of the broaching process are the generally low applicable cutting speeds. Ti(C. This method thus makes it possible to deposit very smooth layers (Rz = 0. CrN and Al2 O3 .4. 4. Ti(Al. The Use of PVD-Coated Cemented Carbides at Low Cutting Speeds Broaching soft and hardened steels offers an impressive example for the cutting edge durability of PVD-coated cemented carbides at low cutting speeds [Köni92a]. TiN.2 Specific Properties of Hard Material Coatings The most commonly utilized wear-protection coatings in metal machining commercially are TiN. Furthermore. By using PVD-TiN-coated cemented carbides in place of uncoated ones. usually 1–25 m/min depending on the broaching machine. CVD diamond coatings are also being increasingly used [Lemm03]. cutting tool material and the target material.1 μm) and is thus suitable for coating mirror-polished surfaces without the need for post-polishing. One further development in multilayer technology is the deposition of nanolayers or coatings with superlattice structures. Multilayer coatings also make it possible to integrate layers with different properties within one coating system.2 mm Material: 17CrNiMo6+FP Cutting tool material: HW6Co Chip thickness: h = 0. especially in the case of coatings with very diverse Young’s modulus. to lower residual stresses.42). high hot hardness or the low friction resistance of intermediate or cover layers [Krie01. transitions between the individual layers signify barriers at which they are forced into energy-dissipating detours.41 Coating prevents formation of built up edges The primary goals of multilayer technology are not only to improve the toughness. Lugs03]. 4. high thermal stability. for example. in comparison to an equally thick monolayer coating. The multilayer structure leads. slowing or even stopping further growth (Fig. Such specific coating functions include. hardness and wear resistance of the coating but also to increase the total thickness of the coating. For cracks that are growing in the coating.150 vc = 63 m/min broaching oil vc = 10 m/min dry 4 Cutting Tool Materials and Tools Uncoated vc = 63 m/min dry PVD-TiN-coated vc = 63 m/min dry 0. which leads to a large increase in strength. These are coating systems consisting of a large number of extraordinarily thin layers. The transitions that are formed between layers of different materials place limits on shifting. Larger layer thicknesses mean a larger coating volume that has to be worn. the thickness of which amounts to . From this fact results the generally longer standing times as well. thus combining the functionalities of diverse coating materials.1 mm Tool geometry: αo γo λs rβ 5° 9° 0° 35 μm Fig. 4. Si) are deposited during the coating process on the tool surface. i.g. With respect to the structure of hard material coatings. The nanocomposites can be deposited as monolayer or multilayer coating systems.42). 4. 4. 4. TiN). . Ti. Balance between high residual compressive stresses (bad coating adhesion) and low blank (no obstruction of crack growth) ones Fig. The boundary layers/grain boundaries in the nanolayer/nanocomposite layers are energy-dissipating barriers to cracks. Examples of this are the embedment of nanocrystalline cubic TiN in an AlN matrix. usually columnar layers. Another way to improve a coating’s properties is to produce so-called nanocomposites.g. These are noncrystalline isotropic multiphase systems.Al)N into a matrix composed of (Al.4 Coatings Crack growth down to the substrate 151 Crack growth along the grain boundaries Substrate Substrate Coating with low internal residual stresses Nanocrystalline structures Crack growth along the boundary surfaces Substrate Multilayer Crack growth is hindered Substrate Coatings with high residual stresses . Cracking and the speed of crack growth are thereby reduced (Fig. This increase in hardness is explained by the phenomenon that diminishing layer thicknesses have altered crystal lattices and thus Young’s modulus as well. Both nanolayer and nanocomposite layers are thus characterized not only by extremely high hot hardness and high-temperature wear resistance but also by favorable toughness attributes. For example. in which two mutually insoluble phases (e. multiphase layers (e. of nanocrystalline TiAlN/AlCrN in an amorphous Si3 N4 matrix [Csel04] or of nanocrystalline (Ti. In contrast to conventional. layers the chemical composition of which exhibits a gradient (e. this is based on the fact that the hardness of a material increases significantly below a certain layer thickness. we differentiate between monophase layers (e.g. TiN + Ti2 N) and graded layers.42 Schematic diagram of the crack growth in hard coatings depending on structure and properties (Source: Oerlikon Balzers) only a few nanometres (Fig.43).Al)N layers with an increasing Al content) [VDI3824]. Al.e.4. (Ti.Ti)N. coating materials deposited as nanolayers are characterized by their significantly higher hardness. As has already been explained in the context of the PA-CVD coating process. AlN has a hexagonal lattice structure at layer thicknesses > 10 nm and a cubic structure when the layer thickness is < 10 nm [Csel03a].g. Ductility of the coating is as important as the hardness of the coating . Ti(C. structure. TiC s resistance to flank face wear is thus higher than that of TiN.Al)N-(Al. such properties as chemical composition. morphology. The data presented can thus only be considered as guides for possible areas of application. 4. (TiAl)N + WC/C. Figures 4. 4. a-C:H.) Super hard layer (CVD-Diamant.44. By changing the deposition conditions.) Hard layer with shear-flexible cover (TiN + MoS2. Besides the hard material coatings here presented and explained in more detail below. On the other hand. (Ti. TiC is used as a monolayer when abrasion is predominant or in multilayer coatings together with TiN.N) or Al2 O3 [VDI 3824]. texture. which depends on the conditions of their deposition.1 Titanium Carbide Coatings (TiC) Coating tools with hard materials began with the deposition of TiC. For the most part. etc. its tendency to diffusion is a bit higher than that of TiN and Al2 O3 due to the relatively small enthalpy of formation.4.4. Bornitrid) Multilayer with functional intermediate layers (TiN-Ti(C. the thermal expansion coefficient and oxidation resistance can be altered within certain limits.N)-Ti(C.N). while its resistance to crater wear is lower.N). there is a number of other hard material layers used for tool coating that are not more fully described here. a-C:H:Me) Graded layer Ti(C. tools were already being coated with titanium nirtide.2. 4.05).2 Titanium Nitride (TiN) Since 1980. TiN-Ti(C.43 Schematic diagram of the structure of current CVD and PVD layer systems The properties of hard material coatings are determined by their chemical composition and structure. nc-(Ti. Due to the high hardness realizable with titanium carbide (3100–3400 HV0. etc.Al)N/a-Si3N4) Fig.Hf)N-CrN Hard layer + low-friction amorphous carbon layer (DLC-layer) (a-C.Ti)N Nanocomposite (nc-TiN/a-AlN. TiC coatings are chiefly applied using the CVD process.N)-Al2O3-Zr(C. Ti(C.2. 4.152 4 Cutting Tool Materials and Tools Monolayer (TiN.Al)N.) Nanolayer (Superlattice) thickness of one layer: a few atom layers to 100 nm (Ti.44 show some characteristic values for layer properties for some exemplary hard material coating systems produced by PVD and CVD processes.25 and 4. TiN is the most frequently employed hard material for coating cutting tools today. residual stresses and thus microhardness. etc.N) (Ti. The coating material is an intercalation compound composed of titanium and nitrogen. . Its oxidation wear is also the lowest of the layers shown in Fig. it provides more effective protection against abrasive wear than TiN.N)-Al2O3-TiN. They are obtained from reported measured values resulting from different compositions. .4. Silver-hued TiN coatings consist of a phase composite of Ti2 N and TiN. (Leaks in hard coatings may lead to the development of local galvanic elements and to pitting corrosion). 4.1 Oxidation temperature/°C 2) > 450 > 350 > 350 > 700 > 600 –3) Thermal barrier effect 4) +++ ++ + ++++ + +++++ Resistance to abrasion ++ +++ +++ +++ ++ ++ Resistance to wear due to adhesion (against steel) ++ ++ + ++ ++ +++ Resistance to wear due to diffusion (to steel) ++ + + +++ ++ +++ Protection of basis material against corrosion 5) + + + + ++ + 1) For the microhardness. 2) The oxidation temperature is the temperature at which oxidation of the coating material begins. to VDI 3824/1 Due to the high interaction between the metal and nitrogen atoms. whereby the Ti2 N phase is dominant. TiN is thermodynamically more stable. this compound is highly stable. the coatings act as a thermal barrier to the heat produced during metal-cutting so that most of the heat can be removed through the chip. coating thicknesses and internal stresses. considerably affecting the characteristics of the coating. The characteristic colour of TiN is goldish yellow.05 1) 2300 3000 3100 3000 1900 2100 HV 0. acc. They are most commonly used when abrasive wear predominates [VDI 3824]. With an enthalpy of formation that is almost twice as high as that of TiC. and thus more diffusion resistant and less inclined to adhesion. TiN coatings are characterized by a high level of toughness.44 Quantities Characterising the coating and the coating performance. TiN’s resistance to crater wear is higher than that of TiC. mean values are indicated. 5) As the aforementioned hard coatings themselves do not corrode. TiN can be deposited with both the CVD and PVD process. Therefore. Fig. they protect the basis material from corrosion. 3) Al2O3 is already an oxide.4 Coatings 153 TiN Production process TiCN PVD/CVD PVD/CVD TiC TiAlN CrN Al2O3 CVD PVD PVD CVD/PVD Coating thickness/µm 1 to 5 1 to 5 1 to 5 1 to 5 1 to 10 1 to 5 Microhardness/HV 0. 4) Being poor conductors of heat. These coatings are harder but also more brittle than pure TiN layers. Al)N coatings is explained by the fact that.Al)N coatings include the development of nanolayers and increasing the content of aluminium. (Ti. In this context. deposited with increasing carbon content in the direction of the coating surface.g. as opposed to stable composite phases. Knot87].Al)N) The (Ti. TiAlN coatings are deposited as monolayer. Often Ti(C. the preferred areas of application are dry machining. (Ti.4. Leye04.N) is a metastable coating system.Al)N coating is increased. i.Al)N coating system was developed in order to improve the oxidation resistance. 4.e. Depending on the composition. VDI 3824]. PA-CVD or MT-CVD process.4. a considerable increase in hardness can be realized. hot hardness and wear-protection properties above the levels of previously used coatings [Quin87. Ti(C. The properties of titanium carbonnitride are adjustable by varying the C/N ratio. To compensate for this increase in brittleness.Al)N is one of the most commonly used high-performance coating systems.N) coatings. Since Ti(Al. as heat). so that residual stresses between the individual coating layers can be reduced.N) coatings are multilayered. Important measures being taken to further augment the cutting edge durability of (Ti.Al)N is derived from the fact that a thin aluminium oxide layer is formed on the coating surface which is constantly renewed during the cutting process. metastable layers decompose into stable boundary phases or form stable oxides in an oxidizing atmosphere as long as the energy required to convert into an equilibrium condition is added (e. Despite its much further improved oxidation resistance compared to TiN and Ti(C. Due to their high level of oxidation resistance and hot hardness. the high oxidation resistance of (Ti. whereby titanium is substituted by aluminium by 20–60 at. this coating also fails beyond approx. the colour of the Ti(C.N)) TiC and TiN can be mixed at any ratio. By integrating carbon atoms in place of nitrogen atoms into the titanium nitride crystal lattice. these layers range from brown (lower Al-content) to black-violet (higher Al-content) [VDI 3824].Al)N coatings have the highest oxidation resistance with a comparably high level of hardness. Ti(C.3 Titanium Carbonnitride Coatings (Ti(C.-%.Al)N coatings are a further development of TiN. The primary goal of these developments is to improve their hot . thus decelerating the progress of wear. With increasing Al-content.N) layers change from coppery to violet. it can only be deposited with the PVD. multilayer or gradient layers [Lemm03. the oxidation resistance of the (Ti.154 4 Cutting Tool Materials and Tools 4. With increasing carbon content. 800◦ C. (Ti.2. Schi74. The excellent wear resistance of (Ti.N) coatings. which is positive for wear resistance but also increases brittleness [Sato78. bluish-grey to grey [VDI 3824]. Titanium carbonnitrides are used industrially both as hard materials in cemented carbides and as wear-resistant thin-films. hard machining and HSC machining.4 Titanium Aluminium Nitride Coatings ((Ti. as it is the case in dry machining or high-speed machining.N) coatings are deposited as multilayers. In comparison to TiN and Ti(C.2.N) coatings are suited for machining steels with high tensile strength and thus for higher cutting temperatures. Berg90]. 4. In comparison to the conventional (Ti. but only with the CVD process for technical reasons. In order to differentiate them from conventional (Ti.2.5 Al-Cr-N Coatings Besides increasing the aluminium content another approach to further improving the oxidation resistance and high-temperature properties of (Ti. those with over 50% aluminium content are designated as (Al. Y or Si. HSC and hard machining of steel and non-ferrous metals by turning.4.Al)N coatings. The electrically nonconductive Al2 O3 layers could thusfar not be fabricated with the PVD.Al)N coating systems is adding small amounts of oxide formers such as Cr. Their typical layer .7 Amorphous Carbon Coatings The concept “amorphous carbon coatings” includes a number of coatings and coating systems. it has become possible to coat cutting tools with crystalline γ-Al2 O3 layers at substrate temperatures of 500–600◦ C. One example for this direction in coating development is the Al-Cr-N coating system. With the help of the pulsed magnetron sputtering process. In the case of Me-C:H layers. drilling and milling [Gey04. Denk04].Al)N coating. 4. but only in combination with other hard materials in multilayer coatings. Due to its excellent wear protection and high performance potential. finely distributed metal carbides are also embedded. attempts are being made to produce Al2 O3 layers with the PVD process as well.4.Ti)N coatings. Al2 O3 is generally not used in monolayer form. Its areas of application include the dry. DLC coatings are manufactured by means of plasma-activated PVD and CVD techniques and deposited at coating temperatures ranging from room temperature to about 300◦ C.6 Aluminium Oxide Coatings (Al2 O3 ) The exceptional resistance of Al2 O3 against both abrasion and diffusion wear and its simultaneous insensitivity to oxidative wear. Because of the low coating rates and consequently long process durations.Cr)N coating system has proved to have a higher resistance to abrasive wear as well as higher hot hardness and oxidation resistance [Gey04].2.4 Coatings 155 hardness. this technique is proven to be uneconomical for coating cutting tools. They are often referred to as “diamond-like carbon” or “DLC” coatings. the (Al. highly cross-linked amorphous layers with different amounts of sp2 (graphite bond) and sp3 bonds (diamond bonds) as well as various amounts of embedded hydrogen. A first step in this direction was the deposition of amorphous aluminium oxide (α-Al2 O3 ) with the help of RF sputtering technology. wear resistance and oxidation resistance. has made Al2 O3 an obvious candidate as a hard material for coating [Sche88]. 4. γ-Al2 O3 can be applied either as a monolayer or multilayer coating in combination with carbidic or nitridic intermediate layers [Hauz05]. Because it is very brittle.2. known from its use in cutting ceramics. The deposition of the thermodynamically stable high-temperature modification of aluminium oxide – α-Al2 O3 – by means of CVD process has been established industrially for over two decades. 4. These are carbon-based.4. CrN) as a top layer. (Leaks in hard coatings may lead to the development of local galvanic elements and to pitting corrosion.) Fig. In cutting technology. to VDI 3824/1 thicknesses are 1–5 μm.05 1) Internal stresses in GPa Graphitisation temperature in °C 2) Resistance to abrasion Resistance to wear due to adhesion (against steel) Protection of bais material against corrosion 3) 4 Cutting Tool Materials and Tools MeC:H a-C:H a-C Diamond PVD CVD PVD CVD 1 to 10 1 to 5 1 to 3 3 to 10 800 to 1800 1500 to 3500 3000 to 7000 10000 0. considerably affecting the characteristics of the coating.g.g.45 shows some characteristic values for the layer properties of the most important carbon-based hard material layers producible with PVD and CVD available today [VDI 3824. these coatings are used above all on tools for dry machining NE metals (e. VDI 2840]. in which case hardness and the Young’s modulus can be adjusted to the mechanical properties of the carbon top layer by successive exchange of nitrogen and carbon.1 to 1. 3) As the aforementioned hard coatings themselves do not essentially corrode (except for MeC). In order to improve their adhesive strength. they protect the basis material from corrosion.5 1 to 3 2 to 6 – 350 400 450 > 600 + +++ ++++ ++++ +++ +++ +++ (+++) with good cooling + +++ +++ +++ 1) For the microhardness. Amorphous carbon layers are characterized by extremely low friction coefficients in the case of solid body friction.45 Quanties Characteristing the coating and the coating performance.156 Production process Coating thickness in µm Microhardness in HV 0. 2) The graphitising temperature is the temperature at which amorphous carbon begins to convert from a three-dimensional lattice into graphite. carbon layers are as a rule deposited on a hard material layer (e. Graded intermediate layers made of CrCN have proved favourable for this. They are obtained from reported measured values resulting from different compositions. Figure 4. coating thicknesses and internal stresses. mean values are indicated. acc. . high wear resistance and low tendencies to adhesion. 4. Further areas of application are in machining graphite and fibre-reinforced plastics. This procedure makes it possible to obtain a largely continuous transition from the hard material layer to the carbon layer in order to optimize the bond between both coating systems [Leye04]. aluminium or magnesium) and in fine machining with great success. it is softer. With high deposition energies. The layers exhibit a high hardness level. The added metals form finely distributed carbides . Tetraedic amorphous carbon coatings have the designation “ta-C” [VDI 3824. The atoms are arranged randomly in amorphous solid bodies. a limit of about 3 at. VDI 2840].g. leading to softer layers [VDI 3824. VDI 2840]. Of all amorphous carbon coatings containing hydrogen. it is above all the nonmodified layers that besides carbon contain only hydrogen. a-C:H:Ti). these therefore have the broadest and deepest range of application.g. a-C:H:W) oder titanium (a-C:Ti. With increasing amounts of hydrogen. tungsten (a-C:W. According to VDI guideline 2840. the integrated metals can also be named specifically. The only bonds are those between a few isolated atoms.4 Coatings 157 The property profile of amorphous carbon layers can be purposefully influenced by process control and by integrating additional chemical elements. Of all amorphous carbon layers. the amount of sp2 bonds is higher. hydrogen-free and hydrogenous coatings. the carbon atoms in the layer are predominantly arranged in one of the two hybrid states. As opposed to a-C:H layers. This potential has led to a number of diverse types of carbon coatings in the last several years. while it is harder if the amount of sp3 is higher. Amorphous carbon layers containing metal are also generally known as “a-C:Me” or “a-C:H:Me” (the abbreviation Me-C:H is also common). e.-% hydrogen is seen as the transition from hydrogen-free to hydrogenous carbon coatings. If. metals are additionally integrated. In the following. The latter is derived from hydrocarbon gases like acetylene. In addition to carbon. As wear-protection coatings on cutting tools. the essential characteristics and properties of some selected amorphous carbon layers will be described in close detail. amorphous carbon coatings can be subdivided into two groups.4. which are employed in the fabrication process. At low deposition energies. These have the abbreviation “a-C”. layers containing metal are produced. The latter contain further elements in addition to hydrogen and are classified in accordance with VDI 2840 into the group of carbon layers containing metal and the group of hydrogenous amorphous carbon layers modified with non-metals [VDI 3824. making the layers softer. where “Me” stands for “metal”. designated with “a-C:H” as well as modified carbon layers that are of any importance. the amount of interconnection between the carbon atoms decreases. and the compressive residual stresses are increased. during vacuum deposition. VDI 2840]. hydrogen-free carbon layers are used above all in the form of the amorphous carbon layer a-C and the tertraedic amorphous carbon layer ta-C. designation and characterization of the coatings are derived from VDI guideline 2840. Since all carbon layers contain a certain amount of hydrogen even without adding hydrogen gas (e. The classification. Depending on the deposition energy. these are electrically conductive and can be manufactured with a technically simpler and cost-efficient DC method. it contains 10–30% hydrogen. Instead of the abbreviation “Me”. The a-C:H coating system represent the origin of coating with amorphous hydrocarbon layers. sp3 hybridizations with a tetraedic arrangement predominate. If the sp2 bond is predominant in the amorphous carbon layer. from residual gasses). Silicon for example helps to increase temperature resistance. MoS2 also has a lamellar structure. Graphite and MoS2 have similar crystal structures. In the case of tapping. lubricating effect of graphite and molybdenum sulphide. nitrogen (N). By integrating metal carbides. friction and adhesion processes. Both coating systems are used primarily for machining materials that tend to adhere strongly with the tool. Even if the layer appears optically to be worn. This is explained by the fact that the coating material still exists in microscopically small recesses on the tool surface where it can still be effective. The bond between two neighbouring lamellae is made between the sulphur atoms. wettability) can be altered. these coating systems can bring about significant improvements in performance even without hard material intermediate layers. Graphite is deposited in combination with WC as a WC/C multilayer coating system consisting of several layers of WC and graphite. By incorporating different nonmetals. fluorine (F) or boron (B). these can be shifted towards one another very easily. These slip planes are responsible for the soft. There is a relatively large distance between the atom layers however. for example in the case of dry machining aluminium wrought alloys. the further wear properties of the tools is then determined by the properties of the supporting hard material system. so the layers of atoms can be easily shifted against each other under the influence of external forces. these layers decrease the friction between the chip and the tool because of their “lubricating” effect and thus help to improve chip removal. further improvements become possible.4. 4.158 4 Cutting Tool Materials and Tools with the carbon in the matrix. After a complete loss of the soft top layer. special layer properties like surface energy (adhesion tendency. There is a very strong chemical bond between the Mo and S atoms forming each lamella. which partially also form carbides. different elements can be built into the layer at the same time. in which case the machining results are determined above all by crushing. where they act as a solid lubricant. In the case of drilling. Hydrogenous amorphous carbon layers modified with non-metals contain nonmetallic elements such as silicon (Si). both coating systems are gradually worn out during the cutting process (loss lubrication). its effect is still partially demonstrable. graphite and MoS2 coatings are applied as top layers on a supporting hard material layer.8 Self-Lubricating Coatings Self-lubricating coatings include coating systems which include graphite or molybdenum disulfide (MoS2 ). oxygen (O). VDI 2840].2. In this way. Moreover. Due to the intermolecular van der Waals bonds between the sulphur atoms of two lamellae. On cutting tools. Because of their properties. It is characteristic of graphite that its C atoms are arranged in a plane resulting in a very high level of internal bond strength due to the small inter-atomic distance. layer adhesion can be improved and the tribological properties of the coating can be affected [VDI 3824. . In the case of the hot filament method. W or Re).4 Coatings 159 4. thick diamond layers are generally deposited on an auxiliary substrate and then detached from it again. Diamond plates produced in such manner are then used as free-standing diamond (e. accelerates graphite formation. chemical etching. The common process presently is a multi-stage pretreatment consisting of irradiation. The substrate is first purified and homogenized with microbeam treatment. Diamond-coated tools can not be used to machine steel and other ferrous materials since the diamond coating is easily worn due to the solubility of carbon in iron. in which case we differentiate between physical and chemical pretreatment and the introduction of an intermediate layer. In the subsequent chemical etching . We differentiate between thin diamond layers (sd = 1–40 μm) and thick diamond layers (sd = 0.2. These tools are used in the machining of plastics. which forms the binder phase of the cemented carbide. purifying and seeding. economically out of diamond [Leye95].9 Diamond Coatings (Crystalline Carbon Coatings) Coating tools with polycrystalline diamond has been available only since 1990 and is as such the most recent technique in diamond tool production. The most important include the hot filament process (HF-CVD). Gas temperatures of over 2500◦ C are necessary to initiate the required chemical reactions [Lemm04. The rim zone is of particular importance when coating cemented carbides. non-ferrous metals as well as in the machining of abrasive materials.g. Industrial deposition temperatures are around 600–1000◦ C. Thin diamond layers are used to coat components (tools. Several processes have been established for fabricating diamond coatings.3–2 mm). On the other hand. VDI 2840]. What all of these processes have in common is the decomposition of carbonic gases (e. acetylene or carbon monoxide) followed by a deposition of carbon on the substrate surface. Application examples for this are drilling tools. end milling cutters and indexable inserts with chip form geometries. Diamond coatings are applied to tools made of cemented carbide or ceramics in a low-pressure diamond synthesis (a CVD process).4. and affects crystal growth and layer adhesion. cemented carbide and ceramic green bodies. Atomic hydrogen plays a key role in the germ formation and growth of the diamond layers.4.g. Cobalt. reduces the germ formation of diamond on the substrate surface. The diamond layer consists of pure diamonds and does not have a binder phase. the best-known process used for depositing CVD thin diamond layers. we often no longer refer to it as a CVD diamond layer but as a CVD diamond [VDI 2840]. such as curved surfaces. the atomic hydrogen required for the process is produced by electrically heated filaments made of refractory metals (Ta. methane. Cemented carbide substrates must therefore be subjected to pretreatment prior to coating. The formation of unwanted graphite must be avoided in the process. as radiation windows) or mounted on supports. usually by means of vacuum soldering in order to produce tools or other components. This coating technology makes it possible to manufacture even tools with complex geometries. the microwave process and the plasma jet process (DC arcjet). for example) directly. Since this can no longer be referred to explicitly as a layer. resulting in the typical appearance of a microcrystalline CVD diamond layer with sharp-edged crystal surfaces [VDI 2840]. Lemm04]. which are much smoother compared to crystalline layers [VDI 2840]. the cobalt that has diffused to the surface reacts with the diamond layer and leads to the separation of diamonds on the boundary surface. there must be diamond germs on the substrate surface that serve as starting crystals and from which the layer grows. acc.46 Growth model of diamond coatings. Due to the extreme differences between the coating and substrate material (Young’s modulus and hardness) this mechanical clamping is of special importance with respect to the adhesive strength of the diamond layer. the cobalt binder phase is removed from the rim zone of the cemented carbide substrate. the substrates are again purified and seeded. With increasing layer thickness.46 and 4. If the etching depth is too small. such that they can no longer easily reach the Microcrystalline (standard) Nanocrystalline (smooth) Multilayer (smooth) Microcrystallin Nanocrystalline Fig. 4. The crystallites grow from the germs with varying speed depending on their crystal orientation.46 and 4. the carbide bonds in the binder phase are weakened. 4. In order that the diamond layer will develop. uniting the positive attributes of microcrystalline and nanocrystalline diamond layers [Uhlm05] (Figs. for example.This is how “nanocrystalline” CVD layers are formed. by applying small diamond crystals with a diamond suspension [Gram04. The depth to which the binder phase is removed depends on the back diffusion of cobalt during the coating process. 4. to VDI 2840 and LEMMER [Lemm04] . If growth conditions are set such that new germs are constantly formed and largescale growth of individual crystallites becomes impossible. Due to the high substrate temperature of 900◦ C and long processing times. Slowly growing crystallites are covered by the faster-growing ones.47). This increases the fracture toughness of the entire system and contains the development of cracks. 4. The rough surface caused by etching serves mechanically to clamp the diamond layer to the cemented carbide substrate. VDI 2840. Standard diamond layers have a microcrystalline structure (Fig. cobalt diffuses through the etched rim zone to the substrate surface during the diamond coating process. Both layer types can also be combined in multilayer structures. This lowers the adhesive strength of the diamond coating. crystallite size remains in the range of nanometres within the entire layer (Figs.47). After etching. the crystallites forming the surface become larger and larger.160 4 Cutting Tool Materials and Tools process.46). If the etching depth is too large. also lowering layer adhesion. Seeding can be performed. Then the tools are etched by means of argon ion bombardment in order to produce a purely metallic surface. Coating follows directly thereafter. B. Even the smallest impurities on the surface of the substrate – be it residual cutting fluid or grease. borides. negatively influence that process and lead to coating adhesion problems. ultrasound-supported purification). Lemm04] interface between the first diamond layer and the substrate. N or . microbeaming and polishing) and physicochemical pretreatment (coating-removal. whereby further impurities can evaporate. If the substrates still need to be stored temporarily before charging.3 Substrate Pretreatment Both CVD and particularly PVD coating processes place high demands on the surface conditions of the components that are to be coated. the parts are then first heated in a vacuum. A distinction is made between oxidic and non-oxidic ceramics.5 Ceramic Cutting Tool Materials Ceramic materials include all non-metallic. VDI3824].4. In order to optimize layer adhesion. 4. nitrides and silicons. Non-oxidic ceramics are carbides. In substrate pretreatment a distinction is drawn between mechanical pretreatment (grinding.4. Gey03]. In the coating plant. the parts are subjected to a multi-staged ultrasound purification process before charging. VDI3824. inorganic solid materials. this takes place in a drying closet in order to protect the substrate surfaces from water deposits (from air moisture). Essentially these are chemical compounds of metals with non-metallic elements of group III A to group VII A of the periodic system of elements. hardening salt residues or oxide deposits – can degas during the coating process.5 Ceramic Cutting Tool Materials Microcrystalline (standard) Nanocrystalline (smooth) 161 Multilayer (smooth) Fig. The largest group of ceramics are the oxides. rust film formation and skin contact [Lugs03. 4. water deposits. potentially causing the entire surface to chip [Lemm04].47 SEM photographs of CVD diamond coatings on a cemented carbide substrate [VDI 2840. grinding and polishing agents. Some authors differentiate again between metallic hard materials (compounds between C. 4. deburring. The pretreatment of the substrate therefore has a key role in creating high and reproducible coating quality [Lugs03. Mentionable among these are the high-speed machining of cast iron materials. The following will describe more closely those ceramic materials that are used as cutting tool materials in cutting technology. finer-grain hard materials are used and titanium carbide is being partially substituted with titanium carbon nitride. they have high mechanical strength (Rm up to 7000 N/mm2 ). 4. which can be derived from the strong covalent and ionic bonds of the atoms [Salm83a. Clau77. reduction of the amount of glass phase at the grain boundaries of non-oxidic ceramics as well as the formation of special microstructures [Kola86. Zr. Si3 N4 . The increased interest in ceramics in production is based not only on the excellent wear properties of ceramic cutting tool materials but most importantly on their toughness properties. In the case of pure oxidic ceramics. silicon nitride ceramics have been able to capture an important market share in the high-speed cutting of cast iron workpieces [Schn99]. In the field of automotive technology. W among others) and non-metallic hard materials such as diamond.1 and 1 μm. The brittle fracture attributes characteristic of ceramic materials. Their length is about 20–30 μm. New developments and improvements in ceramics have been concentrated therefore on further augmentation of the toughness and dependability of these cutting tool materials in production.1 Cutting Ceramics Ceramics have applications in many areas of machining with geometrically defined cutting part geometries. SiC. Ta. Following the usage common in practice. flywheels and like workpieces has become a matter of course. high chemical resistance and high melting temperatures. their diameter between 0. “Whisker-strengthened” ceramics are often used in the area of engine construction for machining engine components made of nickel-based alloys. Horn06]. which have been clearly increased in recent times. Correspondingly. Characteristic properties of ceramic materials are compressive strength. Hard machining is a primary application area of mixed ceramics. we will differentiate between cutting ceramics (oxidic and non-oxidic ceramics) and superhard non-metallic cutting tool materials (diamond and boron nitride). In the case of mixed ceramics. Nb.162 4 Cutting Tool Materials and Tools S and Ti. .5. B4 C and BN. improving the distribution of these phases and by making the microstructure more consistent. Further measures include fibre and/or whisker reinforcement. The use of ceramics in the mass production of brake disks. Whiskers are needle-shaped monocrystals with a small degree of misorientation in the lattice. hard machining or nickel-based alloy machining. Schn99]. research is focused on improving these properties by increasing their zirconium oxide content. the wide variation of their strength properties and the resulting stochastically appearing tool fractures are as before the main causes limiting the use of these cutting tool materials in machining technology (in contrast to cemented carbides for example). 50 shows some physical properties of commercially available ceramic cutting tool materials. 4. Characteristic of non-oxidic ceramics based on Si3 N4 is the needle-shaped formation of the crystals. these ceramic types can have considerably diverse properties. 4.49 shows images of fractures of different cutting ceramics taken with a scanning electron microscope. Figure 4.48).4.5 Ceramic Cutting Tool Materials 163 Cutting ceramics Oxidic ceramics Oxide ceramics Mixed ceramics Al2O3 Al2O3 + TiC Al2O3+ ZrO2 Al2O3 + ZrO2 + TiC Non-oxidic ceramics Whiskerreinforced ceramics Al2O3 + SiC-whisker Silicon nitride ceramics β-Si3N4 β-Si3N4 + hard materials α-Sialon (α + β)-Sialon Fig. which in addition to Al2 O3 also contains metallic hard materials (TiC/TiCN) and whisker-reinforced ceramics.g. Inserts were made for a long time out of pure aluminium oxide. It is typical of oxidic cutting ceramics that they do not have a visible binder phase such as cemented carbides (cobalt) have. Oxidic cutting ceramics include all cutting tool materials based on aluminium oxide (Al2 O3 ).1 Cutting Ceramics Based on Al2 O3 Oxide Ceramics White oxide ceramics are the traditional type of cutting ceramics. Ceramics based Al2 O3 on were already introduced as cutting tool materials at the end of the 1930s [Kola86]. mixed ceramics.5. Depending on their chemical composition. which are only a few μm large in the detail magnification. in which SiC whiskers are integrated in the Al2 O3 matrix. Due .1. One can recognize the globular grains of oxide ceramics. the extremely fine grains of mixed and whisker-reinforced ceramics as well as the SiC whiskers. Figure 4. 4.48 Classification of cutting ceramics Ceramic cutting tool materials can be subdivided into oxidic and non-oxidic cutting ceramics (Fig. A distinction is made between oxide ceramics that only contain oxides (e. Si3 N4 cutting ceramics can be coated with Al2 O3 or with multilayer coating systems made of Al2 O3 and TiN [Schn99]. ZrO2 ) in addition to Al2 O3 . In order to improve wear resistance. 2 4.5 5.5 Thermal conductivity λ W/m⋅K 16.0 Fig. Silicon nitride Mixed ceramic Oxide ceramic Al O 2 3 ceramic Si3N4 Al2O3 +15%ZrO2 +3.7 20 Thermal expansion α 10–6⋅K–1 8 8 - 8 8 3.5% +15% +20%SiC.50 Physical and mechanical properties of different cutting ceramics and their main components .4 Melting point T °C - - - - - - 7.3 Vicker hardness − - 1730 1750 1900 1730 1930 1750 Bending strength σ N/mm2 700 800 900 650 620 800 N/mm2 5000 4700 - 4800 4800 - bB Compressive σ dB strength Young‘s modulus E 103⋅N/mm2 380 410 390 390 400 Fracture toughness KIC N⋅m1/2/mm2 4.3 3.1 4.164 4 Cutting Tool Materials and Tools Oxide ceramic (Al2O3 + ZrO2) Mixed ceramic (Al2O3 + TiC ) Silicon nitride ceramic (Si3N4 + MgO ) Whisker-reinforced ceramic (Al2O3 + SiC-Whisker ) Detail magnification (Al2O3 + SiC-Whisker ) Coated silicon nitride ceramic (Si3N4 + Al2O3) Fig.7 4.N) Oxide ceramic Al2O3 Properties Density ρ g/cm3 4.4 15 32 14.+10%ZrO2 +30% +10%Y2O3 ZrO2 ZrO2 Whisker +5%TiC Ti(C. 4. 4.1 8.0 4.2 3.0 4.49 Fractures of cutting ceramics Whisker-reinf. The result of this is that critical cracks only develop at a higher level of energy. Correspondingly. ZrO2 . In the case of hard turning. As opposed to conventional cutting ceramics based on aluminium oxide and titanium carbon nitride with an average grain size of < 2 μm. which exists in the form of a tetragonal lattice modification in the sintering temperature range (1400–1600◦ C). The smaller the ZrO2 particles are. 4. improved toughness properties and a high level of edge strength and wear resistance.50). gearwheels or sliding sleeves with a Rockwell hardness of 54–62 HRC but also for planing and fine planing cast iron at very high speeds. submicron mixed ceramics are competing with PCBN cutting tool materials in many areas of application as more economical alternatives due to their good cost-benefit ratio [Krel97. The development of mixed ceramics is progressing towards finer and finergrained cutting tool materials with extremely homogeneous textural structures. Clau84. The common effect of all these mechanisms is that they ultimately absorb fracture energy. the lower the transformation temperature. The finer-grained structure increases hardness and bending strength and consequentially the mechanical and thermal loadability.4. various specific mechanisms of action can assert themselves depending on the size of the particles. Zieg86]. the stress-induced transformation of small ZrO2 particles as well as crack diversion. e. which limit the growth of aluminium oxide grains. The hard materials in the matrix form finely distributed phases. the more advanced mixed ceramics have a submicron structure with grain sizes of < 1 μm.g. Compared to pure ceramics. Schn99]. The temperatures in which this transformation takes place depend on the size of the particles. which corresponds to an increase in fracture resistance and an improvement of ductility [Clau77. these ceramics have a very fine-grained structure. crown wheels. such inserts are no longer used in machining. wear resistance and edge strength of ceramic inserts. The speed of crack development is reduced by microcracking. for machining hardened rolling bearing steels. transforms during cooling into its monoclinic low-temperature modification. . Since the transformation from the tetragonal to the monoclinical modification is associated with a volume expansion. Mixed Ceramics Mixed ceramics (black ceramics) are dispersion materials based on Al2 O3 that contain between 5 and 40% of non-oxidic components in the form of TiC or TiCN. The pure ceramics used today are dispersion material containing Al2 O3 as well as about 3–15% of finely distributed zirconium dioxide to improve toughness. crack branching. Fine-grained mixed ceramics are used in hard-fine machining.5 Ceramic Cutting Tool Materials 165 to their brittleness and vulnerability to fracture. for hard-fine turning case-hardened automotive components such as drive wheels. The toughness-enhancing effect of dispersed ZrO2 particles in an Al2 O3 matrix is based on the phase transformation of zirconium dioxide. they are harder and have more favourable thermoshock properties due to their high level of thermal conductivity (Fig. The toughness of these ceramics can be further improved by adding ZrO2 . the contact face. Hot isostatic pressing can be used to subsequently compact sintered ceramic parts. including pressing and sintering auxiliaries. the lateral surface and cutting edge chamfer are ground with diamond grinding discs [Kola86.166 4 Cutting Tool Materials and Tools Whisker-Reinforced Cutting Ceramics Whisker-reinforced cutting ceramics are cutting tool materials based on Al2 O3 with about 20–40% silicon carbide whiskers. bending. graphite moulds instead of cemented carbide ones). As mentioned at the beginning of this chapter. The goal of whisker-reinforcement is to enhance the toughness properties of ceramic cutting tool materials. Fabrication of Oxidic Cutting Ceramics The starting materials of cutting ceramics. and in the case of precision inserts. whisker-reinforced types have up to 60% higher fracture toughness. One method variant is hot isostatic compacting of pre-sintered closed-pore mouldings in an installation. are compressed at room temperature to inserts (cold pressing) and then sintered. . so that whisker-reinforced cutting ceramics can also be used in wet cut processes. the number of manufacturable tool geometries is much more limited. The sintering temperature is about 1600◦ C. hot pressing is more cost-intensive than normal sintering. homogenized in vibration mills and converted to a compressible powder with a high bulk material stability in a spray-dryer. However.g. The whiskers bring about a more consistent distribution of mechanical loads in the cutting tool material as well as a more rapid transport of heat from the thermally highly loaded cutting areas due to their improved heat conductivity. Oxide ceramics. Properties of Oxide Ceramic Cutting Tool Materials Because of their high hot hardness and chemical resistance. This results in improved fatigue limits and thermoshock resistance. the highly favourable wear properties of oxide ceramic cutting tool materials is accompanied by their sensitivity to tensile. After sintering/hot pressing.e. Compared to mixed ceramics. ceramic cutting tool materials based on Al2 O3 are characterized by excellent wear properties. Furthermore. current further development of oxide ceramic cutting tool materials has for this reason been concentrated on improving toughness and thermoshock properties without diminishing wear resistance. Grew85]. Due to the special equipment required for this (e. are metered according to an exact formula. Mixed ceramics with a high percentage of hard material (> 10%) as well as whisker-reinforced ceramics have to be hot-pressed. as well as mixed ceramics with small amounts of hard materials. thus reducing the porosity of the cutting tool material. i. The increase in toughness gained by incorporating whiskers in oxidic ceramics is remarkable. impact and thermal shock stresses. pressing and sintering take place in one cycle. 4. The high compressive strength of ceramics has been a primary reason for their being privileged as cutting tool materials. For example. particularly at high temperatures. Kola86a. it is at 1100◦ C still as great as that of steel at room temperature. the use of oxide ceramics as the cutting tool material requires that the process kinematics be selected such that the cutting forces are effective as much as possible only in the form of compressive stresses [Kola86. While the compressive strength of Al2 O3 at room temperature roughly corresponds to that of cemented carbide.51). is based among other things on their superior hardness and compressive strength compared to other cutting tool materials at high temperatures. creeping processes are introduced which are smaller by several orders of magnitude in the case of Al2 O3 . high temperature strength is limited by the property-determining cobalt phase at temperatures of 800–900◦ C. At higher temperatures. Zieg86]. 5000 Diamond Hardness HV 10 4000 Boron nitride 3000 Mixed ceramic Al2O3 + TiC/TiCN 2000 Carbide Oxide ceramic Al2O3 1000 0 250 Tool steel 500 750 Temperature T / K Fig. 4.5 Ceramic Cutting Tool Materials 167 The excellent wear resistance of oxidic cutting tool materials.51 Hot hardness of cutting tool materials (Source: CeramTec) HSS 1000 1250 . A further property which makes the use of ceramic cutting tool materials advantageous at high cutting speeds is Al2 O3 ’s low level of creeping. In the case of cemented carbides. But because their banding fracture strength is relatively low. while neither steel nor cemented carbide are resilient to pressure at 1100◦ C. 4. oxide ceramic cutting tool materials at 1000◦ C are still harder than high speed steel at room temperature (Fig. Failure of the cutting tool material due to fracture occurs if the stress intensity factor reaches a critical value. case-hardened steels and heat-treated steels. since ceramics can be destroyed immediately in the form of fracture should the marginal conditions be unfavourable. The stress intensity factor KI is a parameter for judging the stress condition at the peak of a crack. In the case of temperature changes of more than 200◦ C. a careful adjustment of cutting parameters to tool geometry is the prerequisite of a successful use of cutting ceramics. chamfered edges should stabilize the cutting edge. If the crack reaches a critical size. The most crucial disadvantage of ceramic cutting tool materials is their brittleness. However. lacking capacity to reduce stress peaks by means of plastic deformation.52). cracks begin to form when the inner material cohesion is exceeded by external stress. This is justified by the reduction of processing times and the high cost of selecting and adjusting the cutting tool materials.168 4 Cutting Tool Materials and Tools The high wear resistance of oxidic cutting tool materials can also be attributed to the good chemical resistance of Al2 O3 . There is no room for error. In comparison to other materials. the cutting tool material is destroyed by brittle fracture. e. unstable crack growth appears leading to fracture of the ceramic tool. Coolants should only be used to temper the workpiece. The main area of application of oxide ceramic cutting tool materials is rough and finish turning of grey cast iron.53).e. Due to their sensitivity to thermoshock. Areas of Application of Oxide Ceramic Cutting Tool Materials It must be stressed that.g. This results in high sensitivity to tensile stresses and low resistance to mechanical and thermal shock compared with metals. coolants should not be used in conjunction with oxide ceramics when roughing and planing. which is associated with delayed load of the cutting edge of the tool (Fig. Another important disadvantage of oxide ceramics are their relatively low resistance to temperature change. crack toughness (fracture toughness) KIC . Due to their low edge strength. . This can only be improved by alloying a component of superior resistance to temperature change. The increased popularity of dry machining however is opening up new areas of application to oxide ceramics in the case of turning extrusion parts with a small machining allowance. Al2 O3 is resistant to oxidation at the cutting temperature used in practice and has only a small amount of affinity to metallic materials. Because of insufficient ductility.50). Low bending fracture strength and relatively high sensitivity to impact and temperature change stresses require a slanted entering and leaving cutting path. In the case of a sudden mechanical overload due to impulse-like stress. 4. i. Cutting ceramics are therefore almost exclusively used in mass production. 4. 4. pure oxide ceramics are being increasingly replaced by silicon nitride ceramics. especially in cast iron processing (Fig. pure Al2 O3 is destroyed. The cause of this is the low number of sliding systems in the crystal structures of ceramic materials. due to narrow workpiece tolerances. the values of KIC are very low for ceramics (Fig. due to their extreme brittleness. Chamfering and external turning Finished part 1. Chamfering and internal turning 3. Gers02. Uhlm04]. for machining hardened rolling bearing steels) for fine turning case-hardened automotive components such as drive wheels.4.52 Entering and exit tool path when turning with cutting ceramics (Source: Ford) Fine-grained mixed ceramics are used in hard-fine machining (e. They are also used for finish planing and fine planing cast iron at very high speeds. Face turning 2. As opposed to machining with cemented carbide tools. .54). crown wheels. In the case of hard turning. in the case of steels with a C content of under 0. Clamping (external) 2. In contrast to otherwise used cutting tool materials (cemented carbide or HSS). Schn99].g. cutting speeds of 800–1000 m/min can be realized with cutting ceramics based on silicon nitride or whisker-reinforced aluminium oxide. 4. gearwheels or sliding sleeves with a Rockwell hardness of 54–62 HRC.35% adhesion occurs on the tool as well as chemical reactions which increase wear and generally make the process uneconomical. Krie02. Whisker-reinforced oxide ceramics have been used with great success for turning high temperature nickel-based alloys (Fig. submicron mixed ceramics are competing with PCBN cutting tool materials in many areas of application as more economical alternatives due to their good cost-benefit ratio [Krel97. Clamping (internal) 2. 4. Face turning Fig. In the case of rough and finish milling Inconel 718 with milling head face cutters. this represents an increase in performance by factors of 25 or more [Momp93. it is possible to increase the cutting speed by a factor of 10 or more. Chamfering 1. Both types of cutting ceramics still however remain restricted with respect to the carbon content of the machined steels. For example.5 Ceramic Cutting Tool Materials 169 1. 53 Influence of cutting ceramics and entering path on the tool life quantity in brake disc manufacturing (Source: CeramTec) Tool life travel path Lc / m 3000 2000 Cemented carbide K10/20 1000 Polycrystalline cubic boron nitride 500 400 300 SiC-whisker reinforced cutting ceramics and SiAlON 200 100 Criteria of tool life: VB = 0.35 mm ap = 3.5 mm ap = 3.12 mm ap = 0.54 Achievable tool life travel paths and applicable cutting speeds in finish turning Inconel 718 with uncoated cemented carbide.3 mm. 4. VBN = 0. 4. silicon nitride ceramic and PCBN .3 mm 20 200 300 400 600 100 30 40 50 Cutting speed vc / (m/min) VB vc = 400 m/min f = 0. whisker-reinforced cutting ceramic.170 4 Cutting Tool Materials and Tools Al2O3-ceramic 300 vc = 1000 m/min f = 0.30 mm Notch wear VBN VB Flank wear Inconel 718 Wet cut Determinant tool life wear criteria Fig.0 mm 250 Tool life quantity N Si3N4-ceramic 200 150 100 50 0 non-chamfered chamfered non-chamfered chamfered Fig.0 mm vc = 600 m/min f = 0. This glass/binder phase has a negative influence on the high-temperature properties of silicon nitride ceramics.4.5. MgO. can be modified and adjusted to the stresses of various machining tasks. . MgO. The tubes are arranged in a plane. Si3 N4 cutting ceramics. In machining processes involving grey cast iron.2 Non-oxidic Cutting Ceramics Among the non-oxidic ceramics (carbides. the properties of the cutting tool material. Cutting ceramics composed of thin ceramic tubules represent a new generation of ceramic cutting tool materials. wear-resistant ceramic material. The diameter and thickness of the tubes. 4. in order to press silicon nitride ceramics completely. where the tubes forming the individual planes can be parallel from plane to plane or can be arranged alternately at an angle of 45◦ /90◦ to each other. Al2 O3 ) are required that form a glass phase and fill in the gaps between the crystals. they have a higher hot hardness and high temperature strength.1. Moreover. However. are characterized by increased toughness and improved thermoshock resistance (Fig. borides. 4. Si3 N4 cutting ceramics can also contain other additives that affect their crystal structure or texture and thus their properties as well.5 Ceramic Cutting Tool Materials 171 Chemical reactions and built-up edge formation when machining light metal alloys make Al2 O3 cutting ceramics unsuitable for machining Al. such as toughness and wear resistance. sintering auxiliaries (Y2 O3 . Mg and Ti alloys. .50). The high safety level of these cutting tool materials has been an especially important factor in the acceptance of Si3 N4 cutting tool materials by manufacturers. Besides sintering auxiliaries. the tool body is formed of small thin tubes consisting of a tough ceramic material and filled with a hard. The significantly higher fracture resistance compared to oxide and mixed ceramics is based on the needle-like shape of the hexagonal β-Si3 N4 crystals as opposed to the globular Al2 O3 grains. Initial machining experiments in turning high-temperature nickel-based alloys as well as in machining hardened steel materials demonstrated the promising wear properties and performance of this new cutting ceramic [Haid05]. The tool is formed with a large number of planes built on top of each other. In the case of these ceramics. they make it possible to use the highest cutting values while maintaining the highest tool life spans and low failure rates. The non-directional growth of the needle-shaped crystals leads to a microstructure made up of mechanically interlinked components which provides the cutting tool materials with excellent strength properties. in contrast to oxidic cutting ceramics.). silicides. materials based on Si3 N4 have been especially successful as cutting tool materials for machining purposes. are variable. In this way. Corresponding to their chemical composition and crystallographic structure. nitrides. first introduced at EMO 2005. the silicon nitride ceramics available today can be subdivided into three groups: • I: β-Si3 N4 + binder phase (Y2 O3 . as well as their orientation and filling. The low thermal expansion of Si3 N4 ceramics compared with oxide and mixed ceramics is responsible among other things for its favourable thermoshock properties. Al2 O3 ) • II a: α sialon (α–Si3 N4 + Al2 O3 + AlN) + binder phase (Y2 O3 ) . They are therefore in direct competition with whisker-reinforced oxide ceramics.52. zirconium oxide or Sic whiskers. TiN. They are also good for high-speed milling of nickel-based alloys with tools mounted with indexable inserts at cutting speeds of 800–1000 m/min. β sialon mixed crystals are stem-shaped.g. high cutting speeds and thus large volume removal rates can be realized when machining cast iron materials in smooth and interrupted cut [Köni86. using Si3 N4 ceramics improves tool life quantity considerably compared with oxide ceramics (Fig. Like the latter. 4. so the use of these materials groups currently remains uneconomical. Including special additives makes it possible to stabilize the β sialons. sialons are harder. large feeds. Some nitrogen atoms are replaced in the process by oxygen atoms and silicon atoms by aluminium atoms. when turning automotive brake discs. They wear very quickly in the case of steel machining. SiC-whiskers) + binder phase The cutting tool materials from Groups I and II are fabricated by means of hot pressing. to omit workpiece chamfering as long as one forgoes maximal utilization of the cutting tool material. For example. In this case. . especially in automated manufacture. sialons can be used for machining nickel-based alloys at much higher cutting speeds than cemented carbide tools. more chemically resistant and have an increased resistance to oxidation. The wear resistance of silicon nitride ceramics is slightly lower than that of oxide ceramics. Most of the Si3 N4 cutting tool materials available on the market today belong to Group I. ZrO2 . The cutting tool materials from Group II are usually designated as sialons. Areas of Application of Non-oxidic Cutting Ceramics The classic field of Si3 N4 cutting tool materials is grey cast iron machining.53). In comparison to the cutting tool materials from Group I. titanium carbide. The amount of additives determines the amount of β sialons in the cutting tool material’s microstructure. hot isostatic pressing or a combination of these methods.172 4 Cutting Tool Materials and Tools • II b: (α + β) sialons (α Si3 N4 + Al2 O3 + AlN) + additive (e. Schn99]. 4. Due to the high fracture toughness of these cutting tool materials. Silicon nitride can receive up to 60% aluminium oxide in solid solution. Group III can include Si3 N4 cutting tool materials. which can sometimes be followed by hot isostatic pressing. sintering. Silicon nitride cutting tool materials exhibit a strong affinity to iron and oxygen under machining conditions. the properties of which can be specifically altered by adding hard materials such as titanium nitride. usually the tougher silicon nitride ceramics of Group I are preferred. The main area of application of sialons is in turning nickel-based alloys under finishing or average roughing conditions (Fig. ytterbium) + binder phase (Y2 O3 ) • III: β silicon nitride + hard materials (e. Improved resistance to fracture makes it possible. 4.54). The manufacturing method in this case is sintering. thereby creating a cutting tool material the structure of which consists of both α and β mixed crystals. While the α sialon mixed crystals have a globular form.g. Köni87. as shown in Fig. Diamond is equally superior to all other known hard materials with respect to thermal conductivity. Following the usage common in practice and in the literature. which both can appear in monocrystalline or polycrystalline form. One significant property of monocrystalline diamond is the anisotropy (directional dependence) of mechanical properties such as hardness. the identification letters of polycrystalline diamond are DP while those of monocrystalline diamond are DM (Fig. Although diamond also exists in nature in polycrystalline form (ballas. As opposed to cemented carbides. 4. boron nitride to non-oxidic ceramics. (a) Natural Diamond Natural diamond is only of importance for the purposes of cutting with geometrically defined cutting edges in its monocrystalline form. Classification of Diamond Cutting Tool Materials To classify diamond cutting tool materials. TiC.2.g. The extremely high bond and lattice energy is the reason why diamond is the hardest of all known materials. A multilayer coating system consisting of aluminium oxide and titanium nitride remains the most popular. 4. which are applied in different combinations and layer thicknesses.5. in which the C atoms are bonded covalently and tetraedically. carbonado). cutting tool materials based on diamond and boron nitride are designated as superhard and non-metallic. 4.5. strength . Diamond solidifies in the cubic crystalline lattice system. where diamond is assigned to monatomic. GJV40) [Schn99].1 Diamond as a Cutting Tool Material Elemental carbon appears in the two crystal modifications graphite and diamond.2 Superhard Non-metallic Cutting Tool Materials In machining technology. especially when machining cast iron with globular graphite (e. these types of diamond are of lesser interest since the synthetic fabrication of polycrystalline diamond is more advantageous both economically as well as technologically. According to the definition.5 Ceramic Cutting Tool Materials 173 Coating silicon nitride ceramics opens up further prospects for their use. we differentiate between natural and synthetic diamond. Figure 4. The service life of Si3 N4 can be considerably increased by applying a wear and diffusion inhibiting coating. which are components of cemented carbide tools and permits a comparison with diamond. The classic coating materials are TiN.55 shows the hardness and thermal conductivity of some materials that tend to be used as abrasives as well as different carbides. both are ceramic cutting tool materials. we will also refer to polycrystalline diamond cutting tool materials with the abbreviation PCD. they allow for higher cutting speeds and thus for shorter machining times and in many cases also for dry machining [Schn99]. Coated silicon nitride ceramics are also suitable therefore for cases that had hitherto been reserved for cemented carbides. TiCN and Al2 O3 .3). According to DIN ISO 513.4. From this we can see that the position of the lattice directions must be known both for grinding monocrystalline diamond as well as for using it as a tool. (b) Synthetic Diamond The fabrication of synthetic diamonds takes place using a catalyst solution in a pressure and temperature range in which diamond forms a stable phase.174 4 Cutting Tool Materials and Tools Thermal conductivity λ RT / (W/m⋅K) Hardness HV Diamond cBN B4C SiC TiC α -Al2O3 10000 5000 2500 100 1000 10000 Fig. 4. While this directional dependence is compensated in the case of polycrystalline materials by the completely random distribution of the individual crystals. The most common cutting edge shapes of .55 Hardness and thermal conductivity of diamonds in comparison to other hard materials or elastic modulus. This also the cause of the cleavability of monocrystalline diamonds in four select cleavage directions. Fabrication and Designs of Diamond Tool Bodies (a) Monocrystalline Diamond Tools Monocrystalline diamond tool bodies are manufactured with both natural diamond and synthetic monocrystals [Heus96]. monocrystalline diamond tools have to be oriented in the tool holder such that the cutting force points in the direction of a hardness maximum. By a careful selection of pressure and temperature. the crystal growth rate and dimensions of the crystals can be controlled within a range of a few micrometers to several millimetres and specific physical properties like purity or porosity can be influenced. Diamond synthesis results in monocrystalline diamond particles that are utilized in machining with geometrically defined cutting edges as synthetic monocrystalline diamond tool bodies or for further processing into polycrystalline cutting parts. anisotropy remains in monocrystalline materials due to the aligned orientation of the crystal lattice. While grinding must always take place in the direction of lowest hardness. Surface quality can be affected considerably by varying κr slightly. high temperature process (60–70 kbar. The tool is adjusted so that the minor cutting edge has a very small lead angle as acts as a broad polishing cutting edge. but due to the curvature of the cutting edge they have unfavourable chip formation conditions and relatively high passive forces. (b) Polycrystalline Diamond Tools Tool bodies made of a synthetic polycrystalline diamond layer were first introduced in 1973 and have replaced monocrystalline diamond tools and cemented carbides in some areas. in which synthetic diamonds are sintered together into polycrystalline bodies in the presence of a metallic catalyst. tungsten or tungsten carbide are . 4.56 Shapes of monocrystalline diamond tools monocrystalline diamond tools are shown in Fig. whereby in the latter case a corner radius is ground onto it in order to improve the surface profile.5 Ceramic Cutting Tool Materials 175 κ'r ≈ 2° κr ≈ 45° Tool with one cutting edge Tool with round cutting edge κ'r ≈ 2° κr ≈ 18° 160° Tool with facette shape Fig.4. whereby two neighbouring edges form an angle of about 160◦ . 1400–2000◦ C). usually cobalt. By selecting a very small minor cutting edge lead angle κr < 2◦ . The polycrystalline diamond coating is fabricated by means of a high pressure. but also silicon.56 [Wein69]. three to five cutting edges are ground on. The grinded tips are brazed on the support material or fixed with special clamps. The starting materials are synthetic diamond particles of a very small defined granulation (grain diameter 2–25 μm) in order to obtain a maximum amount of homogeneity and packing density. the minor cutting edge can be designed as a broad-tool cutting edge which takes on the function of further polishing the surface. To this end. 4. The tool with one cutting edge is used both for drilling as well as for external turning. Tool variants with round cutting edges have the advantage of having a very large useful cutting edge length. In the case of tool bodies with facette shapes. They are used for rotating tools with high requirements on cutting edge quality and service life. Polycrystalline diamond thus does not exhibit the hardness isotropy and cleavability of monocrystalline diamonds. the diamond coating is a statistically isotropic overall body in which the anisotropy of the individual monocrystalline diamond particles is balanced by the random distribution of the diamond grains. this phase is also called the binder.5 mm.57). since indexable inserts made of polycrystalline diamond . Fine-grained types (about 2 μm grain size) have established themselves especially in the automobile industry in cases demanding the best possible cutting edge quality [Bail01]. they increase the number of diamond bridges and with it the impact resistance of the composite material as well as the quality of the cutting edge. Coarse-grained DP cutting tool materials (grain size about 25 μm) are characterized by higher hardness and thermal resistance. which gives the polycrystalline diamond bodies the required toughness [Pret06]. Depending on the grain size of the diamond granulation used as well as the catalyst used. wear resistance increases in the case of primarily abrasive stress during the machining process (Fig. On the other hand. Due to its polycrystalline structure. 4. Changing the composition of the “binder phase” can also increase the limits of thermal stability. They are the most advantageous with respect to wear. which are not critical with respect to the workpiece surface. 10 μm) are very common as multi-purpose variants. the properties of the polycrystalline composite material can be specifically influenced.e. for example by using SiC. “diamond bridges” are formed between the diamond grains which give the polycrystalline bodies their high strength. DP types of average grain size (approx. Cutting edge shaping of the cutting part blank (i. it also does not reach the hardness level of a diamond monocrystal in its “hardest” direction. The small diamond grains accumulate in the gaps formed by the large crystals. cutting edge quality and cutting edge strength in the machining of aluminium composite materials and alloys. The tool bodies are either soldered onto the tool bearer or clamped into standardized tool holders. As the diamond grains consolidate. gaps arise between neighbouring diamond crystals that are filled in by the catalyst. of the cemented carbide base with applied diamond coating) is accomplished by means of spark-erosive cutting and grinding. but their cutting edges are rounder and more jagged than those of finer-grained types. However. As a result. During the sintering process. is either directly applied on a presintered cemented carbide base or over a thin intermediate layer consisting of a metal with a low elastic modulus bonded with the cemented carbide in order to balance stresses between the diamond layer and the cemented carbide base.176 4 Cutting Tool Materials and Tools used [Neis94]. In the literature. the thickness of which is about 0. The polycrystalline coating fabricated in the high pressure/high temperature process. especially since hardness is also influenced by the degree of consolidation between the individual crystals and their bonding to the binder phase. One way to reduce the size of the gaps between the diamond grains – especially in the case of coarse-grained DP types – is using diamond particles with varying grain sizes (2–30 μm). this considerably reduces the thermal conductivity of the cutting tool material compared with that of materials with the usual Co catalyst [Neis94]. Diamond turns into graphite and reacts with the iron in the contact zone between the tool and the workpiece due to the high temperatures arising there.: vc = 500 m/min f = 0.1 mm ap = 1 mm Cutting time: tc = 40 min 177 Displacement of the cutting edge SV / μm 4.5 14 12 10 8 6 4 2 0 DP1 DP1: grain size: 10–15 µm. Co (53%) Fig. Application Areas of Diamond Cutting Tool Materials Cutting iron and steel materials with diamond tools is impossible due to the affinity of iron to carbon. binder: W (47%).Ceramic Cutting Tool Materials Process: External cylindrical turning Material: G-AlSi12 Cooling: dry Cutting cond. Co (91%) DP4: grain size: 0.57 Microstructure and wear of polycrystalline diamond cutting tool materials when turning AlSi-alloy G-AlSi12. binder: β −SiC DP3: grain size: 2–6 µm. acc.5–1 µm. binder: W (12%). . to NEISES [Neis94] correspond in shape and dimensions with commercially available cemented carbide or ceramic indexable inserts. 4. binder: W (9%). Co (88%) DP2 DP3 DP4 DP2: grain size: 6–10 µm. 178 4 Cutting Tool Materials and Tools As a result. Brin96. Further fields include the production of mirrors for lasers and other optical applications. In comparison to cemented carbides. Chry79. Under certain conditions. Obel84. heavy and precious metals. Their field of application is mainly in finishing. Due to the strong abrasive effect of the silicon particles. Zwah00]. the adhesive tendency of aluminium with the cutting tool material has a negative effect on the machining process. For example. Kloc96. Hedr05a. Their high level of manufacturing safety is another essential reason why tools made of polycrystalline diamond are preferred over those of cemented carbide for this machining task. They have extremely high tool lives of up to 80 times higher than those of cemented carbide tools and have excellent surface quality and precision. Halw04. 4. Heus96. fly-cutting. It is in many cases possible to unite both pre-machining and finishing in one working cycle. Stie99. Their use is not restricted to finishing. of tools for making imprints on plastics or of tools for plastic injection moulding [Spen91. the cutting edge wears quickly in the case of both monocrystalline and polycrystalline diamond cutting tool materials. Weck95. since large depths of cut and feeds are impossible due to limitations on cutting edge dimensions and relatively low flexural strength. PCD tools have acquired a secure place in modern cutting processes as highperformance tools [Schü01. coal. Wald92. the use of nearly notch-free polished diamond cutting edges in ultra-precision cutting (turning. Monocrystalline diamond tools are especially suited to cutting light. Köni82. where short cycle times and high safety against unforeseen cutting edge fracture are a must. Moreover. the palette of materials machined with polycrystalline diamond tools comprises different plastics. Beck95.58). the cutting edge cuts in an alternating fashion through the soft aluminium phase and through the hard silicon particles. Take00]. Hedr05. Polycrystalline diamond tools are of special importance in the machining of aluminium alloys containing large amounts of silicon (Fig. One classic area of application is machining hard and soft contact lenses. This is especially the case in mass production on transfer lines. Fall05. tools made of cemented carbide are subject to a lot of wear. when using cemented carbide tools. PCD cutting tool materials can be used with much higher cutting speeds when cutting aluminium alloys with high silicon content. of shaping tools for making impressions on blank CDs. plastics and stone. One example of this is drilling finishing with reaming tools equipped . Ikaw91. Hoff88. Wick04. Brun99]. Besides light. Kasp05. In the case of machining workpieces made of aluminium and other non-ferrous metals. Vogt04. Kass04. Spur84. but also includes roughing. Jäge89. graphite and presintered cemented carbide. heavy and precious metals. planing) can result in surface finishes of between 3 and 6 nm. Since these alloys have a hard/soft structure. Further application examples include the milling of magnesium pressure cast alloys and the high-performance milling of aluminium integral parts in the airline industry with end milling cutters equipped with PCD [Wein69. it is possible to machine cast iron materials with PCD tools as well. hard and soft rubber as well as glass. The use of monocrystalline diamond cutting tool materials promises advantages when the demand for very high dimensional accuracy and surface quality is of foremost importance. fz = 0.4. A distinction is drawn between thin and thick diamond coatings.2. end milling cutters and indexable inserts with chip form grooves. Thick CVD diamond coatings are excellently suited to cutting highly abrasive materials such as aluminium-based metal composite materials. They are suited to machining graphite. In the case of very small depths of cut.Improvement of surface quality . Thin diamond coatings can be deposited on tools with complex geometries such as drills.Difficult regrinds . fz = 0. it is possible to obtain long tool lives as well as high surface quality and component precision in cast iron machining [NN02].023 mm.Complex cutting edge survey .Accurate application 1600 380 2500 100 360 880 100 1130 16 Cutting tool material costs per workpiece / % Tool life quantity / workpieces Tool costs per workpiece /% Cutting speed / [m/min] 100 50 60 Cutting time per 1 m milling distance / % Fig. 4.Special tools (settings.15 mm.58 Comparison of cemented carbide and polycrystalline diamond when milling the aluminium-based alloy GK-AlSi17Cu4Mg with DP. copper. 4. Recent developments are making it possible to produce very sharp cutting edges on diamond-coated tools as well by means of a sharpening process [Hage04].Reduction of tool change time . The fabrication of tools coated with diamond is treated extensively in Sect. z = 24) 36000 1600 1600 DP (ap = 0.4. adjusted feeds and cutting speeds as well as an intensive cooling of the cutting edge. As opposed to polycrystalline diamond.5 Ceramic Cutting Tool Materials 179 HW (ap = 0. containing no “binder phase”. z = 8) DP with optimal adjusted cutting edges (ap = 0. (c) CVD Diamond Tools CVD diamonds are diamond coatings deposited on a base body made of cemented carbide or ceramics in a CVD process. fibre-reinforced plastics and aluminium alloys high in silicon. Examples of this are .1 mm.Reduction of rework (minimized burrs) . thin diamond coatings have not been successful thus far. fz = 0. In the field of superfinishing. The reason for this is that sharp cutting edges are blunted by the relatively thick diamond coatings (about 20 μm).5 mm. z = 8) .5 mm.5 mm. these diamond layers consist only of diamond crystals. form fit) . Wein02. cBN is far superior to diamond. hexagonal boron nitride with wurzite structure and cubic boron nitride with diamond structure. There is also a third modification.2 Boron Nitride as a Cutting Tool Material Boron nitride exists like carbon in a soft hexagonal modification. the lattice of which consists exclusively of carbon atoms. It is stable from atmospheric pressure to about 2000◦ C.180 4 Cutting Tool Materials and Tools the machining of brake drums made of AlSi9Mg into which 20 vol% SiC particles are embedded or of cylinder crankcases made of AlSi9Cu3. cBN has six cleavage planes – two more than diamond. This property is insignificant for the use of cBN in machining with defined cutting edges. Despite their identical lattice structures. Since the maximum crystal size in the fabrication of cBN is limited (1–50 μm). The boron nitride lattice contains boron and nitrogen atoms and can therefore not reach the same level of bonding force symmetry and hardness as diamond. Only after transforming the hexagonal into the cubic crystalline lattice with the help of a high-pressure/high temperature process does boron nitride exhibit those qualities that distinguish it as a cutting tool material (Fig. after diamond. naturally existing hexagonal boron nitride is soft and not suitable as a cutting tool material for machining with defined cutting part geometries.45 g/cm3 ) does not exist in nature.2. cBN grains are sintered with . Hexagonal boron nitride is synthesized by reacting boron halogenides with ammonia. The wurzite lattice is a lattice type with a hexagonal symmetry but with a different atomic configuration than the graphite lattice. In the case of machining GFK/CFK materials as well. It has a density of 2. the second hardest material. With respect to hardness. The transformation of hexagonal into cubic boron nitride is accomplished with pressures of 50–90 kbar and temperatures of 1800–2200 K under the catalytic influence of alkaline nitrides or alkaline-earth nitrides. In the case of machining with geometrically defined cutting edges. whereas diamond begins to graphitize already at about 900◦ C.27 g/cm3 and a melting point of 2730◦ C [Salm83].5. Cubic boron nitride is. this form is somewhere between the other two modifications. Figure 4. More important is the fact that boron nitride is not a chemical element like carbon. hexagonal boron nitride with graphite structure. Cubic boron nitride (ρ = 3. But with respect to its chemical resistance. cBn is mainly used as a polycrystalline cutting tool material (BN). which are reinforced with 15 vol% Si particles and 5 vol% Al2 O3 short fibres [Suss01. especially against oxidation. 4. and in a hard cubic modification.59 shows the phase diagram of boron nitride system with the four phase areas molten. there are essential differences between diamond and cBN. which crystallizes in the same lattice type. In contrast to silicon nitride. but a chemical compound.55). since in that case only polycrystalline tools are used. which has an identical structure to the diamond lattice. 4. much longer tool lives can be expected compared with PCD cutting edges due to the lack of a binder phase [Feuc05]. which crystallizes in the wurzite structure. Uhlm00]. Its fabrication under the conditions of diamond synthesis was first successful in 1957. Töll82. Wern81.1 and 4. forming AlN and AlB2 / TiN and TiB2 . A distinction is made between cutting edge blanks. Clau85. the thermal conductivity and resistance of BN cutting tool materials against abrasive wear increases with . and indexable inserts. Low cBN-containing types are generally 45–65% cBN and have a ceramic binder phase based on titanium carbide or titanium nitride [Krat06]. This leads to an extremely stable bond between the cBN grains and the Co-W binder phase [Krat06].5 mm thick polycrystalline BN layer and simultaneously applied to a cemented carbide base. which are connected by soldering to the tool holder and only receive their final cutting edge shape by grinding. Wern84. W2 CoB2 or W2 Co21 B6 . nickel and tungsten are used as binders.2). The currently most common cutting tool materials based on boron nitride can be subdivided into two groups. Krat06]. Their grain size varies within a range of 0. 4. Types with high amounts of cBN consist of 80–90% cBN and a metallic W-Co or ceramic binder phase based on titanium and aluminium.59 Phase diagram of boron nitride the help of a binder phase in a high pressure/high temperature process to an approx. titanium aluminium nitride. one containing high amounts of cBN (BH) and the other containing low amounts (BL) (Tables 4. The coW borides arise from grain boundary reactions during the sintering process. Töll81.60 shows. 0. Aluminium and titanium react chemically with the cBN. titanium carbide or cobalt. Aluminium. Buzd87.5–10 μm. Cobalt forms complex borides with tungsten in the form of CoWB. Nott82. which are clamped in common clamping holders [Töns81. 4. Wein87.4.5 Ceramic Cutting Tool Materials 181 180 160 Cast 120 100 80 60 40 20 0 Hexagonal BN (wurtzite-structure) Pressure p / kbar 140 Cubic BN (diamond-structure) Hexagonal BN (graphite-structure) 1000 2000 3000 Temperature T / K 4000 5000 Fig. They are as a rule very fine-grained (grain diameter < 2 μm). titanium. As Fig. 3 Table 4.182 4 Cutting Tool Materials and Tools Table 4.2 PCBN types (BL) available on the market for cutting hardnened sintered steel and sintered forged steel in continuous cut.6 0. Fine-grained BL types have a higher cutting edge quality. grey cast iron.60 Effect of cBN-content and grain size on properties of PCBN 10 Thermal conductivity cBN-content / Vo.5–4 1.1 PCBN types (BH) available on the market for cutting sintered steel and sintered forged steel.5 2 1 Ti ceramic Ti ceramic Ti ceramic 0.5–5 0. hard cast iron. to KRATZ [Krat06] cBN content/ cBN-grain vol% diameter/μm Binder phase Knoop hardness/GPa Thermal conductivity/ Bending (W/mK) strength/GPa 45 50 60 24 28 33 34 44 45 0. The main uses of BH types are generally roughing/pre-machining of hardened steel and Cutting edge quality 80 ce tan 50 45 ar We is res Low cBNcontent 0 <1 High cBNcontent 2 6 cBN grain size / µm Fig.5–4 0. sintered metals and high temperature alloys based on nickel and cobalt – materials which are very difficult to machine using cemented carbide tools.2 increasing cBN content and grain size. high speed steel.4 1.-l% 90 . Tools made of cubic boron nitride are preferred for turning.6 1. acc. acc.0 1. an essential prerequisite for producing high-quality surfaces in the hard finishing of components. 4. to KRATZ [Krat06] cBN content/ cBN grain vol% diameter/μm Binder phase Knoop hardness/GPa Thermal conductivity/ Bending (W/mK) strength/GPa 80 85 90 Co-W Co-W Co-W 26–30 38–41 40–43 80–100 80–100 100–110 0. drilling and milling hardened steel with a hardness of 55–68 HRC. they quickly divert the heat arising in the contact zone during roughing. When machining hardened steels with BN cutting tool materials. Due to its relatively high thermal conductivity.5 mm (turning instead of grinding). 4.1 mm ap = 0.5 Ceramic Cutting Tool Materials 183 iron materials. sintered metals and all machining operations made on perlitic cast iron materials. External cylindrical turning Material: Cooling: Cutting cond. They have increased edge stability and. machining hard cast iron. 4. Neis95.61).5 mm tc = 10 min Cutting time: BN 1 80 BN 2 Width of flank wear land VB / µm Process: 100 BN 3 60 BN 4 40 20 0 Crater depth KT Width of flank wear land VB BN 1 BN 2 BN 3 BN 4 Fig. BL types are primarily employed for hard finishing with depths of cut of < 0.4.61 Wear of different boron nitride cutting tool materials when turning 100Cr6 . better wear attributes than types containing higher levels of cBN (Fig. tribochemical reactions between the cubic boron nitride (cBN) and components of the steel materials to be cut lead to intensified wearing of the boron nitride [Bömc89. for these machining tasks.: 100Cr6 (60 HRC) dry vc = 80 m/min f = 0. 6 Tool Designs Various tool designs for separate processes have become available on the market.5). Okam05]. BN cutting tool materials are characterized by higher process safety. TiAlN or AlCrN after the PVD process. . An emulsion is used as a cooling lubricant during the cutting process – dry machining is generally impossible. Gey05. Coating reduces tribochemical wear. In order to combat this wear phenomenon. Usually. When using the appropriate BN types (low-cBN-containing. there is still another problem.Al2 O3 . since different areas of use place particular demands on the design of the tools. Ohas04. There are also solid BN plates coated with CVD.54). In these cases. the higher Fe content in the materials leads to an increase in chemical wear. cBN tools are coated with TiN. In consequence. In comparison to turning tools made of whisker-reinforced cutting ceramics or SiAlon. the following requirements should be considered already in the design phase: • mechanical tool stresses (cutting forces). In order to guarantee tool functionality. • thermal tool stress (friction and deformation heat. 7. Turning tools made of polycrystalline cubic boron nitride are becoming more and more popular for finishing turbine blades composed of nickel-based alloys (see Sect. RNGX120700E). chip formation changes from lamellar to continuous chips. the stronger this wear mechanism limits the performance of the BN cutting tool materials being used for hard machining. tool life and process safety [Daws02. they can be employed at much higher cutting speeds (vc = 250–500 m/min) (Fig. If hardened components must also be machined in the hard/soft transition phase. As opposed to cemented carbides (vc = 20–40 m/min). and thermal and chemical stress on the cutting edge increases. When machining materials of lower hardness (45–55 HRC). 4.6. coolant). the wear and performance attributes of cBN cutting tool materials can be significantly improved by coating them. Barr06].g. The BN cutting edge becomes very hot. while the resultant component have a superior surface quality. small corner radii also are advantageous for the formation of notch wear. 4. increasing component quality. Barr00. The higher the amount of cBN in the cutting tool material. Besides low cutting speeds. • rapid changing and secure positioning of the tool cutting edge.184 4 Cutting Tool Materials and Tools Halp05. binder phase TiC or TiN) and insert geometries (e. This is accompanied by changes in the friction conditions in the contact zone on the rake face. Due to declining material strength. cutting tool materials made of cBN are coated. the solder with which the BN blank is soldered onto the cemented carbide base can melt and the insert can break. the types of wear that affect tool life as a rule are flank and rake face wear for high cutting speeds (vc > 250 m/min) and notch wear on the major and/or minor cutting edge for low cutting speeds (vc < 250 m/min). cBN). 4. Parting tool holder bits already have a ready-made geometry. Round tools are turned after regrinding by a corresponding angle. which must be radially adjusted. special attention will be paid to turning tools. cemented carbide. these tools are placed in a clamping as shown in Fig. also called a tool holder bit.g. since in these cases they can simply be reground. • versatility in use. solid tools are more economical. Included in the solid tool category are tools whose cutting part and shaft have been manufactured with the same material. In the following introduction to tools designs. 9 and 10. 4.4. which is very expensive to produce. The angles required for the parting process are ground up to the tool orthogonal clearance. For example.1 Solid Tools Solid tools are turning tools whose tool body and shaft consist of one material (e. Final grinding occurs only on the flank face. Round tools can be prepared until the circumference of the cutting part has been reduced to 25%. more wear-resistant material (e.62. This tool type. Figure 4.g. is fabricated by grinding the cutting part geometry on to a wide variety of basic shapes. These tools are advantageous only for turning materials that do not require chip formers. whereas they are only used in special cases for turning.6. Further specifications regarding tool systems of other chip-removing machining processes with defined cutting edges can be found in Chaps.63 shows two forming tools in both round and flat design. These types of forming tools are reground only on the rake face. Different designs have become established for different manufacturing processes. After grinding the tool orthogonal clearance. . In the case of tools with an insert. • manufacturing and maintenance costs Two main tool groups have emerged based on these requirements. as opposed to the radial tool. since any regrinding of the flank face would alter the shape. One advantage of these tool holder bit is that theoretically freely selectable geometries can be ground onto them for every regrind. Tools can be designed either as • solid tools or as • tools with inserts. Any necessary height correction is made by subsequent adjustment of the tool holder bit in the clamp. The latter is designated as a tangential tool because the tool can be tangentially shifted. the shaft usually consists of a tougher and cheaper base material and the insert of a harder.6 Tool Designs 185 • simple and fast replacement of worn tool parts. in the case of milling and drilling. high speed steel). turning tools can be fabricated completely out of cemented carbide. since cemented carbide has a larger elastic modulus.186 4 Cutting Tool Materials and Tools Fig.63 Forming tools . in the case of small tools). 4. In order to guarantee a small amount of shaft deflection.62 Parting off tool clamping Holder bit Solid tools are being used increasingly less commonly for turning. whereas usually tools with inserts are used. 4. γ° α° e Workpiece γ° α° Round formed turning tool Flat formed turning tool Fig.g. in the case of boring bars for example. complete cemented carbide shafts are utilized. If soldering the cemented carbide inserts poses problems (e. 2.64. the strength should be increased to 800–1000 N/mm2 . 4.4.6. inserts are most usually screwed or clamped. If tools for large chip cross-sectional areas are required.6 Tool Designs 187 4. unalloyed construction steels with a strength of 700–800 N/mm2 have proven successful. tools with inserts should be classified as being either • tools with fixed inserts (material connection) and • tools with detachable inserts (frictional/positive connection).64 Turning and milling tools with brazed carbide tipped cutters . For turning tool shafts.6. 4.1 Tools with Fixed Inserts Soldering as a technique for joining the insert and tool shaft is now only used in exceptional cases for turning processes. such as in milling and drilling processes or in the manufacture of woodworking tools. 4.2 Tools with Inserts In accordance with joining techniques. Shaft cross sections are standardized in DIN 770-1 and DIN 770-2. High-speed steel is usually only Parallel ground in chip breaker Cemented carbide Shank Soldered joint Brazed carbide cutters Fig. It is more common in other areas. usually inserts made of PCBN or cemented carbide are soldered onto the tool shaft. In the case of soldered tools as in Fig. Today. In the case of a normal design. tolerances are in the range of ± 0.6. they can be adjusted well and quickly to the machining task at hand. whereby workpiece tolerances of about ± 0.67 shows the standard description for clamping holders by means of an example. Inserts made of ceramic materials and PCBN are usually manufactured without a hole and can thus only be fixed in clamping holders and clamping fingers. The manufacturing tolerances of indexable inserts influence the production accuracy of the workpieces to a very large extent when there is a change of inserts (e.2 Tools with Detachable Inserts As opposed to tool systems with soldered inserts. the inserts are attached with clamping devices on the tool holder. For inserts.188 4 Cutting Tool Materials and Tools soldered onto tools that have lower cutting edge requirements (e. since this makes the storage of replacement parts considerably more expensive. There are now intelligent tool dispensing systems provided by different tool manufacturers that make it possible to manage the tools and to provide orders automatically adjusted to the purpose of the operator [Pitt04]. According to the forms described in DIN ISO 1832. insert replacement can be accomplished quickly during manufacture.1 mm can be observed after an insert change without additional positioning. Figure 4. Since clamping holders make it possible to clamp inserts made of diverse cutting tool materials. For this reason. there is a variety of special designs that are non-standard inserts for machining processes with defined cutting edges.65 and 4. that several cutting edges can be used on a single insert. tools with clamped or screwed inserts have the advantage.g. Of particular advantage is the safe and quick clamping of the inserts. the insert is turned or twisted after releasing the attachment. on NC machines).66.13 mm and for precision designs ± 0.g. Inserts without holes generally have a superimposed chip former and ones with holes have moulded chip formers. As shown in DIN ISO 1832. Storage costs are relatively low. Clamping holders for turning are standardized in DIN 4983.2. In accordance with the selected tolerance class.025 mm. among others. since the cutting edge does not need to be positioned with respect to the workpiece. since these are limited essentially to storing the inserts and replacement parts for the tool holders [Corn74]. 4. If a cutting edge has reached the end of its service life. thereby bringing a new cutting edge into action. there are inserts with and without holes. The designation system for customary indexable inserts is standardized in DIN ISO 1832 and shown in Figs. One must bear in mind that the components of different products are not interchangeable with regard to storage. . On these tools. The term “indexable insert” comes from this process. 4. in the case of saw blades for woodworking). it is uneconomical to use different holder designs in one plant for the same purpose. a distinction is made between normal and precision design. 025 ± 0. acc.152 ± 0.05–0.025 ± 0.025 ± 0. .152 ± 0.08–0.013 ± 0.05–0.. • The support area must guarantee that the insert does not bend. • The mounting must guarantee that heat arising in the process is diverted well into the tool holder.025 ± 0. .005 ± 0.05–0.6 Tool Designs 189 Example of insert notation: C N M G 12 04 08 .025 ± 0..05–0..382 ± 0.65 Standard description for inserts.025 ± 0. 1 2 3 4 5 6 7 8 9 10 2 – Tool orthogonal clearance (α 0) 1 – Insert form (εr) 85° A 82° B 86° M C 80° P D 55° R E 75° S 55° K L F 3° B 25° G 5° C 30° N 7° D T H A O 0° P 15° V 35° W 80° 11° E 20° 3 –Tolerance class Acceptable variation in mm for m T A C d E m F C G S H d J1 K1 D L1 V M N d m U d / mm m / mm s / mm ± 0.152 ± 0.025 ± 0.05–0.025 ± 0.152 ± 0.152 ± 0.13 1 Inserts with ground edges on the insert size (See ISO-norm 1832) 2 Depending S Fig.08–0. .025 ± 0.025 ± 0.013 ± 0.013 ± 0.013 ± 0.025 ± 0.013 ± 0.005 ± 0.025 ± 0.025 ± 0..4. • Position change due to cutting forces must be avoided.025 ± 0.. to DIN ISO 1832 Tool holders for inserts must fulfill the following basic tasks: • Indexable inserts must always be clamped in the identical position after replacement.152 ± 0.05–0.05–0.013 ± 0. 4.005 ± 0.202 ± 0.13–0.05–0.202 ± 0.13 ± 0..025 ± 0.013 ± 0. 8 1.78 3. N 10 – internal notations Condition of the edge.94 9. The indexable inserts .190 4 Cutting Tool Materials and Tools 5 – Size of insert 4 – Fixation and/or chip breaker A M R B N T C Q W H X β = 40–60° Drawing or accurate description required l l β = 70–90° β = 40–60° l l l l l 6 – Thickness of insert d 8 – Formation of cutting edge E s s F sharp S chamfered + rounded 9 – Cutting direction s s T chamfered rounded right 01 s / mm T1 02 T2 03 T3 R 1. 4.56 6.4 08 12 16 24 0.68 shows different fixture designs for indexable inserts with (below) and without (above) a hole. to DIN ISO 1832 (continued) • Cutting forces should be transferred to the holder such that the centering of the insert is supported.2 04 0.76 5. special chip former geometry. for example: P = polished W = with (expanded) finishing cutting edge Fig.38 2. acc.59 1.66 Standard description for inserts.98 2. M0 diameter in metric system. • The use of a chip former is required depending on the machining task at hand.35 7.2 1.6 2. The advantage of mountings for hole inserts is that all clamping elements in the holder are protected from chips.52 L neutral 7 – Design of cutting edge rε / mm rε 02 0. Figure 4.18 3.4 R 00 for diameter in inches converted in mm.97 left 04 s / mm 05 06 07 09 4. 4.68 Types of insert fixation (Source: Widia. by toggle levers or pins. to DIN 4983 are unalterably centred and fixed in the insert seat. 4. The simplest clamping is achieved with a clamping bolt. Hertel) .67 Standard description for tool holders.6 Tool Designs 191 Notation example for tool holders: Cartridge DIN 4985 – C S G N R 32 25 M 16 Q Kind of fixation of the insert (clamped from above) C Basic form of the insert S Form of the tool holder G Tool orthogonal clearance of the insert N Design of the tool holder R Height of the cutting edge (here: h = 32 mm) 32 Shank width of the tool holder (here: b = 25 mm) 25 Length of the tool holder (here: l1 = 150 mm) Size of the insert M 16 Code letter for special tolerances Q Fig. for example. which clamps the indexable insert and a continuously Chip former Clamping device Insert Clamping screw Excentric nut Clamping claw Tool shank Underlay Toggle Clamping screw Clamping screw Fig.4. acc. Another possibility is fixing with a clamping claw. In this design. the tool orthogonal clearance of the insert is 11◦ (wedge angle β0 = 79◦ ) and the tool orthogonal rake angle of the holder is + 5◦ .69). Negative indexable inserts have a wedge angle of 90◦ . in the machining position. 4. making cutting edges available on both the upper and lower side of the insert. Primarily for linked machines or tracer lathes. If damage to the tool holder occurs. 4. If there is a positive tool orthogonal rake angle. 4. as in Fig. i. If.192 4 Cutting Tool Materials and Tools Positive insert Negative insert –6° 5° 6° 6° Wedge angle β0 < 90° Wedge angle β0 = 90° 11° 11° Number of useable edges depending on tool type Fig. but in effect they cut with a positive tool orthogonal rake angle . often special designs are used such as short tool holders or cassettes. Inserts with moulded or ground chip formers have the same basic shape as negative indexable inserts. then the tool orthogonal clearance during tool engagement is + 6◦ .e. the insert is referred to as a positive insert and vice versa (Fig. tools can be employed that can change used indexable inserts automatically during workpiece loading without loss of time. the chip former can be adjusted especially well to changing cutting conditions in order to guarantee a favourable chip form.69. Tool holders with clamping fingers are used when the chip former is gradually adjustable and working conditions are mostly constant. Indexable inserts for tool holders with incorporated positive tool orthogonal rake angles are provided with tool orthogonal clearances. In order to shorten setting-up and auxiliary process times in large-batch production. The criterion for this distinction is the size of the tool orthogonal rake angle when fixed. Positive inserts have usable cutting edges only on the upper side.69 Characteristic properties of positive and negative inserts adjustable chip former. There are negative and positive inserts. the individual parts can be replaced with replacement parts offered by the tool holder manufacturer. e. 4. The large tool included angle of square inserts gives them a high level of cutting edge stability. a lower cutting force. As opposed to triangular inserts.7 Tool Preparation Expensive tools.g. deep and round contours can be reproduced. One must bear in mind that moulded/ground chip formers should be adjusted to the respective cutting conditions [Sche78]. broaching tools.70). 4. sawing and turning.71 provides examples of tools with detachable indexable inserts for drilling. Very high surface qualities can be obtained by using round indexable inserts. On automated manufacturing devices.70 Inserts with chip former (Source: Sandvik) due to the geometry of the chip former (Fig. In order to expand the possible range of applications. are as a rule prepared again. with the higher number of cutting edges of the negative insert. often several chip breakers are arranged in a row. Figure 4. The disadvantage of these however is that the smallest workpiece radius to be produced is prescribed by the cutting insert’s geometry. Triangular indexable inserts have less cutting edge stability as a result of their small tool included angle. in which case process interference due to uncontrolled chip breakage is particularly disruptive. Rhomboid inserts have been manufactured especially for copy turning operations. The main function of the chip former is to produce short-breaking chips. indexable inserts are therefore almost always provided with chip formers [Muel93].4. 4. milling. Regrinding is almost exclusively done in . This type of indexable insert thus combines the advantage of the positive tool orthogonal rake angle. provided the cost to do so is lower than that of a new tool. With these.7 Tool Preparation + 18° 193 + 10° + 12° + 12° Fig. Small chip formers arranged near the corner radius guarantee good chip breakage in finishing operations. they can only be used in form turning to a limited extent. 72 gives an overview of standard wear data used as tool life criteria for tools. Figure 4. such as coarse or fine grinding. the hardness and grain size of the grinding wheel must be adjusted to the high speed steel. If a tool is to be prepared. Manually ground faces are imprecise and thus have a lot of influence on tool life and the chip forms produced.73 shows different regrinding strategies corresponding to the type of wear arising. their surfaces are pretreated by irradiation. regrinding must take place in a timely fashion in order to save on grinding and tool costs. Extensive information on the topic of abrasives and method variants of grinding can be found in [Kloc05a].194 4 Cutting Tool Materials and Tools Drilling tool End milling cutter Deep hole drilling tool Side milling cutter Drill out tool Grooving tool Side and face milling cutter Saw blade Fig. Other tools that are typically prepared are solid carbide tools like drills and shaft millers. In order to be able to use sharply ground tools for finishing purposes nonetheless. 4. When grinding high speed steel. Special manufacturing processes in the area of finishing require sharply ground tool cutting edges. Grinding or regrinding tools is carried out as a rule on special 5-axis tool grinding machines. the rake face is reground. Preparing indexable inserts made of cemented carbide or HSS is not economical because of the low material value. depending on which grinding type is used. Before coating the tools. Figure 4. On the other hand. If crater wear is predominant. This means that the cutting edge should not be used beyond a permissible level of wear. This produces a pronounced cutting edge radius on the tool. cBN or precious corundum discs are used as abrasives. the flank face is reground so that the tools maintain the more wear resistant coating on the rake face. . Only in this way we can always grind on the same angle and chip former geometries. serration tools made of HSS and indexable inserts made of PKD or PCBN. if flank face wear is predominant.71 Typical application examples the case of special tools and tool designs with soldered inserts. different grinding strategies have been developed. 0 mm Crater depth KT 0. regrind from the rake face: .2 mm VB 0. Crater wear (rake face) Substrate Coating Coating . Substrate Flank wear Dominating flank wear.5 mm VBmax 0..1 to 0. 4.3 mm Width of flank VB 0.7 mm KT 0...3 to 0.72 Guidelines for the tool life criterion Dominant rake face wear..regrind from the flank: Common processes: Common processes: Drill out Reaming Broaching Grooving Parting off Fig.2 to 1.35 to 1.4.1 to 0.5 to 0.3 mm KT 0. 4...15 to 0.73 Machined surfaces in regrinding Regr ind Regrind ..7 Tool Preparation 195 Cutting tool material Measurand VB 0..0 mm VBmax 0.1 mm Width of flank High speed steel Cemented carbide Assumed wear data wear land wear land Crater depth Width of flank Cutting ceramics wear land Crater depth Fig. 196 4 Cutting Tool Materials and Tools In regrinding. . one must take care during the grinding process that the tool does not become heated beyond annealing temperature in order to avoid rim zone damage. Moreover. Electrolytic grinding is also a very suitable method for pregrinding and finishing tools [Rein69]. If cemented carbide is used. fine grinding and grinding chamfers and curves is done with diamond discs. An error-free regrind must therefore be guaranteed by a correct choice of abrasive and adjusted machining conditions. the grinding strategy and thus the surface quality has a large influence on the realizable tool life travel paths of the tool. The avoidance of cracks caused by heat and force is of especial importance when preparing tools with soldered cutting inserts [Weir64]. regrinding.  197 . analytical models are only partially suitable for describing complex processes such as can be described by FEM. RWTH edition. determination of the element properties. size and distribution of the elements is also determined. C Springer-Verlag Berlin Heidelberg 2011 DOI 10. The user should be aware of the assumptions made by the software and their possible effects on the calculation result. analytic and empirical process models can also be consulted that have the advantage of providing a quick representation of the process. Zien00. The following steps are taken in every FEA [Hueb82. 2.1 Basic Concepts of FEM It is helpful for the user to be acquainted with the basic concepts of FEM in order to avoid errors and to be able to evaluate the results of calculations.g. it was developed to solve stress problems in structural mechanics. The consideration of a process with FEM is also called finite element analysis (FEA). Besides FEM. selection of interpolation functions. Originally. 4. discretization of the continuum. Roll93]: 1. 3. Due to the simplification. solution domains (e. FEM is a numerical method for finding approximate solutions to continuous field problems. the workpiece) are subdivided into a finite number of subdomains – the finite elements. 5. Empirical models are of limited use: they are generally only calibrated to be valid for a limited process range. leading already to highly promising results in the modelling of cutting processes. Klocke. The use of numerical models for simulating cutting processes makes it possible to illustrate complex tools while simultaneously taking plasto-mechanical and thermal processes into consideration. The type. Continuum elements make F. number. assembly of the element equations and solution of the equation system. Redd93.1007/978-3-642-11979-8_5. Manufacturing Processes 1.Chapter 5 Finite Element Method (FEM) Machining processes have been modelled numerically with the finite element method (FEM) for some years. 5. but its use was soon expanded to the large field of continuum mechanics [Bett03]. As the continuum is discretized. One of the fundamental differences between FEM and other numerical methods of approximation is that the solution is first formulated for each individual element. these are the corner points of the element. due to their higher number of nodes. to STEINBRUCH [Stei98] it possible to grasp all normal and shear stresses. form or basic functions. In the case of linear elements. 5. acc. The complete definition of an element type includes the element form. several approaches can be considered. One approach for example is using the principle of virtual work (energy). Interpolation functions serve to approximate the profile of state variables within an element. The order of the polynomial depends on the number of element nodes. Figure 5. the number of unknowns of each node and the continuity conditions at the nodes. stresses). After choosing element types and interpolation functions. the element equations (element matrices) are determined. Because of their differentiability or integratability. Since the interpolation functions represent the properties of the state variables within the element.198 5 Finite Element Method (FEM) 1D Bar element 2D Triangle Quadrangle 3D Tetrahedron Pentahedron Pyramid Hexahedron Fig. temperature) and the secondary unknowns (e. displacement.g. . the type of node variables and the interpolation functions.g. the number of nodes. is made in practice simultaneously with the selection of the element type.1 Types of elements for the discretization of continuum problems. Element nodes function as support points for the interpolation. polynomials are frequently used as interpolation functions. speed. The selection of interpolation functions. To determine the unknowns. which are often designated as shape.1 shows exemplary some element types that are used for discretization. These equations describe the relations between the primary unknowns (e. they provide a more exact solution. the values of the state variables at the nodes represent the unknowns of the discretized problem. Higher order elements have a set number of additional nodes on the element edges or inside the elements. 2 Lagrangian and Eulerian Considerations of the Continuum The continuum can be discretized from different standpoints. An observer on a node of such a mesh would see the states of all particles that pass his fixed observation point. The assembly of element equations leads to system equations that can be solved with the help of the right methods. An observer travelling on a node would see state variable changes of a particular particle throughout the entire forming process. Roll93. 5. called integration points. For highly dynamic applications on the other hand. Explicit methods consider the process under investigation as a dynamic problem subdivided into time steps. The desired quantities at time t + t are determined solely from the values available at time t. which is a combination of the above approaches and permits the mesh a motion independent of the material as long as the form of the domains under consideration remains the same [Koba89. the nodes of an element move with the material. The “arbitrary LAGRANGian EULERian” method (ALE) is becoming more and more accepted.) are also defined. explicit time integration is prevalent in FE programs. the element matrices are combined (assembled) into the global matrix of the problem. The boundary conditions (clamps. where the LAGRANGian and EULERian approach are the most common in FEM [Bath96]. which sometimes requires remeshing. such as crash simulation.3 Explicit and Implicit Methods of Solution 199 In order to approximate the properties of the total system comprising the sum of all the elements. The numerical integration methods used to solve the element matrices require the evaluation of the integrals at certain points within an element. This is done usually with the help of difference formulae. In the case of the LAGRANGian approach. The Eulerian approach considers the motion of the continuum through a fixed mesh. Wu03]. more or less distinct smoothing of the state variables. One disadvantage of the Lagrangian method is the distortion of the mesh brought about by large plastic deformations.5. The positions of the integration points within an element are exactly set and represent the positions at which stresses and strains are calculated [Koba89. etc. This method is especially suited to the investigation of stationary processes and is frequently employed in flow simulations. 5. The Gauss quadrature is a very common method for numerical integration. However. Zien00]. The now necessary interpolation of the state variables from the distorted to the newly generated mesh leads.3 Explicit and Implicit Methods of Solution Many FE programs utilized to calculate large plastic deformations make use of “implicit” methods. external forces. The number of required integration points can be reduced while maintaining the same accuracy by careful selection of their positions. to an undesirable. this method is only stable if the time step t is smaller . depending on the number of remeshing cycles. artificial increase of the material’s density or artificial reduction of the process time. Implicit solvers look for the solution for every time t +t under consideration of the values of the desired quantities both at time t as well as at time t + t [Hueb82]. heat is generated both from inelastic deformation as well as from the work of friction on the rake and flank faces. the possible length of the time step is a function of the sonic velocity c existing in the material. When using implicit methods. the flow curve).1) The maximum possible length of the time step thus depends on the density ρ and the elastic modulus E of the material. This data serve as input quantities for the thermal calculation.g. a very large amount of computing steps is sometimes necessary. The advantage of the length of the time step being up to 1000 times larger compared to the explicit method is therefore accompanied by the computing time required for the iterative equation solution. i. Linear analysis is a simplification because every real physical system is nonlinear. Zien00]. this is accomplished by establishing a completely combined equation system.4 Combined Thermal and Mechanical FEA In cutting processes. In order to take thermal processes into account. mechanical and thermal calculations must be combined.g. “Mass scaling”. heat from plastic deformation and heat exchange with other objects or the environment. or at every time increment (incremental combination) [Kopp99]. The mechanical calculation determines the heat of friction. For solids:  c= E ρ (5. In the case of simultaneous combination. 5. Chun98].5 Nonlinearities Linear analysis is the analysis of a problem that exhibits a linear relationship between the applied load and the response of the system. represents an attempt to increase the possible length of the time step.e.200 5 Finite Element Method (FEM) than the time it takes for an elastic wave to travel a route corresponding to the shortest element edge. The solution of such a non-linear system of equations demands special iteration methods (e. there is no such limitation. 5. NEWTONRAPHSON) [Roll93. These nonlinearities can . Mass effects caused by such interventions have to be compensated by appropriate countermeasures [Roll93. In this way. Since the length of the time step can be in the range of microseconds. The combination can take place at every iteration (iterative combination). Non-simultaneous combination proceeds from a purely mechanical formulation in which temperature merely serves to help ascertain temperature-dependent material characteristic values (e. Among these is.2). • geometric nonlinearities and • nonlinearities in the boundary conditions. The arrangement of the Split-Hopkinson pressure bar consists of two cylindrical bars of equal diameter arranged in a line. Geometric nonlinearities are caused by change in geometry during the calculation. the calculation of residual stresses remaining in the component). The use of a rigid/plastic material law speeds up the calculation and provides satisfactory simulation results for many applications. usually all three types of nonlinearity appear. 5. One can see from Table 5. for example.6 Material Laws The material laws applied in FEM can basically be classified in two main groups: those concerning elastic material properties by considering the material elastically and in case of further deformation plastically. Material nonlinearities result.5. This sudden stress induces an elastic compression wave . the rate of deformation dε/dt and temperature T. the input and output bar. for example. An accelerated mass strikes the input bar. the use of the Split-Hopkinson pressure bar. In the context of a typical machining simulation. when there is a change in external loads or new contact. special test methods are used. 5.6 Material Laws 201 however be neglected in many cases. This testing apparatus specially developed for high speed deformation can reach deformation speeds of up to dε/dt = 104 1/s. or in the case of loss of contact between two objects (e. and those concerning the material as rigid until the plastic limit is reached (Fig. there are three different types of nonlinearities: • material nonlinearities. Further causes of material nonlinearities include material behaviours contingent on forming speed and/or temperature as well as material failure. To this belongs the description of material flow stress as a function of deformation ε.1 how extreme the conditions are in machining in comparison to other manufacturing processes. To determine the flow curves that are valid in these extreme conditions. in which the amount of plastic deformation is significantly larger than the amount of elastic deformation [Roll93]. Basically. As soon as the displacements are large enough to influence the behaviour of the system. from a nonlinear relation between stress and displacement. If we speak of nonlinear problems in FEM. tool and workpiece). Elastic/plastic material models are of importance when elastic effects are not negligible (e. Nonlinearities in the boundary conditions arise. these nonlinearities must be taken into consideration.g. as occurs in the case of metallic materials after leaving the Hookian range. there is a nonlinearity. for example. Material laws for the simulation of chip-removing machining tasks must satisfy the particular requirements of cutting. Large deformations (up to ε = 5) and very high deformation rates (dε/dt < 106 1/s) combined with high temperatures (T < 1500◦ C) are common in cutting processes. Between them is found a sample of a smaller cross-section than that of the bars.g. to ROLL [Roll93] Table 5.5 1–5 10–1 –10–2 10–103 10–102 103 –106 0.16–0.1–0. 5. rate of deformation and temperature for various manufacuring processes.1 Comparison of deformation. With the timestrain curve determined by the strain gauges.7 0. acc. a constitutive material law is required that can be implemented in a FE program.7 0.16–0. the wave is divided due to the change in cross-section in the following way: part of the elastic wave is reflected and the rest goes through the sample and plastically deforms it (because of the smaller cross-section of the sample).2 Classification of the material models for high plastic deformation.202 5 Finite Element Method (FEM) Material models for strong deformation Material models without elastic behaviour Rigidplastic σ Materials model with elastic behaviour σ ε1 . ε 2 ε2 ε1 ε1 ≠ ε 2 ε1 < ε 2 < ε3 ε1 < ε 2 < ε3 ε ε3 ε ε ε Fig.16–0. compressive or torsion load for a broad range . ε 2 ε1 ≠ ε 2 Elastoviscoplastic Elastoplastic Viscoplastic ε3 ε2 ε1 σ σ ε1.16–0.9 aT homologous = T/Tmelting that passes through the input bar and is measured at the first strain gauge. The remaining part of the wave which proceeds to the output bar is measured by the second strain gauge. both the compression speed and the flow curve can be determined [Abou05]. When it arrives at the contact area between the input bar and the sample. This law must be capable of describing mechanical material properties under tensile. to JASPERS [Jasp99] Manufacturing process Deformation Deformation rate/s Thomologous a Extrusion Forging/rolling Sheet forming Machining 2–5 0. acc.5 0. In order to reduce the number of experiments to a minimum and to make flow curve extrapolation possible.1–0.7 0. σ = A · εn · ε˙ m · [−λ (T − T0 )] (5. For empirical material laws. m and λ are material parameters. SHIRAKASHI and USUI have suggested the following empirical relation. n. stress velocity and temperature. deformation. Here. Several of these are built on microstructure-mechanical foundations. to SWIFT) and the dependence of stress on temperature in another way [Moli83]. These equations make associations between the current values of stress. A. The material model frequently implemented in FE programs     .5.3 Split-Hopkinson pressure bar. deformation speed (strain rate hardening) and temperature (thermal softening). σ = K(B + ε)n · ε˙ m · T −υ (5. 5. acc.6 Material Laws 203 Tempered chamber Projectile Input bar Output bar Strike bar Bearing Compressed air tank Mass of projectile: Projectile speed: Compression speed: Specimen Buffer m = 0. T as variables).64 / 2. formation speed and temperature (σ .3) which describes the strain hardening of the material (acc. yet most of the models used in cutting simulation are based on empirical methods. which take into consideration the influence of deformation (strain hardening). dε/dt. The constants contained in these equations can be viewed as material parameters and adjusted to the experimental results with nonlinear regression or the method of least squares. ε.01 kg v = 8–50 m/s dε/dt = 103–104 s–1 Fig. Several models have been developed for simulation. to ABOURIDOUANE [Abou05] of deformation. usually constitutive equations are applied to describe flow curves.2) Which was successfully applied to describe dynamic viscoplastic material behaviour [Shir70]. MOLINARI and CLIFTON introduced a similar description. (dε/dt)0 a reference velocity and Tr .4) originates from JOHNSON and COOK [John83]. an analytically closed formulation for the adiabatic flow curve is not possible with this temperature function. In it. T − Tr m σ = A + Bεn · (1 + C · ln (˙ε /˙ε0 )) · 1 − Tm − Tr (5. A. m are material constants. B. Tm room and absolute melting temperature. . C. Material strain hardening is described in this model according to LUDWIK. n. However. velocity dependence logarithmically and the influence of temperature by means of a power function. simple equation with an exponential temperature dependence.204 5 Finite Element Method (FEM) JOHNSON and COOK also suggested a modified. . the LUDWIK expression and a HALL-PETCH relation.8) 5. which takes into consideration the influence of temperature. a generalized. and T.7) The ZERILLI-ARMSTRONG equation consists of additive components that contain an athermal component (σG : influence of dissolved materials and the initial dislocations density of inclusions). Here. The simplified operation of such specialized program systems usually entails a limitation how much the model can be influenced.7 Software A number of FE programs have become commercially available that are tailored to simulating the cutting process. “general purpose” systems are indeed highly flexible and can be used for a wide variety of applications. λ is also a material constant. which contains two equations [Zeri87]. Based on physical foundations. σ = A + Bεn · (1 + C · ln (˙ε /˙ε0 )) · e−λ(T−Tr ) (5. a thermal and velocity-affected component.6) and the second for materials with bcc lattice: σ = σG + C1 exp [−C3 T + C4 T ln (˙ε)] + C5 εn + k · l−1/2 (5. the first type is meant for materials with fcc lattices: σ = σG + C2 ε1/2 · exp [−C3 T + C4 T ln (˙ε )] + k · l−1/2 (5. combined formulation was proposed by ZERILLI and ARMSTRONG [Zeri95]: σ = C0 + C1 · exp [−C2 T + C3 T ln (˙ε )] + C4 ε1/2 · exp [−C5 T + C6 T ln (˙ε )] (5. Depending on the spectrum of applications. the use of different FEM programs within one company is therefore quite common. dε/dt. thus making it easier for the user to build and carry out simulations. which . On the other hand. In 1995.5) with which the temperature T by ε and dε/dt can be explicitly expressed. but they demand a large amount of experience for setting up the model as well as a larger amount of time. T HIRD W AVE /A DVANT E DGE and “general purpose” systems like ABAQUS or MSC/Marc [Denk04a]. ADVANTEDGE is a program designed specially for machining. the grain size (l: average grain diameter) of the material to be modelled is also applied as an additional parameter. These programs are adapted to the requirements of machining technology. The following programs are used frequently in cutting simulations: SFTC/ TM TM DEFORM . Besides σ . ε. TM The FE program DEFORM was originally developed for simulating forming TM processes. ZERILLI and ARMSTRONG developed a semiempirical model to describe material behaviour. With the help of the preprocessor. To do this. The behaviour of a software in relation to the available computing power is called its scalability. 5. the user makes all the information necessary for modelling the problem available to the software.9 Phases of a Finite Element Analysis (FEA) A typical finite element analysis takes place in three phases from the standpoint of the user: • data preparation with the preprocessor. Both programs are based on the implicit Lagrangian formulation and include an automatic remeshing routine followed by data interpolation from the old to the new mesh in order to avoid TM highly distorted mesh topologies. 5. the large majority of all commercial FE packages are also available for free or proprietary operating systems used in such computers. While a few years ago costly computer equipment was still required to carry out FEM calculations. Generally. In order to handle very complicated problems with a large amount of elements.5. As a result of the low cost of hardware in the PC sector. the STL standard interface (Standard Triangulation Language) is used as the exchange format for threedimensional data models. The effective increase in speed depends to a larger extent on the hardware and software being used. Commercial FE programs make it possible to import object models from CAD volume software over standardized interfaces. often the interface generated for exchanging product data STEP (STandard for the Exchange of Product model data) is used as well. To do this.9 Phases of a Finite Element Analysis (FEA) 205 facilitates setting up and carrying out cutting simulations but is limited with respect to the influence of the user due to its specialization. A software scales well if the computing time required for a task is approximately halved when the available computing power is doubled. computational performance is being steadily improved. • calculation and • evaluation of the results with the postprocessor. The simplifications made and the .8 Hardware With the rapid development of the computer industry. such hardware can also be operated in “clusters”. The computing time for a single problem does not however become reduced in direct proportion with the number of nodes (here: computers) in a cluster. nowadays already the majority of personal computers (PC) used at workstations fulfil the necessary requirements to carry out simple calculations with regard to memory and computing power. Here. the problem must be abstract enough that the software can depict it. The DEFORM software also includes the Euler approach for calculating the quasi-stationary process condition. this function must be supported by the FE software. several computers are networked in such a way that an expansive problem can be calculated on several computers simultaneously. we input the mechanical and thermo-physical material data. If several element types are available. they are meshed (discretized).g. process data. To save computing time. In principle. If the limitations of these tools are reached. It is essential to compare the results with those from experience. Potential sources of error in FE analyses include: • discretization errors from geometry interpolation when meshing and interpolation of the state variables. friction between different objects. speeds) are entered. rough calculations or experimental results. adaptive meshes with locally varying densities are frequently used in order to improve the resolution of local gradients in the state variables. bound cross section). inputting the material data and defining the boundary conditions Modern programs include import filters for common CAD formats. Some programs offer the option of an automatic adaptive remeshing in which the element density is automatically adjusted to existing gradients. For generating the mesh. the use of a larger number of elements leads to more precise results. orthogonal cross section because it is considerable less complex than. it is up to the user to make a selection between them. the results are evaluated in the “postprocessor”. The geometries of the objects involved in the simulation can thus be taken from 2D or 3D CAD data sets of the design or tool construction in electronic form. contact conditions. specialized software packages often specify one element type. However. separately offered commercial meshers can be used. two-dimensional models are used to simulate the chip formation process.206 5 Finite Element Method (FEM) quality of the data that the user inputs into the system is of decisive influence on the quality of the simulation results. Result evaluation also includes critical assessment by the user. the boundary conditions (external loads. Following this. After discretizing. After defining the geometries of all the objects.10 The Use of FEM in Cutting Technology For the most part. The element type and the element density of the mesh have a significant effect on the quality of the simulation result.g. The corner radius of the insert does not engage with the workpiece . meshing. friction conditions). for example. most software manufacturers make tools available that make it possible to mesh geometries quickly. As a rule. Depending on the software. After calculation. 5. • numerical errors (e. preprocessing has the following steps: • • • • defining the geometry. a variety of graphic representations and processing options are possible. external cylindrical turning (diagonal. The FE model treats the free. in numerical integration) and • rounding errors due to the limited precision of the floating point representation in the computer. material data. • faulty input data (e. i. Besides providing a separation criterion in the first two approaches. it is permissible to assume a plane strain state of deformation. This method lends itself to the use of a geometric separation criterion. or • by dispensing with a separation criterion (Fig. For this reason. 5. the FE mesh follows the material. the tool inclination angle λs is equal to 0◦ and the depth of cut is many times larger than the undeformed chip thickness. so that in the case of material deformation the elements are prestressed/distorted. In the Lagrangian formulation. 5. Schw36]. e. exceeding a defined maximum effective strain or a previously set maximum stress. particularly large deformations are seen in front of the cutting edge. deformations gradients and chip separation: The separation can be realized • on the basis of a geometric separation criterion. as soon as the distance between the nodes and the cutting edge is below a critical distance – the length of one element edge as a rule – separation of the mesh occurs. Element erasure is undertaken as a function of equivalent plastic strain or . e. it is necessary to define a separation line.1 Continuous Chip Simulation Most cutting simulations use the Lagrangian method. Definite crack formation in front of the cutting edge is still contentious and has not yet been demonstrated particularly in the case of cutting materials with ductile behaviour.4). a finely structured mesh is necessary for a sufficiently precise representation in the FE model. In fact. the nodes of an element move with the material.10. As long as the tool lead angle κ r is equal to 90◦ . Crack developments have only been observed in the case of cutting brittle materials [Reul00. and the simulation of the cutting process is possible with a two-dimensional FE model. This cuts down on the time required for the simulation considerably. the material is extremely deformed in front of the cutting edge without forming an observable crack such as we see in forming processes.e.5. the result of which is that the elements become highly distorted. there is also the problem of separating the chip from the workpiece. Besides the difficulty of representing large deformations and deformation gradients in the model. • on the basis of a physical separation criterion. Basically. considerable deformation and stress gradients appear in the area of the primary and secondary shear zones. in the case of ductile material. three different simulation methods exist for large deformations. i.g.g. In addition. Especially in cutting processes.10 The Use of FEM in Cutting Technology 207 and can therefore be ignored in the FE model. The separation of the chip from the workpiece can also be realized by means of erasing the elements before the cutting edge of the tool.e. along which the nodes are separated when the separation criterion is reached. the criterion of the distance at which the separation begins as soon as the tool cutting edge has fallen short of a critical distance to the workpiece nodes lying ahead. 5. Oute06]. • The value of the criterion increases with increasing mesh deformity. In the actual remeshing.5 (a) Distorted mesh topology before remeshing (b) New.4 Chip formation along a previously defined split line based on a geometrical separation criterion (on the left) and a physical separation criterion (on the right).w Gs. 5. usually the outer edge of the old mesh is used as the starting point for the new mesh.w Fw Bw vc Cs IKR Es X As Separation criterion tool X Ew Dw Cw Bw Cutting plane Cutting plane d Fig. 5. The goal of a good data interpolation algorithm is to transfer the solutions of an FE .w Gs. When an automatic remeshing routine with subsequent data interpolation from the old to the new mesh is utilized and a purely ductile material is assumed.w Ew Ds Tool X X Dw dcr Bs Cw Fs Es Hs. The new mesh must now approximate the given edge as exactly as possible and balance any tool penetrations [West00]. • If there is a remeshing. After the successful remeshing. it is possible to simulate the cutting process without a separation criterion. A criterion which can be used for automatically remeshing the model and interpolating the data during a calculation should fulfill the following conditions according to HABRAKEN and CESCOTTO [Habr90]: • The criterion represents the quality of the mesh as faithfully as possible. The remeshing routine is invoked as soon as the elements are critically distorted (Fig. the data must finally be transferred from the old to the new discretization.5). acc. to VAZ [Vaz00] a) Chip Tool b) Fig. undistorted mesh topology after remeshing previous material damage [Sche06. One disadvantage of the element erasure method is that material is removed from the model – it must be guaranteed that this has no effect on the result of the simulation.208 5 Finite Element Method (FEM) Geometric separation criterion Physical separation criterion I As Bs Chip Cs Ds vc Hs.w Fs. the value of the criterion is reduced. displacements etc. adiabatic shear band formation or by a combination of both mechanisms. besides dispensing with chip separation criteria. the Euler model is used to simulate static cutting processes. velocities. which causes a softening of the material starting from a plastic limit strain. In contrast to the Lagrangian formulation. This softening leads to a concentration of plastic deformation. In order to represent a chip geometry that is as realistic as possible. The fundamental advantage of the Euler method in comparison to the Lagrangian method is. The advantage of this is that separation criteria are unnecessary. whereby one should take heed that the transition of sections of varying discretization is defined as continuously as possible. entering and leaving processes as well as non-stationary chip formation processes. The information for the density distribution of elements is given with the so-called weight factors in the model. the shorter computing time for calculating a quasi-stationary process state.10 The Use of FEM in Cutting Technology 209 calculation at the integration points (e. such as exist in milling. Here. In the execution of the data transfer by means of extrapolation and interpolation. shear localization begins when thermal softening exceeds mechanical strain hardening. As a rule.2 Segmented Chip Simulation Segmented chips are either produced by cracks and pores. which results in a further increase in temperature and with it further concentration of plastic deformation. stresses etc. Segmented chips can be simulated in two ways: • simulation of the segmented chip by deformation localization based on modified material characteristic values. Generally.g. The process accelerates itself.g. or by an artificial reduction of the specific thermal capacity and thermal conductivity. To avoid this effect. . 5. • simulation of the segmented chip by crack initiation based on fracture and crack hypotheses and • a combination of both approaches Deformation localization can be simulated either by a corresponding modification of the flow curve. the mesher of a FE program offers the possibility of graduating the fineness of the mesh.) of the old discretization as faithfully as possible to the new discretization.5.10. the material moves through a fixed mesh.) and node points (e. By removing the material from the mesh. leading to the formation of an adiabatic shear band [Abou05]. large deformations in front of the cutting edge no longer lead to highly distorted meshes. an iterative adjustment of the free chip edges and chip surfaces is made in the calculation process based on an initial meshing of the chip root. Besides the Lagrangian solution method. faults can arise that lead to a reduction of the size of calculated gradients. temperature. cannot be described by means of the Euler method [Leop01]. a fine discretization in the primary and secondary shear zone is necessary. and no time-consuming remeshing is necessary. the stationary state is modelled: shear bands. forming path. hypotheses that take deformation history into account provide a damage value C. Micromechanical failure hypotheses come from the consideration that a ductile fracture arises as a result of the formation. which serves as an indicator for the probability of failure of a material. 5.80 1. hypotheses that only consider a momentary state of local process quantities (i.50 1.25 with help of high speed deformation experiments at a deformation velocity of 3000 1/s [Klim00]. For larger deformations. (5. Macromechanical hypotheses are classified as ones that depend on the forming path or ones that are independent of it [Zitz95].20 Plastic strain Fig. a localization of the deformations can be recognized.6 were measured up to a strain of ε = 0. which leads to a softening of the material in accordance with the provided flow curve. which is dependent on stress σ . The flow curves in Fig 5. Between the chip segments. such as occurs when cutting hardened steels.e.210 5 Finite Element Method (FEM) 1600 Flow stress / MPa 1400 T = 0°C 1200 1000 800°C 800 600 600°C 400 900°C 200 3.20 0. This damage value is added up over the forming path until a critical strain εc is reached. an inhomogeneous plastic deformation leads to the formation of microcracks. growth and consolidation of micropores. expansion ε and material-specific parameters a (Eq. If the segmented chip is simulated by crack initiation. In this case a distinction is drawn between macromechanical and micromechanical fracture hypotheses. The critical damage value Ccrit of the crack formation is a characteristic parameter of the .40 0. a material softening was assumed such as has been determined in the case of Ti6-4 for lower deformation speeds [Doeg86]. Considered microscopically.00 1.60 0.20 mm with appropriate flow curves which contain an artificial material hardening for high deformation Figure 5. the formation of shear bands can result from this. On the other hand.independent) are of only limited use for predicting the time of damage.75 Plastic strain 0 0 0 1000°C 0. which form cavities under external loads [Brod01].6 Simulation of the segmented chip formation with vc = 25 m/ min and f = 0. suitable fracture or crack hypotheses must be integrated into the FE model.6 shows the simulation of a segmented chip while cutting titanium alloy Ti6-4 with uncoated cemented carbide cutting edges [Mess07]. Since the capability to change form depends on the deformation history.9)). Macromechanical fracture hypotheses describe the part of the form-changing energy introduced that has dissipated up to a crack. In the case of the segmented chip formation under consideration. The stagnation area disappears and the chip segment begins to slide jerkily from the cutting edge.5 25 12.5 50 37. The shear angle gets larger so that the chip segment is further pushed out of the chip root area. εk Ccrit = f (σ . f = 0. As soon as an element in the FE simulation exceeds the critical damage value.25 mm and γ = −6◦ (ADI-900) material [Kloc07]. Because experimental investigations have shown that the formation of segmented chips when machining ADI-900 is based on two different mechanisms. CHUZHOY’s material model additionally assumes material damage as a function of the state values deformation and deformation velocity in order to take into account the accelerated sliding of the chip segments due to high shear stresses and the heterogeneous graphite sphere structure. BROZZO’s criterion was used as a fracture hypothesis. which relates the tensile stress to the largest shear stress [Broz72].9) 0 Figure 5. Starting from a critical shear angle. the supporting cross-section between the chip segment and the base material becomes smaller. it is erased.7 provides an example of a simulation of segmented chip formation based on the integration of a fracture hypothesis when cutting ADI-900. Due to the crack which has developed. after first contact and shear initiation a crack develops on the free surface which runs along the shear zone. a)dε (5. ε.7 Simulation of the segmented chip formation with vc = 70 m/ min.The Use of FEM in Cutting Technology i Initial contact 211 ii iii iv Shear initiation Crack initiation Sliding 90 75 62.5 0 v End of sliding vi vii New segmentation Shear initiation Material speed/(m/min) 5.10 viii Cracking Fig. 5. the original shear zone collapses and splits . 34 ms tc = 1. only one section of the workpiece was treated in the model.3.1 Simulation of the Turning Process Computing power is becoming increasingly cheaper and faster with the rapid development of the computer industry.46 ms Fig. the other part reforms after repeated compression in from of the cutting edge. 5.8 gives an example of chip formation simulation for the external cylindrical turning of a normally annealed heat-treated steel C45E+N.03 ms tc = 2. and the segmentation process repeats itself [Kloc07].8 Simulated first cut in external turning of C45E+N . but also possible tool wear. The tool used was a Al2 O3 /TiN-coated insert of the form CNMG120408. The values for friction and heat transfer were taken from the model for machining C45E+N verified by HOPPE [Hopp03]. The mechanical material properties of the normally annealed heat-treated steel C45E+N were determined by EL-MAGD with the help of tensile and compression tests [Elma06].3 mm vc Z X Y tc = 0 ms tc = 0.212 5 Finite Element Method (FEM) up: one part wanders with the chip segment across the rake face. 5. This makes it possible to simulate cutting processes that do not allow for the assumption of a plane strain state threedimensionally. Figure 5.10. In order to reduce computing time. USUI discovered in experimental investigations that wear velocity dW/dt can be calculated with the vc = 300 m/min ap = 1 mm f = 0.3 Simulation of the Cutting Process 5.10. When wear models are integrated into the simulation. temperatures and chip forms can be predicted. not only stresses. This is followed by the modelling of the worn tool in the fourth phase.10) The material constants C1 and C2 are dependent on the material/cutting tool material combination. Figure 5. This 4-phase wear calculation cycle is continued until a user-defined tool life criterion is reached. the current wear rate dW/dt is calculated in the third phase based on the wear model provided in the user subroutine. sliding velocity vch and surface temperature of the tool ϑ are known [Usui78]:   C2 dW = σn · vch · C1 · exp − dt ϑ (5.5. ALTAN has suggested a numerically combined calibration method in which the specific material constants can be determined based on experimental results for wear velocity and numerically calculated values for σ n . vch and ϑ [Alta02]. Figure 5. and the simulation is stopped. To determine them. Until the wear criterion of VBT = 200 μm is reached. 5. After the collected thermo-mechanical load for the stationary state has been calculated in the first and second phases.9 shows the basic sequence of a wear simulation.tK tK tK+1 Cutting time tc Integration of the wear model PHASE 2 Calculation of the thermo-mechanical load collective in steady state condition dW = σ n ⋅ v ch ⋅ c 1 ⋅ e dt PHASE 3 − c2 ϑ Calculation of the nodal wear rate and the resulting wear Fig. whereby the nodes of the tool contour are shifted as a function of the calculated wear volume W.9 Simulation of tool wear following equation when the normal stress σ n .10 The Use of FEM in Cutting Technology 213 Tool PHASE 1 PHASE 4 Thermo-mechanical FE-analysis of the turning process VB Adaptation of the tool geometry dependant on wear tool chip tool VBK+1 VBK B A Δt = tK+1 .10 provides an example of wear prediction of uncoated cemented carbide inserts. a total of seven . The SEM image of the cutting edge.10. 5.2 mm Flank wear width VB/µm 300 Simulation Experiment 250 200 150 100 50 0 0 50 100 150 Cutting time tc /s 200 250 Vergrößerung 50-fach 50x-magnification 200 µm Work piece Vergrößerung 200-fach 200x-magnification 600 µm tc = 250 s Tool 200 µm Fig. As the comparison of the wear curves of the simulation and the experiment shows. which was taken at cutting time t = 250 s in the experiment.214 5 Finite Element Method (FEM) Ti6-4. the crater broke through the cutting edge and thus led to an acceleration of flank face wear. the simulation deviates only in the last calculation cycle. 5. the tool geometry of an insertedtooth cutter was optimized with respect to chip flow in the product development . depth of cut ap = 1 mm.3. simulating cutting edge displacement in the direction of the flank face was impossible.10 Simulation of the flank face wear and comparison with the experimental results 4-phase cycles are calculated in the wear simulation.11): Based on a numerical chip form simulation. Since wear in the area of the rake face was ignored because of lacking material constants for crater wear in the FE simulation. HW-K20 cutting speed vc = 100 m/min and feed f = 0. 5. makes it clear that width of flank wear width VB increases erratically in the experiment as a result of the displacement of the cutting edge.2 Simulation of the Milling Process The following example shows how cutting simulation has been applied in the industrial sector (Fig. As this point. 10 The Use of FEM in Cutting Technology 215 Z X Y Chip shape simulation for the calculation of the thermo-mechanical load spectrum Ff.c. prototypes are manufactured and tested in several iteration stages in order to identify the ideal design. this type of simulation requires very exact input data regarding force and directions of force. predictions can be made not only about chip form but also . extremely costly and usually lasts up to 8 weeks. The development process is time-consuming.11 Chip form simulation for the development of the tool body and the tool holder (Source: Kennametal) phase. however. By integrating FE simulations of chip formation into the development process. In the typical product development process for indexable inserts for metal machining. 5. Furthermore. the design of the tool holder can be improved with the help of FE simulation.5. which essentially is determined by the lead angle of the cutting edge to the cutting direction as well as the chip former geometry.p CAD-Model of the tool holder: miller Ø 100 mm equipped with 10 PCD die heads Ultimate design of tool holder and cutting insert Fig. which in the past could only be acquired with costly cutting experiments. the number of iteration cycles in the design process can be significantly reduced. By simulating the chip flow. Chip form simulation can be of assistance here. which is fixed in the tool body. By incorporating chip formation simulation into the tool prototype development process.12 FE-model of the drilling process compared with the experiment 10 . modelling and optimizing drilling processes is thus an of Temperature T/°C Simulation 400 Drill 200 Experiment d 20 Workpiece 6 ratio (dQ / d) % Specific feed force kf.3.max / ( d * f ) 4 5 6 7 Drill diameter d/mm 8 9 Fig. 5. in the above example of an inserted-tooth cutter. 5. These can then be utilized to help design the tool holder. From the standpoint of cost and productivity. positive axial and radial lead angles as well as an optimized cutting edge bevel. resulting in a very smooth surface. which can minimize burr formation.216 5 Finite Element Method (FEM) about forces and temperatures.10.max = 2 * Fz. The two-tier cutting edge bevel results in lower cutting forces.012 * d : HW-K20 : rn = 4 µm : dry Experiment Simulation kf. The chip is removed from the surface being produced.max /(kN/mm² ) 32 5 4 28 24 20 1 3 2 1 0 3 5 7 9 Diameter d / mm Material Cutting speed Feed Cutting tool material Cutting edge radius Cooling 1 2 3 : C45E+N : vc = 35 m / min : f = 0.3 Simulation of the Drilling Process Drilling is the most common means used in tool production. we can find an optimal combination of adjusted. This in turn leads to a considerably better surface quality after cutting. For this reason.5. this increases the costs of both implementation and computation exponentially. since there are different cutting speeds along the drill radius and usually very complex. In order to shorten computing time. The cylindrical workpiece model is adjusted to the material of the first cut process by notching a conic section on the cylinder. both the chip form and the calculated feed force are compared with experimental results. A plane strain deformation state cannot be assumed in the case of drilling.12 gives an example of an FE model of the drilling process. the complex geometry of the drill. In order to reach a satisfactory level of accuracy in the simulation results. Due to its complexity. causing a transverse material flow. cutting edge rounding. The high level of agreement between them shows that even for such complex processes as drilling. The drilling process is particularly challenging for 3D-FE simulation. curved cutting edge geometries. the process can not be represented by 2D simulation as can the orthogonal cross section in turning. only a three-dimensional approach is purposeful. the tool and workpiece must be discretized in detail as volume bodies into finite elements. demanding a lot of computing power from the hardware and efficient simulation tools due to the numerous influencing parameters involved (e. However. To verify the drilling model.10 The Use of FEM in Cutting Technology 217 enormous importance in the manufacturing industry. Nevertheless. Figure 5. different contact friction processes. numerical simulation of diverse target figures is possible. the drilling process is considered from the point at which the entire major cutting edge is first engaged. difficulty definable heat transfer and thermo-mechanical material properties) [Kloc06].g. whereby the computing time with today’s computer technology is increased by the third power of the model size. the most crucial area of the major cutting edge should be meshed especially finely [Kloc06]. . which leads to expansion. modern cutting fluid systems make a substantial contribution to the high performance level of many manufacturing processes. cooling and lubricating the machining site as well as carrying away the chips.  219 . 6.Chapter 6 Cutting Fluids 6. Cutting fluid Non water-miscible cutting fluid Emulsifiable cutting fluid Water based cutting fluid Water-miscible cutting fluid Water-soluble cutting fluid Coolantemulsion Coolantsolution Fig. 6. C Springer-Verlag Berlin Heidelberg 2011 DOI 10. The fulfilment of these functions may at first sound simple. watermiscible or water-based. acc.2 Types of Cutting Fluids According to DIN 51385.1007/978-3-642-11979-8_6. RWTH edition.1 Division of the most important metalworking cutting fluids. Manufacturing Processes 1. 6. to DIN 51385 F. cutting fluids are classified as non water-miscible. Water-based cutting fluids are fabricated simply by adding the water-miscible concentrate to water (Fig. Not only must excessive heating of the workpieces. be avoided. Klocke.1 The Functions of Cutting Fluids By fulfilling its main functions. but often requires cutting fluid properties which are not readily combined with each other. This is achieved by removing process heat from the tool/workpiece contact area by cooling and slowing the progress of heat via lubrication. but also temperature load on the cutting tool material be reduced.1). 6. Burning used cutting fluids containing chlorine is now only allowed in special incineration sites.2. 400 °C until ca. Presently. The polar structure of these additives gives the additives good adhesive properties on the metal surface. This Kind of additive Lubrication improving additives EP-additives Fatty oils (animal. 6. Additives that improve lubrication (“antiwear additives” or AW-additives) help to reduce friction at the cutting location. since toxic dioxins can possibly be generated in case of uncontrolled burning. rapeseed oil) or synthetic fatty substances (ester) are added. These salts can absorb high pressures and exhibit only a low level of shear strength. wear protection.220 6 Cutting Fluids 6. 6. the effectiveness of this lubricating film is reduced in temperatures higher than its melting point (120–180◦ C).1 Non Water-Miscible Cutting Fluids Non water-miscible cutting fluids are mostly mineral oils that contain additional active agents to improve lubrication. mostly mineral oils (about 90%) are utilized. To this end. On the metal surface. 800 °C until ca. 120 °C until ca. The temperature spheres of action of the individual additives can be seen in Fig. The reason for this is the lower cost required to procure it in comparison to synthetic ester oils. the additives form metal salts at different temperatures. corrosion protection.2. so additives containing chlorine are largely avoided. This makes disposal much more expensive. on which they form a half-solid lubricating film called “metal soap”. not only are forces lowered but also heat arising at the cutting location is reduced. 180 °C until ca. Compounds containing phosphorous and sulphur as well as free sulphur are used.2 are now of secondary importance in Germany. natural fat oils (palm oil. 600 °C until ca. The chlorine compounds also shown in Fig. EP-additives (EP = extreme pressure) are also added to the mineral oils. low-viscosity ester oils are also used as cutting fluids in machining. In this way. However.2 Temperature range of coolant additives (Source: Mobil Oil) . durability and foaming properties. 1000 °C Fig. In addition to mineral oils. vegetable) Synthetic Fats (ester) Chloric compounds Phosphoric compounds Sulfidic compounds Free sulfur Temperature sphere of action until ca. Although ester oils have much better lubrication properties in comparison to mineral oils because of their polar chemical structure. a higher flash point. There is also an odour nuisance and hygienic conditions for the operating staff are worsened. ester oils are characterized by a lower evaporation tendency.15%. They form a relatively stable film on the boundary surface between the oil droplets and the water. as a rule tribologically active. A distinction is made between ionogenic and non-ionogenic emulsifiers. surface-active additives in the form of phosphorous or sulphur compounds are also added to them to improve these properties further. Compared with mineral oils. Kloc06a].2 to improve lubrication and compressive strength. while a percentage that is too low has no effect. more favourable lubrication properties.6. The high amount of water used (up to 99%) is the cause of the good cooling effect of emulsions. The result is a decreased pH-value and thus also a reduction of the fluid’s ability to protect against corrosion. and deposits are formed that clog the filter and thus lead to malfunctions. The fluid’s lightly alkaline character (pH-value of 8–9) helps protect against rust. The loss of tribological functions of the lubricating medium caused by dispensing with additives should be compensated by appropriate PVD wear protection coatings. the emulsion becomes unstable. alkali soaps of fatty acids or naphthenic acids (ionogenic) and reaction products of alkylphenols and athylene oxide. Infestation by micro-organisms such as bacteria. yeasts and fungi are a major problem in emulsions. oil is deposited on the surface. In addition.2. provide a remedy to this. i. With this in mind. for example. Biocides. the functions of the cutting fluid should be relocated to the tool surface [Krie01. but they reduce the biological degradability of the oils considerably. If this value is significantly exceeded. They have the function of dispersing the oil in the water so that a stable oil-in-water emulsion is produced after mixing with water.2 Water-Based Cutting Fluids Emulsifiable cutting fluids are supplied as a concentrate and thinned with water to become emulsions prior to use. .2 Types of Cutting Fluids 221 advantage of mineral oils compared with synthetic ester oils is however becoming increasingly smaller due to the steady increase in raw oil prices in the last several years. they are skin-friendlier and they are biologically more degradable [Frei00]. nonadditive ester oils should not only be used as a cutting fluid in machining. it can lead to dermatological problems in the personnel. which prevents the oil droplets from coalescing. That means. These additives reduce friction and wear.e. 6. The most important coolant additives are emulsifiers. i. 6. the goal of current research [Murr07] is to develop a quickly biodegradable family of fluids based on renewable raw materials that fulfils all of the cooling and lubricating functions required in a machine tool. Higher alloyed emulsions contain the additives shown in Fig. Examples of emulsifiers include.e. but as lubricating and pressure transmission media in all other tribosystems of a machine tool. which are added to the emulsion at a percentage of about 0. but they further corrosion for that reason as well. the emulsifiers react with the calcium and magnesium salts. yeasts and fungi at a low level. With the hand-held refractometer. then the concentrate. . centrifuges. which leads to the formation of water-insoluble soaps (creaming on the emulsion surface) and reduces the emulsifier content. the water must first be added to the container. is the result of the content of water-soluble calcium and magnesium salts. Their main area of application is in grinding. In the case of the coarsely dispersed emulsions used in metalworking. A more exact method consists in separating oil and water in a test flask by adding hydrochloric acid. This is tested in practice usually with indicator paper. The mixing process should always be carried out with vigorous stirring so that a good oil-in-water emulsion without flocculation is formed. its most important property. The nitrate content should not exceed 50 mg/l. There are further water requirements concerning nitrate and chloride content. centrifugal force purifiers (e. this amount is between 1 and 10 μm [Mang01]. The pH-value of emulsions should in their freshly prepared state be between 8 and 9. Soft water on the other hand promotes unwanted foaming. Potentiometrical determination of pH is only recommendable when higher precision is required. They are not generally used for cutting with geometrically defined cutting edges. The water’s hardness should be between 5 and 20◦ dH. If the water is too hard.179 mmol/l). 6. It utilizes and displays the relation between the refraction index and concentration. The hardness of the water. as they have a negative effect on tool life and the manufacturing product in addition to clogging the pumps. the quality of the water is of decisive importance. Cutting fluid solutions are produced by mixing a water-soluble concentrate with water. impurities such as chips or dust constantly get into the cutting fluids. there are some guidelines that should be considered in order not to impair their stability and performance. The useful life of the emulsion is shortened drastically by this. one can choose between gravitational purification in sedimentation tanks. This can either be done with a hand-held refractometer or an emulsion test flask. the concentration can be quickly determined on the spot. separators). During operation.3 Guidelines on the Use of Coolant Emulsions When coolant emulsions are used. They need to be removed from the emulsion. In order to get rid of impurities. One test point of the finished emulsion is concentration.g.222 6 Cutting Fluids The amount of emulsifier determines the size of the oil droplets. In general. The disadvantage of this method is that it is more time-consuming. As the main component of the emulsion. magnet filters or various designs of belt filters. When applying coolant emulsions. the coloration of which when dipped into the fluid serves a measure of pH. drinking water quality is sufficient in order to keep the initial load on the emulsion by bacteria. It is given in ◦ dH (degree of German hardness) or mmol/l (1◦ dH = 0. the temperature of the flowing chip is lower and thus its strength is . by cooling.3. Prim69]. there should be solid layers with high compressive strength and low shear strength on the metal surface. by ultrafiltration (membrane technology) or by vaporization/incineration. one should always carefully clean the container. By virtue of their lubricating effects. By cooling. this can be accomplished with high pressure additives in the cutting fluid.4 Effects of the Cutting Fluid on the Machining Process 223 When changing the coolant. Disposal of the coolant is a major cost factor.e. it is thoroughly possible that wear on the tool is considerably increased by the cooling and the tool life is accordingly reduced.6. whereby the mechanical energy created to form the chips is almost completely converted into heat in the shear and friction zones. 3. At the same time. The result of these loads are wear phenomena such as mechanical wear and the shearing off of adhered materials. Accordingly. the tool is exposed to very high mechanical and thermal loads during the cutting process. cooling fluids primarily influences adhesive wear. 6. to plastic deformation of the cutting edge. System purifiers or hot water jet devices have proved effective in this task. since nests of bacteria immediately infect the new filling. 6. On the other hand. making it necessary to cool down the cutting area. both of which occur within the entire effective cutting speed range. This is clarified in Fig.4 Effects of the Cutting Fluid on the Machining Process As explained in more detail in Chap. As the cutting speed increases – in the range of reduced built-up edge formation – the conditions for the formation of high pressure lubricating films becomes progressively more unfavourable because the increased chip flow shortens the time for potential reactions between the additives and the metallic surface. This can stop or at least reduce welding. this condition is met by the fluctuation of the built-up edges. Köni66. i. which prevent a direct sliding of materials into each other. the increase in temperature leads to diffusion phenomena between the friction partners or.7). To counteract surface pressure. However. If necessary. and for this reason the composition of the lubricant must be calibrated to the respective operation. 3. which occurs due to the periodical migration of built-up edges within certain speed ranges [Opit70a. Especially wear phenomena associated with adhered material particles in the lower speed range (when using HSS) can be effectively countered with lubrication. in the extreme case. In the range of increased built-up edge formation. significantly reducing the service life of the emulsion. at these speeds begins the range in which tool life is improved not so much by the lubricating effect of a fluid but by its ability to remove heat. as well as diffusion processes and scaling – effects which manifest themselves only above certain temperatures (Sect. The separation of the oil from the water necessary to do this can be accomplished chemically (salt or acid separation). There is much more crater wear in wet cut than in dry cut operations. The most important prerequisite is that the lubricant has access to the contact zone. one must bear in mind that sulphur or phosphorous additives only become effective at certain temperatures. This results in a larger chip curvature. On the whole therefore. the temperatures prevailing during the formation of built-up edges shift to higher cutting speeds. The use of emulsions permits higher cutting speeds and larger feeds and a significantly augmented tool life [Bömc85]. For example. A further aspect to be taken into consideration is the transport of chips from the cutting area.: vc = 240 m/min f = 0. so that a process optimized for uncooled cutting may be incorrectly designed [Opit64]. we can expect clear improvements in service life when the cutting temperature is near the softening point of the cutting tool material without the use of a cutting fluid. On the other hand. it can be advantageous when groove milling with . Figure 6. and the bottom of the chip remains largely unwetted due to its intense contact with the rake face.2 mm 80 Feed: ap = 2 mm 60 Depth of cut: Crater depth KT / µm 200 40 20 Coolantemulsion 10 8 6 4 Dry cut 2 1 1 2 4 6 8 10 20 40 Cutting time tc / min higher.4 shows how effective the cooling effect of an emulsion is in this case using the example of drilling copper alloys with HSS drills. 6. But when cooling lubricants are used.224 6 Cutting Fluids 300 Fig. which manifests itself in increased forces.3 Wear-cutting time diagram for dry cut and with application of cutting fluid Process: Cylindrical turning Material: 42CrMo4+QT Cutting tool material: Coated carbide 100 Cutting speed. so that the contact surface between the chip and the tool is reduced in size. One must also consider machining operations with low cutting speeds. there is an increase in the specific stress on the rake face and consequently more crater wear. a larger temperature gradient is formed in the chip as in the uncooled process. Since the cutting fluid mainly cools the top of the chip. which generally are designed so that the built-up edge range is avoided. 3 mm) / min 40 30 20 G-CuSn12Ni 10 8 G-CuAl10Ni 6 5 4 3 20 30 40 50 60 80 100 200 300 Cutting speed vc / (m/min) Tool: Cutting tool material: Material: Twist drill (d = 11 mm) HS6-5-2-5 G-CuSn12Ni G-CuAl10Ni dry wet dry wet f = 0. When drilling.4 Effects of the Cutting Fluid on the Machining Process 225 100 80 Dry cut 60 Emulsion 50 Tool lifeT(VB = 0.2 mm f = 0.4 Influence of cooling on tool life during the drilling of copper cemented carbide-fitted shaft millers to remove chips from the cutting edge with compressed air or cutting fluid so that no chips are pulled into the cut. The disadvantage that cooling increases temperature change stress on the indexable inserts is more than compensated by this (Fig. 6. .1 mm f = 0. increasing wear.4 mm Drilling depth: l = 30 mm l = 30 mm l = 45 mm l = 45 mm Feed: Fig. the cutting fluid also fulfils the main function of leading chips out of the drill hole and thereby helping to avoid any clogging of the flutes. Some machines are designed in such as way that specially arranged nozzles clean the working space with cutting fluid so that chips do not hinder subsequent operations or impede the clamping of new workpieces.2 mm f = 0.6.5) [Köll86]. 6. those aspects of cutting fluid selection will be mentioned that are of significance from a technological and economical point of view. broaching and high-speed grinding with PCBN. Only the discharge need be replaced. tools and chips (Fig.17 mm Tool diameter: d = 32 mm Width of cut: Depth of cut: ae = 32 mm ap = 30 mm 6. Their service life is practically unlimited.Fig.6). The advantage of non-water soluble cutting fluids is on the one hand their technical properties – particularly their good lubricating effect – but especially their much lower maintenance and disposal costs compared with emulsions. The good adhesion .5 Time course of wear during groove milling of titanium in dry cut and wet cut with external compressed air 6 Cutting Fluids Maximum width of flank wear land VBmax / µm 226 600 Dry 400 Compressed air Cutting fluid 200 100 80 60 40 20 10 1 2 4 6 8 10 20 40 60 Milling time t f / min Cutting speed: vc = 50 m/min Feed per tooth: fz = 0. 6. The strong tendency to form vapour and mist requires a complete enclosure of the machining area with corresponding suction. The use of non-water soluble cutting fluids is however also associated with a series of disadvantages.5 Selection of Cutting Fluids As a summary. Their high cost of maintenance and the working area and environmental problems associated with their use have led to emulsions being replaced with non-water soluble cutting fluids [Kloc96a]. The higher viscosity of oil requires an adjustment of the machine with respect to the cutting fluid and filters. for example in processes for manufacturing tooth systems. above all their favourable cooling and cleansing effect as well as the low purification cost for workpieces. such as higher maintenance and refilling costs. Oils are already being used as cutting fluids in many areas. 6. the superior skin-compatibility of oils is another essential advantage. Emulsions are characterized by a number of properties that predestine them as cutting fluids for use in machining. From the standpoint of occupational health. leading to deflagration. washed. 6. Leakage .6 Reducing or Avoiding the Use of Cutting Fluids 227 Oil Emulsion Consumption 10% 90% Cooling effect low very good Lubricating effect very good low Skin compatibility good problematical Bacteria tolerance good less good WGK 1–2 WGK 3–4 Preparation costs high low Maintanance costs low high unlimited 2 –24 Months low high Resultant wastewater very low very high Machine compatibility good problematical Corrosion protection good low Fire protection requirements high not necessary Effort for workpiece and chip cleaning high low possible not possible Water endangering class Service life Disposal costs Reuse of cutting fluid Fig. a general statement regarding the economical use of this or that cutting fluid must be made on a case-by-case basis. they also represent a considerable hazard to the environment and humanity. In conclusion. The chips must also be spun and. oil vapour and mist can form a combustible mixture with air.6 Reducing or Avoiding the Use of Cutting Fluids Besides the technological benefits provided by cutting fluids.6 Advantages and disadvantages of using oil or emulsion as cutting fluid of the oil to the workpieces leads to large drag-out losses. if necessary. Moreover. Cutting fluid components such as bactericides and fungicides.6. 6. reaction products originating in the cutting fluid and included foreign substances can all become the root cause of illnesses. demands long periods of draining in basins as well as extensive turning. spinning and cleansing of the workpieces. inclined machine bed concepts. in BG rule 143. In order to transport chips.6. 6. such as chip transport within the machine or bringing those chips to the right temperature must be executed by other measures which concern not only the tool but also the machine tool (Fig. For companies. these regulations and requirements mean not only great responsibility towards their workers but in particular growing financial burdens. An approach for this purpose represents the reduction or the avoidance of the cutting fluids use. regulations and the requirements and rules of professional associations are given.228 6 Cutting Fluids and drag-out losses.7 Tool and machine related means of reducing the demand of cutting fluid . Guidelines regarding cutting fluid requirements and associated devices. The handling of cutting fluids and their disposal is therefore being regulated by lawmakers and professional associations with strict requirements. regarding maintenance and disposal of the fluids and regarding the laws. VDI guideline 3397 and the VDMA brochure “Cutting Fluids – Fresh Air at the Workplace” [VDMA 2002]. There are different possibilities for the machine tool. 6. special chip guiding systems Dicontinuous CF-use CF-supply change Optimize filtration New tool materials Tool coatings Alternative machine tool cooling Control feed pump (NC) alternative tool cooling Change chip transport alternative workpiece cooling Fig. conveying systems. 6. water and air. the task of satisfying requirements concerning environmental protection without putting the profitability of the production in danger is becoming increasingly crucial. emissions. For manufacture. among other places. Secondary tasks of the cutting fluid.1 Reducing Cutting Fluids A reduction of the use of cutting fluids aims to make avail of only the technologically necessary amount of cutting fluid. wash water and the disposal of used cutting fluids are a burden on earth.7) [Köni93a]. which have been state-of-the-art for a long time. usually of oil. the direct cooling effect of the oil/air mixture is of only secondary importance.air = 1. can be achieved. The cutting fluid is supplied directly to the cutting area by a duct in the supporting tool and the insert. 6.8). in grooving and parting-off operations. Because of the very small cooling effect of the oil/air mixture. the tools are supplied with the smallest amounts of a coolant and/or lubricant. In the sphere of dry machining. This more effective cutting edge cooling requires not only a significantly lower volume of cutting fluid.oil = 1. Köni93a. there are a number of different approaches for reducing the amount of cutting fluid used. Their task is to reduce friction and adhesion processes between the workpiece. . 6. Besides drills with cooling ducts. the tool is supplied by means of a pump with a medium in the form of individual. One example is the internally cooled tool. In comparison to oils. we can draw a distinction between a minimum quantity lubrication (MQL) and a minimum quantity cooling (MQC) (Fig. This is achieved either with or without a transport medium. Usually. minimum quantity lubrication is generally understood as the supply of a cooling lubricant medium in the form of an aerosol. compressed air) can be used. a positive effect on tool wear. rapid. the medium is atomized into ultrafine droplets with the help of compressed air in a nozzle and supplied as an aerosol to the machining location.g.6. With respect to tools. chip and tool.04 KJ/kgK) and the small quantity applied. They are supplied to the tool and/or cutting area in the smallest possible quantities. They are generally only used when the tool or component must be cooled more intensively than is possible with oils. it is their good lubricating effect that stands in the foreground. oils are used. In the case of “airless” systems. the use of these media are also referred to as minimum quantity cooling (MQC). Due to the low thermal capacity of oil (cp. 6. successive finely dosed droplets. chip removal and the surface quality of the workpiece [Köni93a]. the use of oil as a medium is referred to as a minimum quantity lubrication (MQL).9) [Eise00]. important for form and dimensional tolerance.2 Minimum Quantity Cooling Lubrication (MQCL) A further measure to reduce the amount of the cutting fluid represents the so-called Minimum Quantity Cooling Lubrication (MQCL) [Kloc96a. Due to the much lower lubricating effect of emulsions and water’s complete lack thereof. emulsions or water are used as media for minimum quantity cooling lubrication much less often. The result is less heating of the tool and component in comparison to pure dry machining (Fig. by means of closed cooling cycles. but also has. tool life. but also emulsions. The machine tool’s temperature consistency. In the second case. there is less frictional heat. In this cutting fluid technology. for example.6. water or air.6 Reducing or Avoiding the Use of Cutting Fluids 229 or other alternative media (e. Depending on the type and primary task of the added medium. indexable inserts with “internal” cutting fluid supply are also being used. Wein04]. When oils are used.92 KJ/kgK) and air (cp. By reducing friction. Due to their great importance in dry machining. 6. water (with a corrosion inhibitor). cold air or fluid gases is still a relatively seldom used component of minimum quantity cooling lubrication technology (MQCL) and is thus little known to users.92 kJ/kgK Medium Cooling Lubrication Chip transport Emulsion very good good good Oil very good very good good limited nothing limited Compressed air Fig. MQC technology can however most certainly make a contribution to solving thermal problems in the tool or component in dry machining operations. Air) = 1. The . Oil) = 1.04 kJ/kgK Oil (cp.8 mm f = 0. Water) = 4.2 mm Feed: Fig. the following remarks are concerned exclusively with minimum quantity lubrication technology (MQL). 6.8 Definition of “minimum quantity cooling lubrication” Tool temperature Tmax / °C Material: C45E Drill with thermocouples 400 Dry 200 MQL 300 250 200 Thermocouples near the main cutting edge CF 150 0 40 60 80 Cutting speed vc / (m/min) 100 Process: Drilling Drill: Solid carbide TiN coated d = 11.230 6 Cutting Fluids Minimum quantity cooling lubrication (MQCL) Minimum quantity cooling (MQC) Minimum quantity lubrication (MQL) Emulsion Water (cp.18 kJ/kgK Air (cp.9 Influence of minimum quantity lubrication on the tool temperature Minimum quantity cooling (MQC) with emulsions. less than 50 ml/h of the lubricating medium is used. this means an enormous reduction of the quantities used in MQL technology.6. The number and orientation of the nozzles as well as the spray pattern. This method is used. In the case of internal machining operations. In optimally adjusted MQL systems. which is dependent on the nozzle design. Measured by the fact that up to 6 m3 cutting fluid can be discharged daily in the case of a transfer line with a cutting fluid volume of 60 m3 .channel rotary feedthrough Injector orifice Supply-systems Compressed air supply Oil supply Compressed air supply Compressed air supply Unpressurized tank Mixing chamber Oil Oil Pressure tank Pump Fig. for example. For larger l/D ratios. external supply of the medium is only practical up to length/diameter ratios of l/D < 3. the aerosol is sprayed onto the tool from the outside with one or several nozzles. In the case of external supply. 6. the tool has to be retracted for a rewetting several times if necessary.6 Reducing or Avoiding the Use of Cutting Fluids External MQL supply 1-channel 231 Internal MQL supply 2-channels 1-channel 1-channel rotary feedthrough 2-channels 2. Both systems have their fields of application.10). in sawing. workpieces and chips all remain dry. all have a significant effect on the result. shaft and knife head face milling and also in turning. such as drilling.10 MQL supply systems MQL medium can be supplied to the cutting area from the outside by means of nozzles affixed separately in the machine space or by means of the tool spindle and internal cooling ducts inside the tool (Fig. reaming or tapping. The essential feature of MQL is that. when correctly used. 6. which can lead to a considerable elongation of the machining process. The use of external supply is also problematic in machining tasks in which tools are used that vary significantly with respect . the tools. From the standpoint of the material. In the case of deep-hole drilling. In the case of dry machining without MQL.11). But even in this case. With internal MQL supply. When MQL was used. the use of a minimum quantity lubrication is vital to the realization of a dry machining operation (Fig. tool manufacturers are offering special solutions that make it possible to conduct the MQL medium flowing through the spindle within the tool holder outside and then on the tool circumference lengthwise to the machining location. coating. The media used in MQL are primarily fatty alcohols and ester oils (chemically modified vegetable oils). Internal MQL supply requires tools with cooling ducts. The supply nozzles need to be manually aligned or with the help of positioning systems.12 shows the effect of MQL on the tool’s condition and tool life quantity when drilling into an aluminium wrought alloy. Figure 6. neither wear nor adhered material could be detected after 128 drill holes. 6. The air-oil mixture is produced directly in front of the tool. reaming and tapping with larger l/D ratios. linked with the machine control. the turning and milling of steel and cast iron materials are done to a large extent completely dry. In 2-channel systems. varnishing).10). drilling tools of < 1 mm diameter with internal cooling ducts are already available. . 6. internal MQL supply is indispensable because of the large l/D ratios. which. the machining method and the aftertreatment of the component (annealing. move the nozzles in an axial or radial direction or pivot them by a certain range of angles depending on the tool length and diameter.232 6 Cutting Fluids to length and diameter. In many material/method combinations. the tool was already unusable after 16 holes due to material adhesion in the flute. External MQL supply can however also be indispensable if the tools used do not have internal cooling ducts. In the case of drilling. The classic application area for MQL technology is sawing. The essential requirement of both system variants is that the medium be available at the cutting location at the moment the cut begins in sufficient quantity. In the case of the 1-channel variant. this is especially true for dry machining aluminium wrought alloys. Further advantages of the internal supply of a MQL medium is that positioning fields such as are seen when nozzles are used. tappers or end milling cutters – external MQL supply is absolutely essential. In the case of tools without internal cooling ducts – be it drills. and chip removal from the drill hole is supported. largely independently of the material. Due to the high hot wear resistance of the coated cemented carbide tools available today. for drill hole manufacture and drill hole aftertreatment. Medium selection depends on the type of supply. This is also similar for tools with highly varied dimensions. internal supply of the medium by means of a spindle and the tool is advantageous. the aerosol is produced outside the spindle and supplied by the latter to the tool. we make a further distinction between 1-channel and 2-channel systems (Fig. and from the methodological standpoint. since the medium is continuously available near the cutting area independently of the drilling depth. Currently. the material. oil and air are conveyed separately by the spindle. are avoided. and the integration of the MQL into the machine tool does not require the working space to be restricted by feed lines. 13) [Kloc98. even if this brings about a positive effect in isolated cases. cutting tool material.6. Wein04]. 6. A sensible and economically viable dry machining operation demands a very thorough analysis of given restraints as well as an understanding of the complex relations that interconnect the process.3 Avoiding Cutting Fluids The most decisive step in the avoidance of problems associated with the use of cutting fluids is dry machining. Adam02. heat-treatable steel GG20 – GGG70 Casting Cast alloy Material Drilling MQL MQL MQL MQL / dry MQL / dry Reaming MQL MQL MQL MQL MQL Thread cutting MQL MQL MQL MQL MQL Thread moulding MQL MQL MQL MQL MQL Deep hole drilling MQL MQL MQL MQL Milling MQL / dry MQL dry dry dry Turning MQL / dry MQL / dry dry dry dry dry dry dry MQL MQL MQL MQL MQL / dry dry Hobbing Sawing MQL MQL Broaching Fig. that still use large quantities of cutting fluids today do not technologically require them.11 Application of MQL 6. This cannot however simply be realized by stopping the cutting fluid supply. . the basic question should be posed of whether one can dispense with cutting fluids or not. Tham98. For every current or future machining task therefore. Many machining tasks. 6. bearing steel Maching steel.6.6 Reducing or Avoiding the Use of Cutting Fluids 233 Steel Aluminium Process Wrought alloy High alloyed steel. component and machine tool (Fig. 12 Drilling of aluminium with and without minimum quantity cooling lubrication Process Cutting tool material Qsp Qws .Dimensional accuracy . 6.Machine accuracy .Machine concept Surface Fit Tolerance Workpiece Fig.Hot hardness .234 6 Cutting Fluids 300 128 Number of boreholes 250 200 150 100 50 16 0 Dry MQL Dry Material: AlSi9Cu3 Tool: Solid carbide drill.5 mm Fig.Adhesion . Rimzone . 6.Friction .Surface.Warming processing .Material behaviour .Hot wear resistance .thermal .13 Dry processing – requirements and constraints Machine tool .Chips QW Dry- .Chip transport .5 mm Coating: (Ti.Resistance against Adhesion .Heat . d = 8.Al)N + MoS2 Bling hole: Cutting Speed: Feed: MQL l = 30 mm vc = 300 m/min f = 0. Dry machining also leads however to an alteration of the heat flows between the tool and the chip. aluminium alloys and non-ferrous metals. material or required component precision. Despite promising attempts to expand the field of application of dry machining and to make it more economical by finding appropriate tool geometries. reaming and tapping (Fig. Dry machining tools exhibit much better wear and performance properties than conventional wet machining tools. One example of this is the manufacture of internal threads by thread milling and combination drill taps. 6. tool and workpiece is no longer absorbed by the cutting fluid and hot chips are no longer rinsed out of the cutting area or the machine tool. cutting tool ceramics and polycrystalline boron nitride have sufficient hot wear resistance to be used without cutting fluids. component and machine tool. component and chip as well as the poor chip removal. it is incontestable that a complete relinquishment of cutting fluids will not be possible for all machining tasks. cermets. The consequence of this is higher thermal loading of the tool. In tapping. This reduces the thermal load on the substrate and reduces frictional and adhesive phenomena between the material and the cutting tool material. While we can do without the use of cutting fluids in many cases in turning and milling cast iron materials. improving surface quality and even reducing manufacturing times [Kloc98]. Problems in dry machining include higher thermal loading of the tool. but to create the prerequisites necessary for dry machining from the standpoint of the component and the machine tool. the primary cutting fluid functions of lubrication. friction and adhesion phenomena lead to higher amounts of mechanical tool load. compression. This requires in turn that the hot chips are removed as quickly as possible from the working space by a suitable machine tool concept [Kloc98]. Since there is no cutting fluid to absorb the heat. At present. Since the component’s . which in turn has a negative effect on tool life and component/machine precision. Effects on the rim zone and form/dimension faults in the component represent further potential restrictions on dry machining processes. cooling and cleaning and rinsing. conditions are generally more difficult in the case of processes like drilling. Restrictions may derive from the method. steels. cutting materials provide the best basis for dry machining. Process substitution is a possible alternative. fine boring and reaming tools available with special substrates. coatings and cutting parameters. taps. more heat must be dissipated by the chip with a comparable heat conversion. There are a number of drills. Tool coating is particularly important in this regard. Cemented carbides. Chips caught in or welded to the flute reduce the quality of the drill hole and can lead to tool damage. This means for the cutting process on the one hand that stronger frictional and adhesive processes can take place between the tool and the material.6 Reducing or Avoiding the Use of Cutting Fluids 235 In dry machining. the goal must therefore be not only to lay the technological foundations.6.11). It also means that part of the heat produced in energy conversion locations and dissipated by the chip. Interrupted cuts and the use of coated milling tools made of cemented carbide are favourable prerequisites for manufacturing threads by dry cutting. are omitted. When planning and designing single processes or manufacturing sequences without cutting fluid. coating systems and tool geometries adjusted to the particular requirements of dry machining. But it is also indisputable that cutting fluids will remain necessary for some machining tasks. “Near-net-shape” parts.236 6 Cutting Fluids quality is affected by the amount of heat that flows into the component. For these processes therefore. This same is true for the reduction of friction. Because they shorten the contact time between the tool and the workpiece. This demands components with volumes to be machined that are as small as possible as well as equal machining allowances. the process must be planned such that as little as possible heat enters the workpiece. The operation should take place in one cut if possible. higher cutting speeds and larger feeds contribute just as much here as larger positive tool orthogonal rake angles. adhesion and wear by the use of coated tools. . which reduce cutting work. Dry machining is feasible in numerous manufacturing methods. dry machining is surely the most effective approach in cutting technology to combining ecological objectives with economical advantages. Manufacturing processes will in future no longer only be assessed from the standpoint of the improvement of performance but also with respect to ecological safety. The distribution and number of cuts is very important. Many companies have recognized this and converted at least some components of their production to dry machining or minimum quantity lubrication. Kloc05]. the final contours of which are frequently created with one cut. we must look for alternative. In light of the problems associated with the use of cutting fluids. provide optimal conditions for this. environmentally friendly media that not only fulfil the functions of traditional cutting fluids but are also not potentially ecotoxic [Krie01. cutting edge geometry and cutting tool material of the workpiece.g. machine tool and coolant.g. changes to roughness. width of flank wear land. e. RWTH edition. criteria and parameters.1007/978-3-642-11979-8_7. The following definition applies: Tool life behaviour is the ability of a working pair (tool and workpiece) to withstand a certain cutting process [DIN6583]. tool. chip temperature and form. Machinability is the property of a workpiece or material which allows chip removal under specified conditions. 7. machine tool and coolant. from the beginning of use until the achievement of F. Examples of tool life criteria are: • all measurable tool wear values. tool.g. e. i. These state variables are influenced in turn by the tool life conditions. Klocke. The tool life conditions are all the conditions present during the cutting process or operational test. the type of cutting fluid and thermal marginal conditions In order to judge the tool life behaviour of the system encompassing the workpiece.e.g.1). This is influenced by the cutting edge durability of the tool. Cutting edge durability is the 4x ability of a tool to retain its cutting ability during machining.Chapter 7 Tool Life Behaviour The term tool life behaviour was introduced to describe material and cutting tool material behaviour during the machining process.g. Cutting ability is the ability of a tool to machine a workpiece or a material under specified conditions [DIN6583]. its form.g. by the machinability of the workpiece and by tool life conditions (Fig. • all measurable values of the cutting process.g. e. e. • all measurable workpiece data. clamping. C Springer-Verlag Berlin Heidelberg 2011 DOI 10. clamping. Manufacturing Processes 1.  237 . Machinability and cutting edge durability are both functions of the state variables force and temperature. its kinematics and cutting edge engagement of the environment. e. changes to cutting power. e. To describe the tool life of the system encompassing the workpiece. its static and dynamic stiffness of the cutting process. They comprise multiple components [DIN6583]: • • • • • of the tool.g. Tool life behaviour is evaluated by means of the tool life conditions. its shape and material of the machine tool. workpiece or cutting process caused by machining. e. tool life criteria are used which represent limiting values for undesired changes to the tool. e. 1 Tool life behaviour the tool life criterion under the influence of tool life conditions. In order to describe the tool life behaviour of the system of the workpiece. If the tool life condition for describing machinability is selected.Kinematics .Cutting conditions Environment .Form .Geometry Cutting edge durability (Tool) Tool life criterions Tool wear Resultant force.Cutting tool material Work piece . the clamping. the tool life travel path.Thermal boundaries Machinability (Material) Tool life parameter Tool life Tool life travel path Tool life quantity Tool life volume Fig. Tool life parameters are times. If the tool life criterion is used for description. tool life parameters are used.Cutting fluid . quantities or paths achieved in chipping under specified conditions until a tool life criterion is reached. For example. . the tool life parameter and condition must both be entered in the index. the tool life conditions.Dynamic a. the two following examples illustrate the relations explained above. 7. the tool life parameter and tool life criterion must be taken into account in the index. the tool life volume and the tool life quantity. static stiffness Tool life behaviour Cutting process . criteria and parameters must always be specified.238 7 Tool Life Behaviour Tool life conditions Tool . To conclude this section. These parameters include [DIN6583]: • • • • the tool life. when describing the tool life behaviour via tool life parameter. the machine tool and the coolant in a clear way.Cutting edge geometry . cutting power Surface roughness Chip form and temperature Machine .Material . the tool. the tool life criterion and tool life condition are indicated in the index. This section will focus on the material.2) Specified parameters: ap = 2 mm (tool life condition) and N = 500 (tool life parameter) Due to the interaction between machinability and cutting edge durability. N 500 = 4000 N (7. etc.1 Determining Tool Life Parameters Tests to determine tool life parameters can basically be executed using either slow testing methods or quick testing methods. 7.2 mm (tool life criterion) (b) description by means of the tool life criterion: Fcap 2. Tool wear is highly significant.7. such as mechanical stress or temperature. however. The following sections will consider this more closely. both parameters can be used to evaluate the machining system. tool wear can be defined relatively easily. • tool life (or tool life travel path. 7.2 = 200 m min (7.1) Specified parameters: T = 15 min (tool life parameter) and VB = 0. One must strictly define. different quick testing methods were developed in order to evaluate and compare the cutting edge durability and machinability of different materials while minimizing the required time and materials as much as possible. As an alternative.2 Machinability 239 (a) description by means of the tool life condition: vcT15. In contrast to the time-varying state variables of the machining system.2 Machinability Parameters subjected to state changes during machining can be used as evaluation parameters for judging machinability. whether the object of evaluation is the material (the workpiece) or the cutting tool material. . The following parameters can be used to evaluate machinability: • cutting force. assuming that a parameter is held constant with respect to the tool life behaviour (constant machinability for evaluating the cutting edge durability of different cutting tool materials given constant tool life conditions or constant cutting edge durability for evaluating the machinability of different materials given constant tool life conditions). Such tests are costly in terms of both time and materials.). quantity. while the cutting tool material will be assumed to be constant. VB 0. Long-term cutting tests are executed for detailed descriptions of the stress of tools on machine tools. Using the wear curves in Fig. This process of succumbing to thermal influences is also referred to as bright braking. 7. 7. The test is executed under constant tool life conditions until the cutting edge becomes unusable due to thermal conditions. etc.1 Temperature Tool Life Rotation Test The temperature tool life rotation test was developed for cutting tool materials with low temperature resistance (tool steels and high speed steels). ceramics.3 can be described approximately by means of a general exponential function. 7. this function assumes the approximate form of a straight line (Fig.2.3). It is held using a longitudinal round cut with constant tool life conditions.1. but rather the influence of cutting temperature is the predominant factor for terminating tool life. It is generally sufficient to determine the width of flank wear land VB. The tool life Tc is the time in min in which a tool performs from its first cut to its becoming unusable due to a specified tool life criterion under specified machining conditions.2). 7. 7. 7. It is often sufficient to use a single dominant parameter to evaluate machinability. After different cutting times.2. the crater depth CD and the crater mean CM. In a double logarithmic system. CBN). The measurement results can be represented in a diagram (Fig. Bright braking can be recognized by the formation of bright lines or lines of temper colour on the cut surface or on the machined workpiece surface or by the appearance of surface alterations. . 7.1 Tool Life The tool life Tc of the tool is the most significant parameter for characterizing the machinability of a material. Altered chip forms and noises are also indicative of an advanced stage of damage to the cutting edge.1. The test is executed whenever wear instead of cutting temperature is the predominant influence on tool life leading to the unusability of the tool.4). The test is executed whenever not wear. The curve in Fig. cermet.2 Wear Tool Life Rotation Test The wear tool life rotation test is executed for cutting edge materials with a great temperature resistance (cemented carbide. 7. respective value pairs can be formed for the tool life criterion from the cutting speed and cutting time which together form the tool life curve (Fig. wear is measured on the flank and rake faces until the previously determined tool life criterion has been reached.2.2.240 7 Tool Life Behaviour • the surface value of the workpiece and • the chip form. 7. 7.3) log Tc = k · log vc + log Cv (7.2 Wear of uncoated carbide during machining heat treatable steel 60 Fig.5) . the following equation applies: Tc = Cv · vkc (7.4) The following applies: And after taking the antilog.2 241 800 vc1 > 600 Tool life criterion vc3 vc2 > 400 200 0 0 2 4 tc1 tc2 6 8 10 12 14 16 18 tc3 Cutting time t c /min Fig.3 Tool life curve (heat treatable steel/carbide) Tool life Tc / min 45 30 tc3 /vc3 15 tc2 /vc2 tc1 /vc1 0 0 200 400 600 800 Cutting speed vc / (m/min) A straight line can generally be described by the linear equation: y=m · x + b (7.Machinability Width of flank wear land VB/µm 7. 5 mm 80 60 40 Tool life Tc / min 20 α 10 8 6 4 Tool life graph Taylor equation 2 1 50 100 200 300 400 500 Cutting speed vc / (m/min) Fig. Further research became necessary in his wake to make the tool life equations practically useful in a broad field of application. but also of feed and cutting depth. since it only takes into account the influence of cutting speed on wear.4 Tool life curve in a logarithmical system (heat treatable steel/carbide) This is referred to as the TAYLOR equation.242 7 Tool Life Behaviour 100 Material: C53E Feed: f = 0. The parameter Cv (ordinate intercept) indicates the tool life at a cutting speed of vc = 1 m/min (standard tool life) and the parameter CT (abscissa intercept) the cutting speed at Tc = 1 min (standard cutting speed).5 mm Cutting tool material: HW-P15 Insert geometry: SNGN120412 Tool life criterion: VB = 0. KRONENBERG . This simple tool life function was developed by TAYLOR [Tayl07]. Thus the increase value k can also be expressed as follows: tan α = k = − log Cv log CT (7.4) is designated as a simple tool life function. 7. (7. Tc = C · vkc · fzkfz · akpa (7. It is also known in an expanded form which takes the influence not only of cutting speed.8 mm Depth of cut: ap = 2.6) The tool life function in Eq.7) Research in to these basic relations essentially goes back to the American TAYLOR [Wall08]. 3).2 Machinability 243 was to exert the most influence on this research [Kron27]. With respect to machinability tests in which the resultant force is used as an evaluation parameter. (7. Further research by KRONENBERG also showed that the application of similarity mechanics to chipping leads to the relations named in Eqs. geometrical and kinematic influences of the machining process will remain largely excluded from consideration. the feed force Ff and the passive force Fp .2. one can determine the stress state in the material in front of the workpiece cutting edge from the resultant force (Chap. The specific resultant force or the specific cutting force may also be used as evaluation parameters (Chap. 7.e. In practical applications. • estimating the workpiece accuracy achievable under certain conditions (deformation of workpiece and machine). Resultant force is described by its amount and direction. It must be generally borne in mind that the parameters in these functions are not constants. the magnitude of the resultant force represents an evaluative standard for the machinability of a material. 3). Also.7). drives. Furthermore. the distinction must be made between two different types of cut which respectively cause fundamentally different stress states in the material: • Those that cause a biaxial stress state or one which lies at least in direct proximity to a biaxial stress state. The following will focus primarily on the influence of materials on the resultant force. • determining cutting conditions in the work preparation phase. the force’s effective direction can also have a significant effect on mechanically related changes to the tool or workpiece. • determining processes which occur at the locus of chip formation and explaining wear mechanisms. since greater forces tend to arise during the machining of materials which do not easily chip. designing frames. the cutting force Fc . in line with requirements.7. tool systems.2 Resultant Force Knowledge of the magnitude and direction of the resultant force F or its components. Exceptions to this will be noted where appropriate. cutting force is often used instead of resultant force as an evaluation parameter. is a basis for • constructing machine tools. • Those that are far removed from a biaxial stress state. They can only be assumed to be approximately constant in certain areas.5) and (7. This procedure is reliable when the other components of the resultant force remain negligibly small. guideways etc. . i. The cutting force is the component of the resultant force in the direction of primary motion. In addition to amount. 2. 3 and 8).1 + mz · log h (7.6). force measurements have been represented in relation to chip thickness values (Fig. Salo28].8) kz = kz1.5).5 Gradient of specific resultant force depending on the chip thickness (schematic diagram) . By means of a representation in a double logarithmical diagram in which the exponential function follows a straight line. SALOMON discovered that there is an approximate exponential relationship between specific force and chip thickness [Salo26. 7. It must be borne in mind that extrapolations are not permissible. The following equations apply: log kz = log kz1.10) 5000 Specific resultant force kz / (N/mm2) 4500 4000 3500 3000 2500 2000 vc1 vc2 vc3 1500 1000 vc1 < vc2 < vc3 500 0 0 200 400 600 800 1000 1200 1400 1600 Undeformed chip thickness h / µm Fig. especially in the region of small chip thicknesses. since in this case at least the same exponential function is no longer valid.9) mz = tan α (7. the specification parameters of the straight line may simply be determined by means of the axial sections and the gradient (see Fig.1 Resultant Force Measurements Measurements of resultant force are carried out by means of dynameters.1 · hmz (7.244 7 Tool Life Behaviour 7. preferably in the direction of the axes of the machine tool (Chaps. 7. Since this discovery.2. which measure the average mechanical strain on the cutting tool in three directions which are orthogonal to each other [Schl29]. 7. 2 Machinability 245 Specific resultant force kz / (N/mm²) 10000 α kz1. 7. The most important factors for this are the elastic and plastic deformations of the material in the area of the minor cutting edge [Djat52]. 7.3 Surface Quality Surface quality can also be used to estimate machinability.8 compares calculated and measured roughness values given a constant cutting speed without process disturbances caused by built-up edges.1 1000 100 100 1000 Undeformed chip thickness h 10000 Fig.7). the material which builds up on the rake face is sporadically stripped off und transferred to the workpiece surface. Figure 7. The kinematic roughness is yielded by the relative motion between workpiece and tool and by the edge radius.2. Built-up edges are undesirable.7). Due to mechanical and thermal stresses. 7.7. Low cutting speeds and certain material-tool combinations may lead to the adhesion of material particles on the rake face. They increase tool wear and lead to a poor surface quality (Fig. it is primarily influenced by the form of the cutting edge and the feed. this influence becomes increasingly insignificant. BRAMMERTZ’s theory accounts for plastic deformations and also the elastic spring-back of the material effected after the cutting edge reaches a certain area . During turning. With increased cutting speeds. This is referred to as the growth of built-up edges (Fig.6 Specific resultant force depending on the chip thickness in a double logarithmical diagram 7. 4 0. 7.3 Feed f /mm 0.6 .5 1 20 16 2 Rt = 8rf ε 12 8 Measured roughness 4 Fig.5 0. 7. 28 Roughness R /µm 24 rε = 0. A minimum cutting depth must be achieved in order to guarantee chip formation.25 mm 0.2 0.246 7 Tool Life Behaviour 25 Material: C45N Cutting tool material: HW-P10 ap = 3 mm Depth of cut: Feed: f = 0.8 Comparison of calculated and measured roughness 2 Theoretical roughness (Rt) 0 0 0.7 Influence of cutting speed on surface quality of the material [Bram61]. Otherwise.1 0. The yield point can be used to judge the elastic behaviour of the material to be machined. the material is only deformed elastically by the cutting edge.25 mm 15 Cutting section geometry: γo = 6° αo = 5° λ s = 0° κ r = 70° ε r = 90° rε = 1.2 mm Built-up edge Average roughness Rz / µm 20 10 5 0 0 50 100 150 200 Cutting speed vc / (m/min) 250 Fig. This compromises the achievable surface quality. This means that a minimum feed fmin must also be achieved as to allow a material separation to take place. There is no material separation below the minimum chip thickness value and thus no chip formation.9). He investigated cutting speeds between 8 and 210 m/min.7.9 Schematic sketch of minimum feed and minimum chip thickness Since the material is not only stressed on the finished surface. Sokolowski cites a dependence of minimum chip thickness on cutting speed. as a result of which the workpiece material becomes pressed. 7. since the effective tool orthogonal rake angle becomes increasingly negative with an increasing cutting edge radius. As soon as the cutting edge radius assumes higher values than the chip thickness.2 Machinability 247 Trace of the plane of the rake face Aγ Tool Trace of the plane of the flank face A α fmin ≡ hmin rβ Work piece assumed working plane ≡ Tool-cutting edge normal plane Fig. the influence of the nominal tool orthogonal rake angle becomes insignificant. . In theoretical cutting theory. If the minimum chip thickness is not reached. The literature varies with respect to the minimum chip thickness hmin . but also on the cut surface. This material separation depends on the cutting edge radius rβ and the yield point Re of the material. minimum feed is frequently projected to the tool-cutting edge normal plane Pn and referred to as minimum chip thickness hmin (Fig.25 and 1. In reference to the turning of steel. Further notable influences on surface quality are material inhomogeneities and hardening. König and Klocke cite a factor of two to three of the cutting edge radius or the bevel width. with hmin decreasing with increasing cutting speed [Soko55]. As reference data for hmin . leading to values for hmin /rn between 0. squeezed and conveyed to the flank face (ploughing effect) [Albr60]. The effective tool orthogonal rake angle represents a major influence on minimum chip thickness. material accumulates in front of the cutting edge radius.125. elastic deformation must also be taken into account in this case. 7. 248 7 Tool Life Behaviour 7.10. These may not damage the tool system. A temperature range is . Flat helical chips tend to migrate outside the engagement length via the flank face. snarled and discontinuous chips represent an increased hazard to machine operators.4 Chip Form In the machining of different materials. Favourable Unfavourable 1 2 3 4 1 Ribbon chip 2 Snarled chip 3 Flat helical chip 4 Angular helical chip 5 Helical chip 5 Favourable Good 6 7 8 6 7 8 9 10 9 10 Helical chip segment Cylindrical helical chip Spiral chip Spiral chip segment Discontinous chip Fig. the cutting parameters and the material properties. Figure 7. 7. sulphur and lead. Chip forms can be altered through alloying different chemical elements.5 Cutting Speed The following will evaluate the influence of cutting speed on machinability and discuss this influence via and analysis of specific cutting force. The adhesion tendency decreases with increasing cutting speed. Especially advantageous are chip forms that do not inhibit the machining process. 7. the machine tool and the surface of the processed component. such as phosphorous. Ribbon. Chip breakage is generally favoured by the material’s increasing strength and decreasing toughness. Adhesion and the growth of built-up edges occur with lower cutting speeds.2. Examples of typical chip forms are shown in Fig.11 shows a typical curve for the specific cutting force when machining non-heat-treated steels. Long chip forms make the evacuation of accumulating chips difficult. The spatial requirement for the chips and chip evacuation are also important parameters (chip volume ratio).2.10 Conventional chip forms 7. The formation of the different chip forms depends greatly on the friction conditions in the contact area between the chip and the rake face. different chip forms are formed under the same tool life conditions. the tool orthogonal rake angle. or by means of a targeted heat treatment of the material. thus causing damage to the tool holder and the cutting edge. their metallographic constituents and their mechanical properties. which causes the resultant force to be lowered again. thus increasing the specific resultant force.3 The Machinability of Steel Materials 249 2000 1000 500 Cutting speed range of blue brittleness 1500 Propensity to adhesion Specific resultant force kz/( N/mm²) 2500 Thermal softening 0 0 25 50 75 100 125 150 175 200 225 250 Cutting speed vc/ (m/min) Fig. This is the area in which blue brittleness occurs. steel materials are divided into the following groups: • unalloyed steels. 7.7. • low-alloyed steels (alloy content < 5%) and • high-alloyed steels (alloy content ≥ 5%). 7. As a result. If the cutting speed is further increased. deformability decreases. In the case of unalloyed steels. the temperatures in the shear zone increase sharply and the material is weakened. The following will describe materials frequently processed by chipping methods and their machinability. we must furthermore differentiate between those steel materials that are not to be heat-treated (common construction steel) and those that are (grade and special steel). Such a classification aids both in selecting a material with respect to the properties required for their future function and in determining machining conditions. Steels referred to as common construction steels .11 Material properties depending on the specific resultant force reached in which the specific resultant force again rises until a maximum value is achieved.3 The Machinability of Steel Materials Steels can be classified according to their alloying elements. Depending on their alloy content. Blue brittleness refers to a material condition in which the dislocation mobility is strongly limited by the interaction with nitrogen present in the microstructure. 1 Mechanical properties of metallographic constituents. S235JR.06%. to VIEREGGE [Vier70] Hardness/HV 10 Rm /(N/mm2 ) Rp0. The crystalline structure of steel is mainly composed of the following components: • • • • • • ferrite. Ferrite complicates the chipping process through the following: • its strong tendency to adhere. whose mechanical properties (Table 7.250 7 Tool Life Behaviour (e. Fe3 C) is hard and brittle and thus cannot be machined. Perlite appears in iron materials given a carbon content between 0.3. perlite. 7. Table 7. cementite. S355J2G3) those whose mechanical properties exhibit minimum values. Perlite is a eutectoid phase mixture composed of ferrite and cementite.2 /(N/mm2 ) Z/% Ferrite Cementite Perlite Austenite Bainite Martensite 80–90 >1100 210 180 300–600 900 200–300 – 700 530–750 800–1100 1380–3000 90–170 – 300–500 300–400 – – 70–80 – 48 50 – – Ferrite (a-ferrite.02 and 6. one or more of these metallographic constituents predominate.67%. cubic body-centred.g. • its tendency to form undesirable ribbon and snarled chips due to its high deformability • poor surface qualities and increased burr formation on the workpieces Cementite (iron carbide.1 Metallographic Constituents The machinability of steels essentially depends on their respective crystalline structure. bainite and martensite Depending on carbon content. it is the . cementite can occur either by itself or as a structural constituent of perlite or bainite. which favours material smearing on the tool and the formation of built-up edges and built-up edge fragments.83% carbon. Depending on carbon content and cooling speed. The eutectic point lies at 723◦ C and 0.02%) is characterized by relatively low strength and hardness and by a high deformability. alloying element content and heat treatment.1) affect the machinability of the given steel. maximal solubility for carbon: 0. acc. austenite. Given a carbon content of 2. Construction steels are used in practice when no special requirements are made with respect to their structure. causes less burr formation on the workpiece and improves the surface quality. which is 1/3 lower than in unalloyed steels. It has a face-centred cubic structure. on the one hand. the deformation of the material caused in chip formation causes a hardening of the material forming the surface. the low deformability and greater hardness of perlite causes.06%. . above 2. globular (spheroidal) cementite can also be formed. promotes the formation of favourable chip forms. On the other hand. • • • • Perlite reduces the adhesion tendency and the formation of built-up edges.3 The Machinability of Steel Materials 251 only metallographic constituent. into ferrite or cementite. gluing-points and built-up edge fragments occur more frequently when machining these steel materials. it is a constituent of ledeburite. Essential properties of austenitic steel materials relevant to machining are: • their high deformability and toughness: The high ductility of this material is based on the good plastic deformability of its face-centred cubic crystal lattice. linear cementite appears in perlite. This results in additional stress to the tool cutting edge. which has four slip planes each with three slip directions and thus 12 slip systems in total. which means that built-up edges. manganese (Mn) and nitrogen (N). For the most part. • significant abrasive wear and • high resultant forces. In unalloyed and low-alloy steels below approximately 723◦ C. although its strength can be increased by means of cold-forming. • their relatively low heat conductivity. • their tendency towards strain hardening: in the area of the workpiece surface being created.7. lamellar. depending on the carbon content. gluing-points and built-up edge fragments and to form unfavourable ribbon and snarled chips. Heat treatable steels with a carbon content < 0. The maximum solubility for carbon is 2. The structure exhibits only minimal hardness. Austenite refers to the γ-mixed crystal of iron. Examples of austenite builders are nickel (Ni).8% are referred to as sub-eutectoid. During machining. The result is a strong tendency of the material to form built-up edges. especially in subsequent cuts. It impairs heat removal through the chip and increases the thermal stress of the cutting edge. Austenite is the main constituent of many non-rusting steels and is not ferromagnetic.06%. After to a corresponding heat treatment (soft annealing) however. • their strong tendency to adhere to the cutting edge material: face-centred cubic materials tend to adhere much more than hexagonal or space-centred cubic metals. austenite is transformed into perlite and. Thus austenite is only present at room temperature in alloys. By means of a diffusionless transformation. with turning: γ o > 6◦ ) should be used.2% is rapidly cooled from the austenite temperature range to a temperature below the martensite starting temperature. with their lubrication properties being more significant than their cooling effect. Martensite forms when a steel material with a carbon content of > 0.12).2 Carbon Content The metallographic constituents of steels can be seen in the iron-carbon phase diagram (Fig.8%. only perlite is formed as a eutectoid mixture of ferrite and cementite. Because of the high deformability of the material.8%. the resulting cut surface roughness is high. Due to the rapid cooling.g. The secondary cementite is precipitated from the austenite predominately at the grain boundaries. Among the acicular bainite forms. Since ferrite has an intrinsically large adhesion tendency. bainite consists of carbon-supersaturated ferrite. perlite and secondary cementite are formed. With a carbon content of 0. Steels with a carbon content of < 0. There are two main forms of bainite: • acicular bainite (with continuous cooling and isothermal transformation) • granular bainite (only with continuous cooling) Independently of the form. . only tools with the greatest possible positive tool orthogonal rake angle (e. In order to decrease the adhesion tendency and to improve the surface quality. body-centred martensite lattice forms from the face-centred cubic austenite lattice. built-up edges form with low cutting speeds.25% is essentially characterized by the properties of the free ferrite.8% precipitate ferrite when cooling from the austenite zone.252 7 Tool Life Behaviour Bainite forms in the temperature range between those of perlite and martensite: iron diffusion is no longer possible. Steels with C-content < 0. Because of ferrite’s low strength. In steels with a C-content > 0.3. one distinguishes according to the transformation temperature between lower bainite (strong similarity to martensite) and upper bainite (strong similarity to perlite). with the carbon partially precipitated as carbides (e.25% pose particular problems for grooving and parting off. The remaining austenite disintegrates into perlite below 723◦ C. very hard and brittle microstructure which is difficult to machine. oils are generally used as cutting fluids. The machinability of steels with a carbon content < 0. Fe3 C) whose size (from rough to very fine) is determined by the conversion temperature. Tool wear and cutting temperature only increase slowly with a rising cutting speed. 7. the carbon dissolved in austenite is forced to remain dissolved in the mixed crystal. reaming and thread die cutting. The cutting tools used are subject to • an increased abrasive wear and • high mechanical and thermal stresses 7. and carbon diffusion is already considerably hindered.g. Martensite has a fine-acicular. a tetragonally deformed. as well as for boring. . This results in the following: • a lower adhesion tendency and thus a shift of built-up edges to lower cutting speeds. burr formation occurs to a greater extent. the achievable surface qualities are markedly inferior. This is important to the extent that these materials are frequently deformed through cold extrusion and then finished through cutting.4%). Cold-forming has a positive effect on machinability. especially with respect to chip formation.67 Carbon concentration / mass-% Fig. The strength of the structure increases. the special property of this metallographic constituent is granted a stronger influence on the machinability of the material. • increasing tool wear and higher temperatures in the tool contact zones due to the higher material strength. In this way. 7.12 Section of the iron-carbon phase diagram (metastable system) Due to high deformability and relatively low cutting speeds.35%. • an improvement of surface quality and chip form. The amount of perlite in the structure increases with increasing carbon content (0.8 C E Austenite + Secondary cementite Melt + Primary cementite Austenite + Secondary cementite + Ledeburite Perlite + Secondary cementite + Ledeburite Perlite + Secondary Cementite F Ledeburite + Primary cementite Primary cementite + Ledeburite Cementite (Fe 3 C) 1600 1536 253 Ledeburite 7.The Machinability of Steel Materials Steel A H B Melt δ-Mc + γ -Mc 1147 Melt + Austenite Austenite (γ -Mc) G 911 M P 769 723 α -Mc α-Mc + γ -Mc O S Ferrite + Perlite Q 500 0 Perlite Temperature T/°C Melt + δ-Mc I δ -Mc Cast iron 0.3 K L 2.3 6. Tools with a positive orthogonal tool rake angle should be used. • an increase in abrasive wear.25–0. Machinability can be improved by means of coarse grain annealing given a low C-content and by means of normalizing given a C-content above 0. Moreover. its deformability decreases.06 4. Secondary cementite precipitates at the grain boundaries given a carbon content significantly exceeding 0.83% carbon. In the case of steels with larger amounts of carbon.3. machinability is negatively affected by increased tool wear. Steels with a carbon content of 0.254 7 Tool Life Behaviour A further increase of carbon content (0. At the same time. Problems related to chip form are more rare.1 Manganese Manganese improves temperability and increases the strength of steel (approx.4–0.3 Alloying Elements Alloying and trace elements can influence the machinability of steel by changing the composition or by forming lubricating or abrasive inclusions. also in the form of crater wear. tools with positive orthogonal tool angles γ o up to 6◦ and mildly negative tilt angles λs up to –4◦ should be used. high rake face temperatures already arise at low cutting speeds. Molybdenum. 100 N/mm2 per 1% alloying element). Perlite formation initiates directly from the austenite grain boundaries. The free cementite forms shells around the austenite/perlite grains [Schu04].3. 7.8%) consists of secondary cementite and perlite.3. In the case of steels with a greater carbon or alloy content. Because of its high affinity to sulphur.8% are generally regarded as having good machinability only with respect to surface quality and chip form. Strong crater and flank face wear is to be observed even at relatively low cutting speeds. the high pressures and temperatures which arise cause an extra stress for the cutting edge. Such shells cause very strong wear in cutting processes. . with wear also increasing. Due to increased strengths. After slow air cooling. The following will discuss the influence of some important alloying elements on the machinability of steel. however. these elements and also tungsten form hard special and mixed carbides which impair machinability. manganese forms sulphides with sulphur.3. thereby influencing the machinability of case-hardened and heat-treated steels in terms of structure and strength.5% facilitate machinability in steels with low amounts of carbon due to good chip formation.4–0. Manganese contents up to 1.8%) causes a further decrease of the amount of ferrite with a corresponding perlite increase until only perlite remains at 0.3. the structure of supereutectoid heat-treated steels (C > 0. Tungsten Chrome and molybdenum improve temperability. 7.8%. The chip form improves with increasing carbon content.2 Chrome. For turning. the increasing mechanical stress of the rake face causes increased wear. The cutting edge parts must have a stable design. 7. In addition to the strongly abrasive effect of the hard and brittle metallographic constituents. 5 Phosphorous Alloying phosphorus. area reduction and the impact value. especially when it is included transversely to the strain direction. This allows for the formation of short-breaking chips.7 Sulphur Sulphur is only slightly soluble in iron.3. Iron sulphides (FeS) are undesirable. strain.1%. Vanadium Titanium and vanadium. This results in increased tool wear. 7. This leads to the unwanted “red brittleness” of steel. tellurium. 7.3. While higher amounts of phosphorous lead to an improvement of the surface quality. . but it forms. MnS exerts a negative influence on mechanical properties like strength.3 Nickel Nickel is among the group of elements which expand the γ-phase zone in iron alloys. as they exhibit a low melting point and deposit primarily at the grain boundaries. improved surface quality and a smaller tendency towards forming built-up edges. it forms.3. The positive effects of MnS on machinability are short-breaking chips. 7. which is carried out only in some free-cutting steels.3.3 The Machinability of Steel Materials 255 7. With oxygen.g. the strength of steel materials increases. to an unfavourable machinability. hard Si-oxide (silicate) inclusions. In practice. depending on the alloying components of the steel. especially in the case of austenitic nickel steels (with larger amounts of nickel). selenium) can effectively help to counteract the deformability-related extension of MnS. What are desirable on the other hand are manganese sulphides (MnS).3. 7.4 Silicon Silicon increases the strength of ferrite constituents in steel. They lead to a high grain refinement. in the absence of stronger deoxidation agents like aluminium.3.7.3.6 Titanium. This generally leads. which has a negative effect on machinability with respect to mechanical stress and chip form. they also cause increased tool wear. can increase strength considerably due to the extremely dispersed carbide and carbon nitride precipitations.3. however. even in small amounts. special alloying additives (e. With an increased inclusion length. Nickel increases toughness. Up to an amount of 0. phosphorous has a positive effect on machinability. especially at low temperatures. which have a much higher melting point. By adding nickel.3.3. leads to segregations in the steel which cannot even be removed with subsequent heat treatment and heat deformations and to an embrittlement of the ferrite. various stable sulphides. non-deformable inclusions then found in the steel. 7.4.3. heat treatments can influence microstructures with respect to quantity. as aluminium oxide and silicon oxide. The mechanical stress of the tool can be lowered by up to 50%. 7.3. under certain machining conditions. silicon.g. e. perlite. 7.g. Analogously to manganese sulphides among machining steels. mechanical properties and thus machinability can be customized to the given requirements. Three basic categories of heat treatment may be distinguished: • establishing an even structure in the entire cross-section. In this way. a protective lead film forms between the tool and the material. diminish machinability.9 Non-metallic Inclusions The elements added to the steels for deoxidation. which to a great extent is in a state of thermodynamic equilibrium (e.8 Lead Lead is not soluble in iron. The hard.1). soft annealing structures. bind the oxygen released by steel solidification. This effect is exploited especially in machining steels (see Sect. wear-inhibiting oxidic and sulphidic layers may form after deoxidation with calcium-silicon or ferro-silicon [Köni65. form and the configuration of their constituents. For example. by choosing a suitable deoxidatizing agent. Töns89. According to DIN EN 10052. i. bainite. the machinability of steel can also be positively influenced.g. these calcium aluminates are able under certain cutting conditions to form friction-reducing and/or wear-inhibiting coatings in the contact zones of the tool cutting edge [Köni65. One measure for improving the machinability of steels deoxidized with aluminium is calcium treatment.3. This film reduces tool wear.256 7 Tool Life Behaviour 7.4 Types of Heat Treatment When executed in a targeted manner. Opit67]. in machining conditions. This converts the sharp-edged aluminium oxides present in steel in conventional melting which act abrasively during cutting into globular and. . especially when the oxides exist in the steel in larger amounts or in linear form [Wink83].e. heat treatment is defined as follows: Heat treatment is a sequence of heat treatment steps during which a workpiece is in whole or in part subjected to time/temperature sequences in order to cause a change of its properties or its structure. Because of the low melting point. calcium is added to the steel melt by means of secondary metallurgy. Kloc98a. Zink99]. plastifyable calcium aluminates.3. martensite).3. In certain circumstances. structures consisting of ferrite or austenite) or in thermodynamic disequilibrium (e. aluminium. it is present in the form of submicroscopic inclusions. the chemical composition of the material may be changed during the treatment (thermochemical treatment). However. with the chips becoming short breaking. manganese or calcium. In this process. 2 1.6 2.3 The Machinability of Steel Materials 257 1300 Austenite (γ –Mc) 1200 (E) Homogenisation 1100 de rbi Temperature T / °C 1000 ion G Coarse grain annealing Har 900 den α−Mc + γ−Mc P 800 700 ing Ac3 723 °C +N orm alisi O Ac1 o it os p om c De a fc t ne Ac m Austenite + Secondary cementite ng S Soft annealing (K) Recrystallisation annealing α−Mc Low-stress annealing 600 Recrystallisation annealing 500 (Q) 400 0 0. . carburizing. The temperature ranges for the individual types of heat treatment can be found in Fig. 7. 7.4. depending on the chemical composition of the steel.13 Partial iron-carbon diagram with heat treatment range data • establishing a hardening structure that is restricted to smaller areas of the cross-section at unaltered chemical composition (in particular: surface layer tempering). carbonitriding. can affect machinability in a targeted way with respect to. carburizing hardening). There are different heat treatment methods with a broad field of application which. chip form and tool wear.7. especially in the rim zone.8 1. 7.4 0.0 Carbon concentration / mass-% Fig.1 Homogenization Homogenization is defined according to DIN EN 10052 as follows: annealing at a high temperature with a hold sufficiently long enough to minimize local differences in chemical composition due to diffusion-related segregations. as a result of a change of the chemical composition (nitriding.3.13. for example. • establishing structure types that are starkly differentiated over the cross-section. e. By means of annealing at temperatures just below the PSK line (Fig. they allow a high surface quality of the finished surfaces. High surface qualities remain achievable. Such a structure is soft and easily deformable.3 Normalizing Normalizing is defined according to DIN EN 10052 as follows: a heat treatment comprising austenitizing and subsequent cooling in still air.4.258 7 Tool Life Behaviour 7. 7. while chip formation worsens to the extent that ferrite predominates in the structure. this heat treatment should be suited to achieving an even. depending on the carbon-content. and chip formation generally good. i. Coarse grain annealing followed by isothermal transformation is utilized for subeutectoid steels with a C-content of 0. Soft annealing (+A) is applied in order to take away the high hardness and low deformability from structures with lamellar perlite or lamellar perlite and cementite. a soft and low-stress state can be created as the lamellar perlite and strands of cementite are caused to disintegrate.3. fine-grain perlitic-cementitic structure. to temperatures exceeding Ac3 . In general. Stronger carbide bands cannot be fully dissolved.3. a nearly even. if the carbide network is to be dissolved in structures with higher carbon content. limited by interference by strength properties and due to financial considerations.3–0. either by ferrite (low wear. The goal is to achieve a perlite structure which is as granular as possible consisting of ferrite with globular cementite. 7. if necessary.3. Through normalizing (+N). fine-grain structure is achieved whose machinability is. determined by the predominant structural constituent. A α/γ transformation takes place in the process. Subeutectoid steels are heated to temperatures above Ac3 . 7.2 Coarse Grain Annealing Coarse grain annealing is defined according to DIN EN 10052 as follows: annealing at a temperature usually considerably above Ac3 with a hold sufficiently long enough to achieve coarse grain. however. poor chip formation) or perlite (increased wear. Supereutectoid steels are heated to temperatures 30–50◦ C above Ac1 or.4. The machinability of such a structure becomes more favourable with respect to wear effects on the tool.4% (ferritic-perlitic steel) in order to produce a coarse-grain structure with a ferrite network which is as closed as possible in which either perlite or bainite is enclosed [Hors85. Tool wear when machining such a structure is relatively low. which means that normalized supereutectoid steels can still cause relatively high tool wear.13) – or.4. Schu04].4 Soft Annealing Soft annealing is defined according to DIN EN 10052 as follows: a heat treatment for reducing the hardness of a material to a specified value. improved chip formation) [Hors85]. oscillating at the PSK line – and subsequent slow cooling. However. The application of coarse grain annealing to improve machinability is. . glide lines and the splitting of brittle crystal types (cementite). it may worsen [Opit64]. A further heat treatment for improving machinability used primarily with case-hardened and heat-treatable steels is annealing to a specific tensile strength (+TH). This is usually applied between the individual shaping stages. they can achieve a similarly good machinability as with machining steels with low carbon content. Recrystallization annealing prevents changes to properties associated with this. controlled cooling from the heat of the forge (BY annealing) [Fasc80].3. e. long holding times and rapid cooling. The reason lies in the relatively coarse-grain structure of the steels used and in the fact that the ferrite network also encloses the perlite grains during the shear process.g. In this state. such as increased hardness and strength and decreased strain and toughness. Meta06]. For energy-saving reasons. Recrystallization annealing refers to annealing following cold forming at a temperature below Ac1 . when cold rolling or cold drawing metal sheets and wires [Schu04. 7. materials with Widmannstätten structure may be created by means of high austenitizing temperatures. Temperatures are held in the region of the PSK line for a longer period of time. other types of treatment are required in addition to classic soft annealing which target specific properties or structures.5 Annealing to Specific Properties In practice. and this with respect both to low tool wear and to good chip formation.10–0.g. a recrystallization . If the material reaches its forming limit in the process of cold forming. targeted heat treatments are executed directly using the heat of the forge. Given steels with a carbon content of 0. its machinability approximates that of pure ferrite. A special type of soft annealing is annealing to spherical carbides (+AC) or spherical cementite (also referred to as GKZ annealing).6 Recrystallization Annealing Recrystallization annealing is defined according to DIN EN 10052 as follows: a heat treatment with the goal of achieving the formation of new grain in a cold-formed workpiece by means of nucleation and growth without a phase change. Case-hardened steels are heat treated to a ferrite/perlite microstructure (+FP). Strain hardening becomes noticeable in the structure through the occurrence of displacements. Machinability investigations [Wink83] have shown that heat-treatable steels cooled from the heat of the forge (e. which for steel is usually between 500 and 700◦ C. when the cementite is fully moulded.3 The Machinability of Steel Materials 259 7. e. if necessary oscillating at this temperature.35%. The goal of this type of annealing is to mould the cementite completely as a sphere.g.3. Such a structure is characterized by good chip formation and chip form. without causing a α-γ-transformation of the crystal lattice.7. C45E+BY) may exhibit a more favourable wear behaviour than the same materials in a heat-treated or normalized state. The result is an acicular ferrite with extraordinarily finely distributed lamellar cementite. Since.4. though it exhibits poor usage properties [Schu04]. Differences with respect to chip formation could not be ascertained.4. When hardening steel (+Q). Processes of continuous and isothermal transformation for the heat-treatable steel C45E are shown in Figs. MPI73]. Low-stress annealing is mainly applied to workpieces which exhibit high internal stresses either as a result of irregular cooling following casting. 7. the precipitation of carbon from the γ-mixed crystal which happens at a normal cooling speed counteracted through a high cooling speed. The composition of the microstructure is not newly formed during recrystallization. This prevents the release of existing internal stresses during the further processing of such workpieces and the formation of geometrical deviations resulting from warpage.4. the conversion processes proceed in the intermediate stage and in the perlite stage [MPI61. only the grains are newly formed. 7. especially high toughness and ductility. if applicable.260 7 Tool Life Behaviour must be executed to form new grains. the greater the tendency towards the formation of new grains. however. A microstructural transformation does not take place. 7. deep drawing. the size of the newly formed grains can lie below that of the original grains [Schu04].4.8 Hardening Hardening is defined according to DIN EN 10052 as follows: a heat treatment comprising austenitizing and cooling under conditions conducive to an increase in hardness caused by the more or less complete transformation of austenite into martensite and. The usual temperatures for low-stress annealing steel workpieces lie between 550 and 650◦ C.9 Heat Treatment Heat treatment is defined according to DIN EN 10052 as follows: Hardening and tempering at higher temperatures in order to achieve the desired combination of mechanical properties. .3. is reduced by means of recrystallization annealing.7 Low-Stress Annealing Low-stress annealing is defined according to DIN EN 10052 as follows: a heat treatment comprising heating and holding the material at a sufficiently high temperature and a subsequent cooling appropriate for eliminating internal stresses as much as possible without essentially changing the structure. The cutting edge must have a stable design. planing. forging or another thermal process or after strong mechanical processing through milling. These structure types are not as easy to machine because of their greater strength. Low-stress annealing serves to reduce these stresses. The strain hardening in the deformation zones induced by cold-forming. bainite. martensite forms after falling short of the Ms-temperature (Ms = martensite starting) [Schu04].4. etc. welding.3.14 and 7. At high strain levels. MPI72. At cooling speeds lower than the critical cooling speed. The greater the strain is. Chip formation can be considered good. Transformation in the intermediate stage is basically characterized by that fact that only the carbon can diffuse. turning.3. 7.15. At supercritical cooling speeds. Metallographic constituents: ferrite (white) with globularly shaped cementite. but the structure is finer-grain and more homogeneous. Metallographic constituents: perlite with lamellar cementite. to MPI The strength values of steel can also be increased through quenching and tempering (+QT). Quenching and tempering. Coarse grain annealing. Structure: tempered martensite. . Soft-annealing. The machinability of the tempered structure increases with increasing martensite decay.16): 1. When the material is tempered.14 Time-temperature-transformation diagram for continuous cooling of C45E steel. acc.3 The Machinability of Steel Materials 261 1000 900 Temperature T / °C 800 700 50 30 20 40 50 70 80 F 600 P 179 500 400 B A Ms 2 5 300 200 M 210 100 548 224 274 0 0. 7. 3. 2. These are the same constituents as in coarse-grain annealing. while at higher temperatures coarser cementite grains develop [Schu04]. 7. the martensite formed during hardening is broken down again in a targeted way through re-heating. carbon precipitates in a finely distributed form. Metallographic constituents: coarse-grain perlite with lamellar cementite. Normalizing.15 using the heat-treatable steel C45E as an example. a ferrite network between the grains (white). 4.7. ferrite.1 Ac3 Ac1 60 1 102 10 1 A F P B M Austenite Ferrite Perlite Bainite Martensite Hardness/HV 20 Part of the structure/ % 104 103 10 10 2 s 105 min 103 Time Austenitisation temperature 880 °C (holding time 3 min) heated up in 2 min Specimen dimension / mm: Ø 3 x 15 Material: C45E Fig. 7. Several possibilities for a targeted influence on structure by various heat treatments are shown in Fig. At low tempering temperatures. The allocation of the partial images to the heat treatments is as follows (Fig. acc. to MPI Fig. 7.2 mm Length 30 mm 100 1 102 10 1 103 10 104 2 10 Time 105 10 3 s min 10 106 4 Material: C45E Austenitisation temperature 880 °C.15 Time-temperature-transformation diagram for isotherm conversion of C45E steel.262 7 Tool Life Behaviour 1000 900 Start of ferrite formation Start of perlite formation 800 Ac3 Ac1 Temperature T / °C 700 A 600 500 25 F 28 29 30 33 75 15 P 85 A F P B M Austenite Ferrite Perlite End of Bainite transformation Martensite Hardness/HRC 20 Part of the structure/ % 33 42 B 400 Ms M 300 50 50 % 90 % 200 Specimen dimension Outside diameter 4 mm Insidediameter 3. 7.16 Crystalline structure for different heat treatments (C45E) . (Holding time 5 min) heated up in 1 min Fig. heat-proof and highly heat resisting steels. Pb) form which contribute to reducing tool wear [Zink99].4. positively influencing the surface quality of the workpieces. nitrided steels. Even at low cutting speeds. The result of this is a low level of abrasive tool wear. Reasons for this are the large amount of ferrite in the microstructure as well as the strength-reducing properties of the main alloying elements.4 Machinability of Various Steel Materials 263 7. They can be machined without difficulty on automatic lathes and multi-spindle lathes and are thus highly suited to serial and mass production. lead and phosphorous. tool steels and rust. The machinability of free cutting steels is relatively low for machining purposes. Pb and P reduce this adhesion tendency. Machining steels are predominately cut with coated cemented carbide tools and moulding tools consisting of high speed steel.7. sulphur. The development of wear on these tools is relatively slow.and acid-proof. 9S20) are characterized by high ferrite and low perlite content. When machining low-carbon machining steels at low cutting speeds (vc < 100 m/min). These favourable properties with respect to machinability are given to machining steels primarily by adding different alloying elements.4 Machinability of Various Steel Materials In addition to the categorization of steels according to their alloy content. The categories are: • • • • • • free cutting steels. 11SMn30. they are also categorized in practice with a view to their uses and applications. these being often limited by the workpiece diameter and the respective machine tools (multi-spindle lathes). bismuth and antimony reduce the strength of the material in the shear zone. The high cutting speeds can be used. Depending on their chemical composition and field of application. The achievable surface qualities are considerably influenced by the adhesion tendency of the material caused by the formation of built-up edges. heat-treatable steels. adhesion in the form of built-up edges plays an important role with respect to tool wear. The main alloying elements. free cutting steels are subjected to heat treatments like carburizing hardening or quenching and tempering.1 Machining Steels The free cutting steels are materials which are distinguished by especially good machinability. . as well as tellurium. 7. case-hardened steels. Characteristics in this respect are a favourable breakage. clean workpiece surfaces and low tool wear. The reduced strength results in shortbreaking chips with a low level of compression.g. The low-carbon machining steels in widespread use (e. friction-reducing layers (MnS. 11SMnPb30. MnS layers. The adhesion tendency in the contact zone continues to be reduced and the surface quality is positively influenced. segregations at the macro-scale (macrosegregations) cannot be removed extensively by means of a subsequent heat treatment. The result is a more favourable chip formation with short-breaking chips when machining. The degradation of strength properties caused by phosphorous are responsible for the embrittlement of machining steels at the average shear level temperature of approximately 200–400◦ C. Recent investigations prove the inclusion of MnS as the basis for glide line formation in the shear zone. MnS causes the chips to be short. it is favoured over phosphorous as alloying element. These precipitations increase with increasing temperature.1% phosphorous. globular particles or irregularly rounded particles. phosphorous causes a formation of segregations (demixing) in the steel because of its relatively small diffusion coefficient. The positive effect of MnS on machinability is explained by the fact that both the internal friction of the material in the shear zone and the friction in the contact zone are reduced. furcations form or. These brittle type 1 sulphides are clearly separated from each other in the steel. Type 2 sulphides are not present in steel in enclosed cells. High-temperature diffusion annealing can only compensate microsegregations (crystallization segregations. This is because of the large distance from the middle to the edge of the steel ingot. The form of sulfidic inclusions in machining steel is determined by the quantity of oxygen released upon solidification. concentration differences). • Type 2: In iron melts with an oxygen content of < 0. An essential requirement for the formation of these sulphides is a carbon . this form of sulphide precipitates from melts rich in manganese sulphide at the primary grain boundaries as a MnS phase similar to an eutectic point. this sulphide appears in the form of regularly distributed. This undesirable property can only be partially removed through subsequent heat treatment. in some places. After solidification.01%. Since sulphur reduces tool wear but hardly influences tempering brittleness. In contrast. improves the surface quality of the workpiece and reduces the tendency toward the development of built-up edges.264 7 Tool Life Behaviour As a function of the steel production process. but rather grow as if radiating from individual centres. • Type 3: This type of sulphide crystallizes from Fe melts with a reduced melting point. Machining steels contain up to 0.02% corresponding to the quasiternary system Fe-MnO-MnS in iron-rich melts. Three different types of sulphides are found in steel: • Type 1: This type of sulphide forms as a liquid phase at an oxygen content of > 0. which means that tempering brittleness occurs to a greater degree and the impact value is already reduced from 100◦ C. Smaller macrosegregations can be achieved by deoxidizing the steel prior to casting. Sufficiently eliminating macrosegregations would require annealing times of such a length as would lead to a significant coarsening of the grains. enclosed in cells. Nitrogen and carbon precipitations are promoted by phosphorous and induce an embrittlement of the α-mixed crystal (ferrite embrittlement). the sulphides growing out of the funnel-shaped cells fuse together. In the process. In the cutting speed region above 100 m/min.g.05 and 0.3%. Typically.29% and cutting speeds below 100 m/min. about 0. but these steels may only be stressed in a temperature range below 200◦ C. especially in the region of 250–400◦ C. Machining steels in which lead has been replaced by tin exhibit a significantly reduced tendency towards chip breakage and are thus only recommendable for methods with interrupted cutting.1 and 0. The effect of lead on tool wear depends on the cutting speed. Substituting lead as an alloying element in machining steels gains in importance with a view to its harmfulness to health [Wink83]. tellurium.35% lead. As a result. Similarly to lead.25%) to machining steels can increase tool life by approximately 50–70%. With respect to crater wear. With respect to machinability. The machinability of machining steel improves in an essential way with an increasing quantity of larger manganese sulphides. for example. the onset of abrasive wear on uncoated cemented carbide tools is considerably more aggressive when turning. The strength properties and toughness of steels are negatively influenced. a thin lead film may wet the contact surfaces between the tool and the workpiece. surface roughness and forces arising during machining. Adding lead (about 0. they are evenly distributed. Lead liquefies at relatively low temperatures (TS = 326◦ C). different metallurgical measures are taken to create this sulphide type for free cutting steels. As opposed to sulphur. . The specific cutting forces fall by up to 50% and the chips become short-breaking. However. When machining steels containing lead. Lead does not dissolve in α-Fe and penetrates the steel microstructure in the form of sub-microscopic inclusions. like those of type 1. bismuth and antimony also help to achieve improved chip breakage and bring about a lubricating effect in the contact zone which reduces tool wear. however. such as milling [Esse06]. In comparison with alloys containing lead. Investigations have shown that adding bismuth considerably improves the machinability of lead-free machining steels (e. Type 3 sulphides precipitate in interdendritic regions. by alloying tellurium. Their composition and form correspond to that of the angular. Such sulphides prevent the pressure welding of the ferrite grains by forming scale-like layers and create a protective zone between the chip and the tool [Köni66]. Free cutting steels in common use include 9SMn28. 45S20. At cutting speeds exceeding 100 m/min or with large feeds. The formation of round manganese sulphides is favoured.15–0. lead can be alloyed with almost all steels. there is a reduction of flank face wear on HSS tools. Given a lead-content increase of 0–0. This effect is not to be observed with coated cemented carbide tools. Machinability is improved by adding up to 0. face-centred cubic α-MnS. type 1 sulphides are considered to be the most favourable.4% and an aluminium content between 0.4 Machinability of Various Steel Materials 265 and silicon content between 0.30% Pb is added to machining steels. 9SMnPb28. an increased lead content has a negative effect on the development of flank face wear.7. This reduces the tendency towards pressure welding and facilitates the shearing-off process. 16SMn30) with respect to their chip formation. 35S20. the lead film becomes ineffective and wear accelerates. the effect of lead is not affected by the cutting speed. g. on the other hand. hardening and tempering.3. such as tapping. HC-P10) or with cermets in order to withstand the thermal stress caused when machining at high cutting speeds. the onset of wear on the tool is low. joints. depending on their alloying elements.g. this reduces the strong tendency of the structure towards linearity.266 7 Tool Life Behaviour 7. broaching and shaping. After machining. This lends especial importance to the selection of a suitable chip breaker when turning carbide indexable inserts. Coarse-grain annealing is often applied to alloyed case-hardened steels in order to reduce difficulties caused by their adhesion tendency. for example. by changing the tool geometry (positive tool orthogonal rake angle) and through reducing the feed [Opit64. Common to all of these is a relatively low carbon content (C < 0. boring. The adhesion tendency of case-hardened steels leads to the formation of builtup edges and to bad surface qualities when procedures with low cutting speeds are used. Case-hardened steels are almost exclusively processed by chip removal prior to case-hardening. Beyond this. the machinability of case-hardened steels in a soft state is low. The workpiece rim zones are carburized to obtain . since individual lines may be cut out. especially for reaming and broaching. In order to improve machinability with respect to surface roughness. case-hardened steels are heat treated to a certain ferrite/perlite structure (BG) or to a certain strength (BF). This is because of the formation of manganese sulphides (see also Sect. Since the microstructure of these steels contains mostly ferrite and only a small amount of perlite. The machining of case-hardened steels is mostly achieved with coated cemented carbide tools belonging to application group P (e. 7. connectors etc. so that different cutting speeds are required depending on the heat-treatment condition of the material. 16MnCrS5).2 Case-Hardened Steels The case-hardened steels include unalloyed machining steels. grade and special steels as well as alloyed special steels. Tools made of HSS. gear shafts. The applicable cutting conditions are little influenced by the heat treatment used – provided that machining is executed with cemented carbide tools and the tensile strength of the material is under 650 N/mm2 [Well72]. Because of the high ferrite content and low perlite content in their microstructure. but it appears again when reheating over the transformation point.4. Because of their very high toughness and low carbon content. cutting speeds should be kept above 200 m/min in order to avoid the growth of built-up edges. Chip breakage can be improved by alloying. which is highly disadvantageous for machining purposes. Alloying sulphur reduces the strength of steel. the case-hardening process follows in three steps: carburizing. At the same time. sulphur and lead (e.3). Peke74]. A reduction of strength by alloying lead is only observable from a temperature of over 300◦ C. The best possible surface quality is achieved furthermore by applying suitable cutting fluids. case-hardened steels tend towards forming long chips. react more sensitively to differences in strength. Case-hardened steels are predominately utilized in the manufacture of wear-stressed and variably stressed parts like cogwheels.2%). This linearity can be partially suppressed by means of a rapid cooling during heat treatment. However.and perlitecontent in the microstructure. Abrasion and thermal wear become insignificant when machining heat-treatable structures (predominantly tempered martensite). and can thus vary to a great extent. chrome. Soft-annealed structures with a mixture of lamellar and granular cementite also lend themselves to machining at higher cutting speeds.4. 20MoCr4. 18CrNi8. the hardness values in the rim zone increase up to 62 HRC. on the respective type of heat treatment and the crystalline structure of the steel. which is a result of the respective heat treatment applied. especially when the steel contains larger amounts of chrome. chip breakage can be improved by means of tool geometry and by alloying lead and sulphur. Very high resultant forces arise in the process. At higher cutting speeds. very high surface qualities are achieved in this way (see also Sect. The cutting speed should be .4 Machinability of Various Steel Materials 267 0. The main alloying components are silicon. Increasing the perlite content accelerates the development of tool wear and increases the resultant force. which degrades the surface quality of the workpiece. nickel and vanadium. The influence of the material structure on machinability is generally stronger than that of the alloying elements. The intended use of the heat-treatable steels is of particular importance when selecting alloying elements which promote chip breakage. mixed ceramics and PCBN cutting tool materials are especially suited to finishing highly heat-treated or hardened steels (> 45 HRC). molybdenum.3 Heat-Treatable Steels Heat-treated steels have carbon contents between 0. double hardening). 7. since this can lead to a reduction of strength.2 and 0. The machinability of heat-treatable steels depends primarily on their crystalline structure. Finest-grain carbides. This limits the applicable range of cutting speeds. After case-hardening (direct hardening.5%. the adhesion tendency also increases. 16MnCr5.6–0.4. a cutting postmachining process must be executed in some cases.6). Frequently used case-hardened steels include C15E. The length of the chips when machining heat-treatable steels depends considerably. the end of tool life is brought about through crater lip breakage. manganese. chip breakage does not become a problem. manganese and vanadium (alloyed heat-treatable steels). The development of wear is essentially determined by the ferrite.7. Because the chip is annealed in the clearly high temperatures.9% carbon. As a result of the warpage of the components caused by the case-hardening. Cementite spheroidization (soft annealing) is advantageous with respect to the wear behaviour of steels with larger ratios of carbon.6% and are therefore stronger than case-hardened steels. As a rule. Similarly to case-hardened steels. 7. This allows higher cutting speed to be used. as do other machinability criteria. In the case of unalloyed heat-treatable steels with a carbon content up to about 0. single hardening. regularly formed ferritic/perlitic microstructures have a positive effect on machinability. 2 and 0. Heat-treatable steels are heat treated in order to regulate their mechanical properties with respect to their intended purpose. sulphur additives at a ratio of approximately 0. Among the heat-treatable steels frequently used in practice for machining are C45E. The nitrogen diffusing into the surface layer during the nitriding process forms with the alloying elements Cr. below the α-γ-transformation temperature of the material [MDH80]. cogwheels). For this reason.268 7 Tool Life Behaviour reduced correspondingly. 7. is performed on materials in a normalized state whose machinability is characterized. especially in automobile and aircraft construction (connecting rods. the machinability of steel C60E is improved through soft-annealing. They can only be adjusted for good machinability in few cases. Rough-machining. rotor and crank shafts. for which high levels of hardness are achieved by means of a γ-α-phase transformation and the production of the metastable phase martensite. At higher values.e. For example. 42CrMo4. springs. axles. as well as with aluminium or vanadium (nitride formers).06-0. They are heattreatable and are alloyed with Cr and Mo for improved hardenability. Mo and Al special nitrides. As opposed to case-hardened steel. that of steel C22E through coarse-grain annealing (or cold-forming). Moreover. In some cases. For turning. for which the most important factor is a high chip-removal rate. These materials are utilized for components of medium and high strain. Tools made of high speed steel are used in many cases for boring and thread die cutting.1% effect a clear improvement of the steel’s machinability. Nitriding is carried out at temperatures between 500 and 600◦ C. 30CrMoV9 and 36CrNiMo4. These mostly precipitate in submicroscopic form and cause high latticework tensions. nitrided steel has a very hard surface traced back to the brittle metal nitrides. axle-pivots. In view of abrasion and thermal wear. heat treatment procedures for achieving good machinability vary. Most components made of heat-treatable steel are machined in a heat-treated state. cutting ceramics or CBN as cutting materials. by relatively low wear. the selection of a suitable coating is recommendable here. Material hardness values > 45 HRC require highly wear-resistant cemented carbides. high surface hardness.45%.e.4. The most appropriate cutting edge materials for machining heat-treatable steels are coated cemented carbides for rough-machining and. for planing. i. i.4 Nitrided Steels The carbon content of nitrided steels lies between 0. this improvement is reduced and the steel’s strength is lowered. milling and drilling. high cutting speeds induce a strongly increase in tool wear. The associated strength values exceed those of the annealed state. because of their ferritic/perlitic microstructure. hardening and tempering takes place between rough-machining and planing or precision machining. cermets. . With an even higher carbon content. especially at high cutting speeds. fine. are difficult to machine. When machining nitrided steels in a heat-treated state. regularly distributed carbids. exhibits unfavourable machinability properties. guide strips. The heat-treated structure usually found when machining and the carbides distributed in the structure lead to high mechanical and thermal stresses on the tools. Non-alloy tool steels in a forged or rolled state with a carbon content up to 0. while those with higher carbon contents have lamellar perlite and a cementite network. The martensite gradually transforms into intermediate structures as well as . • hot-working steels and • high-speed steels. which is favourable for subsequent nitriding. etc. The addition of sulphur (34CrAlS5) has a positive effect on machinability. tempered martensite). burr formation may cause a degradation of the surface quality and an impairment of the quality of the component. Nitrided steels containing aluminium are fundamentally more difficult to machine than aluminium-free ones. and usually in a heattreated state. softannealed steels should have more or less regularly distributed cementite grains in a ferritic matrix. If the nitrided steels are machined in a soft state. 1% Ni. such as 34CrAlNi7 with approx.4 Machinability of Various Steel Materials 269 The machining of these steels is done prior to nitriding. the resultant force is relatively high.4. Greater precipitations of ferrite in nitrided steel lead to an embrittlement of the rim zones and to an irregular transition in the core zone. problems related to chip breakage will arise during machining. This structural state (i. such as 31CrMo12.7.9% contain lamellar perlite and ferrite. Nitrated steels with increased nickel content. 7.).5 Tool Steels One generally distinguishes between non-alloy and alloyed tool steels. Coarse-grain annealing for achieving good machinability is not to be recommended with respect to the steel’s later use. Due to the high strength values of heat-treated steels. the cementite network cannot be removed by means of conventional soft annealing. one can expect predominately acceptable chip forms. Tool steels are required for different stresses. In a non-heat-treated state. On this basis. the structure consists primarily of martensite in the rim layers. Nitrided steels are used in a similar range of applications as case-hardened steels (cogwheels. which exhibits a lower adhesion tendency. however. since the ferrite would become even more coarse-grain and the strength would further decrease. Irrespective of carbon content. A short tool life is to be expected.e. In a hardened state. These categories are also useful for describing the machinability of tool steels. the following categories are used: • cold-working steels. When machining tool steels in a normalized or soft-annealed state. This can be partially remedied by means of a quenching and tempering to a higher strength. as with unalloyed tool steels. increases to an equal extent. Cutting materials most often used for machining tool steels are cemented carbides containing titanium carbides and tantalum carbides with medium toughness (e. The amount of carbide formers is bears little importance for the machinability of alloyed tool steels. The resultant force when machining tool steels is determined to a great extent by the special alloy composition and the type of heat treatment used. Carbon also influences hardenability and contributes decisively to tempering consistency and toughness via carbide reactions during hardening and tempering. The adhesion tendency can be reduced by quenching and tempering to greater strengths (1200–1400 N/mm2 ). The carbon content of the steel determines the ratio of carbides. Subeutectoid. the dissolution of carbide formers and the increase in strength associated with this leads to an increase in the resultant force. This is due.5 and 1. toughness and wear resistance. cementite grains are embedded in the matrix as well. a relatively inferior machinability is to be expected because of the increased adhesion tendency and the growth of built-up edges. Selecting alloying additives for tool steels is based first and foremost on their influence on surface hardness. then remnants of the brittle cementite network take the place of the cementite grains. with finely distributed granular carbides. the adhesion tendency of these steels. This increases abrasive wear and the thermal stress on the cutting edge. An increasing carbide moulding degrades the chip breakage. are poorly machinable in an annealed state. from application group P20) as well as cermets. tool steels can also be machined in a heat-treated state (Rm < 2000 N/mm2 ) using cutting edge materials made of CBN. relatively low and increase with the level of carbide moulding. however. which significantly promote abrasive wear.g. . Because of the high deformability of ferrite. if the steel was soft-annealed prior to hardening. the increased adhesion tendency and associated growth of built-up edges have a negative effect on surface quality. tempering consistency. However. Carbide formers only increase the wear effect on the steel in an obvious way when they have dissolved during austenitizing and have not formed any carbides during subsequent annealing. In both cases. especially for higher-alloyed steels.270 7 Tool Life Behaviour fine-lamellar perlite in the direction of the workpiece interior. long chips with bad breakage form when machining tool steels in a soft-annealed state. Non-alloy tool steels with a carbon content between 0. to the marked formation of gluing-points and built-up edges. Following machining in an annealed state. whereby a suitable coordination with the carbon content is necessary. Should this treatment be left out. non-alloy tool steels can also be machined in a normalized state or in the condition of delivery after hot working. especially high-alloyed high speed steels. When machining alloyed tool steels. Disruptions may form at the outlet points of the tool.5% are machined in a soft-annealed state. If machining is executed in a heat-treated state. chip breakage is not to be considered a problem. In the case of supereutectoid steels. as a rule. The cutting speeds which are applicable in machining tool steels are. The alloyed tool steels. hardness penetration depth. which clearly exceed both that of the very heat-resistant. In comparison to PCBN cutting edge materials. Hard turning procedures for machining interiors and contours are still very important. broaching. Small bevels and radii lead to disruptions and wavering tool life. though relatively brittle PCBN and. they exhibit very good working behaviour in these applications [Koch96. The spectrum of chemical and physical properties of commercially available PCBN cutting materials is relatively large. Since then. a very high accuracy of position of the functional surfaces can be realized. resistance to thermal shock and thermo-chemical inertia with respect to the material to be machined. such as turning with interrupted cutting.4. while larger bevels and radii cause larger forces and may induce oscillations [Kloc05c]. Mixed ceramics represent a further application group which can be used for hard machining with continuous cutting. such as crystalline cubic boron nitride (PCBN) and – with some limitations – mixed ceramics. grinding used to be the only way to machine hardened steels. The use of round indexable inserts or the selection of the greatest possible corner radius is recommended for pre-machining with smooth cutting [Abel93]. Joch01]. For turning hardened steel. the surface quality can be improved by modifying the cutting edge geometry. properties that vary especially are heat conductivity. almost the only options are ultrahard non-metallic cutting edge materials. gear skiving and -shaping.6 Hardened Steels For a long time. even more so.7. In addition to clear differences in hardness and resistance to bending. Because of their low hot hardness. it is now used increasingly for large-batch and mass production. that of cutting edges made from mixed ceramics. hard milling. By selecting a large tool orthogonal clearance. one can principally increase the cutting time without changing the amount of free flank wear and simultaneously increase the displacement of the cutting edge. Because . the machining of hardened materials exceeding a hardness of 50 HRC has come to be known as hard machining. When machining hardened steels with PCBN.4 Machinability of Various Steel Materials 271 7. When machining in one clamping. Because of their toughness. however. While hard turning had been implemented almost exclusively in single and smallbatch production up to a few years ago. finest-grain cemented carbides and ultrafine-grain cemented carbides are only suited to hard machining with low cutting times. an optimum radius of approximately 20 μm is recommended. This variance can be attributed to the cBN grain size and to the type and concentration of the binder phase [Kloc05b. hard turning is advantageous if a machining multiple sides can be achieved in a single clamping. This is because of the extremely high requirements on hot hardness and the cutting tool material’s consistency against diffusion resulting from the high temperatures and from cutting pressures in the machining zone. As a rule. the use of mixed ceramics as cutting edge materials places less stress on the cutting edge with the result of clearly smaller chip thicknesses of a maximum 100 μm. For the cutting edge fillet. Wina96]. The development of ultrahard cutting edge materials in the 1980’s led to the machining of these materials with geometrically defined cutting edges. unetched layer (Fig. the maximum amount of compressive residual stresses are shifted to the greater depths of the workpiece rim zone with an increase in flank wear. The stress state in the workpiece caused by this pressure induces a residual austenite transformation and cold-forming in the rim layer of the workpiece [Gold91].17. exceed the local melting temperature of the material to be machined have been recorded [Berk92]. Töns93. right). 7. Phenomena which occur at high temperatures because of transformations and high cooling gradients cause tensile residual stresses which are superposed on mechanically induced compressive residual stresses [Jonk87]. the passive force takes on very high values in this case.272 7 Tool Life Behaviour of the low toughness. The magnitude of the flank face wear on the minor cutting edge has a significant influence on the distribution of stress in the workpiece rim zone. and these cause. via rapid cooling. to high friction forces. The mechanical stress is the result of flank load similar to HERTZian pressure. In consideration of the fact that tool costs are lower by a factor of 10–20. anything but smooth cuts cannot be recommended [Köni89a. mixed ceramics for smooth-cutting represent a widespread alternative to cutting edge materials with a small ratio of CBN. Momp87]. For this reason. the formation of martensite when the α-γ-transformation temperature is exceeded. The normal compression stresses in the region of the wear mark on the side of the minor flank lead to a high mechanical and thermal stress on the workpiece rim zone. High resultant forces and high rake face temperatures arise during hard machining with rough cutting. The martensite is recognizable in the microsection as a “white”. in combination with the relative movement between the tool and the workpiece. A darker zone with a tempered structure can be seen below the white rim layer. Specific cutting forces kc between 4000 and 4700 N/mm2 and rake face temperatures which may even. High contact temperatures are the result. Wobk93]. compressive stresses are induced which increase with increasing flank wear of the minor cutting edge.17 Formation of the rim zone during hard turning . The thermal stress is a result of the friction between the flank face and the workpiece. when hard-machining. These values may even be higher than those of the cutting force [Köni84b. Both high normal stresses and high shear stresses lead. Through this process. in exceptional cases. Metallographic studies 20 µm Fig. 7. As opposed to soft machining. 7.4 Machinability of Various Steel Materials h > 20 µm 273 h < 20 µm Fig. 7.18 Longitudinal section of a saw tooth chip (left) and a ribbon chip (right) of hard-machined workpiece surfaces confirm the influence of tool wear on the formation of a new hardening zone. How the distribution of stress and the new hardening zones ultimately affect the behaviour of the component has not been sufficiently explained yet. Of particular interest are the effects on the fatigue strength of rolling contact stressed functional surfaces. The formation of new hardening zones during hard machining cannot be inhibited, not even by using coolants. Since the use of coolants effects no improvement of the tool life behaviour of the cutting edge material, hard-machining is generally performed in a dry state. When machining with interrupted cutting, the thermo-shock sensitivity of superhard cutting tool materials even forbids the use of coolants [Acke89, Töns91]. Highly heat-treated steels which receive their hardness mainly from their martensitic crystalline structure are practically non-deformable in room temperature and environmental pressure. This deformability behaviour results in a special chip formation, one different from the classic chip formation of unhardened steels. When machining hardened steels, two different chip forms may develop depending on the chip thickness. With high chip thicknesses (h > 20 μm), a “saw-tooth” chip develops [Acke89, Berk92, Naru79] (Fig. 7.18), while smaller chip thicknesses (h < 20 μm) tend to cause the formation of a ribbon chip [Köni93b]. The development of “saw-tooth” chips initiates material separation via a crack formation on the surface which transmits itself into the workpiece under an angle of the greatest shear stresses [Acke89]. The released chip segment slides off between the rake face and the newly developed separating surface. If the chip segment has slid so far that the increasing compressive stresses in turn cause the critical shear stress to be exceeded, the next crack forms. One theory ascribes the connection of chip segments to the fact that the crack is caught up in front of the cutting edge and a plastic deformation takes place due to the compressive stresses and high temperatures. Chip segments on a plasticized material thus slide off [Berk92]. In the case of high chip thicknesses, the transition from a ribbon chip to a saw-tooth chip is a function of the hardness of the material. With the predominately martensitically hardened, unalloyed tool steel C105W2 (1.1645), for example, this transition takes place at approximately 50 HRC [Naru79]. At low chip thicknesses (h < 20 μm), the chip thickness is in the region of the cutting edge radius rn with a tool orthogonal 274 7 Tool Life Behaviour rake angle which acts in a correspondingly strongly negative way. A triaxial stress state with a high ratio of hydrostatic compressive stress is thereby caused in the area of the material directly in front of the cutting edge with no material separation ensuing. The shear stress hypothesis put forward by MOHR offers an explanation of these phenomena. VON KÁRMÁN already indicates the possibility of a plastic deformation of brittle materials under pressure on all sides [Karm11]. If such a state of compressive stress exists, the shear yield stress becomes a decisive criterion. If, however, the strain on the material is characterized by a monoaxial compressive stress state as it occurs on the workpiece surface, the failure of the material is caused by a cleavage fracture. Precision hard turning (vc ≈ 100–200 m/min, f ≈ 0.05–0.15 mm, ap ≈ 0.1–0.5 mm) is used for manufacturing ready-to-use components. Surface roughness values in the region of Rz = 2.5–4 μm can be achieved in series manufacturing in a procedurally reliable way. A further development of precision hard turning tending toward higher component quality is high-precision hard turning (vc ≈ 150–220 m/min, f ≈ 0.01–0.1 mm, ap ≈ 0.02–0.3 mm), which has the potential of achieving surface roughness values which lie clearly below those of conventional precision hard turning processes. The surface roughness Rz achievable when using special machine tools lie below 1 μm. However, one must also expect that increasing tool wear will bring about a degradation of surface quality. The procedure shows potential for creating surfaces with Rz ≤ 3 μm with long tool life. 7.4.7 Non-rusting Steels Non-rusting steels contain at least 10.5% chromium and a maximum of 1.2% carbon. Their main alloying elements are chrome and nickel. Amounts of chromium exceeding 12% render the steel material corrosion-resistant. Nickel expands the γ-region and, in the case of steels with a high chromium content, leads to the stabilization of the austenitic structure, which is generally unstable in the case of low-alloyed carbon steels and which disintegrates into ferrite and cementite below the Ac1-temperature. Nickel starkly decreases the heat conductivity of the steel material. “Chrome steel” is the usual term for ferritic steel types and “Cr-Ni steel” for austenitic steel types. According to their essential usage properties, DIN EN 10088-1 categorizes non-rusting steels into: • corrosion-resistant steels, • heat-resistant steels and • high-temperature steels. Non-rusting steels are distinguished by good persistence against chemically aggressive substances. In general, they have a chromium content of at least 12%. Protection is achieved with a chromium content of more than 10.5% by means of the spontaneous formation of a protective chromium oxide layer [DINEN10088]. With respect to their metallographic constituents, the corrosion-resistant steels are subdivided into ferritic, martensitic, austenitic and ferritic-austenitic steels. Figure 7.19 Machinability of Various Steel Materials 275 40 60 7.4 Nic on ten t/ 80 % 70 30 Iro nc nte l co 20 ke Austenite nt /% Martensite + 90 1 10 Austenite Austenite + Martensite Ferrite Ferrite 0 10 0 10 1 : X5CrNi18–10; 2 0 3 20 Chromium content / % 2 : X6Cr17; 30 40 3 : X39Cr13 Fig. 7.19 Phase diagram of Cr-Ni-steels with metallographic constituent data after quenching, acc. to IGNATOWITZ [Igna97] shows the phase diagram of Cr-Ni steels. One can see from this diagram which structure is to be expected, depending on chromium and nickel content, with a nonrusting chromium-nickel steel after quenching. Point 1 corresponds to the austenitic steel X5CrNi18-10 with 18% chromium, 10% nickel and 72% iron, point 2 corresponds to the ferritic chrome steel X6Cr17 and point 3 to the martensitic steel material X39Cr13 [Igna97]. The ferritic steels are primarily chrome steels with a chromium content of 12.5–18% and a C-content below 0.1% (e.g. X6Cr13, material no.: 1.4000, X6Cr17, material no.: 1.4016). They are magnetic and not hardenable. Martensitic steels are primarily chrome steels with a chromium content of 12–18% and a C-content of 0.1–1.2% (e.g. X12Cr13, material no.: 1.4006; X39Cr13, material no.: 1.4031). Depending on the quality, these steels also contain additional Ni and Mo. They are magnetic and can be heat-treated and tempered by means of a corresponding heat-treatment. With increasing C-content, the hardness of the steel increases when in a hardened state. A hardness of 40 HRC is achievable with a C-content of 0.1% and a hardness of 59 HRC with a C-content of 0.9%. The steels the most used by far are austenitic steel materials. They contain approximately 17–26% Cr, 7–26% Ni, less than 0.12% C and, in some cases, small 276 7 Tool Life Behaviour amounts of Si, Mo, V, Nb, Ti, Al or Co (e.g. X5CrNi18-10, material no.: 1.4301; X6CrNiMoTi17-12-2, material no.: 1.4571). They are not magnetic and cannot be hardened by means of heat treatment. In an annealed state, they are characterized by very good toughness properties which are retained even at extremely low temperatures. Especially given a high carbon content, they tend when cold-formed towards considerable strain hardening. Through cold-forming, therefore, their strength values can be increased dramatically. When strain-induced martensite forms, however, the strain is reduced to a very considerable degree. Among the products made from the abovementioned corrosion-resistant steels are household devices, razor blades, knives, surgical instruments, parts for automobile construction, agricultural and materials handling technology, mechanical and plant engineering and devices and instruments for the food industry, the chemical industry, the textile industry and for shipbuilding [Stah01]. A further group of corrosion-resistant steels are the ferritic-austenitic steel materials, also either referred to as duplex steels (e.g. X2CrNiMoN22-5-3, material no.: 1.4462) or super duplex steels (e.g. X2CrNiMoCuWN25-7-4, material no.: 1.4501). The name “duplex” refers to their two-phase crystalline structure consisting of ferrite and austenite. These steels have optimal properties at a balanced ferrite/austenite ratio of approximately 50/50%. In comparison to normal austenitic steels, the duplex steels contain less nickel (about 4–8%), though usually a significantly higher amount of chromium (about 18–25%). In order to increase their resistance to intercrystalline corrosion, a certain amount of nickel is exchanged for additional nitrogen as an austenite former and molybdenum. The optimal microstructure is created by means of a heat-treatment at 1000–1100◦ C. These steels find the most use in the gas and oil industry, in the petrochemical industry, in chemical tankers and in sewage treatment plant construction. In comparison to austenitic rustproof steels, the duplex materials have a strain limit which is almost double as high (Rp0,2 about 400–550 MPa) given similar or markedly higher strength values. Heat-resistant steels are mainly ferritic and austenitic steels with a high resistance against oxidation as well as against the influence of hot gases and combustion products above 550◦ C. As a rule, the heat-resistant ferritic steels contain at least 12% chromium as well as aluminium and silicon (e.g. X10CrAlSi13, material no.: 1.4724; X10CrAlSi25, material no.: 1.4762). Heat-resistant austenitic steels are additionally alloyed with at least 9% nickel (e.g. X8CrNiTi18-10, material no.: 1.4878; X15CrNiSi25-21, material no.: 1.4841). These steel types are used, for example, in ovens and apparatus construction for annealing bells, baskets and tubes [Stah01]. The high-temperature steels are mainly martensitic and austenitic steel types with a high long-time rupture strength with a long-term mechanical strain above 500◦ C. Among the products made from high-temperature martensitic steels (e.g. X20CrMoV11-1, material no.: 1.4922; X20CrMoWV12-1, material no.: 1.4935) are components for thermal power plants, steam boilers and turbines, as well as for the chemical industry and nuclear technology. High-temperature austenitic steels (e.g. X6CrNi18-10, material no.: 1.4948; X5NiCrTi26-15, material no. 1.4980) are used in the construction of pressure vessels and armatures, pressure tanks and steam 7.4 Machinability of Various Steel Materials 277 boilers, but also in the manufacture of blades, discs, axles and bolts for steam and gas turbines as well as in shipbuilding [Stah01]. Non-rusting steels with a ferritic structure lend themselves to machining relatively well. The onset of wear through abrasion and adhesion is comparatively low. The machinability of martensitic steels is a function of hardness, which is in turn a result of the heat treatment used. Depending on the heat treatment, the structure consists either of martensite (hardened) or tempered martensite with chromium carbides and ferrite (quenched and tempered). Duplex steels are considered to be extremely hard to machine. Characteristic reasons for this are a marked adhesion tendency, high strain hardening of the workpiece rim zone and an unfavourable chip formation. Because of the eminent importance of corrosion-resistant austenitic steel materials, the following will go into more detail on their machinability properties. Austenitic steel materials are machined in either a quenched or a solution-annealed state. In comparison to ferritic-perlitic or quenched-and-tempered steels, they are significantly harder to machine. This is due to their high deformability and toughness, their tendency towards strain hardening and towards adhesion with the cutting material, as well as their lower heat conductivity, which is about 1/3 lower than with non-alloy steels. The latter impairs heat removal through the chip and increases the thermal stress of the cutting edge. In spite of the comparatively low tensile strength of austenitic Cr-Ni steels, the results of these specific material properties are a high thermal stress of the tool cutting edge, pronounced flank face and/or rake face wear, material gluing, notch wear, pitting of the cutting tool material, cutting edge breakages and unfavourable chip forms, which allow for machining tasks to be performed only at relatively low cutting speeds, for short tool lives and with insufficient surface qualities. In the case of coated tools, one can observe in many cases that the adhesion between the material and the hard material coating is stronger than their adhesion on the substrate and that this leads to the delamination of the coating. Austenitic steel materials are machined preferably with uncoated or coated WC-Co cemented carbides of the main application group M. Because of the high thermal stress of the cutting edge, cutting speeds are used which, in comparison to those used in turning ferritic-perlitic steels, are lower by a factor of 2–5. Applicable cutting speeds range from vc = 50 m/min (X5NiCrTi26-15) to vc = 160 m/min (X6CrNiMoTi17-12-2), depending on the alloy. The tool life is between 5 and 15 min. A type of wear which limits tool life when turning austenitic steel materials with uncoated cemented carbides is the formation of a pronounced crater on the rake face (Fig. 3.41, left). Due to this strong crater wear, uncoated cemented carbides can only be used at relatively low cutting speeds (vc < 100 m/min) [Gers04]. The use of high cutting speed is only made possible when turning austenitic steels through the application of coated tools (Fig. 3.41, right). Significantly improved performance in turning austenitic steels requires cutting tool materials with properties customized with respect to the respective machining task (Fig. 7.20). The more the cutting tool material system reaches the physical limits of the substrate and the hard-material coating in the chipping process, the 278 7 Tool Life Behaviour 40 30 A 20 Tool life Tc / min B 10 9 8 7 6 R R 5 4 * * 3 X5CrNi18-10 2 100 150 X6CrNiMoTi17-12-2 200 250 300 100 Cutting speed vc / (m/min) 150 200 250 300 Cutting tool materials: A: WC-7.5Co-6.8MK-0.75Cr3C2 / TiN-TiCN-Al2O3-HfCN B: WC-7.5Co-6.8MK-0.75Cr3C2 / TiAlN R: Reference cutting tool material Tool life criterion: flank wear VB = 0.3 mm, *: plastic deformation of cutting edge Process: external cylindrical turning Cutting parameters: feed: f = 0.32 mm, depth of cut: ap = 2 mm Cutting fluid: emulsion 6% Fig. 7.20 Examples for the performance of carbides with tailored substrates and coating systems compared to a common cutting tool material during roughing of austenitic steel, acc. to GERSCHWILER [Gers04] more carefully the substrate, the hard-material coating and the tool geometry must be customized to the individual machining task [Gers04]. In addition to a high resistance to wear, machining tools used to turn austenitic steel materials are also required to exhibit “performance” in terms of chip formation and chip breakage. Because of their high deformability, the austenitic steels tend more than any others towards the formation of long ribbon and snarled chips. The latter represent a high hazard potential not only for the component and the machine, but also for the operating personnel. It is thus essential, and not only in automated manufacturing plants, that this type of chip formation is avoided through all available means to secure a higher level of machining safety. 7.4 Machinability of Various Steel Materials 279 A safe chip breakage is guaranteed in two ways. Firstly, on the material side, through alloying measures. In many ways, however, there are narrow limits on these measures placed by the usage properties of the material and the component. The more promising way in most cases is thus the optimization of the cutting part geometry. In case of varying cutting depths and/or small feeds, however, this places an extremely high demand on the geometric design of the cutting edge. Especially with small chip cross-sectional areas, the tools must possess chip shaping components which lie close enough to the cutting edge so that they come into any contact at all with the fine chips [Köni90a]. The high ductility of austenitic materials requires tools with the sharpest possible cutting edges. Sharp tool cutting edges facilitate the separation of the material during chip formation and, by reducing the resultant forces, contribute in a highly essential way to lowering the plastic deformation of the workpiece rim zone, to the creation of high-quality workpiece surfaces and to the reduction of burr formation. On the other hand, however, one can also observe that, with increasing mechanical stress, very sharp cutting edges lead also contribute to an increased pitting of the cutting tool material, which in turn leads to accelerated wear. As experiments have shown, a defined cutting edge fillet can stabilize the cutting edge and significantly improve tool life. When finish turning with small chip cross-sectional areas, however, a cutting edge fillet no greater than 30 μm should be selected. When rough turning, on the other hand, a highly stable cutting edge can be designed with a 40–60 μm fillet in order to prevent edge breakages and pittings of the cutting tool material. Burr formations become a further serious problem when machining ductile austenitic steel materials. In the case of components which are to be burr-free, up to 20% of manufacturing costs may result from burr removal [Köni93]. In addition to the costs for burr removal, increasing demands on component quality and workplace attractiveness entail combating burr formation through suitable measures. As a rule, austenitic steels are machined by wet cutting. However, dry machining is thoroughly possible with corresponding process dimensioning. One must consider, however, that due to the lack of the cooling effect of the cutting fluid, both chip and tool exhibit higher temperatures than is the case with wet machining. The results of this are an increased in smeared material on the tool, clearly inferior chip formation, more aggressive notch wear on the major and minor cutting edges and, as a result, increased burr formation and inferior surface qualities. In comparison to wet machining, dry machining austenitic steels can lead to a drastic reduction of tool life, the latter being limited as a rule not by flank face wear, but rather by the degradation of the surface quality associated with notch formation on the minor cutting edge. Machining austenitic steels in an economical way thus requires special measures. In addition to a tool geometry customized to the machining task, suitable coating systems and cutting parameters, the most important of these is the use of a minimum quantity lubrication. This lubrication in particular can reduce rewelding of the material and significantly improve the surface quality. Mediums used for this are, as a rule, synthetic ester oils [Gers04] (also Chap. 6). 280 7 Tool Life Behaviour 7.5 Machinability of Cast Iron Cast iron refers to iron-carbon alloys which have a carbon content above 2.06%. In the iron-carbon two-material system, cast iron becomes a steel (up to 2.06% C). The material is usually shaped by means of casting and a final machining operation. Cast-iron alloy material groups are classified on the basis of the appearance of their fracture faces into white and grey cast iron (Fig. 7.21). Among the white cast iron materials are chilled cast iron and malleable cast iron, both of which solidify according to the metastable iron-iron carbide system and in which carbon is thus present in the form of cementite (Fe3 C). As opposed to this, the solidification of grey cast iron takes place according to the stable iron-graphite system, with the carbon thus being present as graphite. Depending on the form of the graphite, grey cast iron materials are divided into cast irons with lamellar graphite (GJL), cast irons with vermicular graphite (GJV) and cast irons with spheroidal graphite (GJS). Cast iron White cast iron White malleable cast iron (GJMW) Black malleable cast iron (GJMB) Grey cast iron Chilled cast iron (GJH) Lamellar graphite (GJL) Vermicular Spheroidal cast iron graphite (GJV) (GJS) Fig. 7.21 Cast iron classification In addition to the metallic matrix, the machining properties of iron-cast materials are also heavily influenced by the amount and formation of the embedded graphite. Graphite inclusions, in the first place, reduce friction between the tool and the workpiece and, on the other, disrupt the metallic matrix. This leads to improved machinability in comparison with graphite-free cast iron or steel materials. The results are short-breaking chips, small grinding forces and higher tool lives. The metallic matrix of cast iron is, to a high degree, crucial to its machinability. The matrix is influenced by the respective chemical composition (alloying elements) and heat treatment and consists, in the case of materials with low strength, predominately of ferrite. The material becomes stronger as the amount of perlite rises, which also leads to a more significant abrasive tool wear. Cast irons of high strength and hardness often have a bainitic, ledeburitic or martensitic structure and are therefore very hard to machine. 7.5.1 White Cast Iron As mentioned above, the solidification of white cast iron takes place according to the metastable Fe-Fe3 C system (Fig 7.22). If the cooling speed is high, a eutectic Machinability of Cast Iron 281 Steel A I 769 723 α-Mc 500 Melt δ-Mc + γ-Mc 1147 911 B Austenite (γ-Mc) G M P α-Mc + γ-Mc O S Ferrite + Perlite Q 0 D Melt + Austenite E Austenite + Secondary cementite Perlite + Secondary cementite 0.8 Austenite + Secondary cementite + Ledeburite Perlite + Secondary cementit + Ledeburite Melt + Primary cementite C F Ledeburite + Primary cementite Primary cementite + Ledeburite 4.3 2.06 Carbon concentration / mass-% Cementite (Fe3C) Temperature T / °C H δ-Mc White cast iron Melt+ δ-Mc Perlite 1600 1536 Ledeburite 7.5 K L 6.67 Fig. 7.22 Section of the iron-carbon phase diagram (metastable system, fast cooling) (1147◦ C, C-content = 4.3%) forms which is referred to as ledeburite (austenite + cementite). Depending on the carbon content, additional perlite and secondary cementite is found in hypoeutectic compositions and additional primary cementite in hypereutectic alloys. The high ratio of iron carbide in white cast iron leads to great hardness with a pronounced level of brittleness. If the white cast iron is not subjected to further heat treatment, it is referred to as chilled cast iron. If a further heat treatment is executed (tempering), malleable cast iron forms. 7.5.1.1 Malleable Cast Iron The usage properties of malleable cast iron are adjusted by means of a metastable solidification and subsequent heat treatment (tempering). On the basis of the fracture appearance, malleable cast irons are subdivided into white malleable cast irons (GJMW) and black malleable cast irons (GJMB). In order to produce white malleable cast iron, the white cast iron is annealed in an oxidizing atmosphere (approx. 1000◦ C, 60–120 h). The aim is to produce a partially or completely decarburized material by removing the carbon from the casting [DINEN1562]. The degree of decarburization of the material is strongly dependent on the duration of the tempering process and on the section thickness of the casting. A complete, uniform decarburization is only achievable with low section thicknesses; with thicker castings, a rim decarburization and a disintegration 282 7 Tool Life Behaviour of the cementite in the workpiece interior take place. The remaining graphite is then present as temper carbon. In the case of partial carburization, therefore, there is a purely ferritic rim zone structure and a perlitic-ferritic core structure. As opposed to white malleable cast iron, black malleable cast iron is heat-treated in an oxygen-free atmosphere. The cast iron is thus not decarburized. The cementite disintegrates entirely into temper carbon [DINEN1562]. A characteristic property of black malleable cast iron is that, because of the non-decarburizing annealing, there is a uniform structure across the workpiece cross section, irrespective of the section thickness. As a function of cooling speed, a ferritic matrix forms given slow cooling, with the temper carbon evenly distributed in the form of nodular formations. Given a more rapid cooling, a perlitic or even martensitic matrix forms. Perlitic-ferritic mixed matrices are also possible. In contrast to steels, malleable cast iron is excellent for machining purposes. In spite of the good plastic deformability of the different types of malleable cast iron, the manganese sulphides and temper carbons embedded in the steel-like ferrite or perlite matrix cause short-breaking chips which can be removed without difficulty [Wern83]. Moreover, the temper carbon reduces the friction between the workpiece and the tool and interrupts the base metallic material, which leads to low resultant forces and higher tool life. Given identical workpiece hardness, black malleable cast iron is more easily machined that white. This circumstance is attributable to white malleable cast iron’s purely ferritic rim zone structure (without temper carbon). Problems arising when machining ferrite are the growth of built-up edges (adhesion tendency of ferrite), the formation of ribbon and snarled chips (high deformability of ferrite) and the reduction of the surface quality with increased burr formation. Cutting tool materials usually employed for machining malleable cast iron are uncoated and coated cemented carbides, cermets, oxide ceramics and PCBN of the main application groups P and K. As a result of heat treatment, a remarkable consistency is achieved in the production of malleable cast irons with respect to their mechanical/technological material properties, which allows optimal cutting conditions for economic manufacturing. Furthermore, malleable cast irons are characterized by high strength and toughness properties as well as excellent casting processability. Workpieces can be achieve with accuracy of form with high surface quality, even thin-walled and intricate workpieces. Because of their high ductility, malleable cast irons are especially suitable for components which are subjected to dynamic stresses – oscillating or impulsive – and which must resist mechanical forces of great magnitude [Wern00]. White malleable cast iron is both weldable and hardenable. When selecting a hardening procedure, one must consider that white malleable cast iron exhibits a structure which is dependent on section thickness and a carbon content which increases from the outside to the inside. Thus only a thermochemical procedure can be applied for the low-carbon rim layer, e.g. case-hardening or nitriding [Schü06]. 7.5.1.2 Chilled Cast Iron White malleable cast iron without heat treatment is referred as a result of its high zementite-content as chilled cast iron (GJH). It is both very hard and brittle and slow cooling) 7 . Tool cutting edges are subjected during machining to high mechanical and thermal stresses. 7. comes with the disadvantage of an increased susceptibility to breaking.2 Grey Cast Iron In grey cast irons.23 Section of the iron-carbon phase diagram (stable system. The higher material removal rate.23). cemented carbides. Commonly used in chilled cast iron machining are a lead angle of 10–20◦ and a tool orthogonal rake angle of –5 to 5◦ . high demands must be made on the cutting tool material with respect to wear resistance and pressure resistance.7. however. chilled cast iron is very difficult to machine. 7. the carbon is ideally present in the structure exclusively in the form of graphite.25 Carbon concentration / Mass-% Fig. especially with high hardness values. small amounts Grey cast iron 1600 1536 D′ Temperature / °C Melt Melt + Graphite Melt + Austenite 1153 E′ C‘ F′ Austenite + Graphite (+ Fe3C) 911 769 738 K′ Ferrite + Graphite (+ Fe3C) L′ 500 0 0. Materials used for machining chilled cast iron are. because of the final cooling speed. In order to keep the mechanical stress of the tool cutting edge as low as possible when machining chilled cast iron and thus to improve the tool life parameters. however.g. Because the cementite is very hard. The melt solidifies according to the stable iron-graphite system (Fig. Therefore. 7.5.5 Machinability of Cast Iron 283 thus suitable for components which are to exhibit a high resistance to wear with low dynamic stresses. the cutting speed and the feed should be reduced with increasing material hardness. for rollers and grinding tools. and.06 4. In comparison to cemented carbides. the use of cutting ceramics allows for an increase of the cutting speed by a factor of 3–4. e. In reality.8 2. cutting ceramics (oxidic mixed ceramics) and PCBN cutting tool materials of the main application group H. almost exclusively. Töns91]. stress peaks form at the margins of these graphite lamellae. These forms vary from disc-like forms (lamellar) to fissured forms (wormlike.e. Types with a mixed matrix (with different ferrit-perlite percentages) fall somewhere between these limits [Herf07]. Also.2. One can say in sum that the machinability of grey cast irons is not determined by the hardness and strength of the alloy alone. 7. when stress caused by external forces is present.284 7 Tool Life Behaviour Fig. The graphite inclusions take on varying forms depending on the conditions of their origination. The type of metallic matrix can vary broadly according to the selection of chemical composition and heat treatment. These graphite lamellae interrupt the base metallic mass to a very great extent. Cast iron types with a ferrite matrix have the lowest strength and the highest plasticity and toughness.1 Cast Iron with Lamellar Graphite Cast iron with lamellar graphite (GJL) is the most frequently produced type of cast iron. 7.24 Cast iron with lamellar graphite (left) and spheroidal graphite (right) of Fe3 C are found in the structure in addition to graphite.5. vermicular) to globular particles (Fig. which are also controllable through metallurgical measures (Mgcontent). the form. the matrix is usually either ferritic-perlitic or purely perlitic. i. both the mechanical properties and the machinability properties of cast iron materials containing graphite depend in a very clear way on the crystalline structure of the matrix and the form of the graphite inclusions. The following will treat the different types of grey cast iron. As mentioned above. The lamellae then act as internal . The graphite is present in the form of thin. In practice. the types with a perlitic matrix have the highest strength and the lowest plasticity and toughness. In order to maximize the amount of graphite. so-called graphitizing elements such as silicon can be added to the melt. quantity and distribution of the graphite inclusions in combination with the given type of matrix play an important role [Stäh65. 7. Only after an especially long annealing process does a purely ferritic matrix form.24) [Hors85]. unevenly formed discs referred to as “graphite lamellae”. because of the material brittleness. 7. The rim zone structure of cast iron workpiece generally exhibits lower machinability than the core zone.7. Cast iron with lamellar graphite only having a small amount of perlite (about 10%) after annealing treatment. . When machined. They significantly reduce the applicable cutting speeds (Fig. high stiffness. e. as well as to high temperature oxidations. for which reason the tensile strength of this type of cast iron is relatively small (100–350 N/mm2 ). due to the resultant increase in the machining temperature. at a cutting speed three times faster than with a cast iron with a large amount of perlite (about 90%). usually spiral chip segments or discontinuous chips. A major advantage when machining GJL is derived from the formation of wear-reducing manganese sulphide layers on the flank and rake faces of the cutting insert. this is often compensated by means of a reduction of the cutting parameters. The surface quality of machined workpieces made of GJL is a function of the finishing method. On the other hand. gear and machine housings. Cast irons with lamellar graphite are especially suitable for machine beds and stands and bearing. with the graphite consequently forming a lubrication layer on the tool. The hardness of the material is a reference value in the first approximation for the applicable cutting speed. The mechanical strain on the tool cutting edge when machining GJL tends to be low. This effect ensures long tool lives when machining GJL.g. the graphite lamellae act favourable on the chip form. This can be attributed. the manganese sulphides form a layer on the insert which becomes thicker and thicker. increase tool wear and reduce tool life in the same way cementite does in perlite. since they stop any incipient shear early on and induce cracking leading to the formation of segmented or discontinuous chips. for example. due to the embrittlement.25). leading to low resultant forces. This leads to lower wear and higher tool life. fineness and evenness of the cast structure [Opit70]. With increasing cutting speeds. Short-breaking chips develop.e. Other hard metallographic constituents. There is generally no burr formation at the workpiece edges during machining. the material being designated as brittle [Herf07a]. This results in stronger abrasive wear and the formation of a wear notch on the tool cutting edge. instead. This layer reduces the friction coefficient on the one hand and acts as a diffusion barrier and a protection from wear on the other. to the altered graphite structure and microstructure directly beneath the outermost cast layer. there are edge disruptions. favourable damping properties and. the cutting conditions and the composition. GJL is distinguished by excellent self-lubricating properties. These result from the circumstance that the graphite lamellae are cut during the machining process. i. The formation of wear-reducing coatings only sets in with higher cutting speeds (from approximately 200 m/min onwards). lamellar graphite gives the material a high heat conductivity. In addition. with the same tool life. on the one hand. on the other. the phosphide eutectic steadite. In practice. to non-metallic inclusions and. can be machined.5 Machinability of Cast Iron 285 notches (predetermined breaking points). 7. but rather in coral-like form. cutting ceramics and especially PCBN cutting tool materials of the main application group K.5.2. The use of high speed steels is generally limited to tools with very fine cutting edges. boring and milling grey casts. Types K01-K05 and especially cermets are especially suited to precision and ultra-precision machining.25 Influence of the crystalline structure and hardness on the cutting speed during turning of cast iron with lamellar graphite A rich array of cutting tool materials lends itself to machining cast iron with lamellar graphite. The material removal rate can be increased considerably by coated cemented carbide. the graphite doesn’t form in lamellar or globular form. Coated cemented carbide indexable inserts are used for turning.25 mm² 300 Cutting section geometry γo αo λ s χ r ε r rε 6° 6° 0° 90° 84° 1. Silicon nitride ceramics are suitable for rough machining with large interrupted cuts or extreme irregularities in the contour of the workpiece as well as for machining with cutting fluids.286 7 Tool Life Behaviour Tool life criterion: VB = 0.75 mm Cutting tool material: HW-K20 Cross-section of undeformed chip: ap·f = 2. The classic types of cemented carbide for machining cast irons with lamellar graphite are those in the main application group K. worm-shaped in microsection .2 mm vc = 20 m/min vc = 60 m/min Ferrite Cuttingspeed vc/(m/min) 250 200 Ferrite + Perlite Perlite (coarse grained) 150 Perlite (fine grained) 100 50 Quenched structure 0 0 125 150 175 200 225 250 Brinell hardness HB 30/5 275 300 Fig. reaming and thread die cutting.5 · 0. 7. The longest tool lives and highest cutting speeds can be achieved in the process by using cemented carbide with ceramic multi-layer coatings. Typical procedures used are boring.2 Cast Iron with Vermicular Graphite In cast irons with vermicular graphite (GJV). there is a growing general interest in using GJV in other applications than those in automobile construction [Kloc01. no sulphur and no manganese sulphide is present and thus no wearreducing layer. The industrial implementation of GJV in the automobile industry is slow because of the uneconomical and difficult machinability of this material in comparison with cast iron with lamellar graphite. GJV offers a good alternative to traditional construction materials such as aluminium and cast iron with lamellar graphite. higher endurance strength. higher stiffness. the titanium content is less than 0. no wear-protective coating develops when machining GJL. This is because of the magnesium treatment necessary for the formation of graphite. such as engine blocks. In comparison to lamellar cast iron. tool life travel path differences when machining different cast iron materials are essentially determined through differences in the mechanical characteristic values (hardness. Because of these properties. exhaust manifolds. When machining cast iron with vermicular and spheroidal graphite. cylinder liners. With lower cutting speeds (vc < 200 m/min).g. Studies have shown that doubling the titanium content reduces the tool life travel path by about half. The inhomogeneity of . In comparison to traditional motors made with GJL. a lower tendency to warp. higher toughness. Vermicular: worm-shaped). When machining GJV. Particularly when faced with the conflicting interests of high component strength vs. the use of GJV allows for a weight reduction of up to 20%.5 Machinability of Cast Iron 287 (lat. In comparison to cast iron with spheroidal graphite. An essential impact on tool life is made by alloying elements. crankcases. a lower thermal expansion and a better deformation resistance with temperature change. titanium. tensile strength) as well as in the graphite morphology. higher hardness. low component weight. e. with the result of short-breaking chip forms. These highly resistant types of cast iron are particularly suited to the demands of modern direct injection diesel engines. The alloying element titanium forms titanium carbides which are harder than WC-Co cemented carbides and thus cause an increase in the abrasive wear of the tool. With respect to its mechanical and thermal properties and its machinability. cast irons with vermicular graphite can be cast more with thinner walls than GJL and GJS. The rounded edges of the graphite inclusions are reduced by internal stress peaks. which allows for lighter components. These combinations of properties render GJV suitable as a construction material for combustion engines. Röhr06].015% [Kopp04]. at which. because of the low temperatures. chip formation is discontinuous within a broad range of cutting speed. wear-reducing manganese sulphide coatings form on the flank and rake faces of the cutting insert. Leng06.7. In addition to vehicle-related areas of application. cylinder heads. higher oxidation-resistance and higher thermal shock resistance. vermicular cast iron lies between cast iron with lamellar (GJL) and with globular (GJS) graphite inclusions. In GJV types used most frequently at present. a better damping ability. when machining GJL. The main difference between the machinability of GJL on the one hand and that of GJV and GJS on the other is that. GJV has better pouring properties. GJV has higher strength (up to 70% higher). a better machinability. as they appear on the tapered ends of graphite lamellae in GJL. brake and clutch discs. 7. must be kept under a certain limit value in order to prevent a sudden change from mechanical to thermal wear. Generally. PCBN cutting tool materials allow for significantly cutting speeds and a correspondingly higher productivity compared to cemented carbides. aluminium oxide ceramics (CA) show great potential for machining GJV. Initial studies have shown that PKD appears to be an interesting cutting tool material. coated cemented carbides (HC) and. Coated cemented carbide is suitable for machining at conventional cutting speeds. Since PKD cutting tool materials have a higher fracture resistance and strength than ceramics. because of their high hardness and wear-resistance. one can see that each of the two cutting tool materials is suited to a respective cutting speed range. At higher temperatures. with spheroidal cast iron reaching a high ductility [Herf07b]. perlite and precipitated graphite encourage this form of chip development [Röhr06]. Among the cutting tool materials for machining cast iron with vermicular graphite. This increases the strength. In comparison to other cast irons with graphite inclusions. one with which the tool lives achieved when machining GJV are comparable to those for GJL. with the cutting temperatures generally much lower than with continuous cutting. When comparing the tool life travel paths of HC and CA at different cutting speeds. which in turn allows for the application of higher cutting speeds [Pret06]. It is recommendable to use compressed air or minimum quantity lubrication as a coolant/lubricant. This can reduce the resultant force and contact zone temperature. tougher WC-Co-based cutting tool materials can better compensate for the high stress reversals in lower cutting speed ranges than ceramic cutting tool materials. PKD has the capacity to machine workpieces at twice the speed and achieve a 10. while aluminium oxide ceramics is the preferable cutting tool material for highspeed machining. the base mass is less often interrupted by the graphite spheres and the internal notching is almost nonexistent.2. the hot hardness and the chemical resistance of the cutting tool material become decisive factors. In comparison to cemented carbides. Even when using PCBN as cutting tool material. Because of the dynamic excitation caused by the discontinuous chip formation. the optimal cutting speeds for machining GJV lie at an approximate value of 300 m/min. however. however. significantly reduced tool live travel paths can be expected compared to GJL [Leng06. PKD tools have a great potential for milling applications with GJV. giving CA tools good usage behaviour at high cutting speeds and temperatures at which HC tools become weak and thus fall victim to increased chemical and abrasive wear [Röhr06]. because of the absence of an MnS protective layer when machining GJV. If high cutting speeds are used (500–1000 m/min).5. however. clearly lower than those used for GJL (up to 1500 m/min). Reut02]. The machining temperatures.to 20-fold increase in tool life in the process. .288 7 Tool Life Behaviour the material resulting from the different mechanical properties of the phases ferrite. also referred to as spheroidal cast iron. graphite is present in the form of spheroidal inclusions. it is recommendable to apply neutral to positive geometries to PKD tools.3 Cast Iron with Spheroidal Graphite In cast iron with spheroidal graphite. in particular. In comparison to steels of the same hardness and similar strength.g. A special heat treatment in the bainite stage can produce an austenitic/ferritic (also called ausferritic) matrix in cast iron with globular graphite. coated cemented carbides (HC) and oxide ceramics (CA) of the main application group K. which leads in turn . referred to as the growth of built-up edge fragments. especially when fine-machining with cemented carbides. are determined by the ratio of ferrite to perlite in the matrix. The term ADI (austempered ductile iron) is common for this. lubricated graphite inclusions. the strength of the cast iron rises along with the abrasive tool wear. High strength in combination with low toughness is typical for types with a predominately perlitic matrix (e. the material plasticises and is discharged between the workpiece cut surface and the flank face or between the lower side of the chip and the rake face. can occur when dry cutting at high cutting speeds. It consists of acicular ferrite and a high carbon-containing stabilized austenite.26 shows the structure of ADI. the matrix is characterized by increased toughness (e. In addition to its increased strength and hardness. The result of this is low tool life [Stau84]. segmented chips. GJS 400-15) consist predominately of ferrite.g. Increasing the cutting speed lightly reduces the values of the individual resultant force components. GJS 600-3). Because of the high temperatures in the contact zones between the workpiece and the tool.7. optimized tools and fewer steps as a result of a smaller material allowance can compensate for this disadvantage [Schm07]. Only with very sharp cutting edges does a continuous chip develop which turns into a segmented chip given even a small cutting edge fillet. Types with low strength and good toughness qualities (e. Helical chips may form which are slightly brittle because of the reduction in chip strength due to graphite inclusions. On a microscopic level.g. which can be altered by means of a heat treatment. Chips formed when machining cast iron with spheroidal graphite are. such as tensile strength and toughness. Higher feeds. This effect only marginally degrades the measured roughness values. With an increasing perlite content. This ratio of ferrite to perlite is the result of the amount of bonded carbon in the matrix. The machinability is also influenced by the perlite-ferrite ratio in the material matrix. The resultant forces and the resultant effective mechanical stress of the tool cutting edge are relatively low when machining the GJS cast iron. first and foremost.5 Machinability of Cast Iron 289 The mechanical properties of this material. The material was erroneously referred to as bainitic cast iron in older publications due to the visual similarity of its structure with the ferritic carbide structure of steel. This phenomenon. cast iron with spheroidal graphite must be machined at somewhat lower cutting speeds. The special combination of abrasive and adhesive wear behaviour when machining ADI leads to extensive crater wear near the cutting edge. Particularly suited to machining cast iron with spheroidal graphite are uncoated cemented carbides (HW). GJS 900-7). Interrupted cuts can be used without difficulty because of the good damping property of the material. the surface is characterized by disembedded and in part. Figure 7. Surface roughnesses of just under Ra = 1 μm are achieved in the finishing process. For discontinuous processes such as milling. Surface quality is comparable to that of conventional cast iron with globular graphite.1 Aluminium Alloys Mentionable areas of application of aluminium alloys as constructional materials include the following industries: .26 Crystalline structure of austenitic-ferritic cast iron with speroidal graphite (ADI) to a destabilization of the cutting edge. However. ADI’s high fracture strain has a negative effect on this [Klöp07]. cutting strategies specially adjusted to the material are required.290 7 Tool Life Behaviour 50 µm Spheroidal graphite Austenitic-ferritic matrix 10 µm Acicular ferrite Initial grain size Rest austenite Stabilized austenite Fig. Tool geometries specially tuned to ADI machining. the austenite converts into martensite because of mechanical stress. Lower cutting speeds due to high amounts of heat release can be compensated by increased feeds. Segmented chip formation is similar to that of conventional cast iron with globular graphite. 7. which is accompanied by an approximately 200–300 HB increase in hardness. such as drills with a radius or facette shape have a longer tool life. In the rim zone. ADI is basically not more difficult to cut than higher-strength GJS types.6. The average forces arising when machining ADI are comparable to those of other cast irons with globular graphite.6 Machinability of Non-ferrous Metals 7. coated cemented carbide tools of the main user group K. the average cutting forces are superimposed with a high dynamic. cBN and cutting ceramics are used as cutting tool materials [Kloc03]. The use of a cutting fluid is recommendable for continuous cut operations. 7. The use of cutting fluids is beneficial in continuous cut operations. Mostly. the weak edge breaks before the critical flank wear will be reached. Therefore. cutting fluids are not recommendable due to thermoshock stress on the tool [Schm07]. Due to its particular finely-striped grain structure. Aluminium alloys are distinguished by a low density of ρ Al = 2. The boundary surface temperature is very much contingent on the tribological conditions. this percentage can be in the order of 90–95% [Schu96. which generally are located on the grain boundaries. Berk01]. Aeronautic engineering traditionally uses a large amount of aluminium components. most importantly higher cutting speeds. Because of its low strength compared with steel. Adhesion is mostly dominant in .7 kg/dm3 but also by low strength. lower mechanical and thermal stresses on the cutting edge are to be expected for the same cutting parameters. makes it clear how many aluminium components are used [Star96. Materials whose structure primarily consists of intermetallic phases are often abbreviated simply to “intermetallic phases” in common usage. The intermetallic phases should also be mentioned as another important group. obviously their strength must be increased. In the chipremoving manufacture of structural components. which consists of up to 65–81 mass percent aluminium. which are often characterized by a large percentage of machined semifinished products. strength is increased by depositing hard structural components. one must take care that the boundary surface temperature does not get too close to the melting point. the example of a modern commercial aircraft. A distinction is made between wrought alloys and casting alloys. more favourable tool life parameters can be expected than in steel machining. NiAl.6 • • • • • • • Machinability of Non-ferrous Metals 291 automotive engineering aerospace engineering installation and apparatus engineering electrical engineering food technology chemical industry optical industry From the realm of aeronautic engineering. In the case of hardening (hot age hardening or room temperature precipitation hardening). Alloys of both groups can exist in both a self-hardening or hardened state. Alloy technology makes this increase possible with the main alloying elements manganese. The machinability of aluminium alloys depends on their composition and structural state. the semifinished products are manufactured by forming. which are delimited from below by adhesive wear and from above by temperature resistance of the material. Due to the low melting point of aluminium alloys. The low boundary surface temperature makes it possible to use higher cutting parameters than in steel machining. silicon. In the case of wrought alloys. The tool life parameters of machining tools used on aluminium alloys vary depending on the alloy and the structural state. the properties of the formed structure are decisive for machining. Compared with cutting steels. Töns01]. magnesium. In order to expand the use of aluminium alloys. Ni3 Al and TiAl are key examples. Generally. Here. boundary surface temperatures are much lower. around 350◦ C. the properties of which are very different from those of its components.7. zinc and copper. cutting tool materials of the material group DP are superior tot hose of the HW group with respect to tool life parameters and material removal rate. In interrupted cut with low cutting speeds and high material removal rates. abrasion is always active as well. In the case of boring. Cemented carbides are used as cutting tool materials for machining wrought alloys and sub-eutectic casting alloys due to their wear resistance and hardness. increasing especially with a larger distribution density of hard particle inclusions. In contrast to diamond. Cutting tools made of uncoated cemented carbide (HW) and diamond (DP) are the most commonly used cutting materials for machining aluminium alloys. Diamonds. Selection is based on general criteria such as cutting speed. When there is a high material removal rate in an aluminium alloy machining operation. 3. the use of cutting tool materials of the material group DP is not to be recommended for drilling into solid blocks due to compression processes in the area of the chisel edge. these processes use high cutting speeds and small crosssections of undeformed chip. The surface quality depends to a great extent on the wear mechanism of adhesion. a large amount of chips has to be removed. which manifests itself in built-up edges and can be influenced by the process kinematics.292 7 Tool Life Behaviour the lower cutting speed range. Adhesion prevents the attainment of an optimal surface quality. Generally. When machining the wrought aluminium alloy AlCu4Mg1 (2024). here the main stress is on toughness and the ability to manufacture complex tool geometries such as highly twisted end mill cutters with sharp edges. high-speed steel (HSS) can be used advantageously to machine alloys with small amounts of silicon. . plasticized material can escape between the cut surface and the flank face and/or between the chip and the rake face. to improve the tool life in comparison to uncoated tools. cross-section of undeformed chip. Bömc87]. as mentioned already in Chap. Hardened wrought alloys and casting alloys with a silicon content of up to 12% cause increased tool wear with increasing amounts. in polycrystalline form (DP) and in monocrystalline form (DM). All cutting tool materials for main application group K can be used sensibly. The cutting speed must be reduced in these conditions. The specific resultant force of the types AlMg5 (5019). This phenomenon contributes to the deterioration of surface quality. the CVD diamond coating makes it possible. for example. especially aluminium alloys with high amounts of silicon. the chip form is a particularly important criterion for judging machinability. Zoll69. although. Cutting trials have shown that CVD diamond coatings on cemented carbide substrates have the potential to combine the advantages of cemented carbides with those of polycrystalline diamond. Diamond cutting tool materials are often used to create highly reflective surface properties. For all aluminium alloys. Due to the higher boundary surface temperatures. Opit64b. and continuous or interrupted cut [Bech63. are the first choice for machining strongly abrasive super-eutectic casting alloys. For the sake of undisturbed manufacture. Such particles can be intermetallic compounds and nonmetallic inclusions such as impurities of the molten bath. AC-AlSi6Cu4 (AC-42000) and AC-AlSi10Mg (AC-470000) is about 25% below those of the heat-treated steel C35. lanthanum. The same is true of products made of magnesium wrought alloys. silicon and rare earth elements such as cerium. In comparison. These elements cause inner corrosion in magnesium alloys.6 Machinability of Non-ferrous Metals 293 The chip form can be affected by the alloy components. Of the many types of magnesium alloys. The most difficult to cut are especially nonhardenable aluminium alloys and hardenable aluminium alloys in a soft state. copper and nickel are found. neodymium. with respect to both inner corrosion and contact corrosion. The alloy AZ91hp is the most frequently utilized magnesium alloy for die-cast parts. especially for applications at temperatures above ca. When machining non-hardenable and hardenable alloys in a soft state without any percentage of silicon. DINEN1753].6. it can be said that cutting aluminium alloys forms the more favourable chips the harder they are. laptop housings. sand casting is of only minor importance.8 kg/dm3 has predestined magnesium alloys for applications in lightweight construction in particular. It has become clear that magnesium alloy machining is focused primarily on die-cast components. Hard and brittle inclusions such as Al2 O3 and silicon also benefit chip breakage. zinc. In summary. seat frames. In hardened wrought alloys and casting alloys with a silicon content of up to 12%. These alloys should be avoided as much as possible for components requiring machining [Bech62]. Approximately 80% of the cast pieces produced in Germany currently consist of magnesium alloys [Wolf03]. mobile phones and other devices in the area of consumer electronics as well as machine parts and sports articles. manganese. the following will take a closer look at the example of one alloy group.7. Other products using magnesium include manual electronic devices. Another important goal of alloy technology is the improvement of the corrosion resistance of magnesium alloys.2 Magnesium Alloys Magnesium alloys have been used in Germany since the end of the 1930s especially in automotive engineering as light constructional materials in Volkswagen manufacture. Examples include transmission housings.7 kg/dm3 and steel with ρsteel = 7. 100◦ C. The notation “hp” (high purity) means that only trace elements of iron. steering parts. Solidifying the alloy in a temperature range of 465–598◦ C favours inhomogeneous crystalline structures in the cast piece and the . praseodymium and yttrium. The most important alloying elements include aluminium. inner linings. instrument supports and engine mounts in magnesium-aluminium composite structures [Dörn04]. AZ91hp is characterized by good mechanical properties and very good castability [Kamm00. increased silicon content leads to a more advantageous chip breakage. by heat treatment and by the process kinematics. 7. long ribbon chips arise which make the machining process difficult. Alloy engineering aims to improve the strength properties of magnesium. The low density of ρMg = 1.74 kg/dm3 in comparison with ρAl = 2. For this reason. it is recommendable if possible to cut these alloys in a condition of increased strength (cold-twisted/hardened) [John84]. The corrosion resistance of alloys of this AM group is better than that of group AZ. 7. The distance between the lamellae or frequency of chip formation depends essentially on the tribology of the boundary surface. The alloy AE42 contains RE as a misch metal with the composition 50% cerium. Magnesium-aluminium-manganese alloys (AM-alloys) have very good strength properties together with higher fracture strain and notched-bar impact work. copper and nickel. When cutting magnesium.6. e. AM-alloys are used in automotive engineering.1). plasticizing a thin material layer which holds the individual chip lamellae together. Manganese and aluminium combine to form hard MnAl deposits. about 225◦ C in the case of magnesium alloys. especially after machining. This phenomenon can be explained by the material structure. Sharp tool cutting edges and smooth rake faces lead to good machining results. 20% neodymium and 3% praseodymium.27). Above ϑ = 225◦ C. This means that a smart choice of process kinematics can adjust the amplitude and frequency of dynamic stress on the cutting edge to its loadability. With the exception of higher fracture strain and higher creep strength. Because of their good castability. Their particular advantage is improved creep strength for temperatures up to about 150◦ C. which accelerate tool wear. for structural parts. praseodymium and yttrium. Only the base planes act as slide planes at room temperature.g. Further alloy systems that aim to improve mechanical properties and creep strength include: • • • • Magnesium-zinc-RE-Zr (ZE-group) Magnesium-RE-Ag-Zr (QE-group) Magnesium-yttrium-RE-Zr (WE-group) Magnesium-aluminium-RE (AE-group) The rare earth elements (RE) include cerium. but can also be influenced by process kinematics. neodymium. 7. This increases deformability. When rotating tools with large diameters are . In order to classify material properties into a material spectrum. steering parts and rims. This can impair the density of cast pieces. it has similar attributes as the alloy AS41.294 7 Tool Life Behaviour formation of porosities. it changes into a cubic lattice structure. which is indeed contingent in a certain way on the material-cutting tool material combination. 25% lanthanum. The tool geometry is based on those of tools for machining aluminium (Sect. lanthanum. the maximum spindle rotation speed puts a limit on the cutting speed. lamellar chips are formed (Fig. which can in turn be affected by feed and cutting speed. friction causes temperatures to increase. AS-alloys (magnesium-aluminium-silicon) have similar mechanical parameters as AM-alloys. The alloying element manganese bonds corrosive elements like iron. They are inferior however with respect to toughness. material behaviour shifts from brittleness to toughness. Magnesium has a hexagonal lattice structure at room temperature. Beyond a certain temperature. Characteristic of magnesium alloy cutting is the marked stick-slip friction. Near the tool-chip boundary surface. In drilling and milling with small tool diameters. fracture strain at room temperature is a helpful mechanical parameter (A = 7−14% for magnesium alloys). ultrafine-grain cemented carbides of the application group N10/20 and polycrystalline diamond (DP) are the usual cutting tool materials. polycrystalline diamond (DP) and diamond-coated ultrafine-grain cemented carbides. Workpiece stiffness can limit the chip cross-section and material removal rate.27 Chip of the magnesium alloy AM60 used.6 Machinability of Non-ferrous Metals Chip bottom side Material: Cutting tool material: Tool geometry: AM60 HC-N10 (CVD-Diamant) DCGX11T308-AL 295 Chip upper side Cutting speed: Feed: Depth of cut: vc = 1000 m/min f = 0. Milling produces satisfactory surface qualities both in wet cutting when oil or emulsions are used and in dry cutting.0 mm Fig. The melting point of this alloy is in the range of 420–435◦ C.7. They are characterized by especially high wear resistance and contribute to a great extent to process safety. for example. Both alloys are used. With respect to machinability. Tools made of these materials allow for high cutting speeds and feeds. especially in finishing. for transmission housings. HC). so no builtup edge formation is to be expected. When drilling both of these alloys with tools made of cemented carbide. below those of sub-eutectic aluminium alloy machining. Differences in cutting momentum are marginal [Wein00].28 compares experimental results from dry face milling aluminium alloy AlSi9Cu3 and magnesium alloy AZ91hp [Kloc00]. In the case of the cutting . Fitting cutting tool materials for machining include high speed steel (HSS). When cutting magnesium alloys have only a slight tendency to adhesion. The resultant force is relatively low in magnesium machining. This is also true for the rim zones of the pieces to be machined. The possibility of machining magnesium alloys with high and extremely high cutting speeds is a good foundation for creating high surface qualities on machined components. the resultant force of the magnesium alloy is also below that of the aluminium alloy. uncoated and coated ultrafine-grain cemented carbide (HF. 7.5 mm ap = 1. In practice. the maximum allowable tool rotation speeds are the limit criteria. since these are predominately manufactured by die casting. This makes it clear that the thermal load on the tool is relatively low in machining magnesium alloys. Figure 7. magnesium alloys are characterized by the fact that they contain few abrasive components. With a density of ρ Ti = 4.296 7 Tool Life Behaviour 200 AlSi9Cu3 Resultant force Fz / N AZ91hp 150 Hardness of materials: AlSi9Cu3 102HBW 5/250 AZ91hp 67HBW 5/250 vc = 2000 m/min ae = 30 mm ap = 3 mm 100 Tool: Face mill. In general.51 kg/dm3 . The great technical importance of titanium alloys is based above all on their high strength. 7. 7. z = 5 Down milling in dry cut Cutting tool material: DP 50 0 0 0. To achieve this. Even highstrength steels with yield strengths of ca. including the classic alloy TiAl6V4. which is not even closely approached by any other metallic material (Fig. The first alloys were developed at the end of the 1940s in the USA.6. Longer chip pieces break into smaller pieces when they make contact with the tool. the process must be stable. In drill hole finishing.1 0. They are resistant to temperatures of about 550◦ C. 7. 1000 Mpa are still little more than half that of a TiAl6V4 titanium alloy with respect to this ratio. an arithmetical average roughness of Ra = 0.3 mm [Kloc02].4 μm can be safely obtained under production conditions. titanium is the heaviest element in the light metal group.5 μm are often required. even when the highest cutting parameters are used [Wein00]. Ra -values of about 1.28 Comparison of force components during face milling (dry cut) conditions given in Fig. 7. D = 80 mm. Another important property of titanium alloys is their good corrosion resistance. mostly spiral chip segments and discontinuous chips occur. For seal faces of motor vehicle components. longer chips are almost completely absent.29). the chip form poses no problem in magnesium machining.28.2 − 0.3 0. These values can be safely adhered to with feeds of fz = 0.6 μm was measured after wet face milling.3 Titanium Alloys Titanium alloys are still a relatively recent group of construction materials. top surface qualities in the range of Ra = 0.4 Fig. The maximum operating temperature is limited by the increase . but in particular on the ratio of yield strength to density. Due to the formation of lamellar chips. the workpiece and the cutting medium because of their low stability.2 Feed fz / mm 0. C) and β-stabilizing (Mo. the most important alloying element of titanium – extend the α-phase zone to higher temperatures and form a two-phase (α + β)-zone [Pete02]. The existence of these two phases and the associated structural transformation is the foundation for the diverse properties of titanium alloys [Pete02].7.2 [MPa] Ratio of yield strength and density 215 C45+QT 50 Density [g/100 cm³] 600 800 Fig. O. Titanium alloys are used above all in these areas: • • • • • aerospace industry chemical industry medical technology power engineering sports equipment technology Titanium appears in various lattice modifications.7G TiAl6V4G C45+QT 180 87 80 Ti99. In accordance to their influence on transus temperature. of which each is only stable within a certain temperature range. a distinction is drawn between neutral (Sn.2 5. The conversion of the α-phase into the β-phase takes place above 822◦ C.6 46 Ti99. Ta. 7. α-stabilizing (AL. Also worthy of mention is the excellent biocompatibility of titanium alloys.8 Thermal conductivity [W/(m·K)] 100 0 Young s modulus [GPa] 110−140 205 800−1100 370−490 40 55 0 200 400 Yield strength Rp0. Structure is defined here in the case of almost .29 Mechanical and physical properties of titanium materials in comparison to the heat treatable steel C45+QT (reference data) in oxidation beyond 550◦ C [Essl91].7G TiAl6V4G C45+QT 17. The properties of titanium alloys are basically determined by their chemical composition and structure.7G TiAl6V4G Heat dimension [µm/(10m·K)] 120 150 785 Ti99. The high-temperature phase on the other hand has cubic body-centred crystal lattice and is called β-titanium. pure titanium and most titanium alloys exist in the modification of a hexogonally close sphere packing.6 Machinability of Non-ferrous Metals 297 Comparison of thermophysical and mechanic characteristics 451 442 Ti99. which is called α-titanium. Zr). N. the “transus temperature”.7G TiAl6V4G C45+QT Specific heat [J/(100g·K)] 100−145 50 52 56. Fe) alloying elements. Cu. The α-stabilizing elements – which include aluminium. At lower temperatures. V. Cr. Mn. The chemical composition primarily determines the parts by volume of the α und β phases. the always varying structure is created by means of thermomechanical treatment. Ti-0. such as iron or carbon.2%). drastically increases the yield strength. pipes and distance rings between the rotor stages [Adam98.2Fe-0.g. which only contain α-stabilizing and/or neutral alloying elements. Titanium materials can be classified in the following groups: • • • • • • pure titanium α-alloys (α + β)-alloys near-α-alloys metastable β-alloys γ-titanium aluminide alloys Pure titanium (e.18O or 0.40% oxygen. are designated as αalloys. Both structural types can exist in a fine or coarse form. Titanium alloys are used to manufacture guide blades. Pete02. Basically. which represent impurities caused by the manufacturing process.5Sn. with those of globular structures. These titanium types are generally used in low temperatures. and the globular structure. This involves a complex sequence of solution heat treatment. deformation.2Pd). Whereas oxygen is the only element intentionally added in order to meet strength requirements. housings. This microstructure combines the good properties of lamellar structures.5Fe-0. The microstructure of (α + β)-alloys is characterized by a bimodal structure. titanium materials alloyed with aluminium and tin (TiAl5Sn2. . It consists partially of globular (primary) α-phase in a matrix of lamellar α. since what is of chief interest here is good corrosion resistance and deformability. which results from the recrystallization process.5) are also available [Pete02. which arises by simple cooling from the ß-zone. plates. such as higher strength and fracture strain.298 7 Tool Life Behaviour exclusively two-phase titanium alloys primarily as the size and arrangement of both of these phases.40O) and titanium alloys (e. Because they are single-phase. More than half of all titanium materials are melted with this composition. which. If higher strength is required. Schu04]. It is used above all in the aerospace industry. consisting of a combination of forming and heat treatment. impeller blades. Schu04]. these alloys are characterized by high temperature resistance. Moreover. recrystallization. The (α + β)-structure is fabricated by thermo-mechanical treatment. Pure titanium types contain up to 0. aging and stress-relief heat treatments [Pete02]. Among the (α + β)-alloys is the by far most common titanium alloy TiAl6V4 (Ti-6Al-4V).and β-phase. By hardening. pure Ti: 0. α-alloys are relatively low in strength (Rm = 280−740 N/mm2 ).98% Ti. (α + β)-alloys reach very high strengths in the range of Rm = 900–1300 N/mm2 . It was already developed in the early 1950s in the USA and is both the most researched and tried titanium alloy. The high level of corrosion resistance is achieved by alloying palladium (up to 0. Ti-5Al-2. there are other elements. as an interstitial alloying element.g. ultrapure titanium: 99. The two extreme forms of phase arrangement are the lamellar structure. such as high resistance to creeping and fatigue crack growth. They are primarily used in the chemical industry and process engineering. g. which have a different lattice structure than the initial component.1Si) are the classic high temperature alloys. hexagonal. aerospace components and automobile components. These alloys are used to produce foils.6 Machinability of Non-ferrous Metals 299 Near-α-titanium alloys (e. to ADAM [Adam98] .g. copper and iron.7.5Zn). Titanium alumnides are counted among the “intermetallic” phases. 5Al-3V-2Mo-2Fe. 7. tends to strain hardening β -phase. bodycentered cubic. moderate weldability. Ti-15V-3Cr-3Al-3Sn) contain vanadium. used in operating temperatures of 500–550◦ C. Intermetallic phases of the TiAl Crystalline structure of TiAl6V4 Ti-6Al-2Sn-4Zr-2Mo: heterogeneous duplex Material Primary α -phase. from which their specific properties result. Both alloys have a bimodal structure (Fig.30 Typical (α+β) mixed structure as in a compressor blade made of the near-α-alloy Ti-62-4-2. Two essential representatives of this alloy group are the titanium alloys Ti-6-2-4-2 (Ti-6Al-2Sn-4Zr-2Mo) and IMI834 (Ti-6Al-4Sn3. 7. Metastable β-alloys can be hardened to extremely high strengths of over 1400 MPa. The mixed crystals decompose with the other alloying elements eutectoidically at low temperatures. they exhibit a favourable combination of mechanical properties and oxidation resistance. which remain stable at low temperatures. molybdenum. in high-pressure compressors.g. They make it possible to increase operating temperatures up to 600◦ C. This alloy class combines the good creep properties of α-alloys with the high strength of (α + β)alloys. good deformable. Their use is limited however by their larger specific weight. Schu04]. Vanadium and molybdenum form a continuous series of mixed crystals with titanium. Another material group based on titanium are the titanium alumnide alloys (α2 -Ti3 Al. Also. While the standard (α + β)-alloy TiAl6V4 can be utilized only in the relatively cold temperature range of the compressor. These alloys have become very important in engine construction. near-α-alloys can also be used in hotter areas. acc. Ti-6Al-2Sn-4Zr-2Mo-0. brittle. These are defined as compounds of metals with metals or non-metals. poor oxidation properties and the complexity of their microstructure [Pete02. however with a much higher percentage of α-phase compared with the standard alloy TiAl6V4. e. chrome. γ-TiAl) first developed in the 1970s. Ti-4. Metastable β-alloys (e. distinctive adhesion tendency White needles: secondary α -phase Melting point: 1677 °C 100 µm Fig. Their complex microstructure allows for an optimization of the ratio of high strength to high fracture toughness. bad deformable.30). manganese. γ-titanium alumnide alloys are also characterized by their very low fracture strain at room temperature. only a small amount of the arising heat is removed with the chip.31 Heat distribution coefficient of heat flow on the rake face and the thermal tool stress during turning of TiAl6V4 compared to C45E steel.5 mm² 700 500 C45E 300 f . Titanium alloys are difficult to machine because of their mechanical and physical properties. high high-temperature strength. In comparison to machining the steel material C45E. depending on the thermal conductivity of the cutting tool material. when the titanium alloy TiAl6V4 is machined (Fig.21 . ap = 0. about 20–30% more heat must be absorbed by the tool. to KREIS [Krei73] .31). ap = 0. relatively soft and ductile and has a high adhesive tendency (Fig. much more than is the Wear relevant characteristics of TiAl6V4 80 1300 TiAl6V4 Chip 60 C45E 40 Tool 20 0 Thermal conductivity λ / W/(m·K) 42 HW-P10 HSS Ceramic 84 126 HW-K10 Comparison of tool and chip heat distribution while turning steel and TiAl6V4 168 PKD Tool temperature t / °C Heat flow ratio αw / % 100 1100 TiAl6V4 900 f . in the area of A = 0. poorly deformable and has a high tendency towards strainhardening.29). As a result of this. which can be used in temperatures up to about 700◦ C [Pete02]. 1.30). Their strength is high. 7.25 .300 7 Tool Life Behaviour type are characterized by low density. The result of this is that the cutting tool is subjected to high thermal stress. One important physical property for the machinability of titanium alloys is their low thermal conductivity. which is only about 10–20% that of steel (Fig. 7.5–1%. 7. and their fracture strain (A5 = 5–15%) is low. 7. acc. They thus are more temperature-resistant than the above-mentioned titanium alloys and are in direct competition with established nickel alloys with high density. which also crystallizes into the krz lattice type: it is easily deformable. The cubic body-centred ßphase is very similar in its machinability to ferrite. The hexagonal α-phase is relatively hard. Their Young’s modulus is almost half that of steel. high oxidation resistance and creep strength up to temperatures of 650◦ C. brittle. 2 mm² 100 0 20 40 60 80 100 120 Cutting speed vc / (m/min) Comparison of tool temperature while turning steel and TiAl6V4 Fig. This phase affects the active tool cutting edge like the strongly wearing cementite lamellae in the perlite grains of carbon steels. to KREIS [Krei73] Chip Tool . The lamella slides over the rake face while a new lamella is compressed. The dynamic cutting force can amount to about 20–35% of the static cutting force.7. but also to extremely high thermal stress [Krei73]. 7. 7. the frequencies and amplitudes of which depend directly on the cutting conditions. the cutting force is steadily reduced.31. Such discontinuous chip formation subjects the tools to a mechanical and thermal alternate load. In position I. This means that cutting titanium alloys not only exposes the cutting tools to considerable mechanical strain. The simultaneously introduced compression of the newly forming lamella causes the cutting force to go up again (position II). The cause of this is a constant shift between compression and sliding phenomena in the shear zone (Fig. Another characteristic of titanium alloy machining under conventional cutting conditions is the formation of lamellar chips. Cutting force Fc I II Work piece Work piece vc III Chip Tool vc Work piece Chip vc Tool Cutting time tc Fig.6 Machinability of Non-ferrous Metals 301 case when cutting steel (Fig. acc. right). the shear zone is already fully formed. Since the deformation resistance of the titanium material is rapidly decreased immediately after the shear zone is formed as a result of the high shear/deformation speed. until the cutting force or shear force is so high in position II that the shear strength of the material is exceeded and a new lamella is formed [Krei73]. 7.32 Chip formation during turning of titanium. Mechanical and thermal alternating stress can lead to tool fatigue and encourage tool failure due to the formation of cracks.32). The largest amount of crater wear . These cutting tool materials are characterized by high hardness and wear resistance. Especially mentionable in this context are above all the composition of the binder phase. Flank face wear is of secondary importance. thus clearly reducing production time while maintaining the same or even improving cutting quality. silicon. low thermal expansion and low friction between the rake face and the chip (or between the flank face and the workpiece). especially at high cutting speeds. When turning titanium alloys with PCD tools.31). they make it possible to use higher cutting speeds. excellent thermal conductivity compared to other cutting tool materials (Fig. the interactions that effect wear taking place between the workpiece material and the cutting tool material are extraordinarily complex. 7. Of the PCBN cutting tool materials. The range of speeds applicable for tools made of these cutting tool materials is vc = 100−200 m/min. They are characterized by diffusion and graphitization. its quantitative amount as well as the size of the diamond grains [Neis94]. polycrystalline cubic diamond (PCD). the fracture of cutting tool material particles or cutting edge fracture [Krei73. N) react with the workpiece material. the performance capacity of PCD cutting tool materials in the case of titanium machining is highly dependent on the composition of the cutting tool material. Compared with cemented carbides. the tools can however only be used at relatively low cutting speeds. Due to the high thermal and mechanical load. thermally caused cracking. Titanium materials are generally turned with uncoated cemented carbides of the main application group S. CVD-diamond thick films and polycrystalline cubic boron nitride (PCBN). Due to these diverse interactions between the workpiece and cutting tool materials. which greatly reduces the wear resistance of the tools. In the case of PCD types with cobalt-containing binders. surface damage as a result of lamellar chip formation and the potential formation of wear-reducing reaction films on the diamond grains [Bömc89. Cutting tool materials based on Al2 O3 and Si3 N4 do not come into considerations for machining titanium materials due to their low thermal conductivity and the great affinity of aluminium. In isolated cases. Yang99. One alternative to uncoated cemented carbide tools in the case of finish turning titanium alloys are tools made of monocrystalline diamond. Elements of the substrate (Ti) or the finishing material (Ti. above all type containing high level of cBN are good for finish turning several types of titanium alloys. In experiments where the titanium alloy TiAl6V4 was cut using external cylindrical turning. Neis94]. Cemented carbides containing titanium (P-types). The range of usual cutting speeds is about 50–70 m/min for rough turning and 60–90 m/min for finish turning. O. Kita97]. cermets or coated cemented carbides are generally not suitable for turning titanium alloys. oxygen and nitrogen to titanium. it was seen that crater wear was greatly effected by the binder content and the grain size. the lowest amount of crater wear was measured in the case of a PCD variety with SiC as binder. good results have been reported in the case of machining with TiB2 -coated tools. The most dominant form of wear when turning titanium alloys with PCD tools is the formation of craters on the rake face.302 7 Tool Life Behaviour shell-shaped spalling. Due to its catalytic effect. 7. In conjunction with dynamic stress on the cutting tool material by lamellar chip formation. microcracks further the separation of individual diamond grains or entire grain bonds from the cutting tool material compound [Bömc89. Neis94] (Fig.0 mm f = 0. The lower amount of crater wear when turning with SiC-containing or coarsegrain cobalt-containing PCD is attributed in the literature to the formation of a wearinhibiting film made of titanium carbide on the diamond grains. to reduce crater formation when turning titanium alloys with PCD as much as possible. cobalt and diamond have different thermal expansion coefficients.1 mm Binder * Grain size 8% 8% 20 % 6–10 µm 2–6µm 0. PCD cutting tool materials with large diamond grains. Thus. Köni93c. Since the speed of diffusion of carbon to titanium carbide is more than 10 times slower than that of carbon to titanium.7. TiAl6V4 vc = 110 m/min ap = 2. Furthermore. the wear process is significantly reduced [Hart82]. Neis94. The result is that the cutting tool material becomes less resistant to abrasive wear. Kloc07]. Width of flank wear land VB/µm Crater wear KT/µm 100 80 Width of flank wear land VB PCD 1 PCD 3 60 PCD 2 40 Crater wear KT 20 PCD 3 PCD 2 PCD 1 0 200 600 800 400 1000 Path length due to primary motion lc/m Composition PCD 1 PCD 2 PCD 3 Binder phase: WC-Co * Volume percentage Diamond * 92 % 92 % 80 % Face turning. low amounts of cobalt or with a binder phase made of β-SiC should be used.33 Diamond as cutting tool material for turning of titanium alloys .33). 7. It is believed that there is a reaction caused by diffusion between the titanium from the material and the carbon from the cutting tool material in the area of the crater. cobalt promotes the graphitization of diamond. which favours the formation of microcracks. This process can be observed above all in the case of fine-grain types.6 Machinability of Non-ferrous Metals 303 was observed when using the PCD type with the largest quantity of binder and the smallest grain size [Köni93c.5–1 µm Fig. The TiC film that is formed in the area of the contact zone on the rake face adheres tightly to the diamond grains that make up the surface during the machining process. it was possible to reduce the total machining time required for blade finishing by about 50% by using ultrafine-grain cemented carbide milling cutters in conjunction with an adapted machining strategy. Using this method. In up milling. Due to the large volume of material to be removed.304 7 Tool Life Behaviour Weight reduction is an essential criterion for modern airplane engines. in order to finish the blades only very small overmeasures need to be removed. tools made of ultrafine-grain cemented carbide is recommendable due to its excellent wear and toughness properties. One example of a modern component concept is the blade integrate disk or “blisk”. In such cases. even curved blade surfaces can be manufactured in one cycle to nearly the final contour with end milling cutters on 5-axis machine tools by superimposing a wobbling motion. Trochoidal milling is one way to reduce milling time when pre-machining blisk blades (Fig. results from research and industrial praxis have shown that stabilizing the cutting edge with a defined rounding in the order of 5–20 μm leads to a significant increase in tool life not only for long-protruding tools when end milling titanium alloys. of which titanium alloys are an example. ECM machining is preferred. higher speeds and larger material removal rates can be realized with end milling cutters made of ultrafine-grain cemented carbide than with ones made of conventional cemented carbide. machining also requires up to 45 h depending on the size of the blisk. For blisks with a diameter of 600 mm and about 70 blades. surface quality as well as form and dimensional accuracy. but also with the help of an innovative component design. as well as 7 or 8 more hours should an ECM operation be included. For this type of machining. tools with relatively large axial (D) and radial (D/2) depths of cut are used to realize large material removal rates. blisks are produced using end milling or form cutting. the formation of the cutting edge has a key role with respect to tool wear when milling titanium alloys as well. Blisk production has thus proven to be very time-consuming and cost-intensive. especially considering that the blades must meet the highest demands regarding surface quality.6. To manufacture a blisk. 7. the most important of which include improved surface quality of the milled blades and lower resultant forces. Cutting becomes progressively more difficult with increasing blade lengths and thus with increasing milling cutter protruding lengths. For blade lengths up to 80 mm. the . This is an extremely demanding machining task. Blisk pre-milling is done with end milling cutters made of high performance high speed steel or conventional cemented carbides. When milling materials that are difficult to machine.34). rim zone formation and formal/dimensional accuracy. On the other hand. though all the blades must first be premilled to an overmeasure of 3 mm.5). which make it possible to realize higher levels of component precision. Here too. In the example provided (Fig. Cutting blades with these tools at high speeds lead moreover to still further advantages. up to 55 h of pure milling time is required.47). 7. down milling should generally be given preference. Analogously to the remarks given above on milling nickel-based alloys (Sect. the blades must be carved from the solid material. Because of their high level of wear resistance and bending strength. 7. Here too. The main concern in this case is above all high process safety. This can be realized by using materials of lower density. If the tool enters in a straight line. chip formation is preceded by a long friction phase between the tool and the workpiece. such as in up milling. The special characteristic of down milling is that the cutting edge exits with a chip thickness of h = 0 mm. Chip root [Peke78] and tensile residual stresses can develop in the cutting edge as a result of this. Tensile residual stresses can lead to crack formation and then to local cutting tool material fracture and tool failure.045 mm ap = 4 mm (D/2) ae = 8 mm (D) Fig. the .045 mm ap = 0. The tool exits with h > 0 mm except for the fluting from the solid. Potentially adhering chips are only connected with the cutting edge by a thin strip of material and are usually wiped from the tool upon its re-entry. Chips adhering to the cutting edge are partially compressed upon re-entry of the cutting edge into the material on the up milling flank. In down milling on the other hand. If the tool enters on the other hand with an “arc lead”.30 mm High performance finish milling 45 % Micrograin carbide S10/20F vc = 350 m/min fz = 0. Due to the high elastic deformability of titanium alloys.80 mm ae = 0. or they wind up between the cutting edge and the material. 7. The resultant surface quality of the down milling flank is much better than that of the up milling flank.80 mm ae = 0.30 mm Rough milling Roughing Working time Carbide S10/20 vc = 100 m/min fz = 0. also during the phase in which the tool enters the workpiece.6 Machinability of Non-ferrous Metals 305 Blisk production: Working time with conventional milling up to 55 h Material: TiAl6V4 Tool: End mill. the cutting edge enters with h > 0 mm and exits with h = 0 mm. which greatly promotes wear on the flank face. the cutting edge always exits with h > 0 mm until the groove is fully cut. D = 8 mm Finishing Conventional Conventional milling 100 % Planing annular space Finishing High finish milling Performance Carbide S10/20 milling vc = 100 m/min fz = 0.045 mm ap = 0. Chip root formation is not possible when the tool exits the material.34 Reduction of working time by applying high performance milling during finishing a blisk made of the titanium alloy TiAl6V4 (Source: MTU) tool enters with a chip thickness of h = 0 mm. Faulty component surfaces and/or cutting tool material fractures on the cutting edge are the result. This is of great importance when milling grooves.7. 7. high pressure lubricoolant supply . coated tools can also be employed for many machining tasks when milling titanium alloys.36). As this example shows. Compared to the conventional lead. analogously to down milling. Causes of the good wear and performance properties of coated tools in milling could include the lower tool temperature caused by the interrupted cut and the compressive residual stresses characteristic of PVD coatings. The cutting fluid jet supplied to the gap between the chip bottom side and the rake face not only cools the tool intensively. an intensive cooling is necessary. 7. it also improves chip fracture. Because of the high thermal stress on the tool.35).306 7 Tool Life Behaviour 400 Work piece Straight lead Circular-arc lead 350 Straight lead VBmax/µm Circular-arc lead A M M 300 250 200 150 100 50 0 8 16 24 32 40 48 56 64 Number of leads h>0 End mill h=0 h : cross-section of undeformed chip at tool exit while lead Material: Cutting tool material: Cutting parameters: Process: Tool diameter: TiAl6V4 HW-K40 ap = 5 mm vc = 50 m/min fz = 0. the removal of which from the chip formation location and the machine tool no longer presents any problems. Both first cut strategies have a significant effect on the wear and performance of end milling cutters. titanium alloys are usually machined using wet cutting. but above all the cutting speed and thus the process’s productivity [Gold07]. short-breaking chips are formed under the selected conditions.3 mm) compared with conventional cutting fluid supply. especially when turning. the cutting fluid was supplied both conventionally with a pump pressure of 6 bar and under high pressure with 140 bar (Fig. Instead of long ribbon and snarled chips. a cutting fluid jet supplied with high pressure can be used to improve not the tool life. In experiments in turning the titanium alloy TiAl6V4 with uncoated cemented carbides.3 (VB = 0. Due to the long engagement times. Especially in grooving operations.17 mm end milling D = 25 mm Fig. 7. the arc lead results in much lower tool wear (Fig. Under the conditions of high pressure lubricoolant supply the resultant tool life was higher by a factor of 2. One extremely effective method in this context is supplying a cutting fluid under high pressure (p > 80 bar). In contrast to turning.35 Influence of the entering path on tool wear during milling the titanium alloy TiAl6V4 cutting edge will exit the material with h ≈ 0 mm. but also the volume flow. While in the case of internal supply via the machine spindle. At present. The latter must also be optimally adjusted to the respective machining task.6 Machinability of Non-ferrous Metals 307 VBmax /µm 500 400 pKSS = 6 bar 300 200 pKSS = 140 bar 100 Material: TiAl6V4 Cutting tool material: HW-K10/20 Cutting speed: vc = 90 m/min Feed: f = 0. Analogously to turning. In the case of drilling. The thermoshock caused by this promotes the formation of comb cracks and hence tool wear. high pressure units with pressures of up to 1000 bar are available [Fili02]. in the case of external supply in turning or milling. As the first results have shown. the cutting fluid can be supplied with much higher pressure. As in the case of milling steel materials. the cutting fluid is supplied via the spindle and the cooling ducts in the tool. In high pressure lubricoolant supply not only the pressure is important.36 Increase of tool life due to high pressure lubricoolant supply can lead to enormous improvements in chip formation and tool performance. comb cracks.7. The use of the high pressure lubricoolant supply when drilling into titanium-based or nickel-based alloys increases performance significantly. One method variant is supplying the cutting fluid through the insert. It can also be used for drilling and milling operations. the pressure applicable through the pivoting feedthrough is currently limited to about 140 bar.4 mm Depth of cut: ap = 0. The use of the high pressure lubricoolant supply is not limited to turning.5 mm 0 0 5 10 15 Cutting time tc / min pKSS = 6 bar pKSS = 140 bar tc = 4 min tc = 14 min Fig. the resulting tool life is many times higher. The cutting fluid exits near the cutting edge in the area of the flank or rake face. can lead to . cutting fluids are regularly used when milling titanium alloys as well. In unfavourable cases. Compared with conventional supply. the basic problem is that the highly heated cutting edge emerging from the material is abruptly cooled down by the cutting fluid. Whether it is technologically necessary and economically sensible to work with such high cutting fluid pressures is currently being researched. in conjunction with mechanical shock load of the cutting edge when entering the material. cutting fluid pressures in the range of 100–200 bar should be sufficient for most machining tasks. 7. 7. one should take care that cutting fluid supply to the tool is carefully adjusted in order to reduce thermoshock and that the tools are intensively cooled. it is also possible under certain conditions to dry machine titanium alloys. not the least because of the safe chip removal and therefore for reasons of higher process safety. This result changes fundamentally when milling in full groove cutting. In comparison to wet machining. In order to diminish thermal alternate stress and the resulting formation of comb cracks. 7.e. . In these cases.17 mm 2 4 6 8 10 20 40 60 Cutting time tc / min Down milling ae < D D = 25 mm ap = 3 mm Z =1 Groove milling ae = D D = 32 mm ap = 30 mm ae = 32 mm Fig.5/25. the cutting fluid has the essential task of separating the chips from the cutting edge and transporting them out of the slot [Köll86].5 mm wet Cutting fluid 200 * 200 100 80 60 100 80 60 40 ae = 15 mm dry ae = 7. dry cutting also leads here to a significantly lower amount of wear on the end milling cutters used. Numerous investigations into dry milling steel materials have proven that dispensing with the cutting fluids leads to a significant increase in tool life.08 x 26° 40 30 2 4 6 8 10 20 40 60 100 200 400 20 10 1 Cutting time tc / min Material : TiAl6V4 Cutting tool material : HM-S40 Cutting parameters : vc = 50 m/min fz = 0. the question is whether it makes technological sense to dispense with cutting fluids when milling titanium alloys too. In this case. are compressed on the surface of the up milling flank and cause cutting tool material fracture on the cutting edge. The reason why wear is increased when dry milling is that chips adhering to the cutting edge or found in the groove end up in the material between the cutting edge and the groove flank.5 mm dry Cutting edge phase: 0. Comparative studies [Köll86] in down milling TiAl6V4 with ae < D have confirmed the general results gathered when dry milling steel materials (Fig. In the case of down milling with ae /D = 7. In most application cases in praxis however. the lowest amount of wear was recorded when milling with a cutting fluid. When milling in full groove cutting.308 7 Tool Life Behaviour Down milling with ae < D Width of flank wear land VBmax / µm 600 ae = 15 mm wet 400 Groove milling in TiAl6V4 600 400 * Fracture Dry Compressed air ae = 7. as soon as the cutting edge penetrates the material with a chip thickness of h = 0 mm. while in dry cutting its life was extended to 200 min. cutting fluids are used. the tool’s life had ended already after 8 min with wet cutting. In principle. i.37).37 Influence of wet or dry cutting conditions on the tool wear development during milling TiAl6V4 tool failure due to fracture. cooling with liquid nitrogen delivered the best results (Fig. supplying the cutting fluid under high pressure.7. 7. Z = 7 Cutting parameter : vc = 32 m/min fz = 0. in analogy to turning (Fig. There are at points significant differences in performance between the cutting fluid media. 7. 10% CF1. Investigations with various cutting fluid media. 7.36). The use of cold or liquid gases is an extreme form of tool cooling. the amount of oil in the emulsion should not be too high. One example of this is the use of CO2 snow (dry ice) when turning a duplex steel. 10% 40 Water CF2. The boiling point of liquid nitrogen is –195. In this investigation. unsatisfactory results can also stem from the use of an unsuitable cutting fluid. 5% 20 Liquid nitrogen 10 0 0 20 40 60 80 120 90 Cutting time tc/s Material : Prozess : Tool : TiAl6V4 End milling. By means of the intensive cooling.38). a certain amount of oil must however be present in the cutting fluid in order to reduce friction. The use of this cooling medium led to an extreme cooling of the tool and the chips.6 Machinability of Non-ferrous Metals 309 60 Width of flank wear land VB/µm 50 CF3. Similarly positive results can be obtained by the use of cold gases. Since tool cooling is especially important in titanium machining. 7.1 mm ap = 50 mm ae = 15 mm Fig. water and liquid nitrogen had the following results (Fig.38 Influence of cutting fluid on the tool wear development during milling TiAl6V4 One alternative to conventional flood cooling when milling is.38).8◦ C. As the more pronounced wear formation when milling with pure water containing only a rust inhibitor proves. concentrations. a 5% emulsion lead to less tool wear than a 10% emulsion. 10% 30 CF1. In this comparison. down milling D = 50 mm. tool wear and burr formation could be reduced in addition to chip formation and surface . When milling titanium alloys. not only for high pressure compressors but also for the low pressure turbine. As these examples show.310 7 Tool Life Behaviour quality being improved [Wein07].39 Surface formation and chip forms during turning of γ-titanium aluminide with conventional and adapted cutting conditions . This becomes very clear in consideration of the discontinuous and segmented chips that are formed. Their low elongation at break (< 1% at ca. the use of extremely cold media can contribute to the solution of difficult machining tasks.39). 7. is not found in conventional machining. Instead. The cause is the high level of brittleness of the material and its incapacity to deform plastically. Whether the cost associated with their use is economically worthwhile must be decided on a case-by-case basis. Aust99]. needle-shaped chip lamellae 30 µm 60 µm 2 mm 10 mm Fig. The problems include high tool wear. The main problem in machining these materials under conventional process conditions is the formation of defects in the form of microcracks and micro-fractures on the surface of the machined workpieces (Fig. They are for this reason a possible alternative to nickelbased alloys at about 50% the weight. Chip formation such as it is familiar when machining steels or titanium alloys. 7. 700◦ C) and low thermal expansion (λ = 10 W/(mK)) make these alloys problematic for machining. angular. When turning however. tools made of PCD or with CVD thick diamond films can also be successfully employed. Components made of titanium alumnide have predominantly been machined with uncoated cemented carbides (HW-K10/20) [Ecks96. extremely low applicable cutting speeds and insufficient surface quality. Titanium alumnides may have the potential to raise the operating temperatures of titanium alloys to 800◦ C. 6 Machinability of Non-ferrous Metals 311 are formed whose separating surfaces are very rough. With sharp-edged tools and the choice of a small ratio of depth of cut ap to the corner radius rε . cutting speeds of up to 100 m/min can be used. In comparison to other metallic construction materials. as will be explained below. The material to be removed is. Alloys of this group are especially familiar in the form of the two-material system copper-zinc. A distinction is drawn between wrought and cast materials. most copper alloys. it is possible to produce smooth damage-free surfaces by turning workpieces made of Ti-45Al-8Nb-0. known as brass. the machining result can be significantly improved by an adjustment of the cutting edge geometry and process parameters. When cemented carbides of the S10/20 group are used. With respect to machinability. . this organizing principle is not suitable for classifying the machinability of copper alloys because of highly varying machinability of copper alloys of the same type [DKI83].2C. As the basic research has shown. Under these conditions. pried out of the workpiece material. nickel and aluminium. Such alloys are easy to deform when cold and have a high level of deformability. have obvious fracture structures and extensive cracks.6. so to speak. it is generally sensible to classify according to alloy groups. the additive elements of which are adjusted to each other such that they only form a homogeneous mixed crystal. They can range from the superhard two-material system copper/aluminium to types of pure copper with low strength and high fracture stress. the following categorization is suggested: • Pure copper and copper alloys with zinc.7. Although the properties of copper alloys are essentially determined by their chemical composition. Plastic deformation of the material during chip formation has not been observed.4 Copper Alloys Copper alloys are privileged because of their excellent thermal conductivity and corrosion resistance in the following areas: • • • • • • air-conditioning hydraulic engineering recuperator technology food technology chemical equipment and apparatus technology auxiliary equipment Copper alloys are defined as alloys in which there is at least 50% copper. Such surface structures are unusable above all for components requiring high levels of safety such as are needed in engine construction. coiled chips are formed such as one finds in steel machining. Within this classification. tin. are considered easy to machine. This group is considered to be moderately to poorly machinable. 7. tin. nickel. This leads to a positive influence on machinability due to improved chip fracture compared with materials that have not been shaped [DKI83]. which is different from that of the core structure and is called casting skin in practice. Lead.2 Influence of Heat Treatment Machining hardenable copper alloys is preferably executed with a cold-worked material prior to heat treatment. primary forming or shaping • heat treatment. • Alloys of both above groups. Beryllium. however without chip-breaking additives. They have less deformability than the previous group. This rim zone is characterized by a higher level of hardness and strength compared to the core structure. tellurium or selenium are added so that an unproblematic chip breakage can reduce disturbances in the manufacturing process. which form a second mixed crystal. to which are added lead. tin or vanadium. e. This group is especially characterized by the three-material system copper/tin/zinc and copper/nickel/zinc.1 Influence of Semifinished Product Manufacture When machining cast copper alloys. 7. their hardness and strength are increased while their deformability is reduced. cobalt. to which the elements lead.g. notched impact strength and deformability are reduced [Seid65]. This group is the most machinable because of its improved chip fracture. one must consider the structure of the rim zone. The emphasis of the main criteria of machinability changes depending on the machining task at hand. In the case of cold forming copper alloys. This group is made up of automatic alloys. Such heterogeneous alloys are harder than the previously mentioned group. but have higher machinability. Special alloys can contain nickel. hardening • the chemical composition of the alloy 7. selenium and tellurium as insoluble components in order to improve chip breakage. Strength is hardly affected.6. boron and natrium are supplemented as trace additives in order to affect crystallization. antimony and cadmium are contained in free cutting alloys as chip-breaking additives [John84]. e.4. also known as nickel silver.g. sulphur.6.312 7 Tool Life Behaviour • Alloys with the elements zinc. since machining after hardening would cause . there are a few essential points which influence the machinability of copper alloys that should be considered: • the manufacturing process for producing semifinished products. For this reason. aluminium and silicon. The core structure of cast alloys is generally more machinable than that of wrought alloys [DKI83]. However. bismuth. resulting in an acceleration of tool wear. it is difficult to make a general classification of these alloys with respect to their machinability. especially when machining copper alloys.4. selenium and tellurium on the form of insoluble components in copper alloys is comparable to the effects of these elements in automatic steel [Isle73.3 Influence of Chemical Composition Brass and pure copper (Group 1) are difficult to machine because of their high levels of toughness and deformability. For cast alloys. In practice.5 Resultant Force As the cutting speed rises the specific cutting force kc falls in the case of copper alloy machining as well. Since the specific resultant force in the case of copper alloys is generally considerably lower than that in cutting of steel. Further increase of the cutting speed causes the specific cutting force to converge asymptotically towards a constant value. built-up edges can be observed that lead to accelerated wear of the cutting edge [Klei66. They are characterized by high chip compression. Due to hardness and deformability. the harder material state is preferable for subsequent grinding and polishing work.6 Machinability of Non-ferrous Metals 313 accelerated tool wear. Lore74]. Ceramic cutting tool materials are not suited to cutting copper alloys because of their adhesive tendency [grei91]. leading to improved surface quality and lower resultant forces.4. uncoated cemented carbide of the application group K10/20 is recommended in order to guarantee good cutting edge durability. which has a large impact on the tribology in the chip/rake face boundary surface and causes high mechanical stress on the cutting edge. in the case of the cutting speeds commonly used today with cemented carbides as cutting tool materials (> 160 m/min). sulphur. by about a third [DKI83]. In the case of materials that are difficult to machine and tend to adhesion. the tool life parameters are less favourable when machining nickel silver than when machining brass [Vict72]. Mess69]. 7. It has become clear that. the influence of cutting speed can be neglected. the casting method used to produce the semifinished products has a considerable effect on the amount of specific resultant force. such as pure copper or brass containing large amounts of copper. polycrystalline diamond (DP) has proven superior as a cutting tool material not only in terms of better wear resistance but also by its favourable tribological conditions. Built-up edges are formed in brass machining as a result of the wear mechanism of adhesion.6.6. the specific cutting force is reduced. difficulties due to insufficient driving power of the machine tool can hardly be expected in practice.7. Machining experiments have determined that with constant cutting values parameters the amount of specific resultant force is lower for centrifugal casting than for . The effect of the alloying elements lead. these phenomena are known as “snagging”. On the other hand.4. In the case of high speed milling. 7.6.4 Wear When machining tough alloys in the case of continuous chip formation and low temperature at the boundary surface between the chip and the tool. When the cutting speed is increased from 5 to 160 m/min when performing an external cylindrical turning operation on copper. 7. for example.4. The chip formation of pure and homogeneous copper is relatively unfavourable. chip fracture can be significantly improved. which is used for shielding magnetic fields because of its high magnetic permeability [Schu04]. but also induce undesired residual stresses in the rim zone. Brinell hardness and strain. pumps). It is one of the ferromagnetic materials. With additional alloying elements (Pb. Examples include: • chemical industry (cauldrons. nickel.4.000 N/mm2 with RT). Se). at low temperatures. For example.314 7 Tool Life Behaviour sand casting. chip formation also proved more favourable. . S. One example is the softmagnetic nickel-iron alloy with about 15% Fe. 5% Cu and 4% Mo. Mess69]. In contrast to unalloyed copper. an alloy made of telluriumcontaining copper CuTeP can even be machined on automatic machines because of the short-breaking chips. also has excellent ductility and cold-formability. In the case of thin-walled workpieces.6. 7.6. heat exchangers.7 Chip Form The machinability of nickel silver (Group 2) can vary to a great extent depending on the respective amounts of the alloying elements zinc. Nickel has a cubic body-centred crystal lattice and.4. • aerospace (engines). These deformation not only endanger dimensional accuracy. Due to the finer crystalline structure of centrifugally-cast parts. Te. CuZn30: 115. As a rule. acceptable chip forms are obtained however. which are alloyed with at least one other element. • environmental protection and waste management (flue gas desulphurisation plants). Usually. deformations of the workpiece due to the cutting force can appear as a result of the low elastic modulus of copper alloys (e.5 Nickel Alloys Nickel alloys are materials with nickel as their main component. • power production (power plant generators). also familiar under the name Mumetall. They are characterized by good corrosion resistance and/or excellent high temperature strength. although the centrifugally-cast workpieces had higher tensile strength. aluminium and silicon. and thus the chip form as well. 7. 7.g. Large cross-sections of undeformed chip and an unhindered chip flow promote long ribbon chips.6. Some nickel alloys have special physical properties. valves. Nickel alloys are often applied as construction materials due to their superior properties. the use of a cutting fluid improves the surface quality as well [Grei91]. Lowering the cutting force can lead to improved quality. tin.6 Surface Quality Built-up edge formation and flank face wear of the minor flank face lead to poor surface quality [Klei66. the alloy CuTeP has only slightly less thermal conductivity [DKI83]. which can be hardened provided they have a corresponding amount of aluminium and/or titanium in conjunction with heat treatment. 7. nickel alloys can be classified in the following main groups (Fig. Ever71]: • • • • • I nickel-copper alloys II nickel-molybdenum alloys and nickel-chrome-molybdenum alloys III nickel-iron-chrome alloys IV nickel-chrome-iron alloys V nickel-chrome-cobalt alloys Materials of group II are not hardenable by heat treatment. The respective trade name of each alloy to have been offered on the market was assigned to each main groups I–V. for example. DIN17745. adjusted to the respective application area. to EVERHART [Ever71] .7. possible cold-shaping and the type of heat treatment. Groups I. Huntington Alloys). are called “super alloys” as do those of group V.40) [DIN17742. The classification in Fig. yet this classification is still good Main group II Alloy of Type HASTELLOY non-hardenable Main group Ib Alloy of Type K-MONEL hardenable Al Ti Cr Mo Fe Cr Cu Main group Ia Alloy of Type MONEL non-hardenable Al Ti Main group IIIb Alloy of Type INCOLOY hardenable Ni Cr Fe Main group IVa Alloy of Type INCONEL non-hardenable Main group IIIa Alloy of Type INCOLOY non-hardenable Cr Co Al Ti Main group IVb Alloy of Type INCONEL hardenable Al Ti Main group V Alloy of Type NIMONIC hardenable Fig. III and IV comprise both non-hardenable and hardenable alloys. 7. DIN17744. DIN17743. In accordance with their most important alloying elements. acc.40 is not very precise. not all materials classified as nimionic contain cobalt as an alloying element. 7. are essentially dependent on the chemical composition.6 Machinability of Non-ferrous Metals 315 The specific properties.40 Classification of nickel alloys in main groups (Source: Wiggin Alloys. Materials of main groups III and IV. Some elements effect it in several respects. thus have a larger amount of titanium and cobalt than Iconel 718 with the same Nb-content in order to improve the formation and stabilization of the γ phase. formation and distribution in the microstructure. Mark05. which limits the long-term use of Inconel 718 to 650◦ C [Bürg06. Solution annealing first dissolves the γ phase in the mixed crystal matrix. The alloying elements of these materials can be grouped into three categories in accordance with their effect on the microstructure. Co. Kenn05]. Final storage at high temperatures brings about a further improvement of the consistency of the particle size and form of the γ phase in the structure [Schu04]. such as Allvac 718Plus. The γ phase. the γ  phase tends toward a more rapid coarsening and thermal instability. deformability and machinability become increasingly limited. In the case of polycrystalline alloys it is differentiated between forging alloys and casting alloys. In contrast to the γ phase however.Ti). More recent nickel-based alloys. In the case of materials that have an increased amount of niobium. cobalt promotes the stability of the γ  phase. In the cast state however. Udimet 720LI and Allvac 718Plus contain a volume percentage of about 30–40% of γ phase. it is deposited again in a – in comparison to the cast state – more consistent form in the structural matrix. Bürg06]. Chrome improves oxidation and corrosion resistance.Ti)) and γ  (Ni3 (Nb. The γ phase can already originate from the molten bath. wrought alloys are subjected to a multistage heat treatment after melting in a vacuum and solidification. such as Inconel 718. the continuous operation temperature of Allvac 718Plus was able to be raised to 704◦ C. there is also a hardening via the γ  phase in addition to the γ  hardening. During cooling from solution annealing temperature.316 7 Tool Life Behaviour for orientation purposes. Mo and W form mixed crystals with nickel. Helm00. which are chiefly utilized in aeroplane engines and stationary turbines.Ti) and (Ni. Udimet 710 is applicable for continuous operation temperatures of up to 730◦ C [Tors97. A higher amount of cobalt (14. they cause an increase in the alloy’s creep strength at high temperatures because dislocation creep is limited in the γ mixed crystals [Schu04]. In the microstructures of cast alloys however. the γ phase appears in the shape of Ni3 (Al. For this reason. With increasing amounts of γ phase.Co)3 (Al. it exhibits an uneven particle size. causes particle-hardening of the structural matrix that remains effective up to the high temperature range. which can also be substituted with titanium and tantalum. which is coherent to the γ matrix. It is formed by alloying aluminium. such as Inconel .7%) reduces the solubility of titanium and aluminium and thus makes it possible for the γ phase to form in average temperatures. For example. In the case of the nickel-based alloy Udimet 720.Ti)). Besides an increase in strength at low temperatures. Forging alloys. Waspaloy. The main stress of this chapter is on hardenable high temperature resistant nickelbased alloys.Al. The most essential strength-increasing mechanism among high temperature resistant nickel-based alloys is the deposition of intermetallic phases γ  (Ni3 (Al. The elements Cr. such as Inconel 718. It should be noted that every manufacturer provides its products with its own trade name. W and Ta form carbides of various chemical compositions (MC. 3 average and 5 difficult levels of machinability. it is possible to increase the strength of alloys by pulverizing and compacting ultrahard. cast alloys are no longer forgeable and are generally machined using grinding methods. The machinability of these groups is comparable to that of corrosion-resistance austenitic steel.41). which is present in small concentration in the alloys. which are designated by their trade name as is customary for nickel-based alloys. M6 C and M23 C6 ) with carbon. Mo. PM alloys are assigned to machinability-group 4. these carbides are formed chiefly at the grain boundaries. there are larger amounts (up to about 70%) of γ  phase [Schu04]. Here. In addition to cast-metallurgical fabrication. hot-isostatically compressed and then forged in order to decrease the possibility of a larger foreign component inclusion or remaining inhomogeneity. non-forgeable alloys (cast alloys) generally containing high amounts of γ phase. PM technology makes it possible to develop alloys with a higher high temperature strength than forging alloys. One method variant in this context is “gatorizing”. PM alloys are used to manufacture turbine blades for both civil and military aeroplane engines [Adam98]. parts made of nickel-based alloys can also be produced using powder-metallurgical methods. The goal of PM engineering is to avoid all forms of segregation during solidification as well as concentration and structural gradients. cold-shaping (strainhardening) has a positive effect on chip formation and surface quality. They bring about an increase of creep resistance because they inhibit sliding of the grain boundary [Schu04].6 Machinability of Non-ferrous Metals 317 713C and MAR-M-247LC. The powder is encapsulated. In the case of alloys of machinability-groups 1 and 2. One central problem of PM technology is that no foreign components may infiltrate the powder. Machining using methods with geometrically defined cutting edges is possible to some extent. there is practically no difference between wrought and cast alloys of groups 1 and 4 with the same composition. 7. Machinability-groups 3 and 4 comprise nickel-based alloys that are hardenable by heat treatment. The elements Cr. Due to their higher strength and usually high amount of γ phase in the structure. depending on the cast alloy. In polycrystalline alloys. usually fine casting methods are used close to the final contour that do not require extensive mechanical after treatment [Schu04]. group 1 represents easier. nickel-based alloys can be divided into five machinability-groups (Fig. With respect to machining. also called “super alloys”. Because of the larger amount of γ  phase. In addition. since even those of the same size as the powder particles can lead to cracking in the sense of fracture mechanics. With respect to tool wear and potential surface quality. roughing of the materials of both groups should take place in a solution-annealed state and finishing in a hardened state. This is a process in which the powder is isothermally forged using a creep forming process. Representative materials are listed for each machinability-group.7. Cast alloys of group 5 are very difficult to machine because of the large amount of γ . With respect to their machinability. Ti. The varying chemical compositions and crystalline structures of nickel-based alloys have a direct effect on their machinability. Due to the poor machinability of cast alloys. low thermal conductivity. Due to their mechanical. tools made of high speed steel and cemented carbide can only be used at low speeds when machining nickel-based alloys. Non-hardenable alloys of main group I. ceramic materials are being used increasingly – in the case of finishing both ceramic and PCBN cutting tool materials.41 Classification of nickel-based alloys in machinability-groups (Source: Machining Data Handbook.) Ni-Cr-Fe Legierungen V. In the case of finish turning.318 7 Tool Life Behaviour Machinability group 1 2 3 4 Wrought alloy Alloy of main group I. turning operations are still executed to a large extent with cemented carbides. III. Due to the high thermal and mechanical stress.) Ni-(Cr)-Mo Leg. their considerable tendency to form built-up edges and to strain hardening – as well as the abrasive effect of carbides and intermetallic phases – lead to extremely high levels of mechanical and thermal stress on the cutting edge during machining.) Ni-(Cr)-Mo Leg. 7. .) Ni-Cr-Fe Leg. 5 Cast alloy Hardenable alloys of main group II. Common cutting speeds when turning Inconel 718 and Waspaloy with uncoated cemented carbides of ISO-application group HWK10/20 are vc = 20−50 m/min. Huntington Alloys) phase and of carbides. nickel-based alloys are generally included among materials that are hard to machine. IV.) Ni-Fe-Cr Leg. Under the machining conditions shown in Fig. Presently. III. fractured material particles and cracks in the grain boundaries frequently cause problems when manufacturing functional surfaces [Lenk79].) Ni-Fe-Cr Legierungen IV. In the case of pretreatment under average roughing conditions however.) Ni-Cu Leg. 7. 60% higher cutting forces and about 35% higher temperatures were measured on the chip bottom side when turning Inconel 718 with vc = 50 m/min compared to the heat-treated steel 42CrMo4 + QT [Kloc06b]. cutting speeds of up to 100 m/min can be reached with coated cemented carbides.) Ni-Cr-Co Legierungen High temperature cast alloys Examples Examples Examples Examples Examples Monel 400 Monel 401 Monel 404 Monel R 405 Hastelloy B Hastelloy X Incoloy 804 Incoloy 825 Inconel 600 Inconel 601 Incoloy 901 Inconel 718 Inconel X750 Nimonic 80 Waspaloy Allvac 718Plus Nimonic 90 Rene 41 Udimet 720LI Astroloy LC PM René 95 PM IN100 Inconel 713C Inconel 718C Mar-M247LC Nimocast 739 Fig. thermal and chemical properties. their coarse-grain microstructure and low grain boundary strength.42. Their high high-temperature strength in comparison to steel. They are suited both for finishing and for pretreatment of turbine parts under average roughing conditions (vc = 150−300 m/min.6 Machinability of Non-ferrous Metals 319 Temperature at the chip bottom side Tch /°C 800 X5CrNi18-10 250 700 Inconel 718 150 42CrMo4+QT 100 200 150 600 100 Δ Tch 500 50 + 35 % 200 150 50 Δ Tch 100 20 50 + 10 % 400 Δ Fc – 15 % + 60 % 0 0 400 500 600 700 800 900 1000 1100 1200 1300 Cutting force Fc / N Depth of cut: ap = 2 mm. CBN cutting tool materials available on the market can differ considerably with respect to the modification and amount of boron nitride.12−0. α/β-SiAlONs consist of needle-shaped β-Si3 N4 and globular SiAlON crystals. they combine the toughness of β-Si3 N4 -ceramics with the high hardness and wear resistance of SiAlONs in one cutting material [Gers02]. When machining nickel-based alloys with CBN. coated cemented carbide.12–0. Of the silicon nitride cutting ceramic group (CN) especially the α/β-SiAlONs have proved equal to whisker-reinforced cutting ceramics in their wear properties (vc = 150−200 m/min. grain size and the structure of the .42 Comparison of measured cutting forces and chip bottom side temperatures during turning of different types of steel and the nickel-based alloy Inconel 718 In many machining tasks today. the selection of a type of cutting materials suited to the particular machining task is of primary importance.5−2 mm). In this way. tc = 10 s vc = 20 m/min vc = 50 m/min vc = 150 m/min vc = 75 m/min vc = 200 m/min vc = 100 m/min vc = 250 m/min Insert with chip breaker Quartz fibre 2-colour pyrometer Fig. composite ceramics ductilized with SiC whiskers (CW) have shown the greatest performance potential so far. insert geometry : CNMG120412. the use of cutting ceramics is the state of the art. f = 0. f = 0. 7.5 mm).1 mm. f = 0. ap ≤ 0. ap ≤ 4 mm). Of the ceramic cutting materials. dry cut.12−0.3 mm. ap = 0. CBN cutting materials are primarily used for finishing components made of nickel-based alloys (vc = 250–350 m/min.7. feed: f = 0.3 mm.2 mm. CBN inserts should have a small rounding of about 10 μm on the cutting edge. Ceramic and CBN inserts with a neutral tool orthogonal rake angle (γo = 0◦ ) have become standard for finishing nickel-based alloys. The formation of wear notches on the major and minor cutting edges of insert is characteristic of turning nickel-based alloys with cutting ceramics or CBN cutting tool materials.54) is characteristic of turning nickel-based alloys with ceramics and CBN cutting tool materials. physical and mechanical cutting tool material properties affect the wear and performance of CBN tool to a large extent. The resultant chemical. the more important it is to work within a range that is as narrow as possible around the tool life maximum. Notch formation on the major cutting edge leads to the formation of a burr on the workpiece edge.320 7 Tool Life Behaviour binder phase. While flank face wear is the primary wear criterion limiting tool life in the case of turning with cemented carbide tools. 4. 7. Notch wear at the major cutting edge Burr formation at the workpiece edge Notch wear at the minor cutting edge distinctive feed ridges on the workpiece surface Fig. The closer the ascending and descending branches of the tool life graphs are to each other. For nickel-based alloy finishing.43). 7. notch formation on the minor cutting edge leads to a deterioration of surface quality (Fig. This significantly affects tool performance and component quality. The arc-shaped profile of its tool life graphs (Fig. ceramic cutting inserts are usually provided with a small protective chamfer to stabilize the cutting edge. In contrast. tool life is limited at lower speeds by notch wear and only at higher speeds by rake and flank face wear in the case of ceramics and CBN cutting tool materials. fine-grain CBN types with a TiC or TiN-based binder and a percentage of 50–65 vol% of cBN have proven especially suitable [Gers02].43 Notch wear at the major and minor cutting edges significantly affect the performance and surface quality . This is the result of various wear phenomena which are predominant in dependence of the cutting speed. The arc-shaped curve of the tool life graphs indicates that there is a range of optimal cutting speeds. 6 Machinability of Non-ferrous Metals 321 Wear mechanisms that lead to the formation of wear notches include: • fatigue. partially sawtooth-shaped chip edge. In order to reduce notch wear formation on the major cutting edge. by the workpiece edge and the peaks of the feed groove pattern • tribooxidation – caused by the chemical reaction of the workpiece material and/or the cutting tool material with components of the surrounding medium. A clear separation of these mechanisms with respect to their effect on total wear is only possible to a certain extent [Mütz67].7. 7.high pressures .high shear stresses . Depending on the geometry.high temperatures . • abrasion – caused by the solidified. cracking and crack development phenomena – initiated by the high thermal and mechanical alternate stress on the cutting tool material as a result of lamellar chip formation and transverse material flow.high thermal and mechanical stress gradients .45). flowing laterally into the notches. they exhibit not only a high amount of mechanical stability and thus Direction of chip flow Chip lamella Direction of feed motion Transverse material flow following upsetting of the chip lamella Rake face Flank face Direction of workpiece rotation Burr formation at the workpiece edge Abrasion Surface damage Formation of a sawtooth profile at the edge of the chip lamella Abrasion Surface damage Cutting edge sector with : .high thermal and mechanical alternating stresses Surface damage Adhesion Tribooxidation Fig. The effective use of round inserts is explicable in this context. with the cutting tool material particles. • adhesion – caused by the tendency of the kfz-crystal lattice to form microwelding and the mechanical snagging of the chip edge. the ratio of corner radius rε to depth of cut ap should be as large as possible and the effective lead angle in the area of the contact zone end on the major cutting edge as small as possible. A lead angle of κ r ≤ 45◦ has proven effective for turning with tools made of cutting ceramics and CBN. 7. 7. Wear notches arise as the result of the superimposition and mutual influence of these individual mechanisms (Fig.44 Schematic illustration of reasons which cause notch wear at the major cutting edge [Gers98] .44). A small lead angle reduces the chip thickness and thus the effect of abrasion of the chip edge on the cutting edge as well (Fig. In this area. the effect of the highly abrasive chip edge is concentrated not on one location of the cutting edge. especially with small depths of cut.2 mm should be used [Gers02. which can cause a more pronounced plastic deformation of the rim zone. but notch wear proceeds much more slowly. It is thus recommendable to use round indexable inserts if the machining task allows for it.46).45 Influence of the corner radius on wear formation during turning of a nickel-based alloy with PCBN more resistance to insert fracture. One measure that has proven very effective particularly in pre-machining with round cutting ceramics is “ramping”. the disadvantage of round indexable inserts are the high passive forces. but rather it extends along a larger area thereof. Here. 7.8–1. the cutting edge is indeed subject to more wear. As a result of the continuously changing depth of cut. Notch wear formation of the major cutting edge can also be effectively reduced and tool life improved by the selection of an appropriate cutting strategy. the larger contact arc in the area of the minor cutting edge promotes the formation of wear notches. Furthermore.11 mm ap ap Material: Feed: f h = 0. If very high demands are placed on surface quality.06 mm Udimet 720Li f = 0. . However. the material is removed in pairwise pass with continuously decreasing (1st step) and increasing depths of cut (2nd step).322 7 Tool Life Behaviour rε rε VBGW160408F RCGX090700T κ r = 51° κ r = 21° f h = 0. they also generally result in small lead angles. This has been observed especially in the case of turning with round indexable inserts made of cutting ceramics. 7.15 mm Cutting speed: Depth of cut: vc = 250 m/min ap = 0. so that the insert can be used for a longer time (Fig. CBN inserts with corner radii of 0. but. Kloc07a].3 mm Fig. End milling cutters made of superfine and ultrafine-grain cemented carbides have a much higher level of high temperature hardness than HSS tools.16 mm). As a result. high material removal rates and a large amount of manufacturing safety is possible. vc = 180 m/min.8 0. the tools are subject to an extremely high mechanical and thermal alternate stress. whisker-reinforced cutting ceramic. allowing for a significant reduction of machining times. The milling of such alloys is thus an extraordinarily time-intensive machining process. With a cutting edge geometry adjusted to the particular requirements of these materials. both HSS and cemented carbide tools can only be used with relatively low cutting speeds and feeds.15 mm I c = 261 m I c = 1281 m Fig. They can therefore be used at much higher cutting speeds (vc = 20−100 m/min).46 Adapted cutting strategies allow a reduction of notch wear and a longer tool life (Source: Greenleaf) Because of the extremely high stress on the cutting edge.8 mm ff Verschleißkerbe Notch wear 1. milling with HSS tools is extraordinarily quiet.8 ap = 1. 7. f = 0. HSS tools are used with cutting speeds of vc = 5−10 m/min when machining nickel-based alloys because of their low high temperature wear resistance. They are commonly used for average . so that only low material removal rates can be realized.8 mm Inconel 718. Because of the interrupted cut and high heat resistance of the material.8 mm f a p2: 0. Due to the large number of cutting edges being engaged as well as the milling cutter with the fine knurled splines. Coated end milling cutters made of PM-HSS are therefore suited above all for roughing nickel-based alloys. Their excellent toughness properties allow however for the use of relatively large feeds (fz = 0.7.10−0. milling nickel-based alloys requires cutting tool materials of high toughness and wear resistance.6 Machinability of Non-ferrous Metals Conventional: Constant ap 323 Modified: Variable ap (ramping) a p1 : 0. They are formed as a result of a mechanical overload of the relevant section of the cutting edge. This demand cannot always be fulfilled for the given machining task due to the design of milling cutters fitted with cutting inserts [Gers02]. errors in form and dimension on both groove flanks are extraordinarily few. In this way.5 · D). thus corresponding to finishing conditions. Because of the larger potential depths of cut. By superimposing a wobbling motion. one should strive to have as many cutting edge rows as possible in simultaneous engagement. the cutting parameters must be further decreased in order to reduce mechanical stress and the danger of cutting edge fracture and total breakage of the tool. down milling is used. 7. In the case of plunge milling. the tool is primarily stressed in the axial direction [Kloc04]. whose diameter is smaller than the width of the groove. as a function of the ratio of milling cutter diameter to groove width and the chosen feeds in axial and radial direction. Trochoid milling leads to a considerable reduction of mechanical tool loading. Usually. the material volume to be machined is distributed across a larger cutting edge length. superimposes an approximately circular motion. the realizable depth of cut is several times more than that of conventional milling. For the sake of a dynamically stable and quieter milling process. dependent on the radial feed and the corner design of the end . the tools can only be used in full groove cut at relatively low speeds (vc = 20−40 m/min) and axial depths of cut (ap = 0. the end or plunge milling cutter is plunged axially into the workpiece with low radial feed. not only straight but also curved flanks can be created with this method. The radial toolbending forces are. Faults in shape and dimension on the groove flanks are the result. As a result. A further advantage of this method is that the width of the groove that is to be manufactured is independent of the tool diameter and can be produced accurately in one milling cycle. the wrap-around angle is significantly smaller than 180◦ .324 7 Tool Life Behaviour roughing and especially for finishing operations. a very strong force component affects the tools in the feed direction which strongly subjects the end milling cutter to bending. Since in this method only low forces act upon the tools on both flanks when entering and exiting. long-protruding tools. Especially in the case of slender. the performance capacity of the tools is determined to a great extent by cutting tool material spalling on the cutting edges of the indexable inserts when nickel-based alloys are machined. not only the cutting speed but in particular the axial depth of cut can be significantly increased compared with conventional groove milling. lowering tool costs drastically.47) and “plunge milling”. Economical alternatives to conventional end milling of grooves into materials that are difficult to machine include “trochoid milling” (Fig. If grooves must be generated by rough milling in full groove cut into components made of nickel-based alloys. In the case of long-protruding tools. The tools are thereby used more effectively and tool life is much higher with respect to the amount of material machined. In the former case. the tools are subject to very high levels of thermal and mechanical stress. What is special about this method is that. Due to the wrap-around angle of 180◦ between the tool and the workpiece. When end milling cutters and inserted-tooth cutters that are fitted with HM indexable inserts are used. Due to the high static and dynamic stress. the feed motion of the milling cutter. but.2–3 ϕ /° 30–60 180 Fig.47 Trochiod versus conventional end milling milling cutter. Highly positive and sharply formed cutting edges do indeed have good cutting properties. the jaggedness of the . The disadvantage of this method is that the stress is concentrated exclusively on the corners of the tool. the sharper a cutting edge is the greater is its jaggedness caused by cutting edge spalling. In the past.6 Machinability of Non-ferrous Metals 325 Conventionel milling Trochoide milling Angle of contact ϕ 180° Angle of contact ϕ 0° °-6 30 ** ** Direction of **: rotation 2 Feed motion 1 3 1 4 0. By plunge milling. When machining materials that are difficult to machine. 7.5 x Tool-Ø 2 x Tool-Ø 15–40 vc/(m/min) 60–120 < 0. relatively small. popular opinion held that end milling tools should have cutting edges that are as sharp as possible to machine materials like nickel and titanium alloys because of the small realizable cross-sections of the undeformed chip. as a rule. the performance of an end milling cutter can be affected significantly by the micro-geometry of the cutting edge as well. As investigations have shown. very deep and narrow cavities can be produced.5 x Tool-Ø ap/mm <= 2 x Tool-Ø ≤ 1 x Tool-Ø ae/mm 0.7. 49).48 Influence of the cutting edge rounding on tool wear during end milling of a nickel-based alloy with long cantilever length cutting edge – produced as a function of the grinding process.48). 7. an even wear zone is formed on the cutting edge. Gers02].326 7 Tool Life Behaviour Without CER (sharp-edged) CER 5 µm CER 20 µm Material: IN100 Tool: End milling cutter. Cutting edge rounding stabilizes the cutting edge and either evenly smoothens or removes small defects caused during manufacture by grinding in the form of micro-fractures. this means an increase in cutting performance by a factor of 33 [Krie02.48. The cutting edge can be prepared in different ways. A small rounding of the cutting edge in the order of 5–20 μm can significantly increase the wear properties of end milling tools. A sufficiently high level of toughness . cutting tool material microstructure and cutting edge geometry – influences the performance capability of a tool quite considerably. and wear progresses in a continuous fashion. long projecting: l/d = 6 CER-process: Drag grinding Without cutting edge rounding (CER) With cutting edge rounding (CER) Tool life travel path Fig. the use of cutting ceramics still has restrictions because of their limited toughness. 7. brushing. The causes of such a high performance capacity are based on the chemical and mechanical stability of ceramics. jetting with abrasive media or cutting edge rounding with laser beams can be used [Denk05]. loosened cemented carbide crystals or micro-cracks. especially when machining materials that are hard to machine (Fig.1 mm compared with cemented carbide with vc = 40 m/min and fz = 0. However. 7. At a cutting speed of vc = 800 m/min and a feed of fz = 0. 7. In this way. used on the tools shown in Fig. Besides drag finishing.06 mm. Applicable cutting speeds extend to more than 1000 m/min and are thus many time more than those commonly used for milling with cemented carbide. The use of cutting ceramics based on silicon nitride and whisker-reinforced aluminium oxide when rough or finish milling with inserted-tooth cutters has opened a new dimension in the milling of Inconel 718 (Fig. The fabrication process allows for a considerable degree of freedom in process design. reducing the speed of crack development.7 Machinability of Non-metals 327 Cutting tool material: CA + SiC-whisker Matrix: Al2O3 + ZrO2 vc = 1310 m/min fz = 0. Industrial graphite is characterized by good electric and thermal conductivity and is temperature-resistant up to 3000◦ C.7 Machinability of Non-metals The most technically important non-metals that are commonly machined include graphite and fibre-reinforced plastics. 7. 7. The energy required to loosen these crystals delays crack development. corrosion resistance and chemical resistance can be adjusted to a large extent and across a large range by the manufacturing process. The needle-shaped crystals or whiskers embedded into the ceramic matrix can transfer tensile stress and thus reduce cracking. 28HRC Milling with cemented carbide: 59 h Time: Milling with ceramic: 14 h – 76 % Fig. The main .7.7. cracks extending in the matrix are forced into energy-consuming alternate routes. These effects increase the toughness of the cutting tool material and its dynamic loading capacity. [Krie02].8 − 2. It is manufactured by coking coal.49 High-performance milling of Inconel 718 with cutting ceramics (Source: Kennametal) is necessary to avoid cracks and fractures especially in the case of interrupted cut. 7. Reinforcing ceramic cutting tool materials with needle-shaped ß-Si3 N4 crystals or with SiC whiskers meets this requirement and contributes considerably to the improvement of toughness. whereby the resulting material properties can be to a great extent adjusted to the specific application.1 Graphite Graphite is – like diamond – a modification of carbon and is crystallized in a hexagonal lattice structure.0 m/min Trockenschnitt Engine bracket of Inconel 718.10 mm vf = 1. Further properties such as density. At the needle-shaped crystals.076 − 0. thermoshock resistance. Graphite grains are 3–15 μm large and firmly integrated in a coked binder matrix. By increasing the cross-section of undeformed chip and the cutting speeds.50 Qualitative influences on the machining of graphite electrodes .g. the following effects predominate: • Compressive stresses introduced under the tool cutting edge that are reduced by secondary cracking lead to material disintegration. Therefore. heating conductors. machine construction (e.g. there is a chipping of graphite particles and formation of fracture planes in front of the tool cutting edge (Fig. sealing and sliding elements as well as support materials in • • • • • the semiconductor industry (e. Instead. one of these two effects is predominant. and there is also a pronounced formation of ultrafine dust. There is no plastic deformation in graphite upon penetration of the tool cutting edge into the material. energy transfer (e.g. electrodes for spark erosion). This structural composition leads to brittle material properties with a removal mechanism which differs fundamentally from that of steel processing. • Due to leading crack fronts. heating elements in furnace installations).g.328 7 Tool Life Behaviour application areas of graphite are above all in those with high operating temperatures as electrodes. 7.g. 7. sliding contacts). non-ferrous metal production (e. there is a reduced lowering of stress by secondary ap Feed per tooth fz Contact width ae Depth of cut ap Rotational speed n Feed per tooth fz Contact width ae Depth of cut ap Surface finish Rz Rotational speed n Fig. Dimension deviation Δ Depending on the cutting values and engagement conditions of the tool. This implies especially that experiences made in steel machining are difficult or impossible to relate to graphite processing with geometrically defined cutting edges. there is no “chip formation and removal” such as is found when machining ductile materials. the preparation of ultrapure silicon). sliding rings) and the metalworking industry (e.50). 7.51). The only effective protection against the abrasive effect of graphite dust is polycrystalline diamond (DP) as cutting tool material or the use of diamond coatings because of their high levels of hardness (Fig. When roughing with high material removal rates of electrodes soaked in dielectric. TiAlN. Since by increasing the cross-section of undeformed chip the formation of ultrafine dust is reduced.7. Abrasion by means of hard graphite dust (particle jet abrasion) leads to craters and fractures on the rake face. In general. Improving tool life with coatings such as are familiar in steel processing (e. Tool selection in graphite machining is usually made in the context of wear minimalization. wear is reduced by all measures taken to remove graphite dust quickly from the machining area (blowing. this also reduces tool wear considerably. since the resultant abrasive suspension increases tool and machine wear. diamond coated PKD CD10 0 Cutting speed vc / (m/min) Material: EK85 Grain size: 13 µm 300 600 900 1200 1500 1800 2100 Cutting speed vc / (m/min) Tool: End mill D = 12 mm z=2 Fig.7 Machinability of Non-metals 329 25 20 15 10 HM K10 HM P25 HM K10 (TiN) Cermet 5 0 0 300 600 900 1200 1500 1800 2100 Length of cut per edgefz Lfz / mm Length of cut per edge Lfz /mm cracks. The dominant wear mechanism in graphite machining is abrasion due to the flow of ultrafine dust and friction (wear type: particle jet erosion). so crack development and thus the development of larger fracture planes are favoured. The previously described effects influence the obtainable surface quality and tool wear differently. 250 200 150 100 50 0 K10. 7.05 mm ap = 3 mm ae = 12 mm . The obvious use of wet cutting has negative effects however. dust extraction). Friction contact promotes wear on the flank face by the abrasive washing of the cemented carbide matrix. investigations have shown a clear increase in tool life parameters due to the reduction of ultrafine dust.g. feed per tooth fz and depth of cut ap as well as increasing the cutting speed vc by the formation of crack fronts leading from the cutting edge. Surface roughness increases with the grain size of the graphite and with increasing chip cross-section by enlarging the width of cut ae .51 Wear behaviour of different cutting tool materials Cutting parameters: fz = 0. which however was not reproducible in the case of finishing with small chip cross-sections. TiN) is only possible to a certain extent. other machining strategies can and must be employed in order to get the best results. particle jet wear is reduced due to the flatter inflow and graphite removal from the machining location is favoured. Weakening of the cutting edge is of secondary importance due to the low resultant force. As in steel machining. In the case of sinking electrodes such as are used in the manufacture of . as well as a rapid NC control. The goal of roughing is a high material removal rate with high tool life and constant residual machining allowance. machining strategies that are adjusted to the peculiarities of graphite should be implemented in order to guarantee consistent component quality. To reduce turnaround motions. Dust formation demands efficient ventilation and filtering in an encapsulated working space. Increasing the tool orthogonal clearance angle lowers wear. Figure 7. it is clear that the material properties of graphite. Up milling reduces the risk of fracture upon entry of the tool. a distinction must be made between different machining and engagement situations. When machining curved surfaces. both to better surfaces and to higher tool lives. it is recommendable to work completely in an up milling process and to place the tool exit in the allowance still to be machined of a neighbouring surface. During NC programming. an envelope-curve-limited cut distribution should be made. When machining slim webs. as opposed to steel machining. just as down milling upon the tool’s departure. The danger of tool-drift is higher with smaller grain sizes and increasing graphite hardness. diamond-coated cemented carbide milling cutters with cutting edge rounding and a large tool orthogonal clearance or PCD milling cutters should be utilized. up milling with a falling machining direction leads. with its brittle attributes. On steep contour areas. To do this. The machine tool used must be capable of implementing the high potential process parameters even when machining complex geometries. In this case. In summary. To reduce wear. machining with a falling direction is preferable to a rising one. By using a positive tool inclination angle. high spindle rotational speeds and a high axial dynamic are required. since in the former case stress on the milling cutter is in the direction of maximum tool stiffness and cutter-drift can be reduced. Since the material attributes of graphite differ fundamentally from those of metallic materials. The low resultant force in graphite machining allows for a groove cut D = ae ) with large depths of cut. there is a danger of fracture on the workpiece’s edges. In order to avoid these contrary demands. material accumulation in the area of the grooves requires pre-processing along the groove before the neighbouring surfaces can be finished and the groove pulled behind. Especially when thin tools are used.52 shows two graphite electrodes typically used in tool and mould construction. demand a fundamentally different process design in the case of milling. the use of tools with a negative tool helix angle is recommendable in order to lower the risk of fracture due to tensile stresses. The low machining forces and low thermal stress make it possible to use high cutting speeds and feed velocities and thus place special demands on the tool/process kinematics/machine tool system.330 7 Tool Life Behaviour The rounding of the cutting edge associated with coating causes increased chipping of graphite particles and leads to a somewhat worsened surface quality. The goals of finishing are a high surface quality and precision of contour as well as low machining times and high tool life. 52.52 Examples of sinking electrodes for electro discharge machining a computer mouse (Fig. 7. Köni90b.7. milling and sawing have proven the most effective. Since this volume of the compendium deals exclusively with machining methods with geometrically defined cutting edges. only grinding methods are used as the typical finishing method for producing clearances such as bearing seats [Wuer00.7 Machinability of Non-metals 331 Fig. This has a decisive influence on method selection or also. If functional surfaces must be manufactured from plastic. divided electrodes must be produced which are required for the fabrication of grooves in injection moulding tools. with which the component is often fabricated “from solid”. Of this subgroup of manufacturing processes. Fibre-reinforced plastic press parts can be deburred both by methods with geometrically defined and geometrically undefined cutting edges. These are affected during machining essentially by the properties of the fibre type used in the plastic. Frequently. The fibre is characterized by high tensile strength. 7. it is attempted to fabricate components made of FRP close to the final contour so that the ratio of the volume machined to the total volume remains small. As opposed to metallic materials. 7.7. or at least only at unwarrantable cost.2 Fibre-Reinforced Plastics Fibre-reinforced plastics (FRP) consist of a duro-plastic or thermoplastic polymer matrix into which short or long fibres are embedded. Wien87. For example. left) usually complex free-form geometries must be generated. To produce components made of fibre-reinforced plastics. its elastic . high demands with respect to formal and dimensional accuracy or high levels of surface quality cannot be met. these methods will be in the foreground. as a rule post-production operations are required in order to form the components in accordance with their function and geometrical characteristics. Long fibres can be introduced as roving or a sheet material. Wuns88]. To machine a component made of fibre-reinforced plastic. the suitability of tool designs. the mechanical and thermal material properties are of especial importance. for example. which sometimes deviate a great deal from those of the plastic matrix. The properties of the matrix have more of an effect on process management. The low temperature conductivity of the polymer matrix materials is a further problem. high fracture strain and above all by low temperature resistance of plastics.2 μm Shearing with prestressing 2. 7. it has. 7.5 μm 5 μm 1. which can lead to a “frayed” cut surface during the machining process. reinforcement fibres have differing properties under mechanical strain in accordance with their respective structure.54).5 μm Fig. Certain conclusions can be drawn from the results of the shear experiments. with or without axial prestressing. As a result of this. and thus the component is damaged by burns and the tool by thermal strain (Fig. whereby the fracture plane of carbon fibres is somewhat rougher. This further lowers the already low temperature resilience of the matrix. As opposed to thermosetting plastics. Its “machinability” is characterized by the low elastic modulus. depending on the fibre type. In addition.332 7 Tool Life Behaviour modulus and this low fracture strain compared to the matrix. which stay in a solid state until their decomposition temperature is reached. However.5 μm 2. shear or bending strain. highly varied thermal parameters. the thermoplastics used for composite materials soften beyond a temperature of about 200◦ C.53 clarifies this by looking at the fracture behaviour of single fibres in different load cases. low strength. regarding the “machinability” of the respective fibre types. Glass and carbon fibres exhibit brittle fracture properties under tensile. The much tougher aramid fibres tend on the other hand to deflect the cutting edge under shear or bending stress.53 Fracture behaviour of different fibre types 11 μm . These fibres can only be satisfactorily cut when prestressed and are frequently divided axially [Böns92]. the process heat introduced to the material can only be poorly removed from the active site. Figure 7.5 μm F Aramid fibre 6. Glass fibre Shearing without prestressing 2 μm Carbon fibre 1. This results in high surface roughness values as well as the formation of very small particles made of fibre and matrix packets. If on the other hand it is machined with a 90◦ -fibre orientation. 7. Selection . An efficient suction and filtering of these particles is therefore absolutely necessary. it is advisable to seal off the working area. which lead to a high level of machined surface quality. Wuer00]. is characteristic of machining fibre-reinforced plastics [Rumm96. In addition. When machining under the 0◦ -fibre orientation. the surface is always heavily damaged by the cutting edge. milling and drilling [Köni91.7 Machinability of Non-metals 333 Surface Cutting particle 0°-Fibre orientation Fibre fracture Matrix fracture 20 µm 45°-Fibre orientation Crack deflection Fibre-/Matrixpackage 50 µm 90°-Fibre orientation Fibre fracture Matrix fracture 10 µm Fig.54 Failure mechanisms during machining of fibre-reinforced plastics Due to its inhomogeneous structure. particle formation of single fibres and matrix fragments is predominant. Due to the high levels of mechanical stress. Measurements of these particles have shown that respirable sizes do exist (particle < 5 μm) that can have a negative effect on the health of the operator. Tras92]. Machining with a 45◦ or 135◦ -fibre orientation leads to a superimposition of the above particle formation types. causing a large amount of dust. The development of very fine particles made of fragments of the base material and of fibres. The rim zone of the machined area near the surface is then characterized by microcracks. the “machining” of fibre-reinforced plastics involves the formation of highly diverse “chips” compared with metallic materials. the particles formed are primarily fibre and matrix packets.7. The quality of the machined surface is dependent to a large extent on the orientation of the fibres in the polymer matrix. The most common machining method when manufacturing components made of fibre-reinforced plastics are turning. Because of the large number of possible material combinations. For a high-quality machining of aramid fibre-reinforced laminate. Such tools must therefore have a high level of resilience against abrasion and of toughness. it is advisable to use a two-way mill twisted in contrary directions.56). In many cases.7. As in the case of glass fibre and carbon fibre-reinforced plastics. To use these tools successfully. elastic rebound of the fibres embedded in the base material can be considerably reduced. Untwisted two-edged milling cutters with sharp cutting edges should be used to mill components made of directed long fibres with a large fibre amount.2. their length and their percentage in the composite material. This prestressing can be achieved with special tools that have been specially designed for this purpose (Fig.56 shows a distinction made on the basis of the thickness of the component to be machined. cutting speeds of 800–1200 m/min with moderate feeds per tooth have yielded successful machining results. and the cutting edge radius should be in the range of the fibre diameter (~10 μm). the high hardness of glass fibres and in particular of carbon fibres leads to pronounced wear phenomena on the tools employed. but they involve much higher acquisition costs. With respect to the geometry of the cutting edge. the cutting edge must be very sharp. 7. it must have a very low level of raggedness. tools twisted in two ways towards the middle were developed that make it possible to change the distribution of the machining forces acting on the workpiece. To make a precise separation of the fibres possible. The grinding of a chamfer on the flank face of the milling tool has also proved effective. For thin workpieces. A classification into areas of application is made according to the type of fibres. Suitable cutting tool materials are ultrafine-grain cemented carbides (HF). generalizations can be made regarding optimal cutting parameters. In this way. the tool must be precisely aligned with the workpiece. polycrystalline diamonds (DP) or diamond-coated cemented carbide tools (HC) of application group K10. DP-fitted milling tools are superior to cemented carbide milling cutters with respect to realizable tool life parameters and surface quality. the use of tools made of cemented carbides of application group K10 have also proven effective here.1 Machining Method: Milling During machining. reducing thermal and mechanical stress during machining. In the case of this tool .334 7 Tool Life Behaviour of the most proper tool should be made in analogy to machining metallic materials with reference to the technological. For thicker components. standard cutting edge geometries can be used so long as it is taken into consideration that the fibres can only be acceptably cut when prestressed. Further tool requirements include a high level of cutting edge sharpness combined with a small cutting edge radius and a high level of rake and flank face surface quality in order to minimize friction on the workpiece [Köni90b]. economical and ecological limitations. Figure 7. since only such a tool can separate the fibres cleanly. 7. 7 Machinability of Non-metals Electroplated grinding bolt 335 Diamond toothed cemented carbide mill Cemented carbide mill with DPedges Double-edged cemented carbide mill Glass fibre Carbon fibre Short fibre Long fibre Fig. The following describes difficulties involved with particles arising when machining fibre-reinforced plastics (dust. As . CRP) Special tool geometry Low cutting edge radius rβ < 10−15 µm Slight cutting edge chipping Very high sufrace quality at rake and flank face Rz < 0.7.5 μm) dusts. remain in the air long after machining is finished and sediment very slowly.5–10 μm) and alveolar (< 4. fibre particles).8 µm High wear resistance Carbide special mill. the permanent alternate stress prevents the fibres from eluding the cutting edge.57 clarifies the high concentration of particles that remain in the working space even after the machining process and that is only gradually reduced. 7. Figure 7. Especially the small particle sizes.55 Tools for machining fibre-reinforced plastics (GRP. right-left twisted Carbide special mill with contrary fixed DP-edges d > 3 mm Fig.56 Tools for machining aramid-fibre-reinforced plastics variation as well. thoracic (4. alternating twisted to the middle d < 3 mm Carbide special mill. 7. special cutting geometries should be used in order to guarantee separation of the fibres while prestressed (Fig. Potential surface roughness depends heavily on the type and length of the fibres.336 7 Tool Life Behaviour 250 Concentration / (mg/m³) Total dust Thoracic dust 200 Alveolar dust 150 100 50 0 0 100 200 300 400 500 Time t/s Milling time Fig. so this limit must be strictly observed. Besides a small cutting edge radius. Standard tools cannot be utilized to machine aramid fibre-reinforced plastics. 7. it is necessary just as in milling to differentiate in accordance with the materials to be machined.5 mg/m3 was not yet reached. It is important to know that even after 500 s. but they can only be used for manual drilling operations to a limited extent. Tools that are equipped with DP cutting edges are far superior. The experiment shows that it is nonetheless absolutely necessary to isolate the working area and to suction off and filter the resulting particles.2 Machining Method: Drilling In the case of drilling. depending on the orientation of the fibres.58). all fibre lengths are always cut. To drill CFP and GFP. In this case. 7. there is a highly aberrant surface formation in the drill hole. highly wear-resistant tools with standard cutting edge geometries can be used as long as the cutting edge radius is as small as possible. so that.2. 7. The percentages of these sizes in the entire volume of dust can be influenced by tool selection and the cutting parameters. . a 90 s long machining process was conducted and the machine was immediately switched off. glass-fibre and carbon fibre-reinforced plastics or aramid fibre-reinforced plastics. Cemented carbide drills with very positive rake angles have become established for this.7. In many cases. When the tool is rotated. These particle sizes are highly carcinogenic.57 Dust particle size during milling of fibre-reinforced plastics an experiment. solid cemented carbide tools produce good results. high levels of surface quality on the flank and rake faces are required so that friction between the chip and the tool and between the workpiece and the tool reduces potential adhesion of the material on the cutting edge. the legally prescribed fine dust limit of 1. 7. 35° Side clearance angle 10° . 30° Side clearance angle 10° . there are no fibre fractures in aramid fibre-reinforced plastics... This is due to the tough material properties and poorer adhesion of the aramid fibres to the base material. only very poor surface qualities can be realized. in the case of fibre-orientations of 105–135◦ with a simultaneous compressive load. macroscopic cutting wedge geometry Side rake angle 30° . 12° Drill point angle 120° CFRP und GFPR Special.58 Tools for drilling fibre-reinforced plastics It is characteristic of the material that.7. 140° AFRP Fig. .. As opposed to CFP and GFP.7 Machinability of Non-metals 337 Low cutting edge radius High surface quality at rake and flank face High wear resistance Conventional macroscopic cutting wedge geometry Side rake angle 20° .. 14° Rear clearance angle 0° Tool cutting angle 125° .. but the fibres are pulled far out of the cut surface and bent against the cut surface. widths of flank wear land of VB = 200−800 μm are permitted for flank face wear when cemented carbides are used.Chapter 8 Process Design and Process Monitoring 8. C Springer-Verlag Berlin Heidelberg 2011 DOI 10. first a depth of cut ap should be selected that is as large as possible provided it is not F. Variation in the machinability of a material of the same standard designation but different batch can be larger than three to one in extreme cases. Generally. For an optimal cutting value determination.3 · f (8. Specification of these limit values refers to the description of the cross-section of undeformed chip using the theoretical parameters uncut chip thickness h and undeformed chip width b and must be converted in accordance with geometrical relations that are true for the respective cutting process into the technical parameters depth of cut ap and feed f (Chaps.1 Finding Economical Cutting Parameters 8. Klocke. Manufacturing Processes 1. feed and the expected tool life parameters for the given tool life criteria. 9 and 10). depending on the case at hand.1) In addition to tool wear. the other main evaluation parameters of machinability – the cutting force.1007/978-3-642-11979-8_8. cutting speed. RWTH edition. The tool life criterion for crater wear should be determined as a function of the feed f with the following equation: KT = 0. This ratio is the more unfavourable the larger the permissible wear and the higher the cutting speed [Kluf83]. as tool life criteria.06 + 0. the tool life parameters and wear properties of the tool are the most important criteria for evaluating the tool life behaviour of a material/cutting tool material combination [Köni71]. When determining the cutting parameters. the limits of the system workpiece/cutting medium/tool/machine must be taken into consideration.1.1 Cutting Parameter Limits The further development of workpiece materials and cutting tool materials and the increasing automation of machine tools demand an occasional correction of machining standard values.  339 . In order to obtain the largest possible volume removal rate within a given tool life. surface quality and chip form – also serve. Standard values should be determined under consideration of machinability and provide reference data concerning depth of cut. coated indexable inserts always have a cutting edge rounding due to manufacturing conditions.1 Maximum of undeformed chip width Process Design and Process Monitoring Tool included angle εr /◦ Chip width b 80–90 60 35–55 2/3 · cutting edge length 1/2 · cutting edge length 1/4 · cutting edge length already defined by the machining allowance to be lifted in one cut. the machine’s power can be exploited more fully. these roundings stabilize the cutting edges just like the chamfer of the rake face. After determining the depth of cut. For example. which is a function of the stiffness of the system workpiece/cutting medium/tool/machine and is intended to guarantee chip removal especially in the case of rounded off or chamfered cutting edges. tools with a large tool included angle εr are used. For roughing. The maximum depth of cut apmax is limited by the stability of the tool’s cutting edge. Following SOKOLOWSKI [Soko55]. In addition to the maximum and minimum permissible feed of the machine.25 < hmin < 1. The rules of thumb given in Table 8.1 serve as the standard values.2) The lower feed limit is determined using the minimum chip thickness hmin . Larger depths of cut or undeformed chip width can lead to cutting edge fractures.3) This minimum chip thickness can be limited both by the material (e. the feed should be set as large as possible in order to minimize the primary processing time. for turning for example: 0.g. which have a propensity to strain hardening) and by the tool cutting edge form. In the case of roughing and when interrupted cuts require a large amount of cutting tool material toughness. returning and reengagement is economised. . the radius of which is between 20 and 60 μm. in the case of austenitic steels. the elastic portion is greatly increased during the separation process and chip removal can no longer be guaranteed. In the case of very small chip thicknesses or chip widths. A large depth of cut reduces the number of required steps.1.2). so that besides the primary processing time (Sect. In this way.125 rn (8. the following inequality can serve as a guide: fmax ≤ rε (8. the following can serve as a standard value.340 8 Table 8. 8. tool limits must again be considered in selecting the feed. the secondary processing time for lifting. The smaller influence of feed on wear formation compared with cutting speed and the fact that specific cutting force is reduced with increased feed both speak for this sequence in determining the cutting parameters. In order to avoid unmachined areas on the workpiece. 1 Finding Economical Cutting Parameters 341 1. The images on the edges of the respective solution field show what chip forms can be expected.3 0.2 Ribbon chips 0.1 Chip form recommendation for inserts with chip breaker Another parameter which limits feed is the predefined surface roughness. for the applied workpiece material and for the applied cutting tool material The cutting speed is determined in accordance with the previously defined tool life parameter on the basis of a preset tool life criterion by a tool life parameter function (Chap. Figure 8. so the feed should not be set too high in order to maintain a certain level of surface quality (Sect. 8.0 Discontinuous chips 0.1 shows solution fields of favourable chip forms in feed as a function of the depth of cut for two indexable inserts of the same dimensions with different chip breakers. The chip form is of great importance for an undisturbed manufacturing process (especially in the case of automated manufacture with NC and CNC machines) and for the protection of the machine operator. When machining workpieces with rim zone structures that are especially difficult to cut (e. 7) or with standard value tables. for the applied process kinematics. This depends primarily on the feed. forging. The cutting speeds given in the standard value recommendations are usually applied to stable cutting conditions on pre-machined workpieces.7 0.6 0.8.65–0.8 Feed f / mm 0. for the applied tools.g. rolling or cast iron skin) the given cutting speeds should be multiplied by a factor of 0.4 0.1 Snarled chips 0 1 2 3 4 5 6 7 8 9 10 11 Depth of cut ap / mm Fig.8.5).5 Long cylindrical helical chips 0.9 0. Such solution fields are applicable only: • • • • • for the applied manufacturing method. 3. . machine investment costs. proceeding from the aims of management policy: • minimal manufacturing costs KFmin • minimal allowed time for a process temin In the case of finishing.2 Optimizing the Cutting Parameters Due to labour costs. by the use of automated tool changers). Moderate cutting conditions result in a long tool life. little tool change and low tool costs.1. two optimization targets stand in the foreground. to VDI 3321 .2 Impact of cutting parameters by values of labour costs. This allows for higher potential cutting speeds. the optimal value function is derived from the High cutting parameters Short tool life High application intensity High wages Expensive tool Expensive machine Low application intensity Long tool life Low cutting parameters Fig. In roughing.2 illustrates the relation between cutting parameters and labour costs. the constantly dropping costs for tool cutting edges and improving wear properties. The necessity of shortening the tool life in the case of capital-intensive machine tools is understandable when one considers the costs. the cutting parameter recommendations of cutting tool material manufacturers currently refer to a tool life of 15 min. tool life reduction by increasing the cutting conditions leads to lower manufacturing costs. a target value must be selected. Depending on the machining task. Corresponding to the required optimization target. Figure 8. which lead to high labour and machine costs depending on the volume machined. Since labour and machine costs have increased considerably while tool and tool change costs have risen much more slowly (e. The following is oriented formally and content-wise towards the guideline published by VDI regarding cutting parameter optimization [VDI3321]. On the other hand. Improving the cutting tool materials also results in improved wear resistance. acc.342 8 Process Design and Process Monitoring 8. lower workpiece tolerances.g. When optimizing individual machining processes as well. marginal conditions stemming from the production process such as machine availability or cycle times of linked production plants must be considered when determining the machining parameters. Here. predefined surface qualities or other parameters that are important for the functional reliability of the component must be observed. other optimization targets are required. 8. long machining times also result. 4) The additional time tv is the total time required for all irregular events such as procuring necessary resources. The workpiece change time tWST is the time that passed until a workpiece is changed.5) The main process time th is the time in which direct progress in the sense of the production order is made via machining [VDI 3321]. Set-up time is based on the batch time m: number of workpieces per machine: nWM = m (8. measuring.7) tn = tWM nWT Tool change time tW is the time that passes until a tool is changed.1 Finding Economical Cutting Parameters 343 manufacturing time or manufacturing cost equation. additional time tv and recovery time ter : te = tg + tv + ter (8. Since the addends of the auxiliary process times sometimes do not accrue for every workpiece or every process. The basic time tg is the sum of the main process time th and auxiliary process time tn : t g = th + tn (8.6) j To calculate the main process time. The number of processes required until completion is taken into account by the number of feed paths i and the cut length j. the feed paths Li and cut lengths Sj are considered in conjunction with the respective feed velocity vf . adjusting. they are considered proportionately. The auxiliary process time tn is the time during which all indirect processes arising during the machining operation (e.9) .8) The tool change time is based on the tool life T or the tool operating life nWT : number of workpieces per tool life: nWT = T tc Here. The recovery time ter takes all pauses into consideration during which the machine tools are not in operation. (8. Time per unit te is composed of the basic time tg .8. pro rata tool change and workpiece change) are executed [VDI3321]. The following is valid for the main process time: th =  Li  Sj + vfi vfj i (8. tightening. and both the position correction and positioning for re-entry have taken place. This time is partially contained in the auxiliary process time tap [VDI3321].g. the cutting time tc is the time in which the tool is actually cutting. The following is valid for the auxiliary process time: tr tW + + tWST (8. . E usually has highly differing values. L as hourly wage and pL as the amount of non-wage costs. KWu as the tool residual value. KWs costs per regrind and KWv1 and KWv2 as pre-adjustment costs outside and inside the machine.1.2.14) with KWT as tool costs for the tool operating life nWT including pre-adjustment costs.16) with KWP as the indexable insert acquisition value. ns the number of potential regrinds. the following is true for the time per unit te per workpiece or process: te = th + 1 tc · tr + · tWZ + tWST m T (8. H.1 Optimal-Cost Cutting Speed The manufacturing costs per workpiece KF comprise labour and non-wage labour costs KL .11) Labour costs are calculated as follows: KL = KLH · te = L · (1 + pL ) · te (8. pure machine costs KM . The following is true for tools with indexable inserts: KWT = KWP KWH KWE + + + KWv1 + KWv2 nTP nTH nTE (8.12) with KLH as labour and non-wage labour costs per hour.15) with KWa as the tool acquisition value.13) The proportionate tool costs are defined as KW = KWT tc = · KWT nWT T (8. The tool operating life nT with i = P. KWH as the tool holder acquisition value and KWv1 and KWv2 as pre-adjustment costs outside and inside the machine.10) 8. Machine costs are derived from the machine-hour rate and the time per unit te : KM = KMH · te (8. tool costs and residual factory overheads Kx . KF = KL + KM + KW + Kr (8. For the tool costs per tool life for re-grindable tools we have: KWT = KWa − KWu + ns · KWs + KWv1 + KWv2 ns + 1 (8. number of workpieces per tool life T and tc as cutting time.344 8 Process Design and Process Monitoring Finally. 3 Manufacturing costs as function of cutting speed Primary processing costs . one can first select the optimal-cost depth of cut apok at the maximum. tool costs and costs contingent on the main processing time as a function of cutting speed. Due to the shortening of the tool life with increasing speeds. The influence of the depth of cut on tool wear is minimal.18) with KLH as the labour and non-wage labour costs. manufacturing costs can be summarized in the following manner: KF = KL + KM + Kx + KW = KML · teB + KW (8. feed f and cutting speed vc should be evaluated differently with respect to their optimization. raising the tool costs.3 shows graphs of the manufacturing costs. By using the machine and labour cost hourly rate.1 Finding Economical Cutting Parameters 345 The residual factory overheads are calculated in the following way: Kx = KxH · teB 60 (8.8. In the first step in cost-optimization. Steadily increasing the cutting speed cannot lower the manufacturing costs further. more frequent tool change is necessary. Manufacturing costs Toolcosts CMmin voc Cutting speed vc Fig. KMH as the machine hour-rate and Kx as the residual factory overhead. The cutting parameters depth of cut ap .17) with KxH as the residual factory overheads per hour and teB as the occupation time on the machine. Thus at very high cutting speeds the proportionate tool costs can become the largest addend of the manufacturing costs. The machine and labour cost hourly rate KML is defined as: KML = te · KLH + KMH + Kx teB (8. 8.19) Manufacturing costs CM Figure 8. feed and cutting speed: T = Cv · vkc · Cf · fzkfz · Ca · apka logT / min High speed steel k = –5 k = –8. the cutting speed vc is freely selectable across a wide range utilizable for optimization.4 clarifies the dependence of the slope of the tool life straight lines on the cutting tool material when machining iron materials.1).5 k = –12 Cemented carbide k = –2. 8. The description of tool life behaviour is limited in general to a range linearized in a double logarithmic system in which this behaviour can be described with sufficient accuracy (Sect.1. levelly falling tool life straight lines on the other hand for more high-temperature resistant cutting tool materials (e.4 Tool life straight lines for cylindrical turning. Mathematically. The expanded tool life function from a formulation made by TAYLOR contains all three parameters – depth of cut.g. 7. which is designated by the tool life parameter.25 k = –8 log vc / (m/min) Fig.1 and Fig. 8.5 k = –4 k = –2 k = –1. a regression analysis is executed applying a general exponential function. to VDI 3321 . 8. Cutting speed has a considerable influence on the wear behaviour of the tool. Figure 8.346 8 Process Design and Process Monitoring Although manufacturing costs have an absolute minimum as a function of feed and cutting speed. the second step is to select the maximum technically possible feed as the optimal-cost feed fok . In due consideration of the cutting parameter limits (Sect. Steeply falling tool life straight lines are characteristic of temperature-sensitive cutting tool materials (e. which in turn influences the tool change costs. Rising machine and labour costs as well as shorter auxiliary process times cause a shift in the economical cutting speeds towards the lower range of tool life parameters.2.5 (8. high speed steel). As opposed to the parameters ap and f. acc. the feed is of only secondary importance as a freely selectable optimization parameter since the optimal feed is above the technically possible feed for most practical cases.g. cutting ceramics).4).20) Ceramic k = –2. 1 Finding Economical Cutting Parameters 347 Since both the optimal-cost depth of cut apok and the optimal-cost feed fok have already been selected. taking into consideration the process being used.4). the time amounts have to be substituted by the cutting speed. tc (8. the following equations are valid for the main process time and for the cutting time using Eq. we obtain:  t  tc c KFo (vc ) = KML · th + · tw + C1 + · KWT T T (8. T = Cv · vck (8.27) π · D · sj 1 = C3 · vc · fok vc (8. (8.23) Here it is advisable to consider the dependencies of the individual amounts of cutting speed.26) tr tw + + tWST nWM nWT (8.25) Since the optimal value function should be a function of cutting speed.6) and substituting the constant items with the constant C3 : th =  Ci  sj + = C2 + tc vfi vfj (8. All time parameters that are not a function of the cutting speed are viewed as constants and substituted in the following way: C1 = tr + tWST + tv + ter nWM (8. In the case of a machining method using a rotational main motion with constant cutting parameters. into Eq.22) KF = KML · (th + tn + tv + ter ) + · KWT T Using the known structure of the auxiliary process time we obtain:   tc tw tr KF = KML · th + + + tWST + tv + ter + · KWT nWT nWM T (8. it is sufficient to use the TAYLOR’s simplified tool life equation to derive the optimal value function of the costs.19). (8.28) i tn = j tc = j · . (8.8.21) The optimal value function fow is set up by inserting the tool life Eq.24) Inserted. (8.21) and the main process time Eq. we shall dispense with the proof. the manufacturing time must be optimized with respect to the cutting speed.5 clarifies the difference between cost and time optimization. (8. the second derivation must be greater than zero. Figure 8. After re-substitution. we obtain the following for the optimal-cost tool life: Tok   KWT = −(k + 1) · tw + KML (8.32) To make the cutting speed minimal-cost.1. . Eq.29) Now the methods of a general extreme value task can be used so that the necessary and sufficient condition are met. (8.34) 8. Eq.33) into the tool life equation. For the first derivation according to cutting speed we first obtain:   dKF0 C3 · KWT 1 C3 tw − (k + 1) · = KML −C3 · 2 − (k + 1) · · (8.33) After inserting the optimal-cost cutting speed. At this location however.348 8 Process Design and Process Monitoring The tool life equation must also be valid.31) The necessary condition for extreme value tasks demands that the first derivation becomes zero so that the following is true for the optimal cutting speed:  dKF0 = 0 → vc = dvc k − (k + 1) · C3 · (KML · tw + KWT ) KML · C3 · Cv (8.2 Optimal-Time Cutting Speed To determine the optimal-time cutting speed.30) dvc Cv vc (k+1) vc Cv vc (k+2) This expression can be simplified as follows: KML · C3 C3 · (KML · tW + KWT ) dKF0 =− − (k + 1) · dvc vc 2 Cv · vc (k+2) (8. so for the optimal value function we obtain:   1 tw 1 + C1 + C3 · · KWT + C3 · KFo (vc ) = KML C2 + C3 · vc vc T vc · T   C3 1 C3 tw = KML · C2 + C3 · + · + C1 + · KWT vc Cv vc k+1 Cv · vc k+1 (8.2.21). the following is valid for the optimal-cost cutting speed: vok     −(k + 1) · tw + k  = Cv KWT KML  (8. 26) and (8. the following demands must be met: Pspindel > Pc = Fc · vc D Mspindel > Mc = Fc · 2 (8.39) One can assume 5% per 100 μm width of flank wear land as a standard value for the correction factor Kvk . obtained via:  tw (8. 8.36) After determining and optimizing the cutting parameters.Finding Economical Cutting Parameters 349 Costs respectively time 8. to VDI 3321 This means that a cutting speed should be selected which allows for a short main process time and a long tool life.35)) into the tool life Eq. Degn00]. this can be carried out for the optimal-time cutting speed. .21).38) The cutting force can be determined by the calculation basis given by SALOMON [Salo24]. which is used here in KIENZLE’s notation [Kien52] in expanded form with Kvk as the correction factor for tool wear [Lang72. f and vc must be evaluated for their realizability with respect to available spindle power and spindle moment. Fc = kc1.35) vot = k −(k + 1) · Cv After inserting the optimal-time cutting speed (Eq. acc. (8.33)).5 Difference between time and cost optimum. VDI3206. (8. In so doing.37) (8. we obtain for the optimal-cutting time tool life: Tot = −(k + 1) · tw (8. the cutting parameters ap . Analogously to the determination of the optimal-cost cutting speed (Eqs. (8.1 Busy time Manufacturing costs C Mmin Primary processing costs Tool costs Tot Toc Tool life T Fig. and thus the ratio of cutting time to tool life only comprises a small amount of the auxiliary process time.1 · b · h1−mc · Kvk (8. The acquisition value KAW does not remain constant over the entire term however. room costs KR.6 clarifies the calculation of the imputed interest for the amortization term. 3200 h/a or three-shift operation ca.42) KZges = KVE + 2 2 . 8.g. The imputed amortizations KA are comprised of the acquisition value of the machine KAW . The sales revenue KVE can be depreciated over the entire term. The following is valid:   KAW + KVE KAW − KVE ·i·t = ·i·t (8. and energy costs KE are taken into consideration.41) The duration of amortization t is relevant for determining the imputed amortization KA . Figure 8. 4800 h/a). imputed interest KZ.350 8 Process Design and Process Monitoring If the spindle power is insufficient. The financial basis for the duration of amortization can be found in such AfD lists. When determining the machine hour-rate. two-shift operation ca. the depth of cut or feed should be reduced accordingly. maintenance costs KI. but gradually decreases.1.3 Calculating the Machine Hour-Rate The machine hour-rate describes the costs to be calculated of a machine tool per hour. The imputed interest over the amortization term KZges can be interpreted geometrically as surface area under the cost function. The decreasing value of capital assets is designated in fiscal terms as allowance for amortization (AfD). In the case of multi-shift operation. since they turn out differently for different assets.40) The yearly machine runtime TN amounts for example to 1600–1800 h/a for single-shift operation. The machine hour-rate KMH can be calculated as follows: KMH = KA + KZ + KI + KR + KE TN (8. It should be taken into consideration that the symbol I always indicates the rate of interest per year. The duration of amortization is calculated from the minimum of the expected technical or economical service life. the • • • • • imputed amortizations KA. the amortization duration t and the sales revenue of the machine KVE after expiration of the amortization duration: KA = KAW − KVE t (8. the runtime is increased proportionately (e. 6 Imputed interest calculation 351 Costs 8. we distinguish between manual (external) and computer-aided cutting data determination. In the case of manual cutting parameter determination and optimization. resulting in: KZ = KAW + KVE ·i 2 (8. 8. give the process planner only very rough reference values for the target cutting parameters [VDI3206. documents from various sources are utilized: • • • • guidelines of different standardization committees catalogues and cutting parameter recommendations of the tool manufacturers trial results from research papers in-house data banks for cutting parameters Machining standards. such as are published by the Verein Deutscher Ingenieure (VDI) (English: Association of German Engineers) for various machining processes as VDI guidelines.4 Planning Methods and Tools According to the type of data determination. In this case.39) relate to 1 year. Usually. On the other hand. it can be included in the rate of payment.7).5 · (KAW – KVE) · i · t KVE KVE · i · t Amortization period t Since the costs of Eq. interest. energy costs. the imputed interest for 1 year KZ must be inserted in this equation. If the manufacturing labour is dependant on the machine run-time.43) Amortization.Finding Economical Cutting Parameters Fig.1. machine type and machine-hour rate that often . we refer to it as a cost per man-hour. (8. VDI3208. 8. VDI3209].1 Cav 0. The boundaries here. 8. room costs and fixed maintenance costs are all fixed quantities. are fluid (Fig. tool consumption and the variable maintenance costs are variable quantities. cutting speeds are recommended for broadly associated material groups independently of chip cross-section. depending on the computer use. tool life behaviour. to GEBAUER [Geba80] lead to an uneconomically high tool life. etc. More exact results can be obtained from test reports. Uninterrupted. 8. CIMSOURCE. Based on these sources.Standard value tables .Process optimization with continuous sensors and adaptive control adaption Fig. Figure 8. which collect cutting data from industry and research and make these available to companies in a form specifically tailored to them. 8. WinTool. Many tool manufacturers today offer software systems (e. acc.).352 8 Process Design and Process Monitoring Methods for process optimization Manual fixed value optimization Manual Computer aided offoff-line optimization line optimization Computer aided on-line optimization Indicators . internal cutting data collections are often made as part of a company’s process preparation. Information centres for cutting data. These data bank systems contain cutting data for set tool life parameters for a comparable workpiece material/cutting tool material combination and under consideration of geometrical influencing variables. with the help of which the determination of the cutting parameters can be combined directly with the selection of a suitable tool. deviations in the composition of .Process optimization by end of tool life .8 shows the input mask of a data bank as an example of such a type of cutting parameter selection. Process disturbances are. CoroGuideTM .7 Methods of process optimization. unavoidable. These collections make reference to the accumulated practical machining cases in manufacture and also take into consideration the long-term experiences of specialists and skilled workers.Formulas .Process optimization with intermittent sensors . Stochastically occurring tool failure. are another modern phenomenon.Nomograms . fully or partially automated manufacturing processes guarantee high productivity and optimal capacity utilization. however. which however are valid in most cases for special machining conditions and can only be applied to other machining cases with reservations.g.2 Process Monitoring Secure process monitoring has a crucial role in manufacturing technology. Such a data bank makes it possible to process large collections of previously determined machining data under consideration of many influencing parameters and to provide improved target values. . Monitoring of the drilling process has been widely researched and finds extensive use in industry.8 Example for technology data organization (Source: CIM GmbH) the machined material or faulty planning/programming prior to machining are all potential causes for such interruptions. Tool monitoring has been developed to varying extents depending on the various machining processes with defined cutting edge geometries. Process monitoring systems make it possible to monitor cutting tools. Accordingly. that the machining process must be monitored in order to minimize consequential costs of process disturbances.8. In the case of milling. workpiece or machine tool damage can result from tool fractures if the proper countermeasures are not promptly applied.2 Process Monitoring 353 Fig.e. extensive tool. be it manually or automatically (i. Common to all monitoring systems is that process-relevant information is measured with the help of specialized sensors. fracture and wear monitoring is much more costly than in the abovementioned processes due to the more complicated process kinematics. there are only a few monitoring solutions that exist for milling processes. This means. The goal of process monitoring in manufacturing technology is to recognize these process disturbances in a timely fashion and to introduce proper countermeasures. For example. The tools are thereby monitored for sudden tool fractures or overloads and for increasing tool wear. with the help of monitoring systems). The most common sensors and sensor principles today will be introduced in the next chapter. There are also different approaches from research and practice for realizing a reliable tool fracture and wear-recognition monitoring system for turning processes. among other things. 8. in order to determine the measurement parameter. • The measurement method is defined as a special kind of procedure used in the measurement independent of the measurement principle. permeability etc. of which most can be classified in six physical functional principles. . longitudinal/torsional strain. In order to standardize usage and to lend semantic clarity to the terms used. While for direct methods. current.2 shows an allocation of typical measurement parameters and the physical functional principles that primarily underlie them. concentrations etc.2 Relation of measurement parameters to physical functional principles Physical functional principles for process description Mechanical (motion.1 Sensors for Process Monitoring A number of sensors are available on the market for monitoring manufacturing processes. charge. heat flow. Direct measurement methods record the measurement parameter of interest by means of a comparison with a normal of the same physical parameter. torque. bond energies of molecules) Associated measurement parameters Position. velocity. the problem with indirect measurement methods is in the determination of an assignment rule. • A measurement process is the practical application of a measurement principle and a measurement method. A further distinction is drawn between direct and indirect measurement methods. magnetic current etc. Table 8. longitudinal/torsional stress. emission.354 8 Process Design and Process Monitoring 8. conductivity etc. stiffness) Thermal (kinetic energy of atoms and molecules) Electric (electric field) Magnetic (magnetic field) Radiation (electromagnetic radiation) Chemical (forces between atomic nuclei and electrons. intensity. Indirect measurement methods. the validity of which should not be affected Table 8. displacement. Permeability. Energy. • The measurement principle is the physical basis of the measurement. the essential fundamental concepts of metrology will first be defined [Pfei01]: • The measurement parameter is the physical parameter that is to be measured. • Measurement is the execution of activities planned for the quantitative comparison of measurement parameters with the unit. capture an auxiliary parameter which stands in a known and describable relation to the measurement parameter [Pfei01]. acceleration. force. reflection. specific heat. Chemical components.2. pressure etc. it is often difficult to reach the measurement location with suitable sensors. Voltage. Temperature. • The measurement result is the value taken from the measurements estimated as the true value of a measurement parameter. thermal conductivity etc. 45). the relation between strain and relative resistance change can be described by Eq. amounts . The adhesive is of great importance. For this reason. Yet the advantages and disadvantages of the direct and indirect measurement methods cannot be generalized and must be weighed for every particular case. The strain gauge is fastened to the shifting position on the measuring body. The OHM resistance R of a metallic conductor is a function of the material (ρ e ).9 Process emissions for monitoring of cutting process ou st ic em Resultant force is si on Ac ou st ic Acceleration He at Ac em iss io n by changes to the process parameters. (8.8. a strain gauge consists of a resistance wire. The following is concentrated on the monitoring of cutting processes. 8. the resistance change is dependent only on the length and cross-section change. torque and acceleration.44) In the simplest case.2 Process Monitoring 355 ra dia tio n Fig. which is frequently incorporated into a supporting matrix. Assuming that the material properties of the strain gauge are not changed by the strain (ρ e = constant). for example for a constantan strain gauge. Strain sensitivity k.2.9 shows the process emissions relevant for this area. 8. the following sections will examine the workings of strain gauge strips and piezoelectric sensors. In general. length (L) and cross-section (A) according to the following relation: R = ρe · L A (8. Numerous sensors use the strain of an object as an auxiliary function for measuring force.1. because the strain or compression of the measuring body must be transferred unaltered to the strain gauge via the adhesive connection. Figure 8.1 Strain Gauges A strain gauge converts a mechanical strain into an electrical resistance change. 45) In order to reduce the dimensions in comparison to the single-wire form. 8.10). Also valid is the relation given in Eq.46) UM = U0 · (R + R1 ) (R + R4 ) − (R + R2 ) (R + R3 ) (2R + R1 + R2 ) (2R + R3 + R4 ) (8. Strain gauges are not limited to the use of resistance wires. The WHEATSTONE bridge circuit is shown in Fig. usually meander-formed designs are utilized (Fig. For a balanced bridge circuit with R1 = R2 = R3 = R4 = R and a feed voltage U0 .11 Wiring diagram of a WHEATSTONE bridge R4 . 8.46) Depending on whether two or four active strain gauges are used. 8.11. this relation can be simplified in the form of a quarter. With the help of a bridge circuit.044. (8. However. For a quarter bridge with Carrier foil Fig. The normal operating range of strain gauges extends up to a relative strain/compression of about 1%. foil and thin-film materials are also employed. it is possible to measure the arising resistance changes in the form of a voltage variation UM . 8.356 8 Process Design and Process Monitoring to k = 2. R/R = k · ε (8. the deflection areas cause a sensitivity of up to 3% transverse to the measurement direction. UM = 0. Semiconductor.10 Sketch of a strain gauge strip with measurement meander Connectors Measurement meander R3 R1 UM U0 R2 Fig. half or full bridge. The resistance changes arising during the use of strain gauges are very small and are only a fraction of its total resistance. 1. This makes it possible to raise sensitivity ever further and. (8. in case all the strain gauges are exposed to the same temperature. This should be taken into consideration when cutting crystals (Fig.2 Process Monitoring 357 R1 = R and R2 = R3 = R4 = 0 and R << R the Eq. By deforming the crystal lattice along the polar axes. 8.g.8. tourmaline and ferroelectrical ceramics. In the case of piezo-actuators. R (8. Piezoelectric coefficients range from 2 to a present maximum of 1500 pC/N.12).48) UM = U0 · 2R In the case of the full bridge. (8. R (8. generating an electrical dipole moment.47) A half bridge can be used to increase sensitivity by applying two identical strain gauges so that the amount of strain of one is equal to the amount of compression of the other. in which a mechanical change in length is produced by applying voltage.49) UM = U0 · R 8. the positive and negative lattice elements are displaced relative to one another. all four arms of the bridge circuit are fit with strain gauges. i. e. 8. R2 = −R and R3 = R4 = 0. Quartz (SiO2 ). Fig. a temperature compensation is also possible. R1 = R.2 Piezoelectric Sensors Piezoelectricity is caused by the electromagnetic interaction between the mechanical and electrical state of crystals that have no centre of symmetry.48) is true in this case. This charge transfer can be converted into a voltage signal with the help of a charge amplifier. Their piezoelectric coefficient is direction-dependent and indicates sensitivity in the direction of the corresponding axis. Piezoelectric materials have one or more polar axes.47) is valid. Equation (8.2.49) is valid here with R1 = R4 = −R and R2 = R3 = −R. the inverse piezoelectric effect is utilized. UM = U0 · R 4R (8.e.12 Possible cuts through a synthetic crystal (Source: Kistler) . It is used for simple measurements with a single strain gauge. Eq. Qx = d11 · Fx · n (8. Unstressed crystal Stressed crystal Fx – O + + – y + – – + x – Si – + + +++++++++ Fig.14 Shear effect in piezoelectric materials Fx . (8. 8. 8. with d11 as the piezoelectric coefficient in the x-direction. in this case a shear force arises in the direction of the charge pickup (Fig. 8. The amount of charge can be calculated for the longitudinal effect with Eq. but only by mechanical series connection and electrical parallel connection of several plates.13 Longitudinal effect in piezoelectric materials 8 Process Design and Process Monitoring Unstressed crystal Stressed crystal Fx + Si + O – – – – x y + – + – + + ++++++++++ Fx In the case of the longitudinal effect. Fx as force in the x-direction and n as the number of crystal plates. a charge transfer occurs on the force contact surface as a result of a mechanical deformation along a polar axis of the crystal perpendicular to the pickup of the charge (Fig. The charge difference should not be increased with the geometrical dimensions of the crystal plates.14).13).50). 8.50) In the case of the shear effect. stress occurs as in the longitudinal effect along the polar axis of the crystal.358 Fig. However. with d11 as the piezoelectric coefficient in the x-direction. the amount of charge should be influenced by the dimensions of the piezo-element and can be calculated with Eq. Fy as force in the y-direction and b/a as the geometric ratio.3 Force Sensors For measuring forces and torques in manufacturing technology. inductive and capacitive position transducers are utilized. occasionally.2.52).15). stress is applied in the direction of a neutral axis of the crystal.8. Qx = 2 · d11 · Fx · n (8. piezoelectrically generated charge transfers from flowing off rapidly and distorting the reading. In general. 8.1.51) In the case of the transversal effect. Especially when the sensor is arranged in the main force .15 Transversal effect in piezoelectric materials The charge thereby released can be computed using Eq. the isolation materials must have a high level of stiffness for transferring mechanical stresses in order to support the piezo-element against the housing. In this case. 8. piezoelectric sensors and. (8. (8.2 Process Monitoring 359 Unstressed crystal Stressed crystal Fy O + – y + – x Si – + + + + + + – – + – + - Fy Fig. with d11 as the piezoelectric coefficient in the x-direction. The isolation effect must also be guaranteed during use under high temperatures and under the influence of moisture. especially in the case of static and quasistatic measurements.52) Another essential component of piezoelectric sensors are the isolators. Fx as force in the x-direction and n as the number of crystal plates.51). Isolators are subject to extremely high requirements in order to prevent small. Furthermore. 8. Fundamentally. one can measure either in the main force connection or in the secondary force connection. all systems convert a stress into an electrically evaluable magnitude. and the charge occurs orthogonally to it on a polar axis (Fig. Qy = −d11 · Fy · b a (8. the sensors require a high level of pre-stressing. that the sensor itself is not damaged by an overload. 8. 8.16 shows the structure of a piezoelectric sensor used for measuring force components. 8. The plates are arranged mechanically in a series and connected electrically parallel so that the charge is proportional to the torque applied.18 shows a four-component dynamometer used for measuring three force components and torque. two basic principles can be used. on the one hand. on the other hand. If shear-sensitive force sensors are used. the sensors can sustain only extremely low tensile stresses. a corresponding number of additional quartz plate pairs of varying orientation are used. Figure 8. In order to measure torque. one must be sure that. When longitudinally sensitive force sensors are used. Figure 8. The high stiffness and high overload stability are advantageous in this regard. the stiffness properties of the entire system are not negatively affected by the integration of the sensor and. However.360 8 Process Design and Process Monitoring connection.17 Schematic view of assembly of shear sensitive quartz plates in a torque sensor . Circular covering plate Quartz plates Electrode Housing Connector Base plate Fig. Since the shear stresses are non-positively transferred. so that they must be sufficiently prestressed with a compressive stress in the case of tensile or tumescent stress. the torque applied can be directly measured (Fig. In the case of sensors designed to capture several components. Two longitudinally sensitive quartz plates serve as the sensor elements. it is possible to make inferences about the torque from non-parallel sensors under consideration of their reference coordinates.16 Schematic view of a piezoelectric dynamometer Shear sensitive quartz plate M Fig.17). 2 Process Monitoring 361 Fig. in which a small piezoelectric force sensor for measuring cutting force has been fit beneath the cutting edge. above all piezoelectric force Fig.19 Rotating dynamometer for milling processes (Source: Kistler) . In this way it is possible to analyze reliably dynamic process parameters even in high-frequency areas. Besides strain gauges. Piezoelectric measuring systems are characterized by their high levels of stiffness. this is often designated as the capacity of piezoelectric measuring systems for highly dynamic measurement. In this case. Figure 8. measuring in the secondary force connection has as a rule the advantage of simpler sensor integration into the machine’s structure. In common usage. This principle can also be applied to rotating tools. Figure 8.8. a signal transmission between the rotating and stationary parts of the measuring system is required. the measurement section has a high characteristic frequency. 8. Such systems are currently still in prototype stage. 8. If however high dynamics are unnecessary in the measurement.18 Dynamometer for drilling processes (Source: Kistler) Similar systems exist for turning and milling processes as well. If beyond this the oscillating mass located in front of the sensor element is small.20 shows the schematic of tool holder for turning.19 shows a rotating four-component dynamometer for milling. 8. it is then possible to monitor tool fracture and overload.20 Turning tool with integrated dynamometer 8 Process Design and Process Monitoring Insert Intermediate element Slim-linesensor Insert holder Fig. turning and milling machines – are another possible way to measure resultant force. 8. Force measurement rings – integrated into the main or feed spindle bearings of drilling. Figure 8. If this is chosen correctly. it is also possible to apply strain gauges on the outer bearing ring or in a special bearing sleeve. 8.21 Quartz transverse measuring pin (Source: Kistler) measurement pins are suitable as sensors in such cases (Fig.22 shows a piezoelectric ring sensor.22 Multicomponent force sensor for integration into spindle bearings (Source: Kistler) . These indirectly measure force via the strain of the structure into which they are integrated. 8. In some cases.362 Fig.21). Fig. A general disadvantage of measuring in the secondary force connection is that it is necessary to analyze the machine structure in order to determine the optimal assembly location. 23 Schematic assembly of an accelerometer . so that thermally caused errors in measurement can also occur. 8. the pre-stressing is altered by temperature changes. usually piezoelectric accelerometers are used. Furthermore. The susceptibility of the sensor type to these sources of disturbance is much lower in the case of shear-sensitive accelerometers (Fig. The main components of an accelerometer are therefore the seismic mass. sensor element and housing (Fig. The factor of proportionality is the inert mass of the body upon which the force is acting. It is especially important to take this fact into consideration in the case of experimental modal analysis.1.24. A sensor element acting on normal forces is shown in Fig. 8.4 Accelerometers For measuring mechanical process dynamics in machining. The basic functional principle is based on the proportionate relation between force and acceleration. the large surface contact between the sensor element and the base plate leads to the stresses of the measured object being transferred via the base plate to the sensor element. 8.2 Process Monitoring 363 8. shear-sensitive piezoelectric ceramics exhibit no thermally caused charge transfer (pyroelectricity). shear forces or flexural forces. Every mass additionally mounted on the measured object changes its vibrational properties (characteristic frequency) and thus in certain conditions call into question the result of a measurement.8. As a result of the design. One example Seismic mass Sensor element Connector Fig. in which a charge transfer is produced by normal forces. This brings about a measurement error. Besides their lower basic strain sensitivity.24).23). 8. In detail.2. we differentiate between three different accelerometer designs. The mass of the accelerometer must be much smaller than that of the structural mass to be analyzed. 24 Accelerometer with shear sensitive sensor element Sensor element Connector Fig. increasing sensitivity over a larger seismic mass results in a lower characteristic frequency and thus to a more limited available frequency range.364 8 Process Design and Process Monitoring Seismic mass Fig. in which f0 is the characteristic frequency. .25. improper use results in marked sensitivity to mechanical destruction due to beam fracture. the sensor element and the seismic mass are identical. (8. sensitivity. 8. This can been seen in Eq. 8. the mass and characteristic frequency. 8. For example. In this case. Upon displacement of the beam due to acceleration. Besides the low characteristic frequency and the pyroelectric attributes of the sensor. Simultaneous fulfilment of all goals represents an optimization problem due to physical limitations. The sensor is selected primarily in accordance with the measurement range.25 Accelerometer with a piezoelectric beam Tap a Piezoelectric ceramic beam Charge amplifier of a mass-reduced accelerometer is shown in Fig. a charge transfer is caused. The active component of the accelerometer consists here of a piezo-ceramic bending beam. c the stiffness of the sensor element and m the seismic mass.53). 53) In comparison to quartz. No additional seismic mass is required to detect . the frequency range extends from audible sound (ca. Figure 8.2 Process Monitoring 365 Tool turret Tool Tool holder Magnet Accelerometer Fig. 30 MHz).2. ceramic elements are generally not as stiff.26 Application of an accelerometer at the tool holder f0 = 1 · 2π  c m (8. a triaxially measuring accelerometer should be applied close to the action point. The surface waves arrive via a thin membrane with integrated docking element at the sensor element. Depending on the type and magnitude of the emission source. 8.1. e. In order to capture the dynamics of manufacturing process as freely as possible of disturbances in practice. 8. This property can be utilized in order to reduce the seismic mass by a factor of 100 at equal sensor sensitivity. However.27 shows a diagram of possible sources and causes of the development of acoustic emission (AE) in machining processes. which is surrounded by a damping mass. directly under the turning tool.28 shows an AE-sensor in cross-section. 8. Figure 8. the sensor was thus fastened with a magnet on the tool holder.8. 16 kHz) to the high ultrasound range (ca.g.5 Acoustic Emission Sensors There are also dynamics in the province of acoustic emissions [Eise88]. so this approach leads to losses with respect to the characteristic frequency.26. This ideal solution is not always realizable within the machining space of a machine tool. In Fig. some piezoelectric ceramics provide charge differences that are 100 times higher under otherwise identical conditions. Thus. 8.27 Sources of acoustic emission Piezoelectric element Damping mass Membrane and docking element Fig. the area . still other piezoelectric or elastic coupling possibilities exist that can lead to undesired resonances during the vibration measurement [Saxl97]. In practice however. AE-sensors are constructed such that the mechanical oscillation direction is either perpendicular or parallel to the electric polarisation. Piezoelectric ceramics are especially suited to AE-measurement because of their high sensitivity.366 8 Process Design and Process Monitoring 5 1 Plastic deformation in the shear plane 2 Friction at the rake face 3 Crack initiation and crack growth So un d 6 pr 4 Friction between tool and workpiece op ag at io n 7 5 Collision of the chip on the tool or workpiece 1 3 6 Chip breakage So 4 un d pr 7 Friction by redirecting the chip on the rake face op ag at 2 io n Fig.28 Schematic assembly of an AE-sensor the AE signal. 8. The inertia of the sensor element itself is sufficient to produce a measurable charge transfer at typical frequencies between 50 kHz and 2 MHz. 54) . When applying AE-sensors therefore. The power consumed by the motor of a machine tool is composed of an effective and an idle component. High-frequency signal components are more strongly attenuated than low-frequency ones. The effective power is obtained from these using Eq.54). which is directed either at the workpiece. the tool holder or the tool. current I and phase shift φ between both quantities. Köni89b. Another area of application is in the recognition of unfavourable chip forms in automated turning processes [Kutz91. Köni96. Reub00]. Kett96]. Because of its proportionality to the torque emitted by the motor. Surface sensors are attached to the machine structure and register the surface waves produced by the AE-signals.g. Kett96. Dorn89. In order to guarantee constant and reproducible coupling conditions. the effective power is often used as a signal within the control system for quantifying the motor load.6 Effective-Power Sensors Changes to the resultant force components lead not only to dislocations or deformations but also to a change in current and power consumption of the main and feed drives. Sensor selection and positioning are decisive criteria for a successful process monitoring. Fluid acoustic sensors measure the AE-signal via a fluid jet (e. 8. We make a distinction between one. the surface making contact with the sensor element must be machined to a high level of surface quality. fracture and wear can be recognized in turning as well as in milling and drilling operations [Diei87. The extent of this signal attenuation is also strongly dependent on frequency. since the use of other sensor principles has remained problematic due to the small cross-sections of undeformed chip and low resultant forces. Typical frequency ranges are. 50–400 kHz or 100–900 kHz. cutting fluid). The principle of effective-power measurement is based on determining the voltage U. one is faced with the basic conflict of goals between attachment near the process in order to receive signals that are as little attenuated as possible and a positioning which protects the sensor from interference from hot chips or cutting fluid and does not obstruct tool or workpiece change. Pw = √ 3 · U · I · cosφ (8. usually external effective-power measurement tools are used. Köni92b. two and three-phase systems. often with associated evaluation software and visualization unit. One example from the sphere of grinding operations (see Manufacturing Processes. for example.2. When monitoring cutting processes. AE-signals are attenuated at joining points and material inhomogeneities in an order of about 11 dB per point of intersection [Dorn93. whereby three-phase systems have the highest resolution because they execute the measurement in all three phases. Kloc05a]. Volume 2) is the recognition of burn by means of acoustic emission analysis [Saxl97].2 Process Monitoring 367 of use of sensors is limited to suitable frequency ranges. Mori80. With AE-sensors.1. (8.8. It is important that the optimal position for sensor assembly is determined. Monitoring solutions based on acoustic emission are suitable for finishing processes as well. Their suitability as an efficient process monitoring tool depends to a decisive extent on the ratio of the power input generated by the machining process and the total power of the drive. signal adaptation is very simple.368 Fig. The primary area of application is the recognition of tool fractures and collisions in the workspace. Current systems also have electronic signal conditioners. To read the current. Reliable monitoring is particularly difficult in the case of small . hall sensors are used that measure the current along with its phasing via the magnetic field surrounding the conductor in a circular shape. Hall sensors are offered in various performance classes. Effect-power measurement systems are characterized above all by the fact that they do not affect the mechanical properties of the machine tool.29 Wiring diagram for the assembly of effective-power measurement system 8 Effective-power measurementtransducer Process Design and Process Monitoring Frequency converter I1 I2 I3 L1 L2 L3 3~ External effective-power modules are installed in machine tools between the frequency converter and the motor. Furthermore. 8. The machining torque can be measured during the operation without the integration of external sensors into the electric flux of the machine. The initial adaptation can be achieved by varying the number of conductor loops that are led through the hall sensors. Figure 8. During integration into the machine tool. Sufficiently large force changes are necessary for efficient wear monitoring. the systems are inexpensive and easily retrofittable. Small machining torques can thus no longer be reliably measured in the case of spindles with a collectively large power input.29 shows a typical wiring diagram for a three-phase system. for example. detectors and a measuring amplifier (Fig. The principle of the quotient pyrometer is extensively discussed in the dissertation of Müller [Müll04].30). Resistance thermometers utilize the temperature-dependence of the resistance of a conductor/semiconductor for temperature measurement. precise and absolute information on temperature. Variations in intensity that are not caused by temperature. the resistance of which increase with temperature in an easily reproducible manner. by partial impurification of the optics. The wavelength range is between 0.7 and 1000 μm. e. 8.8. The method is hardly suitable for measuring dynamic machining torques due to the high inertia (of the motor armature. Furthermore. The inert masses of the power train represent a low-pass filter that considerably impedes the swift measurement of dynamic magnitudes because of the low cutoff frequency. i. which provide highly dynamic. In the pyrometer housing are the optics. Besides the single-cutter system and the twin-cutter system [Gott25.g. The electronic housing contains the control of the Peltier element cooling and the power supply. Data acquisition and evaluation can be automatically executed with an attached laptop. it is also possible to integrate a thermoelement into the tool [Küst54] or the workpiece [Hopp03]. Usually metallic materials are used. clamping mechanism and so on) in front of the sensors. resistance thermometers and thermocameras. Thermocameras allow for a non-contact. measurement by quotient formation is for almost completely independent of the degree of emission of the material so long as there is no significant dependence on wavelength. The majority of commercial infrared cameras use however only the spectral range of medium and long-wave infrared from 3.7 Temperature Sensors The cutting temperature is an important process-characteristic quantity for evaluating thermal stress on the workpiece and cutting tool material surfaces in the contact zone. have no influence on the measurement result. in the case of finishing or of drilling with small diameters. They function according to the principle of thermography. filter.5 to 14 μm. Vier70]. Thermoelements make use of the thermoelectrical effect between two metals.e. 8. their resistance decreases with increasing temperature. The glass fibre is sheathed with a protective tubing. In addition to thermoelements. but rather. the intensity of which is a function of temperature. quotient pyrometers are also used for measurement purposes. Some semiconductors have negative temperature coefficients. extensive measurement of temperature. In the case of a quotient pyrometer – also referred to as a ratio pyrometer or 2-colour pyrometer – intensity is not only measured with one wavelength but by the ratio of the intensities of two different wavelengths.1. Every object emits a band of infrared radiation. particularly platinum and nickel. . Infrared radiation is captured fibre-optically and sent to an evaluation unit.2 Process Monitoring 369 cross-sections of undeformed chip or small process forces and torques.2. The result of the measurement is a temporally and spatially high-resolution temperature signal such as is shown in Fig.32 using an example from drilling. 8.42 mm) Radiation ≈ 1 mm Fig.31 Temperature measurement of machined workpiece surface . In order to measure at optically difficult to access and thermally/mechanically heavily stressed areas. 8. To do this.370 8 Surface of the measured object Process Design and Process Monitoring Colour divisor Light sensor Lens L Light duct IR-filter Measuring amplifier Fig.31 shows a typical application on an indexable insert for measuring temperature on the machined surface of the workpiece. Measurement data acquisition and evaluation can be carried out with a PC. Figure 8. the measurement location must as a rule be prepared with a hole.30 Functional principle of a quotient pyrometer (Source: WSA RWTH Aachen) Metallically sheer surfaces have small degrees of emission. it is possible to position a fibre-optic cable very close to the contact zone between the tool and the workpiece. into which the fibre can be inserted. Workpiece Chip Carbide tipped cutters Quartz fibre of the 2-colour-pyrometer (D = 0. 8. the methods used either accompany the process or are intermittent.05 mm = ½ f 700 600 500 400 300 200 300 18.88 18.32 Example of a highly detailed temperature signal In the case of the system described here. measurement errors do occur in the case of differing emission degrees for both of the wavelengths on which the measurement is based.86 200 18.2 Process Monitoring 371 800 0. these signals should exhibit a close correlation to the process. signal processing is first required.1 Drilling distance/mm Fig.9 18.1 mm = f 0.7 18.8 18. Depending on the required system reaction speed. When cutting fluids are used. the lower limit of measurable temperature is about 200◦ C. A continuous.2 Signal Processing and Monitoring Strategies In order to develop process monitoring strategies.94 Drilling distance/mm 19 19.9 18.05 mm = ½ f Temperature /°C Temperature T /°C 700 600 500 400 800 0. While losses of intensity by partial impurification of the optics do not distort the measurement result.2. A higher amplification factor results in a higher resolution but also reduces the measurable temperature range and the measurable maximum temperature.8. 8.92 18. Independently of whether the signals are obtained from external sensors or by machine tool control readouts. The intermittent measurement permits reaction after reinstatement . process-concomitant measurement offers the most rapid possibility of detecting events and introducing a suitable reaction. The potential temperature resolution depends on the selection of the amplification factor. measurements are extremely unstable due to the different optical properties of the cutting fluid and the fibre material if the optic is wetted. 8. This measurement path is characteristic of many other manufacturing processes. The signal-to-noise ratio (SNR) is thus low. The signals generated by the sensors generally have low energy contents. signal reception and processing are physically and temporally separated. In order to improve the SNR therefore. signal reception and signal processing occur simultaneously. In the case of online processing. Signal processing has the exclusive function of extracting from the total received information that information which relates to the process parameters to be monitored. Figure 8. A basic distinction is drawn between online and offline signal processing systems. making disturbance-free signal transmission and evaluation more difficult.33 Measurement path for acquisition of acoustic emission . so it is possible that disturbances arising outside the measurement cycle are not reflected in the measured signals. Signal acquisition and assessment Process Sensor /-s A A A MZ Documentation of process developments A-D converter f Bandpass fu Pre-intensification Reaction on detected process solutions Highpass fg P1 FZ fo f P2 Main amplification RMS-ractification Lowpass fg f Fig.372 8 Process Design and Process Monitoring of the measurement cycle at the earliest. or bandpass filtering takes place. Therefore. lowpass. the signal is subject to preamplification. 8. it is necessary to determine signal processing strategies with an eye to the signal properties.33 shows an example of a measurement path for the acquisition of AE-signals in a machining process. Depending on the monitoring task and the type of sensor signals. either highpass. The properties and characteristics of the systems used determine to quite a significant extent the basic properties of the signals that are picked up. No new information is generated during signal processing. Often the signal data is then filtered in accordance with the frequency range that is relevant for the given monitoring task. In offline processing. 55). amplification takes place in the main amplifier. In the effective value rectification. After signal filtering. (8. seff   T  1 = s2 (t)dt T (8. preprocessing is applied to a signal with constant frequency (Fig. or they might be extra disturbance signals that were introduced into the measurement chain from outside. sliding. Reub00]. Burst fraction Voltage U Bandpass filtered signal 0 Continuous signal fraction Voltage U Rectified signal 0 Voltage U Lowpass filtered signal 0 Time t Fig. especially in the case of AE-signals. the effective value of the sensor signals can be rectified following Eq. a time-dependent. 8. quadratic average – the RMS (root-mean-square) – is formed from an output signal [Saxl97.34).55) 0 For the sake of simplicity. Examples of disturbances include irrelevant machine vibrations or external electromagnetic disturbance fields.34 Filtering and RMS-rectification . 8. Optionally.8. The irrelevant signal components might contain information assigned to the process but which is not relevant for the application at hand.2 Process Monitoring 373 The essential function of signal filtering is to filter out those signal components from the raw signal that do not correlate with the process parameters to be monitored. Principally. 8. a tolerance zone and revolving thresholds. Monitoring by means of static thresholds is the simplest type of signal-based monitoring. In the case of time-consuming monitoring processes. further filtering or amplifying of the signals may be necessary depending on the case at hand.35 shows some classic examples. In the simplest case. the analogue measurement values are digitalized in an analogue/digital (A/D) converter. a trend. it is possible to consider signals in the time range or frequency range. After the effective value is rectified. evaluated with the help of software and further processed in digital form. Table 8. signal evaluation in the time range has the advantage that it does not require a time-consuming transformation into the frequency range that demands a large amount of computer capacity.374 8 Process Design and Process Monitoring RMS signals have only positive signal components and are used to determine monitoring parameters. Action limits and tolerance zones are useful for identifying disturbances.35 Action limits and tolerance zones Time Time . Figure 8. In order to make information-technological processing of the signals possible. it is also possible to identify characteristic signal profiles. Wave analysis makes it possible to combine information coming from both ranges. This average is Table 8. process disturbances lead to significant signal changes that can be determined directly from the temporal signal profile. The signal level for the undisturbed process profile is determined in preliminary tests. This signal level is then used for monitoring in the form of a stable limit. a sliding average is calculated from the recorded signal over a defined time period. The measurement values can then be acquired.3 Characteristics of signal analysis in the time and frequency ranges Time range Frequency range Temporal signal profile Transformation of the signal into the frequency range Spectral signal composition Discontinuous signal evaluation in temporally limited signal sections No information regarding temporal signal changes in the transformed signal section Limited information regarding the temporal signal profile by analysis of several consecutive signal sections Tolerance zones Static thresholds Signal Stable limits Signal Signal Continuous signal analysis or in temporally limited sections Almost no information regarding signal frequencies No time-consuming transformation Time Fig.3 compares general attributes of the three methods [Reub00]. In the case of monitoring with dynamic thresholds. Besides a static limit value. Beyond that point. After reaching the end of tool life (i.8. breach of the action limits can be used to produce an automatic reaction of the machine by altering the process parameters (Fig. Fig. 6.5 3 Finishing 2. Irrespective of the fact that the appearance of grinding burn at the first two evaluation points can not be detected by exceeding the threshold value.5 Averaged AE-RMS 5 4. the total amplitude differences are not large enough to lead to reliable information about the development of grinding burn.2 Process Monitoring 375 URMS / V associated with a percentage addition or deduction. the graphs are almost parallel and differ by a signal difference of about 1 V.36 for the roughing and finishing phases of the machining of a total of 1650 gearwheels. The advantage of this strategy compared with a stable limit is that it is also possible to monitor signals that do not proceed uniformly or whose level constantly changes even in an undisturbed process. making it possible to monitor the signal profile dynamically. Grinding burn is a form of undesirable thermal rim zone damage and is described more thoroughly in volume 2 of this series. The profiles of the RMS values of the signals are shown in Fig.5 Grinding burn 2 0 200 400 600 800 1000 Amount of gearwheels/pcs.36 Analysis of acoustic emission in time interval 1200 1400 1600 . 8. When profile grinding tooth flanks. a structure-borne sound sensor is affixed near the workpiece. 8.37). This practical example will be taken up again when dealing with signal analysis in the frequency range. Besides the pure detection of deviations. Saxler provides an example of signal analysis in the time range in his dissertation on the structure of a system for recognizing grinding burn via acoustic emission analysis [Saxl97]. after grinding 1550 gearwheels) are found the highest RMS values of the acoustic emission within the tool life with the exception of the first two evaluation points.e.5 Roughing 6 5. Of particular interest is that both graphs have asynchronous profiles until the 900th gearwheel. 8.5 4 3. 376 8 Process Design and Process Monitoring Process Machine Drives Actuators Fixed parameters Material Workpiece Geometry Machine Clamping External sensors Force Path Speed Acceleration Sensors NC-kernel Internal signals Path Effective-power Speed Temperature Acceleration RPM Power External control Variable parameters Feed Cutting speed Cutting fluid pressure Control algorithm Revision Cutting strategy (Cutting depth) (Tool geometry) Coupling module Storage Definition of interfaces Drives Actuators Fig. the variable of the control loop is varied such that the control variable reaches a level that is as constant as possible. This is necessary because frequent false alarms are a major problem in the industrial use of process monitoring systems in the case of the devices used today. The learning curve could represent.37 Concept for intervention in the manufacturing process For this purpose. New developments in process monitoring are focused on model-based monitoring methods or simultaneous evaluation of different sensor signals in order to obtain a maximum amount of security from interruption. Müll76. One classic example of an adaptive control constraint in machining is the maintenance of a constant level of torque by adjusting the feed in the case of an alternating overmeasure. Gath77] and adaptive control optimization [Esse72. The parameters feed and speed could then be freely selected within defined limits so that the performance follows the learning curve despite wear. adaptive control constraint [Gies73. Adaptive control optimization involves varying one or more variables so that the control variable follows a learning curve. there are two basic control strategies. For adaptive control constraint and . Häns74. the profile of spindle performance over the feed path and be stored for a specific machining situation as an optimal process sequence. 8. In the case of adaptive control constraint (ACC). for example. Otto76]. This will be dealt with later (wavelet analysis). Signal evaluation in the frequency range is especially sensible when different periodic signal components are superpositioned. Roughing and finishing signals during tooth flank profile grinding undergo an FFT within a frequency range of 200–400 kHz. Generally. which is characterized by its fast and efficient transformation algorithm. frequency analysis also makes it possible to identify process disturbances. when analyzing process information in the frequency range. The output of the system can be characterized by several variables. often control algorithms are therefore implemented that combine and evaluate one or more pieces of sensor information. Fig. Roughing –49 –50 Grinding Burn –51 Finishing –52 0 200 400 600 800 1000 1200 Amount of gearwheels/pcs.8. information can be obtained regarding the appearance of signal components at different frequencies and across frequency shifts. the profiles show an increase of about 1 dBV in the averaged acoustic emission amplitude. Analysis of the measurement data gathered from the finishing phase demonstrates a clear relation between the averaged acoustic emission amplitudes and the appearance of grinding burn (Fig. Throughout the tool life of the grinding wheel. for example the appearance of chatter vibrations or significant changes on selected cutting edges in milling operations. The method most used in industrial practice today is the fast Fourier transformation (FFT). This is often the case in machining operations since dominant process frequencies such as turning or tooth engagement frequencies as well as their overtones are superpositioned by the characteristic frequencies of the machine and tool. often analyses in the time and frequency ranges are combined. 8. one must bear in mind that the time information of when a certain signal event occurs is lost. Often. 8. The most well-known method of frequency analysis is based on the FOURIER transformation. For this reason.2 Process Monitoring 377 –48 - Averaged AE-Amplitude U /dBV adaptive control optimization. Process disturbances of this kind are easier to detect in the frequency range than in the analysis of process signals in the time range.38).38 Analysis of acoustic emission in frequency range 1400 1600 . From the representation of amplitude performance spectra over frequency. Saxler’s system for recognizing grinding burn via acoustic emission analysis again provides a practical example for signal analysis in the frequency range. 8. The result for the shortened signal section is represented analogously to FFT. the values of the amplitude peaks are usually directly drawn upon for evaluation. short-term changes in amplitude are not as level. Figure 8. The characteristic values are calculated either from very narrow frequency bands or from the amplitude spectrum in one relevant frequency band.378 8 Process Design and Process Monitoring Both in the time range and in the frequency range. a Amplitude A Characteristics from the amplitude spectrum Time t Amplitude A Frequency-transformation maxpeak maxpeak Maximum of amplitude AFB AFB Integral of amplitude spectrum in a frequency band Frequency f Fig. it is possible to form significant signal characteristic values and to set them in relation with each other. the smaller the signal sections should be selected. No information about the time or temporal sequence of the appearance of the frequencies contained in the signal exist within the analyzed signal section. This is the functional principle of the short-time FOURIER analysis (STFT). In the first case. One possibility of increasing the time resolution consists in selecting the width of the signal section so narrow that every signal segment can be considered as quasistationary and then to consider the sequential progression of several successive signal sections. The Fourier transformation’s usefulness is limited in the case of time-critical signal analyses and highly dynamic processes. in the second often integrating characteristic values are ascertained from the profile of the amplitude spectrum that correlate with the energy content of the signal. In comparison with FFT.39 compares example signals from the time and frequency ranges and illustrates the corresponding characteristic values [Reub00]. The faster the relevant signal changes arise and the shorter the permissible delay time for their recognition.39 Transformation from time into frequency range and characteristic values . One weak point is in the delayed preparation of the evaluation result because information about included signal characteristics can only be made after complete transformation of the signal section. However. The maximum delay results in case a disturbance-related signal characteristic appears at the beginning of the signal section under consideration and is only recognized after the end of that section. 25) for high frequency signal fractions t j Amplitude Shifting parameter t Amplitude Acquisition of temporally signal changes by shifting the wavelet function . si = 1) for slow signalchanges and low frequencies Small scaling parameters si (e. while the variation of the scaling parameter correlates with the frequence-related evaluation (Fig. Different frequency ranges of a signal can be investigated with different temporal resolution. We differentiate between continuous wavelet transformation (CWT) and discrete wavelet transformation (DWT).2 Process Monitoring 379 wide signal section is necessary for a high frequency resolution capacity in order to obtain information about the low-frequency signal components that is as complete as possible. there is a very good temporal resolution and at low frequencies very good spectral resolution. the output signal is multiplied by a wavelet function. si = 0. (8. but requires a larger amount of computing time and memory requirements. the wavelet algorithm adjusts the size of the section to the respective frequency band under consideration.40)... While in the case of the FOURIER transformation only a constant signal section can be selected for the entire frequency range of the signal.56)). This makes a larger amount of discrete values available for the transformation. Time Fig. 1 ψτ . It has a shifting parameter τ and a scaling parameter s (Eq. One approach to solving this goal conflict is raising the sample rate when digitalizing the signal.. So in the case of highfrequency signal components..g. Wavelet analysis is the logical development of the notion of analyzing a signal in real time and completely in its contained frequencies.40 Shifting and scaling of the wavelet function .g. In CWT. Another advantage is that the transformed signal can be fully reconstructed by means of an inverse wavelet transformation. Time . 8.s = √ ψ s  t−τ s  (8. Stretched wavelet function Forged wavelet function Scaling parameters Big scaling parameter si (e.8.56) The shifting parameter contains the time information in the transformation range. 8. The wavelet function ψ has a constant number of vibrations and is thus a wave packet. the temporal signal profile is transformed several times while varying both parameters. which is possible. Only the highpass-filtered signal components are coded with wavelet coefficients. in agreement with the Shannon sampling theorem. often the discrete wavelet transformation (DWT) is used since they require less computing time and provide extensive possibilities in signal profile evaluation and presentation. Pattern recognition is an important strength of the algorithm. 8. as the shifting parameter contains the current location along the time axis.380 8 Process Design and Process Monitoring The signal is temporally resolved by shifting the wave packet along the time axis. The advantage of this procedure is that the amount of individual signal values used in every transformation level for the computation algorithm is adjusted to the decomposed frequency band. a wavelet coefficient represents. the difference of two individual signal values. in the case of “downsampling” during wavelet transformation the number of discrete signal values in each processing level is clearly reduced. simply considered. The product of the respective signal section x(t) with the wave packet creates a function whose integral corresponds with the wavelet coefficient c (Eq. the wave packet is expanded or compressed so that its length becomes a measure for the analyzed frequency range. It represents exactly that signal information that is contained in the frequency band resulting from highpass filtering of the respective decomposition level and can thus be assigned to a certain frequency band. it is necessary to adjust the wave packet to the current signal characteristics by selecting a suitable basic form. (8. “Downsampling” compresses the signal by purging the number of discrete individual signal values of information represented in the wavelet coefficient (making it redundant) after highpass/lowpass filtering. s) = √ |s|   x (t) ψ t−τ s  dt (8. a vector of wavelet coefficients is thereby formed which has half as many inputs as the decomposed signal. 1 c (τ . This saves not only . It is thereby reduced by half of the individual signal values. The individual layers of analysis are called decomposition layers.57)). In accordance with the principle of multiple solutions.57) The calculated coefficient is a measure for the similarity between the analyzed signal section and the shifted or scaled wave packet. The signal vector resulting from lowpass filtering forms the basis for the next evaluation level. In every decomposition level. For practical applications in signal transmission. In the case of DWT. By varying the scaling parameter. The output signal of the lowpass filtering is prepared for the next evaluation level (Fig. While transformation into the frequency range is bound to a constant number of signal values as a function of the spectral resolution. The key difference between CWT and DWT is that in the case of discrete wavelet transformations the signal is broken down into individual frequency ranges by repeated highpass and lowpass filtering.41) [Reub00]. In order to exploit this property. for the half-band frequency range considered in the next evaluation level without loss of information. .. In the investigations. A further practical example is given by REUBER in his dissertation on process monitoring finish milling operations on free formed surfaces [Reub00]. The dissertation of PLAPPER provides a practical application of wavelet analysis [Plap04]. wear-dependent characteristic values.8. In it. Due to its steep flanks. Downsampling Highpass Shifting Downsampling Signal for the next level Value of coefficients output information ofWaveletkoeffizenten decomposition level Wavelet coefficients 300 200 100 0 -100 6 12 18 24 30 36 Amount of coefficients Fig. Input signal of decomposition level Half-band filtering of input signal Lowpass H(d) 2 L(d) Shifting 2 . A clear retrace was possible with wavelet analysis. the distinct sensitivity of the algorithm with respect to deviations from the constant signal patterns of the disturbance-free process is exploited to generate dynamic. 8. Some defects on the guideways made their presence known in such an unsteady fashion in the internal control signals of the machine tool that they could not be traced back to the overrun frequencies in the time signal by means of a Fourier transformation.41 Signal decomposition by discrete wavelet transformations memory capacity but also computation time. the good pattern recognition properties of the method is utilized to detect local defects on the guideways of ball screw guides. which is advantageous particularly in the context of online monitoring. Three characteristic values were investigated . a wavelet function of the DAUBECHIES family [Daub93] was used with the regularity 8 (degree of differentiability).. In it.2 Process Monitoring 381 H(d) 2 L(d) 2 D1 Signal H(d) 2 L(d) 2 D2 H(d) 2 D3 L(d) 2 .. the function approximates the profile of the resultant force signals very closely and also makes it possible to separate the individual frequency ranges effectively. 8. the integral of the amplitude spectrum in the frequency range of 350–650 Hz and variance of the wavelet coefficient in the respective wear-sensitive decomposition level (Fig. Comparison of these characteristic values shows that variance of the wavelet coefficients exhibits a significantly higher level of sensitivity than the characteristic values from the time signal and the amplitude spectrum.42).Sensitivity /% 382 8 1800 1600 1400 1200 1000 800 600 400 200 0 Process Design and Process Monitoring Partly weared tools Tools at end of tool life Progressive weared tools Rise of characteristic values at defined end of tool life (VB = 0.15 mm) 115% Time based characteristic value variance 930% 170% Frequency based characteristic value integral 350–650 Hz Wavelet based characteristic value variance db 84 Fig.42 Comparison of wear sensitivity of dynamic wear characteristic values with respect to their sensitivity to wear: variance of the temporal signal profile. 8. The pattern recognition effect represents wear-relevant signal developments much better than they can be recognized with the increased intensity of signal amplitudes in the frequency range or from the temporal signal profile. . 1 into • • • • turning.  383 . the objective is to realize a high level of dimensional F. 9. drilling and sawing.1 Classification of processes with rotary primary movements A propos of sawing. 9. milling. a rotational cutting motion and an arbitrary transverse translatory feed motion [DIN8589a]. it should be noted that a purely rotational main movement is only executed in the case of circular sawing. 9. For kinematical classification.1007/978-3-642-11979-8_9.1 Turning Turning is a machining process with a geometrically defined cutting edge. Turning methods can be classified from various standpoints. For example different objectives of the machining task lead to the distinction between finish and rough turning. In the case of hacksawing and bandsawing. the tool diameter theoretically assumes an infinite value (translatory cutting movement). a high material removal rate is reached. RWTH edition. Klocke. one always takes into consideration the relative movement between the workpiece and the tool. C Springer-Verlag Berlin Heidelberg 2011 DOI 10.Chapter 9 Processes with Rotational Primary Movement Machining methods with geometrically defined cutting edges in which the main movement is rotational are subdivided in accordance with Fig. In the case of rough turning. Turning Milling Drilling Sawing Fig. Manufacturing Processes 1. In the case of finish turning. In it.3) show various calculation possibilities for the nominal cross-section of undeformed chip.2 and Eq. In the case of automated and NC operations.1) b≈ sin κr h ≈ f · sin κr (9. the values can be approximated with the following equations: ap (9. several tools can be engaged simultaneously during the machining process in order to reduce manufacturing times and to increase the material removal rate.2 right shows the cross-section of undeformed chip A. A = b · h = ap · f (9.2 Tool-in-hand system and nominal cross section of undeformed chip ap A f Pp . only the most important process variants will be explained in detail.384 9 Processes with Rotational Primary Movement accuracy and surface quality via small cross-sections of undeformed chip. (9. • • • • • b is the width of undeformed chip h undeformed chip thickness ap depth of cut f feed κ r tool cutting edge angle Neglecting the inclination. The subdivision of turning process variants according to DIN 8589-1 will be presented in the following. Since some of the process variants that appear in the standard are of secondary importance.2) Figure 9.3) h Po Tool orthogonal plane Po Workpiece → vf b Assumed working plane Pf κr Pf κr Ps Cutting edge plane Ps Tool back plane Pp Tool reference plane Pr Fig. The flexibility of this manufacturing process allows for economical use from prototype and mass production. 9. Figure 9. In transverse parting-off operations. one must bear in mind that the cutting speed changes with the tool diameter when machining Transverse face turning Workpiece Tool Transverse parting off Cylindrical face turning Workpiece Workpiece Tool Tool Fig. which are manufactured from a bar.4 Process variants of face turning. The cutting path of all transverse face turning variants lies on an Archimedean spiral. Under heavy strain. the cutting path is in the shape of a coil (helical line).1.3 according to the design of their tool holder.1 Turning 385 Right-hand direction of cut Left-hand direction of cut Neutral direction of cut Fig. according to DIN 8589-1 . 9.3 Styles of insert holders The turning tools of the various process variants are classified analogously to Fig. In the case of cylindrical face turning variants on the other hand. 9. During face turning processes. especially in the case of small parts. 9. 9.1 Face Turning Face turning is a turning method used to produce an even surface orthogonal to the axis of rotation of the workpiece. the tools tend to clatter due to their geometric design. amongst others. Face turning operations are usually carried out with automatic lathes. the tools are designed to be slender in order to minimize loss of material.4). Process variants include.9. transverse face turning and transverse parting-off for sectioning workpiece components or the entire workpiece [DIN8589a] (Fig. 9. Both minor cutting edges are tapered toward the tool shaft in order to avoid jamming. To do this.g. Longitudinal cylindrical turning is the most common method variant. In the case of lathes with continuous rotation speed control.1. Another advantage is that the long rod material need not be guided by steady rests since the rotating tool stabilizes the position of the workpiece.5 Process variants of cylindrical turning. 9. The feed movement is made by the workpiece and the rotation movement by the tool. according to DIN 8589-1 Tool . On conventional lathes. and the protrusion lengths of the workpiece are very short. 9. for example. a certain cutting speed range is maintained. in the manufacture of cold drawn steel.386 9 Processes with Rotational Primary Movement with a constant rotation speed.2 Cylindrical Turning Cylindrical Turning is used to produce a cylindrical surface that is coaxial to the axis of rotation of the workpiece. by multiple. Surface qualities in the range of Rkin = 2 − 10 μm can be obtained. This process variant is predominantly used for removing oxide and roller coatings as well as the surface cracks of rolling and forging blanks such as is required. The use of this method extends from finishing very small parts (e. The depth of cut is generally kept small (ap < 1 mm). cement mills with lengths of up to 20 m). 9. for example.g. Longitudinal cylindrical turning Workpiece Tool Centreless rough turning Workpiece Transverse turning Workpiece Tool Fig. in the clock and watch industry) to heavy roughing forged turbine blades or drive shafts for plant engineering (e. The most important variants of cylindrical turning are longitudinal cylindrical turning and centreless rough turning (Fig. the minor cutting edge angle κr is kept in the range of 0 < κ r < 2◦ . which will be used to exemplify many different machining phenomena as in Chap. gradual adjustment of the rotation speed to the machining diameter [Degn00]. Centreless rough turning is much more productive than longitudinal cylindrical turning and reduces the need for subsequent machining due to its high surface quality and dimensional accuracy. the cutting speed is kept constant.5). The surface quality of intermediate products can thereby be improved and impermissible shape deviations avoided. feeds of up to f = 15 mm are used. In steel machining. Centreless rough turning is cylindrical turning with several major cutting edges arranged on a rotating tool. 3. This combination leads to a very high material removal rate. The feed is limited by the length of the minor cutting edge and dependent on the demanded surface quality. the translatory motion is mechanically linked to the rotation motion. 9.6. thread chasing and thread die cutting [DIN8589a].6 shows a few important process variants that fall under this category: thread turning. 9. On conventional lathes. After thread turning. shown in Fig. this link is made electronically. the thread is manufactured by only one profiled cutting edge in several passes until the required thread depth is obtained. In the case of thread turning. In the case of numerically controlled lathes.7).7 Helical turning: part and full profile tools. Thread turning Workpiece Tool Thread chasing Workpiece Thread die cutting Workpiece Tool Fig. Thread chasing is similar to thread turning with the exception. 9. It is characteristic of this process variant that the pitch is produced by the feed. Feed corresponds to the pitch of the screw thread. offset by the pitch.6 Process variants of helical turning. Full profile tools on the other hand are shaped in such as way that the corresponding thread depth is directly cut from the material so that the output workpiece must not be prepared beforehand (Fig. Part profile tools can only be used when the workpiece is brought to the required external diameter before thread turning. since only the pitch is cut and the external surface is no longer machined. according to DIN 8589-1 Fig. 9.1 Turning 387 9.1. chaser Tool . that several cutting edges of a tool. are engaged simultaneously. Figure 9. the depth of the thread must be checked.9. Thread turning tools are available as both part and full profile tools.3 Helical Turning Helical turning is used to manufacture helical surfaces with profiling tools. it is possible to reset the die head without changing the direction of rotation.8. For internal thread turning chasing. as they allow for both a better use of space and a solid tool design. Chasers are also used in die heads.388 9 Processes with Rotational Primary Movement By arranging several profile cutting edges beside each other on one tool.4 Profile Turning Profile turning is used to produce rotation-symmetrical workpiece shapes by reproducing the tool profile. according to DIN 8589-1 Tool . are face profile grooving. and for left-hand threads a tool with a right-hand thread. 9. of which every subsequent one is shifted back by the infeed. We differentiate between three types of thread die heads depending on the type and arrangement of the cutting edges: • radial chasers. These modifications alter the process kinematics to the effect that this variant should in fact be considered a helical broaching technique (Chap. which allow for a radial resetting of the chaser after the thread die cutting process. • tangential chasers and • round chasers Chasers are also offered with part profile and full profile. round chasers are usually preferred. a tool with a left-hand thread must be used. Using a full profile tool makes a higher material removal rate possible. Face profile grooving Workpiece Transverse profile grooving Transverse profile turning Workpiece Workpiece Tool Tool Fig. To chase right-hand threads.8 Process variants of profile turning. transverse profile grooving and transverse profile turning [DIN8589a]. 10). Tool manufacturers sometime also designate chasers as thread turning tools of multi-point design (2–3 teeth as a rule). shown in Fig. Round chasers have to be designed as threads themselves so that they do not destroy the manufactured pitch.1. Thread die cutting represents a further development of thread chasing – thread chasing with tangentially distributed cutting edges. In this way. Profile turning variants are classified according to their process kinematics. 9. the thread can be manufactured completely in one pass. The most common methods. 9. Chasers can be designed as flat or round thread chasers. 1. NC-form turning Kinematic form turning Copy turning Workpiece Workpiece Workpiece Tool Tool Tool NC Calibre Fig. Non-circular turning is a special method used to manufacture non-round workpiece surfaces by periodic control of the cutting direction [DIN8589a]. Profile tools made of high speed steel are very common.9 Process variants of form turning. In order to avoid potential clattering during grooving processes due to instabilities in tool clamping. grooves should have a limited width of cut. b = 15 mm (in special cases up to 30 mm) and be up to a depth double the size of the chip width (in special cases up to triple the size is possible).9 into NC form turning. Form turning is categorized as in Fig. the feed axes were kinematically linked via a transmission. a distinction is drawn between cylindrical and transverse non-circular turning. copy turning and kinematic form turning [DIN8589a]. the feed movement is realized by electronically linked feed drives. 9. “Overhead clamping” of the tool can also be beneficial to chip flow. As shown in Fig. Copy turning involves deriving the feed movement from a reference shape. a moulding or a masterpiece. the grooving tools are equipped with chip breakers in order to prevent jamming of the chips in the profile. tools made of both high speed steel and cemented carbide are used. Pure copy turning was developed further when machine tool controls were made available that could store a contour that had once been applied. In the case of large cross-sections of undeformed chip and deep profiles. Kinematic form turning was often used in the past to produce ball heads. according to DIN 8589-1 Gear box . In NC form turning.1 Turning 389 In the case of profile turning. as they are very tough. 9.10. easy to manufacture and to regrind. These are called teach-in processes.5 Form Turning Form turning is used to produce workpiece shapes by controlling the feed movements. 9. NC form turning is the state of the art today. 9. In this case. This process variant has also been replaced by NC form turning.9. When using internal turning to produce deep contours however. 9. The direction of cut is perpendicular or sometimes transverse to the tool’s axis of rotation.11 Internal turning. Figure 9. Boring Workpiece Undercutting Workpiece Tool Tool Internal Grooving Workpiece Tool Fig. which depends on the size of the contour to be machined. For this reason. The rotation movement of the workpiece and the feed movement of the tool have a fixed transmission ratio.10 Process variants of non-circular turning. should be taken into consideration when selecting the cutting parameters.6 Further Process Variants Up to this point. 9. the turning tool can be advanced with an advancing rotational motion of the workpiece. according to DIN 8589-1 9. . the protrusion length and the shaft diameter. stability problems can arise due to the long protrusion length of internal turning tools. This can also mean however that the tool is partially no longer being engaged (e.1.g. 9. according to DIN 8589a By control.11. these process variants can also be used for internal machining as shown in Fig. In principle.12 shows some typical tools used in internal turning. square turning). selected process variants were basically explicated using the example of external machining.2 Milling Milling is a machining production method with a circular cutting movement of a usually multi-tooth tool for producing arbitrary workpiece surfaces. 9.390 9 Processes with Rotational Primary Movement Transverse non-circular turning Cylindrical non-circular turning Direction of instantaneous primary motion Workpiece Workpiece Tool Tool Fig. 15. the cutting speed vector vc and the feed velocity vector vf both point in the same direction at the cutting edge exit point. Depending on the tool’s rotation and feed direction. it is called face milling (Fig.9. form milling and profile milling. Fig. Down milling is shown in Fig. If the workpiece surface is produced by the front face of the tool with the minor cutting edge. hobbing. it is advisable to relate the movements collectively either to the tool to the workpiece. left. the cutting edge exits with a undeformed chip thickness of h = 0 mm. 9. The feed direction angle between both velocity vectors is ϕ = 180◦ . a milling process can contain elements of both up and down milling.e.12 Internal turning: tool design (Source: Sandvik Coromant) Milling processes are categorized in DIN 8589-3 in accordance with the surface produced. milling processes in which the surface is manufactured by the cutting edges on the milling cutter periphery are called peripheral milling (Fig. No definite chip removal occurs at that point. Depending on the position of the milling tool relative to the workpiece. In the case of up milling. 9.2 Milling 391 Fig. Analogously. so a clear classification is not always possible. In the case of pure down milling. In order to distinguish up and down milling from each other. circular milling. the tool shape (profile) and kinematics as (including others) [DIN8589c]: • • • • • slab milling. The feed direction angle is ϕ = 0◦ . In order to distinguish both process variants however. At the cutting edge’s point of exit.14).13). . we distinguish further between up and down milling. the cutting speed vector vc and the feed velocity vector vf point in different directions. 9.15 shows the movements performed with reference to the workpiece. often the cutting motion is often carried out by the tool and the feed movement by the workpiece. 9. In practical applications. the minimum undeformed chip thickness is undershot beyond a certain angle of engagement. 9. i. we make a distinction between a positive (γf and γp > 0◦ ) and a negative (γf and γp < 0◦ ) cutting part geometry. 9. Correspondingly.13 Kinematics of face milling Tool reference plane Pr vf fz Ps ≡ Pp n Tool ω ϕ vc ae Pf ≡ Po Workpiece Fig. . The tool orthogonal rake angle γ is composed of a radial component γf and an axial component γp . 9.392 9 Processes with Rotational Primary Movement vf Tool orthogonal plane Po n Tool cutting edge plane Ps Tool kr ap Assumed working plane Pf Workpiece fz Tool reference plane Pr Tool back plane Pp Fig.14 Kinematics of peripheral milling and solely compressive and frictional processes are occurring. the cutting edge enters the workpiece with a undeformed chip thickness of h = 0 in the case of pure up milling. As in all other processes. 9.2 Milling 393 vf vf vc vc Down milling Up milling Fig. Such “S-contact” can be Workpiece Milling cutter γf > 0° γp > 0° S Insert γf > 0° γp < 0° Reference plane S-Contact T γf > 0° γp = 0° T-Contact T S ST-Contact U γf < 0° γp < 0° γf < 0° γp > 0° U-Contact Fig. Different contacts can result depending on the geometric conditions determined by the milling cutter diameter.9.15 Up and down milling In all milling processes. size of cut and cutting part geometry (Fig. at least one cut interruption occurs per cutting edge during each tool rotation. as opposed to other processes like turning or drilling. the cutting edges.16 Forms of contact in milling V V-Contact γf = 0° γp > 0° S V SV-Contact . 9. are not constantly being engaged. Because of the constant cut interruptions. Rather. 9. the contact conditions between the tool and workpiece are of particular importance for the wear properties of the cutting tool materials in addition to the cutting conditions. It is particularly inauspicious if the cutting edge point that is the most sensitive to impact is the first point to make contact with the workpiece.16). Köll86]. greater importance is attached to the exit conditions with respect to wear caused by fractures [Kron54. 9. for machining cast iron. high-speed steel and tough cemented carbides of machining application groups P15 to P40 are used. Lola75. This phenomenon must be taken into account when determining milling strategies. in up milling). In order to improve cutting edge stability. plastic and hardened steels types K10 to K30 are used. Peke79. All other types of point or line contact are regarded as intermediate stages between S-contact and U-contact with respect to impact sensitivity [Kron54. In comparison to the entry conditions. Beck69. Hosh65. The cutting tool materials used for milling were developed with an eye to increased thermal and mechanical alternate stress and are thus usually not directly comparable with the cutting tool material types used for turning.394 9 Processes with Rotational Primary Movement avoided by varying the cutting edge geometry and feed rate accordingly. “Helical chip milling” (γf < 0. 9. Damm82]. The use of tougher cemented carbides may increase the length of tool life until failure.17). For steel-working. which can cause comb and parallel cracks and thus lead to cutting edge fracture.g. Cutting interruptions mean thermal and dynamic alternate stresses for the cutting tool material. Fig. γp > 0) has become established as a method for improving chip removal. tensile stresses can arise in the cutting edge in the unencumbered state because of resilience. In the case of tool exit with finite undeformed chip thickness (e. The cutting tool materials used must therefore be very tough. but that alone cannot prevent premature failure in the case of unfavourable exit conditions. which is often problematic in the case of rotating tools (Fig. additionally stabilizing protective chamfers are fitted near the corner and on the cutting edge. Okus63. Peke78. NE metals. in which the cutting edge point that is furthest removed from the minor and major cutting edges is the first to make contact with the workpiece. temperatureresistant and have high edge strength [Vier70].17 Helical chip milling cutter in use (Source: Walter) . which lead to cutting edge fracture. Beck69. The most favourable type of contact is “U-contact”. chilled cast iron. . cermets have also found increasing use. face milling. When fine milling steels (HB < 300). 9. 9. plain milling.1 Process Variants.9. Supereutectic Al alloys. milling processes are usually named according to the type and form of milling tools used.18 Slab milling processes 9. side and face milling. In practice.g. and the workpiece surface is created by the minor cutting edge. In this case. e. the width of cut ae is much larger than the depth of cut ap . profile milling etc. this milling process is also referred to as edge milling. For rough milling grey cast iron.2. fibre-reinforced plastics and the milling of graphite electrodes for spark erosion are typical applications of PCD-coated tools when used for milling. heat-treated steels and hardened steels. the workpiece surface is created with both the minor and major cutting edges. while in the case of hardened or high-strength heat-treated steels (> 45 HRC) PCBN is also suitable. Specific Characteristics and Tools Milling processes are used most frequently to produce level surfaces (linear feed movement: slab milling).1 Face Milling In the case of face milling. If the lead angle is κr = 90◦ .1. Si3 N4 ceramics can be used successfully with a high material removal rate. Figure 9.18 shows the most important slab milling processes.2. and designates the slab milling tools associated with them. Plain milling cutter Face milling cutter Shell end mill Side and face milling cutter End milling cutter Fig.2 Milling 395 Further developments in cemented carbides and coating technologies have made it currently possible to use coated cemented carbides when milling cast irons as well as steels. end milling. Oxidic and mixed ceramics are suitable cutting tool materials for finish milling grey cast iron. which differ by their kinematics and engagement conditions. case-hardened steels. For each range. The direction of the active force Fa depends on the feed direction angle ϕ. 9.1 · |h| −mi for 0.01 < h < 0.8.5) with: i = c. 3. cutting tool material and cutting conditions). 9.0. KIENZLE’s resultant force equation [Kien52] can also be applied to milling.01 · |h| −mi0. we have: 1−mi i | = ki · b · |h| |F (9.1 for 0.1 < h < 1.396 9 Processes with Rotational Primary Movement vc −Ff vf −Fcn ϕ n −Fc −Ffn −Fa ϕ = 0° Fig.0.19 Cutting force components at head face milling.0 mm is subdivided into three sections (Fig.0 mm |ki | = ki1.4) with: i = c.1 mm |ki | = ki1. the resultant force F involved in milling can be broken down into an active force Fa on the working plane and a passive force Fp perpendicular to the working plane (see also Sect. which is defined by the specific resultant force ki and the exponent mi : −mi0.01 for 0.20).001 < h < 1. the respective resultant .19. a line can be ascertained. 9. according to KAMM [Kamm77] According to DIN 6584.1 · |h| (9. cn and p Due to the wide range of undeformed chip thicknesses covered by milling. cutting normal force Fcn and passive force Fp . The components of the active force can be related to the direction of the cutting speed vc (cutting force Fc and cutting normal force Fcn ) or to the direction of the feed velocity vf (feed force Ff and feed normal force Ffn ) as in Fig.001 < h < 0. The range of undeformed chip thickness of 0. the KIENZLE relation is only valid for certain areas. cn and p As long as the characteristic values are known for the existing marginal conditions (material.01 mm |ki | = ki1. For the components of the resultant force F – cutting force Fc .1). The tooth pitch of the tool depends on the form and size of the tool. dimensional accuracy and performance milling cutters with soldered cemented carbide cutting edges [Sack76]. level and right-angled surfaces. grooves with square crosssections and long slots.440 2 101 8 6 kc1.9. To estimate the resultant force however. Larger face milling cutters are designed in two parts because of their great weight for better handling during tool change. The size and number of teeth of the milling tool are selected based on the dimensions of the workpiece surface to be machined and the drive capacity of the machine. . characteristic values are often utilized that have been established in turning processes.0. solid face milling cutters made of HSS or cemented carbide are used.0. Face milling heads up to a diameter of D = 250 mm are mounted in the usual fashion on the tool spindle.1 = 2100 MPa 1– mc = 0. Large cutter heads for cast iron machining can thus be fitted with up to 200 inserts. beyond a tool diameter of D = 10−16 mm tools with clamped cemented carbide indexable inserts and in case of increased requirements on surface quality.20 Specific cutting force in face slab milling [Kamm77] force component Fi can be calculated for milling.764 4 2 Range 3 1 10−3 2 4 Range 2 6 8 10−2 2 4 Range 1 6 8 10−1 γp αf 2 4 6 8 100 λs κr h / (mm) Material: Ck45N Cutting tool material: HW-P25 vc = 190 m/min γf αp εr −4° −7° 6° 23° −6° 75° 90° Fig.1 = 0. The base body remains on the spindle during tool change.2 Milling 397 102 8 6 kc1. 9.150 kc / (GPa) 4 kc1. To mill very small. Short-breaking chips require a small chip space and thus a small pitch. on the available machine power and on the chip formation of the material. face milling heads are partly manufactured with an uneven pitch on the periphery. In order to prevent chattering of the tool/workpiece/machine system with the frequency of the cutting edge engagement.01 = 0.01 = 75 MPa 1– mc 0.1 = 998 MPa 1 – mc 0. so only the ring with the clamped cutting edges is changed. to improve chip removal. the milling axis can be tilted by 0. Depending on the requirements. Finishing with geometrically defined cutting edges is becoming increasingly important because of the potential of single-machine processing. However. round). Generally. Hertel) Another potential way to increase efficiency and universality is the use of face milling heads with tool cartridges (Fig. κr = 75◦ . 9. The same is true for machining materials with high strength and toughness.21 Face milling head with tool cartridges (Source: Kennametal. sizes and geometries (e. the face milling cutting conditions are selected lower than in turning processes. negative. It affects to a large extent the size of the active and passive forces and thus the stability of the milling process. a negative cutting part geometry is more advantageous for preventing cutting edge fracture. Face milling is used both for pre-machining and also increasingly for finishing purposes. In the case of welded constructions or larger material inhomogeneities. Smaller cross-sections of undeformed chip are chosen in particular in order to keep the dynamic stress on the cutting tool materials low and to prevent tool fracture. four-corner. In order to avoid re-cutting the face milling cutter due to elastic form changes in the overall system. κr = 90◦ . Milling with cutting ceramics is generally performed with a negative cutting part geometry. helical chip geometries.). tool cartridges can be inserted in a base body that can receive various kinds of indexable insert (three-corner. with moulded chip breakers etc.398 9 Processes with Rotational Primary Movement Fig. positive. 9. welded gearboxes) or in milling operations on milling and boring machines with a widely projecting spindle.5–1◦ . Finish face .g.g. In general.21). steel-working employs positive cutting part geometries and. The tool cutting edge angle amounts to κr = 45−75◦ in face milling (special case: corner milling κr = 90◦ ). this sets the feed direction. especially when machining thinwalled parts (e. g.2 Milling 399 milling is especially used as a finishing process for large even surfaces with special surface quality and smoothness requirements when other finishing methods (e.e.g.23). which are equipped with a small amount of teeth (1–5) and work with very small depths of cut and high feeds (Fig. 9. • Wide finishing face milling cutters. so the chamfer of the minor cutting edge lies parallel to the workpiece surface. e. Indexable inserts for finishing have an active minor cutting edge.22 Processes for fine milling (Source: Siemens) Principle of fine milling with ceramics Insert ap = 0. Such machining problems usually arise in heavy machine construction. There are three kinds of finishing tools (Fig. grinding or shaving) are uneconomical or impossible. i. 9. in these tools the cutting edge angle is κ r = 0◦ . when producing joining surfaces. The feed per tooth should not exceed 2/3 of the length of the active minor cutting edge nad.23 Wide finish milling (fine milling) with ceramics (Source: Feldmühle) . 9.22): • Conventional finishing face milling cutter which work with small depths of cut and feeds per tooth and are fitted with a large number of teeth. the minor cutting edge nads are equipped with large radii to simplify tool pre-adjustment.9. in the case of special wide finish milling tools Lsa = 10 −15 mm. This helps to obtain a very good surface quality Finish milling Wide finish milling Combined process: Finish milling and wide finish milling Fig. 9. The chamfer length of the minor cutting edge is generally Lsa = 2−3 mm. machine tables and guideways on machine tools and when milling sealing faces in engine and turbine construction. In the case of such tools.05 mm Workpiece fz Single-tooth milling cutter in use Fig. g. the jerk forces are larger than in conventional finishing face milling. • Face milling with finishing cutting edges and wide finishing cutting edges that combine the advantages of both methods. The tool is in this case only fitted with one or two wide finishing cutting edges that are radially set back and that protrude axially by 0.1. the cutting force acts upon the workpiece (Fig. all finishing face milling tools should be ground and lapped prior to use on the machine tool and after fitting with the cutting edges in order to obtain the necessary face and run-out accuracy (< 5 μm) corresponding to the required workpiece surface quality. up to vc = 300 m/min in steel-working with cemented carbide) in order to obtain a high surface quality.24 Peripheral up. If no special. which can result in an axial displacement of the tool. surface finishes of Rt = 5−10 μm are obtained. in the case of grey cast iron Rt = 1−5 μm. The width of the finishing cutting edges should correspond to about one and one half times the feed per rotation. Ceramics are the predominate cutting tool materials in this case. 9.2.and down milling Vorschub Fa . a thin steel sheet) can be lifted from the clamping surface or be induced to chattering.24).03–0. while in peripheral up milling it is directed away from the workpiece so that an unstable workpiece (e. The cutting speed is selected high in finishing (e. There is peripheral down milling and peripheral up milling. 9. However. For finishing. the workpiece surface is created by the major cutting edge. the pre-adjustment of the cutting edge has increased significance. 9. similarly to shell turning. Faulty insert pre-adjustment increases kinematic roughness and shortens the tool life sometimes considerably. In the case of steel. Up milling Down milling vf vf n n fz fz fz Ff Fc Ff ae Fc fz Fa Fig.05 mm to produce a high surface quality. finely adjustable tool holding fixtures are used.400 9 Processes with Rotational Primary Movement in cutting.g. In peripheral down milling.2 Peripheral Milling In the case of peripheral milling. When the cutting edges have a coaxial adjustment. 9. 9.26). 9. Besides the usual HSS tools. but an axial force arises then which can lead to tool or workpiece displacement.25).26 Modular shell end mill (Source: Sandvik Coromant) 9. Should sharp-edged profiles with good dimensional and formal accuracy be prepared. Pitch-induced axial forces can be compensated by mutually bracing a right-inclined and a left-inclined plain milling cutter of the same design (Fig. 9.9. Fa n Fa Fig. This can lead to material compression and thus to the formation of a poor surface. combined peripheral face milling cutters or plain milling cutters are used (Fig. in up milling the cross-section of undeformed chip is slowly increased.2 Milling 401 In the case of peripheral down milling. which are relief-ground on the front face of all cutting edges (formation of a tool orthogonal clearance).1.3 End Milling End milling is a continuous peripheral face milling process which uses an end milling cutter. high dynamic stresses come into play because one whole cutting edge enters into or exits from the material at a time. 9. This disadvantage can be overcome with a double helical gearing with opposite pitch. Such tools are very expensive both to acquire and to prepare however (Fig. cemented carbide peripheral milling cutters or plain milling cutters are finding increasing use. the dynamic load can be reduced. In the case of helically toothed tools.25 Combined plain milling cutter with opposite hand helix Fig. a table feed drive that is free of play is necessary in order to prevent vibrations and impacts.2. This process is advantageous when manufacturing mould surfaces .25). While in the case of peripheral down milling the lead takes place with an approximately full cross-section of undeformed chip. A distinction is drawn between right-cutting and left-cutting tools as well as between right-hand spiral. In principle. 9. which can lead to increased wear via fracture. End milling cutters have to be designed in many cases with a large degree of slenderness (l/D > 5–10) depending on the application (e. pockets. conically or as a custom design depending on the machining task. The mill form can be designed cylindrically.28). depending on the selection criteria regarding workpiece materials and . 9.27 End milling cutter (e. left-hand spiral and straight-toothed tools (Fig. chatter vibrations during the process. sometimes with fastening thread).g. Köll86]. in die construction) as well as forming grooves. in the case of tools capable of drilling the face cutting edges must reach as far as the tool centre. especially in the case of hard. both chattering and bending of slender tools lead to dimensional and shape inaccuracies in the components. they are equipped with a parallel shank (with side-clamping and/or fastening thread) or with a taper shank (Morse taper or steep-angle taper. Hann83. Measures taken to avoid these phenomena should be sought in an optimization of the tool and cutting part geometry. End mills correspond to shell end mills in their construction.402 9 Processes with Rotational Primary Movement End mill with morse taper End milling cutter with parallel shank Right-hand direction of cut with right-hand helix End milling cutter with morse taper Right-hand direction of cut with left-hand helix Die-sinking milling cutter with parallel shank Right-hand direction of cut with right-hand helix Conical die-sinking milling cutter Right hand cutting with right hand helix Fig. depending on the contact and engagement conditions. 9. Additionally.g. engagement conditions and milling strategy as well as of the cutting conditions [Schr74. Köni80. brittle cutting tool materials.29). 9. The advantages of these chip dividers include improved chip removal and cutting fluid access as well as reduced stress on the cutting edges (Fig. This causes on the one hand. for clamping. slots and cavities of all kinds and sizes. milling deep engravings in dies and moulds). The design of individual milling cutter geometries and chip divider forms differ depending on the manufacturer. Profiling of the cutting edges in the case of roughing tools leads to a division of the chips into smaller chips.27). all the cutting tool materials are potentially applicable in end milling. The front face of the tool is generally round or half-round. HSS end milling cutters are classified into tool applications groups in accordance with DIN 1836 depending on the material to be machined (Fig. which are subject to highly diverse stresses along the cutting edges.2 Milling Group of application 403 Field of application N Machining of materials with normal strength and hardness H Machining of hard.4 Profile Milling Profile milling is milling with forming tools to produce profiled surfaces. Especially mentionable in this context is the use of different cutting tool materials in a single tool. 9. 9.28 Tool application groups Profiled cuttting edges Profile Group N Group H Flat profile (F) Rounded profile (R) Fig. hard tough and/ or short-chipping materials W Machining of soft tough and/ or long-chipping materials Tool Fig. A more flexible adjustment of the cutting tool material to the machining task is possible by using tools with indexable inserts.g. . e.30). This can be advantageous. radii.9. Besides solid steel tools. in the case of ball path milling cutters.29 Cutting profiles of roughing-end milling cutters (Source: Fette) stability. for milling grooves.1. 9.2. for example. 9. High speed steel is still predominately used – often coated – as well as cemented carbides. gear wheels and gear racks as well as guideways. tools with soldered cutting edges and clamped or bolted indexable inserts are used (Fig. 9.31. gears are employed as components of an exact and effective transmission of motion. the tools are designed in one part (form milling cutters) or in multiple parts (gang milling cutters).2.33).5 Hobbing In almost all areas of technology. the gears are . Increasingly however. 9. 9.404 9 Processes with Rotational Primary Movement Fig. 9.30 End milling cutter with inserts (Source: Sandvik Coromant) Form cutter Gang cutter Fig. cemented carbide indexable inserts or soldered cemented carbide cutting edges are being used (Figs.31 Profile milling cutter Profile milling tools are adjusted to the form of the profile to be produced. In most cases. For high-precision gears.1. Profile milling cutters are in many cases manufactured as solid HSS tools due to the favourable machinability and inexpensive price. a peripheral face milling process is the result. As shown in Fig. 9.32 and 9. 9. hobbing is the dominant machining process for producing externally toothed cylindrical gears.32 Gang cutter for machining of grey iron profiles (Source: Walter) Fig. This results in the hobbing methods axial. radial-axial. fa nw vc Fig. Depending on the hobbing method.33 Gang cutter for machining of machine beds (Source: Walter) manufactured primarily by machining. superimposed over these are translatory motions of the tool in the axial and tangential directions as well as in the radial direction in order to reach the depth of cutting. The kinematics of the process will be explained briefly with the help of Fig. Because of its high efficiency.9.2 Milling 405 Fig. the coupling of a worm with a worm gear is simulated. whereby a worm interrupted by gashes represents the tool and the worm gear represents the workpiece to be manufactured.34 Kinematics of hobbing . 9.34. In hobbing. 9. 9. The rotary movements of the hob and the gear serve to remove the chips. For wear-related reasons.35. hobbing in the same direction and in the opposite direction. In climb cutting.406 9 Processes with Rotational Primary Movement tangential. The parameters of a hob can be seen in Fig. whereby the entire body must be constructed from high-quality HSS or cemented carbide. climb cutting is generally preferred. The pivoting angle or lead angle of the hob results from the direction and size of the helix angle and the pitch angle of the hob worm. The hob teeth are shaped so that tool orthogonal clearances and the potential of regrinding the rake face without altering the tooth profile are created. 9.35). the hob can be shifted along its axis in discrete steps. These hobs are especially suited to manufacturing gears Section A-A β2 A da2 vc Recess fa b η = β2 γ0 fa ± T Shift Entry A da0 Fig. whereby the pitches of the tool and working gear are aligned in the same direction or in opposite directions. axial hobbing is currently the most commonly employed process. and diagonal hobbing. 9. 9. In order to distribute the tool wear arising during the process evenly along the hob for more a efficient use of the tool.35 Terms of the hobbing process Full cut . There are two further process variants used to machine helical gearings. The hob is a cylindrical screw interrupted by chip flutes which bring about the gash. In industrial production. The number of worm threads on the cylinder determines the thread number of the hob. Inserted blade hob and cutters with indexable inserts on the other hand consist of a base body made of a more inexpensive material. Hobs are classified into three different groups with reference to their construction type (Fig. where only in full cut does one obtain the theoretical maximum circular cut lengths and material removal rates (Fig. the cutting speed and axial feed motion are directed opposite relative to the workpiece. Solid steel hobs are manufactured from solid material. We distinguish between climb and conventional cutting by means of the direction of the axial feed fa .36). The traverse path of the hob can be subdivided into the tool inlet and outlet phases as well as that of full cut. which shows the tool in the machining sequence. The feed in one direction is defined as the distance travelled per workpiece rotation. This displacement takes place as a rule after each machined workpiece and is also called “shifting”. 9. especially in dry hobbing. Saacke.37 clarifies the hobbing process by looking at the creation of a tooth gap. Figure 9. By the establishment of (Ti. This hob type is only suitable for workpieces beyond a modulus of 5 mm because of the resulting poorer cutting quality. In the case of cutters with indexable inserts. the material of a tooth gap is machined in the successive engagements (hobbing positions) of the individual teeth of a hob thread as can be seen in the sketch in the bottom left of the illustration. i. Due to the process kinematics and the resulting shifting between the tool and workpiece. . Every cutting tooth makes a cut after one tool rotation in a further tooth gap determined by the hob thread number.36 Hob types (Source: Fette. At the same time. With respect to the selection of the substrate/coating system. coated gear tools are reconditioned. which is made of cemented carbide. For reasons of accuracy. it is always removed one chip with the same cross-section. the tool must have as much wear resistance as possible against abrasive and thermal wear mechanisms in conjunction with a suitable hard material coating. Saazor) with large diameters and large modules. the tool reground and finally recoated on the rake face after the tool operating life is reached [Klei03]. a sufficient amount of substrate toughness is of especial importance in hobbing because of the interrupted cut. is used upto four times and not reground. In this way.Al)N coatings is it possible to perform dry machining processes at cutting speeds up to vc = 200 m/min.e. and the tool’s operating life can be held constant. the tool cutting edge. ground cutters cannot be over-coated more than 5–10 times because of the coating application on the already coated rake face. The evolvents on the tooth of the workpiece are approximated by profiling cuts. The coating is removed. but in the same hobbing position. even with tool systems based on HSS [Wink05]. 9.2 Milling 500 407 500 500 mm Diameter da0 400 300 300 (HSS) 200 100 0 160 140 (HM) or (HSS) 20 60 Full steel hob Inserted blade hob Cutter with indexable inserts Fig.9. More and more. Fastening is done with lateral clamp rings. we can work with consistently high cutting speeds in all tool cycles. Besides the potential of realizing larger constructive tool orthogonal clearances. the inserted blade hob also has a relatively large potential usable tooth thickness (large number of regrinds). especially shortly before the middle position. The penetration areas are highlighted in the image. a large amount of the gap volume is machined. Sulz73. changes to the structurally given tool orthogonal clearance and tool orthogonal rake angles occur during the cutting process [Sand72. during the cutting process.408 9 Processes with Rotational Primary Movement Section A Cross section of undeformed chip Section B Generated cut B Involute Generated cut deviation Hob tooth Fig.39 illuminates the cause of this.39). 9. variously thick and variously formed chips result (Fig. This means that. 9. 9. exist the largest crosssections of undeformed chips. 9. Figure 9. the Rake face Hob profile Hob profile Hob profile Direction of primary motion Hob profile vc Fig. the effective cutting speed vr results from the cutting speed of the hob vc and the hobbing speed vA . the tooth gap is mostly profiled and the cross-sections of undeformed chip are reduced.37 Ratio of engagement in the hobbing process Due to the differing penetrations between the hob and the workpiece in the particular hobbing positions. so that here. During the first tooth engagements. Sulz74. In the following hobbing positions. At the cutting point under consideration on a certain hobbing position. as in other gears manufacturing processes. this leads to an enlargement of the structurally given tool orthogonal clearance but simultaneously to a reduction of the tool orthogonal rake angle. During hobbing. On the entering tool cutting edge (Fig.38). Köni79].38 Chip formation in the hobbing process Upper chip side 3mm . Since both cutting tool materials are in competition with one another.40 shows a simulated chip geometry in a certain hobbing position. Figure 9. The advantages are designated with bright points. upon which the effective tool orthogonal clearance is small and the effective tool orthogonal rake angle is larger than the corresponding structurally determined angles. 9.41 shows the advantages and disadvantages of both tool systems. In addition to cemented carbides.9. . The calculated chip geometries form the basic data of a simulation-supported assessment of the stresses affecting the hob during the machining process. The 3D chip diagram reproduces the distribution of cross-sections of undeformed chips along the uncoiled cutting edge along the cutting arc. PM-HSS cutting tool materials have also become established as substrate materials for hobs in dry hobbing processes. is smaller. By recreating the axis motions of the hobbing machine. The effective tool orthogonal rake angle. In the penetration calculation. whereby the beginning of the cut is in the foreground. the cutting path of the hob teeth are generated so that they cut the workpiece planes. Fig.39 Relative velocity and effective cutting geometry effective tool orthogonal clearance is larger on the entering face than the structurally given one.2 Milling 409 cut C-C C vA vA C vc vc vA vr vc vr vr workpiece vc γeff γeff nw Tool EF AF αeff αeff αkon αkon Fig. the disadvantages by dark points. A penetration calculation can be applied to simulate the cutting process [Wink05]. There are different ratios on the exiting face. 9. It is clear that the chip thickness (top chip thickness) in the top region of the hob can be much larger than in the edge region. From the intersection we can calculate a penetration range that corresponds to the chip geometry of the respective hobbing position in the process [Weck02. the workpiece is analyzed into a certain number of parallel planes. Weck03]. on the other hand. 9. However. PM-HSS hobs can realize larger maximum head chip thicknesses. cemented carbide tools can realize clearly higher cutting speeds. it is only economical to use them if a significantly higher productivity or longer tool life is obtainable. Because of the much higher wear resistance of cemented carbide. Because of the higher tool costs.40 Tension geometry in hobbing HSS HM 30 µm Source: Samputensili Tool original price Preparation charge Wear resistance Cutting speed Chip thickness Hardiness Process reliability Fig.41 Cutting tool materials for dry hobs One of the main disadvantages of cemented carbide hobs in comparison to the HSS variant is the higher cost of acquiring the tools. 9.9 Processes with Rotational Primary Movement Cut-b end h andlin g 410 h Length of the incoming flank Length of the outbound flank Head Cutting edge handling Fig. They are generally at least three times more expensive. Given the high tool price. the use . to DIN 3972 or by pre-milling with protuberance.9. In large batch production. Sulz00]. The tooth base can be free milled in two different ways: by pre-milling with tools corresponding to reference profile II acc. provided the tool life is not significantly higher. cutting edge fractures occur occasionally in the case of cemented carbide hobs [Kloc99a.1. On the other hand. such fractures cannot be detected with process monitoring systems and lead to increased tool wear. After centring. 9. the tooth gaps pre-milled with protuberance have a rounded transition in the tooth base after skive hobbing (Fig. The process kinematics are identical to those of hobbing. not least because of their high toughness. This problematizes an economical use of cemented carbide hobs given the multiple reconditioning of the tool that is required. In order to execute a skive hobbing operation. which acts as a negative inclination angle on the tooth flanks. a sharp edge appears in the tooth base after skive hobbing. process safety is particularly important. it is very important for the sake of consistent cutter wear and the output that the hob tooth is exactly positioned or “centred” in the tooth gap. While HSS hobs. make a relatively high level process safety when their wear behaviour is monitored [Coop99. The skive hobs are designed with a negative top tool orthogonal rake angle. Centring is made more difficult by the fact that the flank allowance fluctuates along the workpiece periphery and the tooth width due to pre-gear cutting deficiencies. Although dry hobbing used to always be performed with cemented carbide tools. Primarily. 9. the same amount of material is removed on both workpiece flanks (right and left flanks) in the ideal machining case. Fig. This guarantees that the first contact between the cutting tooth and the workpiece surface is not made directly on the cutting edge but in the stable cutting part area behind it (see also contact conditions. Otherwise. If this occurs only sporadically. In skive hobbing. .2. Hard finishing with geometrically defined cutting edges requires that small cross-sections of undeformed chip are selected. In case 1 the pre-processing of the gaps is done with hobs of reference profile III.2 Milling 411 of cemented carbide hobs is uneconomical in this case. the danger of fracture is increased [Faul86].42). deformation caused by the heat treatment is removed and the surface quality improved. The concept of skive hobbing is derived from the “peeling cut”. which can have a negative effect on tooth base strength. 9.16). PM-HSS tools have become continuously more popular in the last several years because of their superior toughness properties and lower tool costs [Coop99].6 Skive Hobbing Skive hobbing is a continuous process using geometrically defined cutting edges for machining pre-milled gear teeth in a hardened state. Kölk99]. the tooth gaps of the pre-milled gear teeth must be prepared to such an extent that the top of the skive hob does not engage and only the flank cutting edge is cut. faulty alignment for skive hobbing or as a result of deformation due to hardening. cutting parameters and the hardness of the workpiece to be machined. workpiece geometry.42 Gap profiles of different rough cutters Although only very thin chips are removed during the process.412 9 Processes with Rotational Primary Movement Negative head rake angle γK Hobbing 1 Roughen BP III Hobbing 2 Roughen with Protuberance Fig. 9. . Köni95]. The limits of quality in finishing are basically determined by the feed marks and profiling cut deviations (Fig. Skive hobbing is used as a finishing process or can also serve as a preparation process for a subsequent grinding operation. Furthermore. In the case of skive hobbing. For this reason. The grinding costs are thereby levelled by lowering the amount of deformation due to hardening prior to the subsequent hob grinding process. gears that cannot be ground because of their geometry (large grinding wheel diameters) are also made by skive hobbing [Koep94]. This production sequence is especially of interest for large-module workpieces (larger module 10 mm). which are inconstant – especially in the entrance and exit areas – and also change their signs. With the help of a honing operation following the skive hobbing process. Favourable wear behaviour is exhibited by the ultrafinegrain cemented carbides of ISO application range K10 to K20 in conjunction with a hard material coating [Kais92. the obtainable length of tool life is depends greatly on the cutting edge.37) characteristic of hobbing. they can be removed at least in the case of small gears. very high resultant forces are in play. 9. a high static and dynamic stiffness must be required of the hobbing machine in addition to geometric and kinematic accuracy [Faul86]. We work simultaneously with very rapid workpiece rotation and slow cutter rotation. eccentricity e. there are two process variants in turn milling.43. • by very slow workpiece rotation and fast cutter rotation [Köni84a. both rotation axes are parallel to each other as shown in Fig. The self-propelled cutter works like a turning tool with a longitudinal feed parallel to the workpiece. DIN8589c defines turn milling as a variant of screw milling. Köni84b. The workpiece speed can be lowered so far that the quickly rotating cutter edges successively cut towards the target radius almost continuously. High surface quality (low facet formation) can be obtained in turn milling only with extreme milling cutter and shaft speed settings. left). For the sake of high cutting performance.1.44). Workpiece nw ap nw nF vfax Fig. If the tool axis and the rotation axis of the workpiece are arranged perpendicularly.43 Axle rotation in turn milling Tool nF vfax .7 Turn Milling Turn milling is a machining process in which the principles of turning and milling are combined in such a way that (generally) rotation-symmetric workpieces are machined with an inserted-tooth cutter on a rotating workpiece. In this way. In centric turn milling. 9. Due to the arrangement of the milling tool. In the case of orthogonal turn milling.2 Milling 413 9. axis-parallel turn milling makes both internal and external machining possible. 9. Köni85. In principle. right. 9. 9. Köni86]. 9. while in the case of eccentric turn milling they are offset relative to each other by a certain amount. Alternately. it is called orthogonal turn milling (Fig.9.44): • by lowering the feed per workpiece rotation – and thus the cutter engagement – to a few 1/100 mm. In accordance with this characteristic. we obtain a process very similar to turning. the milling cutter executes a screw-shaped motion relative to the workpiece due to the axial feed movement and the rotation of the shaft. the workpiece axis and tool axis intersect at one point. in axis-parallel turn milling.43. We also draw a distinction between centric and eccentric turn milling (Fig. the feed per workpiece rotation – and thus the engagement width of the milling cutter – should be maximized. This can be realized with a centric cutter setting (e = 0) (Fig.2. 6) (9.44 Centric and eccentric turn milling In order to produce cylindrical surfaces. this is impossible for kinematic reasons. the axial feed can be increased beyond the limit set by the length of the face cutting edge by shifting the milling cutter by the eccentricity e without hazarding cylindricity faults. Figure 9. 9. the surface quality is not as good as in the centric process variant. Under these conditions. In the case of an eccentricity of approximately e= the maximum of feed of dF − Ls 2  fax = 2  2 D − e2 2 (9. A great advantage of centric turn milling is that sharp-edged workpiece shoulders can be created (Fig. .43). Köni84b]. In the second case.7) is reached.45 shows an example of this process. Köni84b. a broad-tool finish tool geometry with a face cutting edge lead angle of κ r = 0◦ is unavoidable. In addition. In eccentric turn milling. Köni85]. 9. Centric turn milling is thus of great interest for practical process design despite lower performance values. Superimposition of the rotary motions of the workpiece and tool as well as of the feed motion results in highly complex cutting kinematics [Köni84a. since from now on the face cutting edges create the workpiece target radius with their full length. Further augmentation of the eccentricity again reduces the adjustable axial feed until finally no cylindrical surfaces can be produced [Köni84a. Shoulders are principally rounded in this case. The feed per workpiece rotation fax is thereby limited to the length of the face cutting edge Ls [Köni84b]. the process is comparable to conventional face milling.414 9 Processes with Rotational Primary Movement f f r r r f r e dF e=0 ls ´ e = dF /2 – l´s e < dF /2 – ls´ e = dF /2 Fig. It is recommended therefore to implement roughing operations with an eccentrically and finishing operations with a centrically positioned milling cutter [Köni86]. the face cutting edges are extended with additional inserts in order to maximize the feed. 9. tangential: e) [Köni84a]. To avoid cylindricity faults. as the material is moved into the front face of the milling cutter by means of the workpiece rotation. these also have a lead angle of κ r = 0◦ . Lifting pins and crankshaft cheeks can be machined with externally or internally geared tools (Fig. Kauf92]. 9. The facet-like surface typical of turn milling can clearly be seen (Fig. Beyond that.9.2 Milling 415 Added front side cutting tool Range cutting edges nF Cutter nW Workpiece Chip-chambers Fig. Turn milling can be used to create circular profile curves (e. An effective and economical cutting process is also possible by using conventional milling heads. various mould parts can be created (e.45). slab milling or thread cutting [Köni84b. radial: ap . The particular advantages of turn milling are safe chip fracture when machining long-chipping materials as well as the high cutting performance in a smooth longitudinal cut at low workpiece rotary speeds. This is especially significant in the case of large or unbalanced parts.45 The turn milling process (Source: Wohlenberg. mutually independent feed motions (axial: fax . In the case of the special tool used here. the face cutting edges participate actively in the machining process and each produce one element of the workpiece surface.46). TiN-coated cemented carbides have proven to be particularly effective. A flexible full-range processing of such components in one clamping is possible by exploiting the additional rotary axis for drilling. on camshafts) and cylinder forms that run eccentrically to the component rotary axis (e. longitudinal grooves) by adjusting the shaft rotary motions with the three possible. . The cutting conditions are generally somewhat higher than those of conventional face milling with TiN-coated cemented carbides [Wand92]. In steel machining in a smooth longitudinal cut.g. lifting pins on crankshafts).g. Steel materials that are difficult to machine can on the other hand be very effectively machined with TiCN-coated cemented carbides by means of turn milling [Stal94]. 9. Seco) As opposed to conventional face milling.g. 9. drill depth. been drilled or punched. 9. increased wear at the sharp-edged cutting edges and friction of the lands against the drill hole wall. unfavourable heat distribution at the action point. 9. difficult chip removal. which will be explored in more detail in the following.46 Crankshaft machining with rotary milling (Source: Walter) Extruder screws are almost exclusively produced by turn milling or planetary thread milling [Sten64]. The peculiarities of drilling include: • • • • • a cutting speed that falls to zero towards the drill centre.416 9 Processes with Rotational Primary Movement Fig.1 Profile Counterboring Counterboring differs from drilling basically by the fact that one does not drill into solid material but rather a prepared hole which has.47 along with the common respective directions of motion [DIN8589b].3 Drilling The term drilling signifies the machining method with a rotary main motion in which the tool is allowed only one feed motion in the direction of the tool rotary axis.3. Spherical actuators for pipeline valves as well as ellipse and eccentric forms can also be manufactured via turn milling. for example. dimensional accuracy and surface quality have led to the development of a series of different drilling processes. The most important process variants are shown in Fig. DIN 8589-2 distinguishes between to process variants: • planar countersinking • planar insertion . is counterbored to a dimension smaller than specified or to finished dimensions. 9. Different objectives with respect to material removal rate. In comparison to spiral drills. For planar insertion of pre-cast or predrilled drill holes. 9. Such special tools.47 Process variants of drilling. screw threaded design gives the spiral countersink much higher stiffness and thus leads to much higher working precision. HSS countersinks are used that are manufactured according to standard with angles of taper σ of 60. .3 Drilling 417 Trepanning Profile center drilling Reaming Tapping Drill out Profile reaming Profile center drilling Non-circular drilling Profile counterboring Fig.48 belongs. For example. the shape and size of which is adjusted to the respective standardized screw type. For manufacturing drill holes of fastening screws. to which the form countersink in Fig. an even surface protruding on the workpiece lying perpendicular to the rotary axis of the cutting motion is created. Automation in manufacturing often presumes the use of tools that are adjusted to a particular machining task. screw head counterbores of HSS or cemented carbide design are employed.9. whereby an internal cylindrical surface is formed simultaneously. then to counterbore the cylinder for the head of a hexagon socket screw and finally to chamfer the drill hole edge. Piloted counterbores are suitable not only for planar insertion but also for planar countersinking the front faces of eyes and hubs. For deburring. according to DIN 8589 In the case of planar countersinking. 9.48). 90 and 120◦ (Fig. usually three-lip spiral countersinks are used. Planar insertion on the other hand is used to produce an even surface recessed in the workpiece lying perpendicular to the rotary axis of the cutting motion. the three-blade. 9. chamfering and inserting the seat of spherical screw heads. can shorten production times quite considerably. it is customary in mass production for a screw connection to drill one through-hole with a form tool. since several working cycles can be consolidated in one spindle stroke. analytical models have been developed to help calculate tool stresses.49 Spiral drill with taper shank.2 Rotary Drilling 9. acc. Only a more exact consideration reveals the complex geometric formation. Simply put. 9.48 Counterboring tools ο Point countersink Pilot countersink Form countersink 9. 9. and it is today the machining tool that is produced in the largest numbers and is the most widespread [Häus79. For some time. as it is the most important tool for creating cylindrical drill holes from solid material or for enlarging a preset drill hole diameter in drilling out.3.2. the spiral drill is composed of the shaft and the cutting part (Fig.3. The about 150 grind types [Tika93] and numerous material-specific drill profiles represent the attempt to do justice to multifarious machining task with respect to quality and performance. especially of the drill bit. Tika93].49).418 9 Processes with Rotational Primary Movement Fig. This requires an Cutting part Drill diameter Neck Point length Taper shank Cone length Neck length Cutting length Flute length Total length Fig. 9.1 Centre Drilling The spiral drill occupies the position of greatest importance among drilling tools. It is estimated that it takes up 20–25% of machining operations. to DIN 1412 Straight shank . but rather deforms the material plastically and forces it to the major cutting edge. automatically executed drill point grinding adjusted to the particular problem can lead to an economical machining . the chisel edge is also part of the major cutting edge. Point angles of 130◦ result in improved free drilling in the case of resilient (“clamping”) materials. Taking the cutting conditions (effective speed) into consideration. which is not constant along the major cutting edge but decreases from its highest value on the corner (γf ) towards the drill centre and becomes negative in the transition to the chisel edge (Fig. Since according to the definition the major cutting edges point in the feed direction.9. the tool orthogonal clearance must be selected such that the effective rake angle is positive.52) [DIN 1414a]. although it hardly cuts due to its highly negative tool orthogonal rake angle. moreover. 9.53). it can be seen that only a careful. The only differentiating factor however that is used is the side rake angle γf . the tool cutting edge moves along a screw line.3 Drilling 419 Major cutting edge Hauptschneide Chisel edge Querschneide Freifläche Flank face βff αff α γff δ Fig. the kinematics of the drilling process and the stresses arising during that process. An upper boundary of the clearance is also given however by the weakening of the cutting part and the rattling tendency.50 Cutting part geometry of a spiral drill acquaintance with the cutting part geometry. The latter is varied because of the differing chip fracture of different materials and categorized into the main drill groups N for normal materials. In summary. Point angles of 90◦ are used for drilling hard. usually heavily wearing plastics in order to make the transition from the major cutting edge to the lands less sharp than is the case with very large point angles. By means of the interaction of the cutting motion (rotation) and the feed motion. usually a point angle of 118◦ is chosen. The shape and pitch of the chip flutes determine the size of the tool orthogonal rake angle γo . Figure 9. thus reducing edge dulling correspondingly. 9.51). 9.50 shows the cutting part geometry of a spiral drill. 9. To machine steel materials. which is identical to the helix angle δ with sufficient accuracy. H for hard materials and S for soft materials (Fig. the chip clogging problem can be countered with a further enlargement to a point angle of 140◦ in the case of long-chipping light metals (Fig. 9. clearance and the cutting speed against the drill diameter 40° 20 vc Clearance angle α 30° 10 20° Cutting speed vc 10° γfe 0 0° 10 9 αfe 8 7 6 5 Drill diameter r 4 mm 1 0 –10° Major cutting edge –20° Chisel edge –30° –40° –50° Rake angle γ –60° Typ N Normal workpiece material (Such as ) δ = 18 to 30° Typ H Hard and brittle workpiece (Such as gray cast iron) δ = 10 to 15° Typ S Ductile workpiece material (Such as aluminium) δ = 35 to 45° δ δ δ process. The rake faces are parts of a taper sleeve (Fig. 9. the conical relief point has asserted itself as the most consistent and suitable type.54).52 Spiral drills for various materials . The advantages of these drills include easy manufacture and preparation as well as their low sensitivity to high mechanical stress. δ = Helix angle Fig.51 Rake angle. 9. For the larger part of all machining cases.420 9 Processes with Rotational Primary Movement Fig. acc.3 Drilling 421 γf e γ β Cutting edge Cutting edge 2 α αf η e f f/2 ng direction η Feed direction Effective cutting direction Fig. 9. to DIN 1412 .9.54 Combination of spiral drills with special polished section from model A till E compared to taper sleeve grinding. 9.53 Motion sequence of the major cutting edges by double edged drilling tool Form A Form B Taper sleeve grinding (Polished cone-shaped section Form A–D) Picked out chisel edge Picked out chisel edge with adjusted major cutting edge Form C Form D Form E Split point Pickedout chisel edge with Drill point angle 180° with faced edge corners centered tip Fig. both major cutting edges simultaneously cut to their full length. In addition.1·D (used in general for Type N beyond 14 mm diameter). The compressive chisel edge is converted into two small major cutting edges with much better cutting properties. the hard. abrasive casting skin of which stresses the sensitive corner to a particularly large extent. and the corners can support themselves immediately on the drill hole wall with the lands. However. a second taper sleeve grinding with a smaller drill-point angle provides a remedy in that it helps improve heat conduction and counters increased wear by increasing its surface area. Grinding B is used in cases of high drill stress such as encountered when machining austenitic manganese steel or when drilling thin-walled aluminium sheets to reduce deformation. since in this case the tapered sleeve grind causes difficulties. In this case and generally only when special demands are placed on the drilling tool. After total penetration of the centring cone. • Form D: The taper sleeve grinding with point-thinned core and bevelled corners was specially developed for machining grey-cast iron workpieces. 9. The drill exits again by the entire major cutting edge. • Form C: A taper sleeve grinding with a split point in which case the chisel is completely eliminated. used when centric drilling must be guaranteed or when round and burr-free drill holes are to be made in aluminium sheets. • Form E: drill-point angle 180◦ with centred tip.54: • Form A: the taper sleeve grinding with a point-thinned core improves to a great extent the centrability of the drill and decreases the axial force corresponding to the chisel edge shortened by about 0. This is especially suitable for deep drill holes. the drill point is equipped with a special grinding that either complements the taper sleeve grinding (e. core point thinning) or completely reshapes the drill point (centre point. whereby a ring-shaped disc is cut out with minimal burr formation.422 9 Processes with Rotational Primary Movement The low self-centring and associated shape and position errors are disadvantageous.54). 9. The four-face grinding. Here. The following describes the most important point grindings in accordance with Fig. Fig. a taper sleeve grinding with a secondary face.5 mm diameters or with cemented carbide drills. is mentionable despite the fact that it is not standardized inasmuch as it is used when drilling below 1.g. This type also guarantees good centrability and reduced feed force. such that the resulting high feed forces have an unfavourable effect on machining accuracy. • Form B: the taper sleeve grinding with point-thinned core and corrected rake angle makes it possible to adjust the rake angle to the machining task. the chisel edge length is increased with increasing drill and core diameters. it is customary to reduce the rake angle by about 10◦ . . resulting in a very stable wedge without hindering chip transport because of a diminished helix angle. 3 Drilling 423 The special working conditions of a drilling tool place high demands on the cutting tool material with respect to hardness. One essential tool life criterion in drilling is reaching pre-given limits in the case of dimension and shape faults (relative disruption). chisel edge wear and corner wear and leads ultimately to relative or absolute disruption of the tool. cooling and stiffness. total tool wear is composed basically of • • • • • flank face wear. the cutting speed vc can only be increased by a factor of three. crater wear. tool holder and tool. the tool holder has the highest share. high speed steels for drilling tools. The advantages of cemented carbides are their high hardness. Solid cemented carbide drills are also used however. The tools are hardened. power. topically treated (nitrated) and often equipped with wear-preventing coatings. wear resistance and insensitivity against thermal alternate stresses. the HSS tool becomes completely unusable because of thermal induced failure of the tool cutting edge or because of tool fracture. Wear on the corner and on the lands is frequently responsible for this. HSS is used as a cutting tool material. contain 6% tungsten. 5% molybdenum. HSS drills are still preferred. Besides this increase in cutting conditions. compressive strength and high-temperature wear resistance. the low torsion and bending stiffness of the tool and chip transport [Spur60]. interface. In the case of the spiral drill. In comparison to internal turning. toughness. land wear and corner wear. In the case of absolute disruption. not the least because of their lower price. Beyond that. Frequently. If the minimum requirements cannot be fulfilled. 2% vanadium (HS6-5-2) and for higher stresses 5% cobalt (HS6-5-2-5).9. In current practice. According to DIN 1414-1. Drilling as a machining process has several peculiarities which we will examine in the following. Due to their high hardness and low toughness compared to HSS tools. cemented carbide drills are much more torsion-stiff than HSS tools. One example for precision requirements is the concentricity of the drilling process. Due to the maximum cutting speed on the . Relative disruption is characterized by the fact that beyond a certain drilling length the machining output no longer correspond to the requirements. which is the result of the comparatively low cutting speed. land wear. Cemented carbides have the same hardness at 1000 ◦ C as high speed steel at room temperature. the machining process can continue with a feed f that is at least 30% higher. drilling with spiral drills produces a greater surface finish on the drill hole wall. Due to their high Young’s modules. their use is technically meaningful and economical only on machine tools that fulfil the minimum requirements regarding accuracy. As a rule of thumb. Moreover. the tool life travel path Lf can be extended by a factor of three. spiral drills are subject not only to flank face and crater wear but also chisel edge wear. The total radial deviation measurable at the cutting edges of the drill is the result of the sum of each radial deviation of the machine spindle. unequal drill-point angles. the dimensional accuracy of the drill hole is dependent on the symmetry of the grinding.424 9 Processes with Rotational Primary Movement outer diameter. which however does not have a dominantly negative effect on the process sequence nor makes itself perceptible in the output. the low cutting speed in the chisel edge area often causes built-up edge formation. since straying of the tool can only be prevented when there is an extensive balancing of the radially acting passive force [Spur60]. As in all machining processes with geometrically defined cutting edges. the cutting speed for steels is in the range of 10–40 m/min. The feed influences tool wear much less than the cutting speed. which has an especially large effect when smaller diameter drills are used. A series of studies has shown that here the influence on the resultant force is small. This is where HSS tool often fail. On the other hand. Different passive forces Fp1 and Fp2 subject the drill to bending and lead to enlarged drill hole diameters in the output.55 Spiral drill forces. When drilling with HSS tools.55). In the case of extremely low or extremely high cutting speeds on the other hand. according to [Spur60] Fp2 Ff2 Ff1 Fp1 2*H Fc2 . 9.55. the corner is especially stressed. On the other hand. Such passive forces arise primarily for tool-related reasons. so it will no longer be considered here. a considerable increase in the feed force and cutting moment has been noted. The forces acting on the spiral drill are represented in Fig. 9. d Fc : Cutting force Ff : Feed force Fp : Passive force d : Tool diameter H : Distance of force-application to drill axis Fc1 Fp1 Fp2 Fig. especially in the case of large drill diameters. asymmetrical point thinning. such as • • • • unequal major cutting edge lengths. the resultant force components increase degressively with the feed (Fig. 9. unequal tool orthogonal clearances. Exhaustive investigations have shown that the surface quality of the drill hole wall cannot be significantly affected by the drill point grinding. 3 Drilling 425 Table 9. Torque and Power When Drilling with Spiral Drills Analogously to the ratios in drilling.2. cutting edge shape. • inconsistent cutting edge sharpness and • radial deviations [Spur60]. we can approximately calculate the cutting force. 9.3 Calculating Forces.9. which should be referred to in consideration of the resultant force more than the drill diameter. chisel edge and land.g.1 should be referred to when considering the percentage proportion of force of the major cutting edge. For drilling.3. The torques and feed forces can be determined sequentially only for the major cutting edges. . The more narrow the chip space is for chip volume specified by the cutting conditions the larger are the required cutting moment and the necessary feed force.2 shows the relevant relations of the cutting parameters in drilling and the potential calculation of the cross-section of undeformed chip from the feed component per cutting edge fz and the depth of cut ap or from the chip thickness h and chip width b. 9. The guidelines in Table 9. it is necessary to introduce a process factor fB in order to take into consideration the altered influences on the forces that occur in drilling as opposed to turning (e. the chisel edge and the land of a spiral drill Major cutting edge Chisel edge Land Torque (%) Feed force (%) 65–75 10–14 15–20 17–25 65–75 7–8 • asymmetrical spiral flutes. feed force and torque for centre drilling and drilling out with spiral drills with the help of the KIENZLE equation (Table 9. Among the feed forces that arise during turning. the chisel edge and the land of a spiral drill with respect to torque and feed force: These percentages were determined by means of step drill experiments.3). Table 9. The increased cutting and feed forces compared to turning (at otherwise identical marginal conditions) are due on the one hand to friction of the chips in the bottom of the groove and on the drill hole wall and on the other hand to the length of the chisel edge.2.2 Cutting Parameters in Drilling The cross-section of undeformed chip A has a major influence on the resultant force in drilling.3. cutting speed etc). for the major and chisel edges as well as for the major edge.1 Force components of the major cutting edge. considerable amounts of force are also added due to friction on the land and compressive processes in the area of the chisel edge. which in turn . The cutting part is made of cemented carbide. h = fz · sin(κr ) z 2 sin(κr ) A = fz · ap = b · h d·f 4 Cross-section of undeformed chip A= D d ap b h z Drill diameter [mm] Pre-hole diameter [mm] Depth of cut [mm] Chip width [mm] Chip thickness [mm] Number of cutting edges A= f fz κr σ (D − d) · fz 2 Feed [mm] Feed per cutting edge [mm] Lead angle [◦ ] Drill-point angle [◦ ] 9. but also by the fact that in deephole drilling a cutting fluid is fed directly to the cutting edges under pressure and that its rinsing effect is the sole transport mechanism for the incoming chips.3.2. Deepholes are drill holes with a diameter between about 1 and 1500 mm and a drilling depth of about three times the diameter.4 Deephole Drilling Deephole drilling is a machining process used to produce or process drill holes. Not only is deephole drilling distinguished from common drilling by its asymmetrical cutting edge arrangement.426 9 Processes with Rotational Primary Movement Table 9. It is not possible to make a general distinction between deephole drilling and other “conventional” drilling techniques by means of a universally valid definition or the like.2 Calculation of the cross-section of undeformed chip in drilling Centre drilling Drilling out Cutting parameters h h A fz fz b ap b ap A d D D ap f σ f z = . a very large ratio of drilling depth to diameter can be obtained. With all deephole drilling methods. b = . κr = . Further advantages of deephole drilling compared with customary drilling with spiral drills include above all the higher quality of the drill holes and its excellent cost-efficiency. so high cutting speeds can be reached. power required in drilling Force application Centre drilling Drilling out H = D/4 H = (D + d)/4 ap = D/4 ap = (D − d)/4 D D D/2 H Fcz Fcz d ap ap fB = 1 (D − d) Cutting force per cutting Fcz = · f z · kc · f B 2 edge F f B = 0. torque.9.3 Drilling 427 Table 9.3 Forces.95 D Fcz = · f z · kc · f B 2 Process factor fB cz Feed force per cutting edge Ffz Torque Ffz = D · f z · kf · f B 2 Ffz = (D − d) · f z · kf · f B 2 Fcz · z · D 4000 For z = 2: Fcz · D Md = 2000 9554 · Pc Md = n Fcz · z · (D + d) 4000 For z = 2: Fcz · z · (D + d) Md = 2000 Pc η Fcz · vc Pc = 60000 Pc = Md = Md = Md · n 9554 Fcz · vc (1 + d/D) Pc = 60000 Power Pa = fB Process factor drilling kf H Lever [mm] Fcz Cutting force per cutting edge[N] D External drill diameter [mm] Md Torque [Nm] d Internal drill diameter [mm] Pa Drive power [kW] z Number of cutting edges Pc Cutting power [kW] fz Feed per cutting edge [mm] n Speed [min–1 ] vc Cutting speed [m/min] η Efficiency kc Specific cutting force [N/mm2 ] Specific feed force [N/mm2 ] . The BTS process (Boring and Trepanning Association – BTA) was invented at the end of the 1930s in order to prevent chip scratching on the drill hole wall during transport and the resulting damage to the surface quality. • the BTA drilling process. 9. core drills. whereby both solid cemented carbide drill heads as well as drill heads fitted with cemented carbide are employed. 9. who reversed the process characteristics of single-lip drilling and supplied the cutting fluid from outside by means of a ring-shaped crack between the drill pipe and the wall (Fig. Basically. The following three process variants are used for the industrial production of deep drill holes: • the single-lip drilling process (ELB process).57). countersinks and step drills.428 9 Processes with Rotational Primary Movement Workpiece Tool Drill bushing Bead Shank Cooling lubricant supply Cooling lubricant chip mixture Fig. Figure 9. according to Sandvik makes it possible to increase the material removal rate.56). which in turn limited the material removal rate. the . • the ejector drilling process. The single-lip drilling process is used in the diameter range of approximately 0. The solution was finally discovered by the “Boring and Trepanning Association”. This stabilizes the start of drilling. the shank and the clamping sleeve.56 shows the essential characteristics of this method. the drill is guided on the top face of the workpiece by means of a drill bushing (Fig. In most cases. the single-lip drill consists of three components: the drill head. cemented carbide is used as the cutting tool material. Single-lip deephole drilling tools are used in manufacturing as solid drills. whereby full drilling is the most common case of operation in practice.56 The ELB process for diameters 0. The characteristic trait and main advantage of the single-lip drilling process is that the cutting fluid is supplied by means of one or several drill holes within the tool and the cutting fluid/chip mixture is safely led away by a longitudinal groove (bead) on the outside of the tool shank. 9. Due to the shape of the lead and the large drilling depth/diameter ratio. The attempt to cover the flute of the single-lip drill outwards resulted however in a drastic reduction of available chip space. Reflow occurs together with the chips through the cutting jaw and the drill pipe.8–40 mm.8–40 mm. whereby these limits depend to a large extent on the available machine power. the BTA process has the disadvantage that a complicated drill oil supply apparatus is required which takes over the sealing of the drill pipe. The process requires machines that are much different than standard drill machines. according to Sandvik diameter of which should not be less than 6 mm. The upper diameter for full drilling tools is around 300 mm and for countersink tools around 1000 mm.58). Reflow is made possible by the arising low pressure at the cutting jaw (ejector Workpiece Tool Drill tube Cooling lubricat supply Internal tube Ejector effect Cooling lubricant chip mixture Fig. The cutting fluid enters the drill head from the side.57 The BTA process for diameters of 6–300 mm. The ejector deephole drilling process is utilized for diameters of about 18–250 mm. Part of the cutting fluid is introduced into the internal pipe via a ring nozzle. 9.58 The ejector process for diameters of 18–250 mm.9. Cutting fluid supply is accomplished by means of a ring space between the drill pipe and an internal pipe (two-pipe process). according to Sandvik . it is a variant of the BTA process (Fig. rinses it and flows back into the internal pipe with the chips. Compared to deephole drilling with single-lip drills.3 Drilling Workpiece 429 Tool Cooling lubricant chip mixture Cooling lubricant supply Drill tube Fig. According to VDI guideline 3209. 9. 9. Kurzlochbohrer. sealing against the exit of the cutting fluid is omitted. heat development and wear are reduced accordingly.430 9 Processes with Rotational Primary Movement effect). The cutting edge.59). Wendelbohrer Twist drill 60 Diameter (mm) ELB-Tiefbohren ELB .deep hole drilling BTA . HM-Bohrer.59 Application of the deep hole tools compared to “conventional” drilling tools. As opposed to the BTA process. is subdivided such that two cutting parts are arranged at a time alternating left and right up to the centre. Deephole drilling tools dominate the entire field of inner contours that can be manufactured by drilling (Fig. and friction. 9. 9.deep hole BTAdrilling Bereich der konvenArea of conventional drilling tionellenBohrbearbeitung Diameter (mm) 1500 1500 Fig. Other drilling methods and tools are used only 200 Length/ Diameter Length/Diameter ELB Deep hole Tiefbohren drilling Short hole drilling Überschneidungsbereich zum Kurzlochbohren BTA Sintered carbide drill. The consequence of this is that the stress on the guide beads is reduced by about 10% to a maximum of 50% of the otherwise expected forces. A further peculiarity is its cutting edge distribution for reducing the forces acting on the guide beads as well as the double-sized cutting jaw thus required. Short hole drill. otherwise continuous from the periphery to the centre. to VDI guideline 3210 . acc. 9.3 Drilling 431 in the range of smaller drilling depths (up to a length/diameter of ca. cost-efficiency. ideal cooling and lubricating conditions. surface quality and geometric contouring accuracy. high alignment accuracy. metals of all kinds as well as other materials (e.g. boring and reaming – by one single operation. Difficult-to-machine materials can as a rule be machined effectively with deephole methods. the dominance and versatility of deephole drilling processes is often not perceived. Some advantages of deephole drilling are: • • • • • • • • • • very high machining performance. possibility of processing hard-to-machine materials. Due to its high productivity and the drill hole quality obtainable. even with short drilling depths. The process variants (including machining methods) of deephole drilling are characterized by the drilling task and the correspondingly adjusted drilling tools . especially in the area of overlap between shorthole drilling (conventional drill technology) and deephole drilling. pre-drilling. tools are being used that have the characteristics of deephole tools or are operated under conditions that resemble a deephole drilling operation. minimal drill hole inaccuracy. 6 and a diameter of up to ca. short primary processing times. Increasingly. • drill start guidance of the tool in a drill bushing or a guiding bore. Numerous examples of machining reveal the presence of deephole technology in the area of smaller tool diameters as well. Deephole processes have the following typical characteristics: • the use of special cemented carbide tools with one or sometimes more cutting edges that lie asymmetrically to the tool axis. minimal burr-formation when drilling out and when overdrilling cross-holes. By means of deephole drilling. deephole techniques are used now almost exclusively. deephole drilling is being used increasingly for manufacturing tasks in which the ratio between the drill hole depth and the drill hole diameter is larger than 6. • self-commutation of the tool by means of a three-point mounting in the drill hole through guide beads and the minor cutting edge (cylindrical grinding chamfer). Since these dimensions are predominant in general mechanical engineering. in which naturally most application cases for drilling are to be found. 60 mm). replacement of several operations – e.g. Especially in the field of deep drill holes and drill holes with large diameters. large drilling depths in relation to the diameter (up to a maximum of 250 times larger). plastics) can be processed both in the mass production of small parts and in the single-part production of large-scale machine parts. high drill hole quality with respect to diameter tolerance. • continuous high-quantity cutting fluid supply under pressure resulting in constant chip removal without chip removal strokes. only rotation-symmetrical parts with small masses can be used. the machines are applicable for a broad range of arbitrarily shaped parts. all deephole drilling processes can be employed for drilling out and core drilling. Form drilling tools serve to produce a definite drill hole bottom defined by the tool cutting edge geometry or contour transitions in the case of step drilling.60 Deephole drilling process variants (Fig. 9. In the case of a rotating tool and a stationary tool. The most frequently utilized variant of drilling is full drilling. they are most suitable for economical large-batch production. since there is a danger that even a small unbalance of the rotating . VDI guideline 3210).432 9 Processes with Rotational Primary Movement Through boring Blind hole Drilling in full metal Drilling in full metal Drill out Drill out Core drilling Core drilling Form drilling Form drilling Fig.60. With an automatic loader and potentially as multi-spindle machines. Deephole drilling machines can be constructed in a way that the primary motion can be carried out by either the tool or the workpiece or by both. Moreover. For both rotating and stationary tools. 9. 000 1/min. under normal conditions the surface qualities obtainable are in .4 kW. : 700 mm. 9. On the other hand.62). which corresponds to a diameter range of 50–80 mm of a maximum deviation of 9 μm. : 2. Since the process combination spiral drilling/reaming can be substituted with deephole drilling.–max.9. the cutting tool material. This approximate indication is grounded in the fact that the machinability of the material is still clearly part of the obtainable tolerance (Fig.9–15 mm. Figure 9. the composition of the cutting fluid. non-ferrous metals permit an ISO tolerance of IT6 under optimal conditions.3 Drilling 433 Fig. : 24. The limits of use of deephole drilling processes are essentially determined by the following factors: • • • • • the machinability of the material. 9.) maximum drill depth with 1 steady rest maximum spindle speed drive power : 0. VDI guideline 3210 summarizes these limits in the form of standard values for full drills. the stability of the tool and the machine. Some of the technical data of this deephole drilling machine are: • • • • full drilling rage (min.61 shows a deephole drilling machine by the company TBT Tiefbohrtechnik. core drills and countersinks and distinguishes according to the process and tool whether ISO degrees of tolerance above or below IT9 can be reached. For example.61 Deephole drilling machine (Source: TBT Tiefbohrtechnik) workpieces may lead to poor drilling results. the best possible tolerance when machining nitriding steels is IT8. the accuracy of the machine. 4 0. .63 shows furthermore that under especially favourable conditions even Ra values of 0. 9.62 Tolerances obtainable in deephole drilling (Source: TBT Tiefbohrtechnik) Non-ferrous metall Aluminium Tool steel Cast iron Tempered steel Carbon steel Cementation steel Nitrided steel Hole quality IT 13 12 11 10 9 8 5 6 7 RZ =100 RZ =25 RZ =6.8 0. Figure 9. which otherwise require a superfinishing operation such as honing.6 0.02 5 N12 N11 N10 Spiral drilling Reaming Broaching Honing Deep-hole drilling 12.3 RZ =1 N9 N8 N7 N6 N5 N4 N3 N2 N1 6.3 3.5 25 50 Average thoughness Under normal conditions achievable Under the best conditions achievable Fig. We can see from these considerations that deephole drilling is especially economical when subsequent processes can be reduced and/or high-alloyed materials are to be processed.2 0.05 0. 9.1 0.434 9 Processes with Rotational Primary Movement Fig.2 1.63 Obtainable surface quality in comparison to other machining methods (Source: TBT Tiefbohrtechnik) the same order of magnitude (fine finishing).1 μm are obtainable. 9. 9.3 Drilling 435 The following overview summarizes the areas of application in which deephole drilling can be used advantageously [Grüb74]: • • • • • • high material removal rate requirements machining materials with high alloy components that are hard to machine materials with a tensile strength of over 1200 N/mm2 high tolerance and surface quality requirements large drilling depth in relation to the diameter substitution of several single process steps (full drilling. drilling out. The multi-blade reamer is guided by the minor cutting edge arranged on the periphery. whereby the functions of machining and guiding are divided among independent active elements (Fig. whereby position and shape errors cannot be influenced. 9.3. The single-blade reamer is guided by a guiding bead arranged on the periphery. according to Mapal .64). 9. With respect to kinematics.9. a distinction is drawn between reaming with singleblade and multi-blade tools.64 Reaming tools. 9. reaming is equivalent to drilling out with small chip thicknesses (Fig.65 Drill out tools with PKD – equipping and also with ISO – indexable inserts. According to DIN 8589-2. Fig.65).5 Reaming Reaming is a fine finishing process and serves to improve drill hole quality.2. reaming) with one process step. according to [DIN 8589-2] Tool Workpiece Reaming Form reaming Fig. 6 Internal Thread Production Internal threads can be manufactured by means of primary shaping. coating and geometry.3. 9. Insert changeability makes it possible to adjust the tool to different materials and machining tasks by means of an appropriate choice of substrate. Besides the available technology. which makes it much easier to determine the diameter. the cutting tool material used is usually tool steel or HSS. 9. Usually. which repeats after half of the circumference. A distinction is made between manual reaming and machine reaming. Performance can be enhanced by employing coated tools in reaming as well. Among the process variants used. reamers are manufactured with an even number of teeth. cutting manufacture via tapping takes the leading position because of it is the most widespread. a distinction should be drawn between the cutting part where machining occurs and the guide part. strengths and surface quality. In the case of manual reamers.2.66).2.436 9 Processes with Rotational Primary Movement The cutting edges of multi-blade reamers can be arranged parallel to the axis or on a helical line. the thread type and the number of units required as well as the required tolerances. Screw taps consist of a shank and a screwed portion. 9. essential considerations when selecting the manufacturing method include the material to be machined. In order to prevent clattering vibrations. Tool Straight flute Workpiece Twisted flute Fig. forming or cutting.3. Drill holes with grooves are reamed with spiral tools in order to avoid cutting engagement impact of the cutting edge. Drill hole qualities of IT7 or better are obtainable.7 Tapping Tapping is drilling out for the manufacture of an internal thread that lies coaxially to the rotation axis of the cutting motion (Fig. while machine reamers use high speed steels or cemented carbides. In the case of the screwed portion. whereby two cutting edges face each other at a time. an odd distribution of cutting edge distances is selected. 9. which is responsible for stabilizing the tool [Zura90].66 Screw tap Section . and thus three chip flutes as well.2. for example. The structure of a screw milling cutter is basically similar to that of a “screw tap”.8 Thread Milling Under certain conditions. Thread milling is a special screw milling process. A screw milling cutter can mill threads of various diameters. Large groove cross-sections facilitate this and are especially important when machining long-chipping materials. spiral chip spaces are necessary to facilitate chip removal and to reduce the danger of chip jams. The surfaces of the screw taps can be processed to increase their wear resistance. power input) [Fosh94]. they guarantee cutting fluid supply to the cutting location. It can be used. screw taps with even grooves. It is not Fig.3 Drilling 437 Screw taps for through borings have a lead that guarantees good tool guidance. However. To process short-chipping materials. Moreover. to obtain very good surfaces on the screw flanks and to produce large numbers of units economically.3. 9. 9.9. which consists as it were of a single spiral-shaped tooth. intermediate cutters and finish cutters) are used to produce the thread in order to minimize tool load and the risk of fracture.67). tools costs. distorting the finished thread profile. the consecutive teeth of a screw milling cutter do not form spirals but rather are arranged without offset (Fig. are sufficient. The tooth shape corresponds as a rule to the form of the thread to be formed. they lead to a weakening of the load-bearing tool cross-section. hard chrome plating and coating with hard materials. The cutting tool materials employed in tapping are high speed steels and cemented carbides. This is the case when the thread to be milled is not at least three times larger than the milling cutter’s diameter. For the sake of good tool centring. In the case of manual screw taps. The grooves serve to receive the chips and convey them outside. 9. The most important processes used are nitriding. which are easier to manufacture. Threads with large diameters can often only be fabricated via milling (number of units.67 Screw tap with coil (left) and milling cutter without coil (right). The potential cutting speeds are relatively low and depend considerably on the combination of cutting tool material. thread milling can be a good alternative to other thread manufacturing processes. Without a correction of the profile. according to Fraisa . In the case of long-chipping materials. a set of two or three tools (pre-cutters and finish cutters or pre-cutters. The screwed portion is subdivided into cutting studs by grooves. As opposed to the screw tap. the tool would cut freely. In many cases it is necessary to correct the tooth profile. at least three cutting studs are required. workpiece material and cutting fluid. The following.68 provides a graphical depiction of a milling cycle.68b). The screw milling cutter is finally radially advanced to the required standard diameter of the thread (Fig. The material is displaced on these forming edges. The rotating milling cutter positioned in the centre of the drill hole is axially lowered to the desired thread depth into the core drill hole (Fig. slightly tapered calibration section serves to smooth the fabricated thread flanks and to guide the tool. 9. 9.68f). The thread moulder is separated axially into three sections. The screw milling cutter is thus designated by the standard thread diameter that corresponds to this pitch. The largest amount of forming work takes place on this area.68 The thread milling cycle possible however to vary the pitch. so that there is a defined axis distance between the tool and the drill hole axis. which serve to receive the lubricant.438 9 Processes with Rotational Primary Movement a b c d e f Fig. It has a conical lead part on the top which extends across several thread turns. While . Figure 9.68c). The profile of a thread moulder is. The next section of largest external diameter has the function of removing the thread flanks from the mould. 9. thread moulding is a non-cutting (forming) process in which the internal thread is created by impressing a tool (the thread moulder or groover) into the workpiece [Fieb95].68d). Finally. 9. The tool is then adjusted to the drill hole diameter (Fig. 9.2.3. the screw milling cutter is backed out of the thread radially via a circular arc (Fig. To form the thread. resulting in increased bending strength due to the larger cross-sectional area.68e). 9. The starting situation in the case of thread moulding is also a pre-drilled hole with a diameter corresponding approximately to the flank diameter of the thread. whereby the workpiece or the tool is axially shifted around a pitch. 9. a polygon with three or more flattened corner areas. The thread moulding process does not require chip flutes.9 Thread Moulding According to DIN 8583-5. 9. there is a movement cycle of somewhat more than 360◦ on a coil (Fig.68a). driven back to the cutter axis and lifted axially out of the thread (Fig. in contrast to the round profile of a screw tap. hacksawing and circular sawing. typical of formed threads. used for separating or slitting workpieces. the saw blades can be seen as a tool with an infinitely large diameter (Fig. The saw band is supplied on large rolls and. This process is distinguished by low cutting losses due to the small width of the bands.4 Sawing 439 Fig. when sawing larger material cross-sections.69 left). of band length (ZpZ). In comparison to “tapping”. 9. As a result. The cause is to be found in the strain-hardening that occurs during forming.1 Bandsawing Bandsawing involves a revolving. the displaced material flows into the tooth gaps of the thread moulder. unending saw band cutting with a continuous. 9.4 Sawing Sawing is cutting with a rotary or translatory main movement with a multi-blade tool of low cutting width. the tooth number has to be reduced in order to obtain a sufficient chip space for the arising chips. However this results as well in low tool stability against cut deviation. since even in the process variants hacksawing and bandsawing.4. mostly linear cutting motion [DIN8589g]. This causes the formation of ears. These are the most commonly encountered process variants in praxis. 9. . thread moulding results in threads of higher strength. The following will be organized according to the type of tool used into bandsawing. 9. Sawing is classified as a process with a rotary main movement.69 Thread tools and several formations of flanks Thread grooving Tapping the forming edges of the tool penetrate into the workpiece material and the thread flanks form to the required dimensions. in the area of the thread crests (Fig. the ends are joined together with a butt-welding process. in which there is a translatory cutting motion. The size of the chip space depends on the dimensioning of the tooth base radius r and on the number of teeth. Figure 9. The number of teeth is commonly given in reference to 1 in.71 shows the cutting part geometry and terminology of a saw tooth. after the cutting to the desired length.70). 9.9. Possible HSS cutting tool materials include HS6-5-2. In the case of larger tooth numbers. In practice. 9. In the case of unlimited teeth. Tool life criteria that define the end of a sawband’s usage are a certain wear condition. measured as the size of the engagement of one cutting edge in the cross section perpendicular to the feed direction.72). the blunt edge made of soft tool steel is joined by means of electron beam welding with a hardened band made of HSS. corresponds to the workpiece width. 9. HS2-10-1-8 or HS10-4-310. shaft offset is also used (Fig. The engagement size ae . In the case of bimetal bands. 3). Saw bands are made either of tool steel or bimetal. bands with soldered cemented carbide plates have also become established.70 Different sawing methods In order to prevent jamming of the saw band in the cutting channel. the single teeth are bent or set to the left and to the right alternately from the cross section (see Chap. the depth of cut ap corresponds to the width of undeformed chip b and in this case to the width of the saw blade. The cutting motion is parallel and the feed motion perpendicular to the longitudinal axis of the band. into which finally the saw teeth are inserted. tooth fracture or a maximum permissible deviation of cut due to excessive . The standard offset sequence for separating metals is right/left/straight.440 9 Processes with Rotational Primary Movement Tool Workpiece Workpiece Hack saw Bandsaw Tool Workpiece Circular saw Fig. 9. 9. 9.4 Sawing 441 Tool γο r fz H t βο vc αο ve Workpiece ae fz H r t Tooth feed Tooth height Base radius vc vf ve Cutting speed Feed rate Active speed Tooth pitch Fig.72 Sawband restriction types (Source: Wikus) Waves restriction (WS) vf . after RENG [Reng76] Standard restriction (SD) Group restriction (GS) Right-left-restriction (RL) Fig.71 Title and part geometry on the cutting saw band. material is removed only in the forward stroke.73 Tooth segments of a saw blade (Source: Ohler) . the saw blade is mechanically or hydraulically lifted. In contrast to other cutting methods. Blades made of tool steel are generally only used for manual hacksaws. gangsawing. In the return stroke. 9. Fig. On the other hand. and jigsawing are all hacksawing processes [DIN8589f].4. From this definition.73). The saw blades are made either of solid HSS or of HSS segments that are riveted on a blade body.70). hacksawing. usually linear cutting motion (Fig. i. we can already see that hacksawing is a process with a discontinuous cutting motion. the cutting loss is relative small.e. the tool life is not given as the measure for the tool life parameter of a saw. According to DIN 8589-6. 9. 9. 9. with machine hacksaws. This high-performance process is employed for linear cuts on low-cost materials. 9.442 9 Processes with Rotational Primary Movement passive force. but the cut surface (corresponding to the workpiece surface cut until reaching the end of tool life) [Reng76].3 Circular Sawing Circular sawing is sawing with a continuous cutting motion using a circular rotating saw blade (Fig.2 Hacksawing Hacksawing is a process variant with a repeated. The result is that the material removal rate is lower compared with bandsawing or circular sawing.4. as much material is lost due to the relatively wide kerf. The material removal can be improved further by means of soldered cemented carbide cutting edges. At the same time.4 Sawing 443 Both tool steel and HSS as well as cemented carbides as used as cutting tool materials. The fact that a tooth in the section of the chip breaker flute of the previous Chip cross section of the after cutting tooth Front Incisor After cutting incisor Chip flute Fig.74 Cutting edge geometries of circular saw blades Chip cross section of the front incisor . 9. If such measures are neglected. the blade body is made of construction steel upon which the individual HSS segments are riveted (Fig. the chips jam in the chip space and can damage the tool.73). In the case of larger saw blades. the chips are fractured such that the pre-cutting teeth protrude at least by the amount of chip thickness and their major cutting edge length is smaller than the total width of undeformed chip. it is absolutely necessary to break the chips in such a way that they are narrower than the kerf.74 shows the two most prevalent possibilities [Schm80]. In this way. Saw blades made of one material are made of tool steel or HSS. there are fundamental differences among construction types of saw blades. By subdividing the teeth into pre-cutting and post-cutting elements. In order to guarantee the cutting capacity of circular saw blades. Figure 9. 9.9. One alternative is grinding-in offset chip breaker flutes into the otherwise identically formed teeth. For cost reasons. one narrow and one wide chip is produced per tooth that can escape towards the centre into the chip breaker flute without jamming in the kerf. 12 − 0. In order to increase the circular saw blades’ stability and running smoothness.28 mm and with cemented carbide saw blades values of vc = 90 − 150 m/min und fz = 0. values of vc = 18−30 m/min and fz = 0. The advantage is that one tooth with a chip breaker flute can take on the same chip cross-section as the pre-cutting and post-cutting teeth of the other lead version. A laser radiation process is also possible. For separating general construction steel with HSS saw blades. This is done either by hammering or by rolling a concentric pressure zone into the side surfaces.22−0. it is necessary to introduce characteristic stresses into the blade in a specific fashion.18 mm are common as cutting data.444 9 Processes with Rotational Primary Movement tooth must remove a larger chip cross-section has only an insignificant effect on the progression of wear. . 1 Broaching Broaching is a machining process with a multi-toothed tool whose cutting teeth lie in a row. The cutting motion is translatory. F. Klocke.1. for which differently designed machined tools and tools are required. circular broaching.1007/978-3-642-11979-8_10. Tooth graduation perpendicular to the direction of the cutting speed replaces the feed motion. C Springer-Verlag Berlin Heidelberg 2011 DOI 10. high surface qualities and precision are obtainable and tolerances of up to IT 7 maintained. helical broaching. planing and shaping as whose process variants. We also distinguish between internal and external broaching. since usually several teeth are simultaneously in engaged. shaving.Chapter 10 Processes with Translatory Primary Movement Cutting processes using a geometrically defined cutting edge that carry out a translatory main motion include.  445 . among others: • • • • broaching. RWTH edition. Manufacturing Processes 1. as the tools can always only be used for one cross-section of undeformed chip [Kraz77]. DIN 8589-5 draws a distinction between: • • • • • face broaching. Broaching can realize a high material removal rate in one stroke. 10. This method can only be utilized economically in serial production due to the high costs of tool production and preparation. each being separated by the thickness of one chip. profile broaching and form broaching. The final contour is usually created in one stroke. Moreover. as shown in Fig. pulled/pushed through a borehole (internal broaching) or pulled/pushed along the external surface of the workpiece (external broaching). The broaching tool is. 10. in special cases also helical or circular [DIN8589e]. The following description will be restricted to the most important of process variants. The tools are either designed as solid units or fitted with indexable inserts [Wege85. The subsections differ in feed per tooth fz . 10.1 Face and Circular Broaching Figure 10. Cutting fluids are almost always used in broaching processes in order to guarantee chip transport from the chip space in addition to the lubricative effect [Falk70]. cemented carbide or CBN are also used as cutting tool materials. Cutting fluids are not used in this case [Klin93]. The teeth in the calibrating section possess the geometry required for the workpiece. 10. Merk80].04 mm and in the calibrating section equal to zero. TiCN). In certain cases. In the roughing section. since acceleration and deceleration must take place in one stroke. usually oils are used due to the low cutting speeds.1 Contact ratios and process variants of broaching To process unhardened steel materials. The standard cutting speeds in steel-cutting are between vc = 1−30 m/min. Technologically comparable methods will be treated jointly.25 mm depending on the workpiece to be machined. Broaching tools have a roughing.446 10 Processes with Translatory Primary Movement Internal broaching External broaching Workpiece Tool Workpiece Tool vc vc Workpiece holder Undeformed chip thickness Fig. When machining hardened steels. in most cases tools made of high speed steel are employed.0015−0.1. Powerful broaching machines can reach cutting speeds of up to vc = 120 m/min so that the surface quality is improved by avoiding built-up edge formation [Schü65. To increase performance and wear resistance. the feed per tooth fz is in the range of fz = 0.g.1−0. To this end. while in the finishing section is it fz = 0. the tools can be coated with hard materials (e. TiN.2 shows the typical structure of a broaching tool. Opfe81]. high cutting speeds of vc = 60−70 m/min and cemented carbide tools are used to reduce the high cutting forces and to increase tool life. such as in large-batch production of grey cast iron or broaching hardened steels. a finishing and a calibrating section. Further cutting speed increase is only possible to a limited extent because of the process kinematics. . The height . since only workpieces with a low height can be machined. 10. The pitch t of the broaching tool and thus the size of the chip space as well depend on the height of the workpiece. which is comprised of the individual components of all the cutting teeth in action. which in turn limits the maximum size of the chip space. only a small pitch is required. In the case of small machines and short tools.3 provides an overview of various broaching tools for internal and external machining. The total cutting force. Hoff76]. The cutting pattern determined by the arrangement of the cutting edges along the tool axis is called the offset. the previously first calibrating tooth now becomes the last finishing tooth. A large workpiece height and unbroken chip forms require large chip spaces. If the cut is perpendicular to the broaching surface. chip formation of the material and the maximum potential tool length. The chamfer width bfα is parallel to the axis in the calibrating section and inclined by the tool orthogonal clearance of the first flank face α f in the roughing and finishing section. If the broaching surface is machined from the side on the other hand. In this way.2 Internal broaching tool (schematic) The calibrating section makes it possible for the expensive broaching tools to be reground and thus be economically employed. is crucial for the minimal cross-section of the broaching tool shank. Regrinding the tool shifts the entire tool profile by one tooth towards the calibrating section. Possible tool lengths are in the area of L = 100−10000 mm. Internal broaching tools are manufactured up to a diameter of D = 500 mm (e. The tool rake angle γ and the tool orthogonal clearance α of the second flank face are adjusted to the material to be machined.10. the individual teeth are equipped with chip breakers in the form of tooth grooves [Weul85. To obtain chip fracture. for the internal gearing of hollow wheels). Both offset types can be provided on one tool simultaneously depending on the workpiece geometry [DIN1415].g. Figure 10.1 Broaching 447 Finishing tooth Rear pilot A Roughing tooth A Sizing tooth (fz = 0) B t B bfa αf α fz γ Fig. it is called offset in depth. it is called lateral offset. 448 10 Processes with Translatory Primary Movement Fig. 10. In the case of a non-integer ratio. . an example is shown in Fig. of the workpiece height to be broached) and the pitch t is an integer.4.5 Broaching stroke m= Cutting length l Pitch t Fig. we obtain a constant total tensile force. 10.e.4 Total cutting force during straight and diagonal toothed external broaching and variation of the total cutting force are highly contingent on the pitch t and the inclination λs . 10. the tools are highly stressed dynamically by periodic variations [Vict76]. If the ratio m of the cutting length (i.3 Internal and external broaching tools (Source: Forst) Total cutting force b Tool lS1 lS0 m=3 lS2 Broaching stroke t Total cutting force lS0 = 0° lS1 lS2 t b t tan(λS2) ≥ b tan(λS1) < lS1 lS0 lS2 m = 2. Since. On internal broaching tools.1 Broaching 449 Tools with an inclination λs not equal to zero show a slowly increasing cutting force profile and. 10. 10. so that a partial device can be dispensed with. This has the advantage that the tool can be subdivided into several bushings to avoid excessive deflection during hardening. including hollow wheels for automatic transmissions. The efficiency of broaching can be fully exploited by using broaching tools that make it possible to broach involute profiles in one pass. In the case of external broaching. in the 150–300 mm diameter range primarily bushings clamped to a pin. Figure 10. which has become increasingly important because of its high cutting performance and obtainable surface quality. a lateral offset is provided in the rear of the tool so that one can calibrate to the finished dimensions with a feed motion in the direction of the tooth flank. in the case of pure offset in depth (in which the profile of the tooth flank is formed by the minor cutting edges) the profile’s dimensional accuracy is insufficient. In the case of the gearwheel broaching processes used today. thereby compensating the lateral forces.5 shows a few application examples.2 Profile Broaching One important variant of internal profile broaching is gearwheel broaching of involute-geared hollow wheels. lateral forces can arise that sometimes cause tool misalignment. sliding sleeves and components with internal splined hub profiles. an inclination λs not equal to zero can be realized by a coil-shaped cutting edge. and the quality of the broached gearwheel depends above all on the precision of the tool. Double helical gearing is characterized by the fact that the teeth of one level have the opposite inclination. inclinations of up to λs = 5◦ are used. In practice. so that the grinding allowance can be kept small. if the construction is favourable. minimal cutting force variations.10. solid tools are used. For diameters up to 150 mm. The tooth thicknesses of the cutting discs are graduated so that the Fig.1. Complex broaching tools can be composed of cutting discs that are bolted on a holder as a block.5 Examples for internal broaching (Source: Forst) . all tooth gaps are machined simultaneously. Spiz71]. The great advantage of calibrating bushings is that they can be used to produce helical gearings among other things (spiral broaching) [Bung74].450 10 Processes with Translatory Primary Movement Fig. one can also work parallel in several clampings if the machine power is sufficient. 10. the tool is the moving element as in conventional broaching.and finish broaching can be executed either on one machine in one clamping (finish broaching tool as an attachment on the pre-broaching tool) or in two clampings with two separate tools (pre. the chip spaces can be arranged both in a ring-shaped or in a helical manner. the use of this method is limited till now solely to the manufacture of dog gears.and finish broaching in two clampings). . becoming ever smaller by regrinding. These plugs are perforated in case of an exact seating of the discs so that two discs can always be pinned to each other. “Pot” or “tubular” broaching is used for the external profile broaching of close surfaces. The tool consists of a hollow body in which are inserted the strip-shaped broaching tool segments. full-dimension cutting broaching bushing as shown in Fig. It also has major cutting edges running parallel to the involute form and is used separately or as an attachment on a broaching tool in order to work in one clamping. Each disc has two cast iron plugs to keep the discs in perfect alignment. To increase output.6. In the case of helical geared broaching tools. 10. The workpiece is pressed by the broaching tool so that all contours to be produced on the periphery of the workpiece are produced in one stroke [Schw75. In gearing technology. In this broaching technique. Pre. forward always by one position and to append a disc with full dimensions only on the respective end of the broaching tool. but they cause considerably higher tool manufacturing and sharpening costs. In another process variant. Another tool concept is the one-piece.and finish broaching can be distributed among two separate machines in order to exclude a mutual influence of the machining processes and to guarantee improved quality [Schw71].6 Broaching tools for finishing internal toothed centre gears (Source: Forst) final tooth cuts to the finished dimensions. Pre. It is possible to shift the discs. the workpiece carries out the cutting motion. In the case of helical chip spaces. cutting force variations are much smaller. The full-dimension cutting broaching bushing is mounted in a floating fashion so that it can centre itself in the pre-broached profile. 1. this process variant is especially suitable for large-batch and mass production. Ansc86]. 10. and then the turbine disc is rotated by one pitch at a time.10. Individual manufacture or hard finishing of batch sized with up to 100 parts is not economically possible with the gearing processes presently available [Peif91]. since internal gearings can only be produced economically by broaching in large-batch production.7) and rotation external cylindrical broaching (Fig.3 Form Broaching Because they involve shaping with a correspondingly built tool and due to their kinematics. Minimal deviations from the cylinder shape cannot be avoided as a rule because of the kinematics [Müll86. external cylindrical broaching (Fig. 10. To this end. External cylindrical turning is a process that has been known for a long time but was first utilized industrially in 1982 by an American automobile manufacturer for roughing of crankshaft main bearings [Whit84].7 External cylindrical broaching with a reciprocating tool . Because of the complex tools and very short manufacturing times. In Crankweb nWST vc nWST Pin bearing fz b vf Tool Fig.8) should be classified among manufacturing methods in the broaching subgroup as form broaching [DIN8589e]. the fir-tree profiles are broached separately. External cylindrical broaching is a combination of external cylindrical turning and broaching. The demand for a hard finishing of internal gearings after case-hardening could only be met in a few cases.1 Broaching 451 Another important area of application of external profile broaching is the manufacture of fir-tree profile grooves in turbine discs. broaching or honing. By substituting the turning tool with a multi-blade broaching tool. 10. The dimensional and shape accuracy of the workpieces is high. we can combine the advantages of turning (continuous process) with those of broaching (multi-blade tool) [Berk92]. Internal gearings with adjacent functional surfaces are not suitable for finishing due to an insufficiently large tool oversize by profile grinding. 10. Tika86].5−0. Problems with chip breakage can be avoided by using indexable inserts with chip breakers. The active direction of the feed runs perpendicular to the rotation axis of the . 10. these deviations are in the range of 5–10 μm.7 μm [Müll86. If ultrahard cutting tool materials are used.452 10 Processes with Translatory Primary Movement Crankweb nWST b nWZG Pin bearing vc nWST fz nWZG Tool Fig. Since the calibrating section is responsible for the manufacturing tolerances to be complied with. 10. Müll87]. this tool structure allows for a simple tool preparation as well as a fast and cost-efficient adjustment for highly diverse profiles and machining tasks. rhombic. The active motion in external cylindrical broaching with a linear tool motion is produced by a rotating workpiece and a translatory feed motion vf of a multi-blade tool.8). The tool is fitted with indexable inserts which are arranged according to the external contour to be created and fastened with bolt clampings in a space-saving fashion (Figs. Both uncoated and coated cemented carbides as well as ceramics or CBN cutting tool materials can be used [Ansc86. Overall. and especially with CBN as cutting tool materials. The single modules are bolted onto a base body. The discontinuous tool engagement results in chips of finite length. finishing and calibrating section. the inserts are fastened in adjustably cassettes. Long tool lives can be realized with mixed ceramics. triangular and round ISO indexable inserts with modified cutting part geometries are used. Quadratic. The indexable inserts in the preparation modules are not adjustable.8 External cylindrical broaching with a rotary tool the case of diameters typical of crankshafts. even in the case of hardened steel materials. Measurements at the crankpin resulted in surface characteristic values of Rt = 6−8 μm and Ra = 0.7 and 10. The broaching tools used are built in a modular fashion and consist (as are conventional broaching tools) of a roughing. both external cylindrical turning and rotation external cylindrical broaching are applicable. the amount of depth of cut ap is determined by the number of cutting edges and the pitch of the tool or by the feed per cutting edge fz . gearshafts or sliding sleeves).10. the preferred area of application is still machining the main and pin bearings of crankshafts (Fig. On the one hand. the rake angle γ 0 and the first orthogonal clearance α f of the tool are not identical to the effective rake angle γ 0e and the effective orthogonal clearance α fe . The tool cutting edge engages with the workpiece eccentrically and moves through the workpiece during the cutting process with feed velocity vf on the line of action (Fig.g. External cylindrical broaching with a linear tool motion is basically suitable for external machining of wave or ring-shaped workpieces as well as for manufacturing profiled and out-of-line rotation-symmetric outer edges (e. The cutting speed vc is created by the rotation of the workpiece. Two effects are caused by the kinematics of the external cylindrical broaching process.9). For this reason.9 Kinematics of external cylindrical broaching with a reciprocating tool according to BERKTOLD [Berk92] workpiece and tangential to the workpiece. since the chip thickness is a function of the angle of engagement ϕ [Berk92].7). 10. the angle of engagement ϕ changes during a cut – and thus the effective angle as well. the chip cross-section changes during a tooth engagement. Similarly to conventional broaching. On the other hand. Currently. . 10.1 Broaching 453 Workpiece nWST αfe Line of action ϕ vc ϕ vf αf fz Tool Schneid e h γ0e γ0 ϕ Fig. 10. however in exceptional cases axis intersection angles of  = 3 − 20◦ are possible [Beck00]. gears machined by shaving are not hardened. The complex tool and very short manufacturing times allow for an economical use of external cylindrical broaching mainly in large-batch and mass production. finishing and profiling elements. External cylindrical broaching with a linear tool motion (Fig. The tool life of the entire tool is accordingly high. On the other hand.10 shows the principle of gearwheel shaving. Gear shaving is a process using geometrically defined cutting edges that is used to finish pre-teethed gearwheels. in which the crossed axes of the tool (shaving wheel) and the workpiece cause a relative cutting motion. One example of this is the widespread practice of gear shaving (also called “soft shaving”) [DIN8589i]. so it is possible to do a complete processing (excluding grinding) of individual bearing positions including plane surfaces and recesses in one cycle is possible. hardening. With the help of a suitably designed machine. The result of this is that tooth flanks can be machined relatively effectively and using relatively small forces in comparison to hard finishing. The tool and workpiece axes are thus not parallel and form the “axis intersection angle” . The latter generally has values between  = 10 and 15◦ . the flanks of which are interrupted by flutes and have a different helix angle than the workpiece. 10. Customarily.8) fulfils the demand for a compact design. turning the cut-ins and plane surfaces. Figure 10. The tool is a gearwheel. The axis intersection angle results in a relative speed of the shaving . several pin bearings lying in a rotation axis can be broached at the same time as well by means of a height offset of the workpiece. external cylindrical broaching with rotary tool motion (Fig. Several main bearing positions can be machined at the same time by means of a simultaneous engagement of several tool arranged adjacently to each other. the tangential cut is obtained with a rotary tool motion by the circular feed motion of a round tool. External cylindrical broaching tools consist of a large number of cutting edges that are each only in action briefly during the working stroke. As opposed to the former machining sequence – spinning the bearing pin. The rolling kinematics during shaving resembles that of a helical roller gear. In external cylindrical broaching. The individual cutting edges are graduated along the periphery of the tool by the feed per tooth fz respectively. 10. 10.7) requires (especially when machining crank webs with a high radial allowance) very long tools that increase the allowances of the external cylindrical broaching machines and their required floor space to a disproportionate extent.2 Shaving Shaving is a manufacturing process used for post-processing. It serves to improve gearing quality and surface quality [Lich64]. the second process variant. levelling and grinding – it is possible to combine or reduce individual manufacturing steps.454 10 Processes with Translatory Primary Movement Tools are used that have roughing. and so the high sliding speed here is equal to zero. The relative speed in the direction of the workpiece flank during shaving corresponds to the cutting speed vc .1) The chips are removed as a result of the tool and workpiece being braced with each other radially with high force and simultaneously shifting on each other. Figure 10.5 mm) und cutting speed (vc = 30−80 m/min) [Kloc03a].2 Shaving 455 Sectional view A-A rw0 Shaving wheel Shaving wheel vw. the tool is propelled while the workpiece. and chips are formed due to the penetration of the cutting edge and the workpiece flank. above all the small chip thicknesses (hcu. Because the shaving wheel flanks are only grooved radially in order to form the cutting edges but the remaining tooth flanks are not relief-ground.rel v uw2 A A vuw0 vuw rw2 Σ Workpiece Workpiece Fig. as it leads to a levelling of the roughness peaks on the tooth flank and thus improves the surface quality of the tooth flank [Busc75]. are generated as the workpiece flank shifts on the tool flank. High sliding speeds. we therefore obtain a negative tool clearance.11 shows the surface structure of a shaved gearwheel flank. runs freely behind. Upon entry of the cutting edge in the workpiece flank. as a rule.max = 5−10 μm).rel in the direction of the workpiece flank. 10. the result is a constructive tool clearance of αcon = 0◦ . cutting lengths (lmax = 0. It is calculated form the circumferential speeds at the rolling circles vuw0 and vuw2 and the axis intersection angle as well as the helical angles β 0 and β 2 : vw. which leads to plastic deformation of the material lying underneath. In this way. This is desirable in shaving. Shaving has a few other technological peculiarities compared with other machining processes with geometrically defined cutting edges. which change with tooth height.10 Principle of gear shaving (Source: Gleason Hurth) base vw.10. The maxima of the high sliding speeds . There is no rolling on the so-called “pitch circle”.1−0. The high efficiency of the shaving process is the result of using a adequately large number of cutting edges. rel = vc = vuw0 sin Σ sin Σ = vuw2 cos β2 cos β0 (10. 10. point contact predominates between two flanks due to the axis intersection angle. a very sharp cutting edge is necessary for a clean chip formation. This offset is usually about 0. yet there is no simultaneous shifting of the base in the direction of the workpiece flank. Even a small amount of wear leads to a disturbance of chip formation and thus to lower workpiece quality. it is therefore necessary to shift the point of contact between the tool flank and the workpiece flank (the axis intersection C) during the process. The workpiece would thus be machined at the same locations at each rotation and uncut sections would remain between them. very high quantities can be produced within the tool life due to the large number of available cutting edges (1000–10. however with different signs. In this method.000 workpieces per regrind cycle) [Busc75].456 10 Workpiece tooth Processes with Translatory Primary Movement Geometry of the single cut Machined workpiece surface Pitch circle Ghost lines in direction of cutting speed Feed marks Fig. very small chip thicknesses exist during shaving. 10. That means that the base . which results from the axis intersection angle.2 mm per workpiece rotation in practice.11 results from the penetration between the tool cutting edge and the workpiece flank. There are various ways of doing this as shown in Fig. In the extreme case of underpass shaving the diagonal angle is ε = 90◦ . 10. the shaving wheel is shifted relative to the workpiece parallel to its axis. the base are offset relative to each other on the shaving wheel flank.11 Cut geometry and surface structure during shaving [Schr07] are located at the tooth tip/base. In the case of parallel shaving. During shifting. the axis intersection is indeed shifted along the workpiece axis as the tool is moved. The tool must be moved at least along the entire width of the workpiece. In the case of “diagonal shaving”. In the case of crossed helical gear transmissions with uncorrected gearings. we obtain the structure on the tooth flank as shown. As already described. In order to machine the entire workpiece flank. The cut geometry shown in Fig. Nevertheless. In order to machine the entire workpiece flank evenly nonetheless. By superimposing the nearly constant sliding speed in the direction of the tooth flank. the shaving wheel is not moved parallel to the workpiece axis but under a diagonal angle ε in order to reduce this path and thus the machining time as well. the point moves along a curved path on the workpiece flank.12. line contact must be made between the flanks by “hollow grinding” the shaving wheel flank. As a rule.3 Planing and Shaping Planing and shaping are machining processes using a repeated. much higher forces act on the machine than in other shaving methods. the workpiece is. because it is necessary to design the shaving wheel for the specific workpiece. 10. these processes are used to cut larger. Both plunge and underpass shaving are primarily suitable for machining larger batches. The arrangement of the base on the shaving wheel flanks thus depends on the teeth number ratio between the tool and the workpiece.12 Comparison of different shaving methods must be arranged on the shaving wheel flank in such a way that the desired offset exists according to the teeth number of the workpiece. 10. in principle. Since the axis intersection is not shifted in this method. In this process. The most economical shaving method by far is plunge shaving. Accordingly. In the case of older shaving machines in particular. this process meets its limits when machining larger gearwheels (beyond mn = 3 mm).10. diagonal shaving is used as a rule [Beck00]. In this way. . and the productivity of this method is significantly higher than in other shaving methods. the only translatory motion carried out by the shaving wheel relative to the workpiece is a radial infeed. a shaving wheel can only be used for exactly one workpiece geometry. usually linear cutting motion and an incremental feed motion perpendicular to the cutting direction. even surfaces to size [DIN8589e].3 Planing and Shaping Parallel shaving 457 Tangential shaving Diagonal shaving Tool at the end of machining Plunge shaving Diagonal angle ε C2 Σ C2 C1 C1 C2 C1 C2 C Tool at the beginning of machining Workpiece Feed parallel to workpiece axis Feed in diagonal angle ε Feed orthogonal to workpiece axis Feed exclusively radial Fig. Because many more cutting edges are involved in the cutting process in plunge shaving. simultaneously machined along its entire width. In the case of smaller batched. the tool makes a lifting movement. the tools are manufactured with tool steel.1 Face Shaping and Face Planing Figure 10. will be treated in more detail. the working stroke. We will dispense with a separate treatment of the individual methods in this context. The infeed motion can be executed both by the workpiece (by lifting or lateral shifting of the table) or by the chisel (by lifting and lowering the plunger head). round. .13 Work movement during shaping and planning The methods differ in the creation of the relative movement between the tool and the workpiece (Fig. In order to prevent collision between the workpiece and the tool during the return stroke vR .3. In shaping. relationships will be explained that can be applied to round. The feed f is realized by the workpiece table.13). the tool moves over with workpiece with cutting speed vc . In shaping. helical. profile and form shaping/planing. the idle or return stroke with vR .458 10 Processes with Translatory Primary Movement Planar shaping Planar planing vR vc f vc ap ap vR f Fig. Due to the technological similarity of these methods. Gear shaping and gear planing. In analogy to other machining processes with geometrically defined cutting edges. 10. reallocating the impact instead to the more stable cutting edge.g. Since in the case of this method large cross-sections of undeformed chip are required for economic reasons. because of their importance in gearwheel manufacture. the following will focus on shaping. profile and form shaping as well [DIN8589d]. of application group K40) due to process-related abrupt stresses as well as the low potential cutting speeds.13 shows the motion sequences in face shaping and planing. the concepts of planing and shaping are not strictly distinguished. the process mostly uses cemented carbide cutting edges with a large negative inclination. In the case of shaping. This inclination causes not only a pulling cut but also prevents the lead impact from affecting the chisel tip. the workpiece is guided past a stationary tool. helical. with speed vc as well as the return motion. On the basis of face shaping. In the case of planing on the other hand. high speed steel or tough cemented carbide (e. a distinction is drawn between face. the tool carries out the cutting motion. In practice. 10. 10. such machines are indispensable. The chip volume per time unit is small in comparison to other machining methods. the difference between the machine run time and cutting time is considerable. Vertical shaping machines are used to machine workpieces with difficult-to-access. To prevent clattering and potential hooking of the chisel in such conditions.2 Gear Shaping Gear shaping is a method for machining gearwheels and serves primarily as an internal gearing manufacturing process (Fig. Shaping machines. 10. while finishing operations make use of pointed. vertical or sloped external and internal shapes. it is often offset such that its cutting edge is behind the chisel support plate.14 Different types of shaping tools There is a diverse array of tool forms. High traction planing machines permit the use of multiple chisel mountings so that the primary and secondary processing times can be reduced. For economic reasons.15). of which the stroke length is normally up to about 1000 mm. In finishing. 10. generally straight and curved chisels are used. since in ever stroke masses must be accelerated and decelerated.3. Gear shaping thus has a role of .14.3 Planing and Shaping 459 vc vc Fig.10. the frequently required tool change (single-blade tool) and the large machine assembly space (planing) should also be considered. wide chisels (broad-tool chisel). Roughing takes place with cutting speeds between vc = 10−30 m/min and with large feeds and chip thicknesses in order to exploit the machine’s power. 10. The cutting speeds realizable in shaping are low. this method is only used to a limited extent. cutting speeds up to vc = 60 m/min are employed with small feeds. Chisels must often protrude extensively depending on the workpiece. Due to the return stroke. which is faster than the working stroke. Especially for machining irregular shapes with a short cutting path and small run-out. The advantages of this method in comparison with for example milling include not only the simple and resultantly cheap tools but also the minimal heating of the workpiece. 10. some of which are shown in Fig. For roughing. and the single routes and overrun routes in which no chips are removed. are especially suited to machining small workpieces. Besides the abovementioned disadvantages of shaping. 10. so even gears on profiled shafts or with large coupling collars can be processed. the radial and rolling feed are superimposed so that machining is done in a spiral shape. The spiral infeed can take place both with a constant and with a degressively sinking radial feed (CCP process). The axis offset AV corresponds to a lateral shift of the shaping wheel axis perpendicular to the symmetry axis of the gear shaping machine and is permanently set before the beginning of the process. This offset helps prevent collisions between the shaping wheel and the workpiece during the return stroke motion. which is classified as a continuous gearing process. Disc gear shaping wheels and bell gear shaping wheels are used for manufacturing large external and internal gearings. Vuce06]. the distance travelled on the circular pitch per double stroke DH is defined as the sum of the working stroke and the return stroke. As the rolling feed. Figure 10. To produce the rolling motion. Just as is the case in hobbing. gear shaping can be used to produce spur gears and helical gears. In one process variant of gear shaping.16 shows the kinematics of the process. the gear and the tool are conjointly driven. this method fulfils not only high performance requirements but also high standards with respect to production accuracy so that in internal gearing manufacture we can often dispense with subsequent finishing. which are shown in Fig. Besides internal gears. The special advantage of this method compared with hobbing is the small run-out of the tool. In the case of gear shaping.460 10 Processes with Translatory Primary Movement Fig. Currently. the rolling motion is superimposed with an additional periodic rotation corresponding to the helical angle. 10. Moreover. which can be mechanically fixed or electronically controllable. several shaping wheel shapes are available as tools [DIN4000. the workpiece executes a radial infeed motion in order to obtain the required plunge depth.17. At the beginning of the machining process and between several cutting cycles.15 Examples of gear shaping (Source: Gleason-Pfauter) special importance from the standpoint of the increasing use of planetary gears. the gear (workpiece) is created by a gearwheel-shaped cutting disc (tool). The cutting disc’s teeth also exhibit the corresponding helical angle. In the case of helical gears. . This additional rotation of the tool is realized by a inclined guide. gear shaping is also suitable for fabricating herringbone or double helical gears. 3 Cutting speed vc Gear shaping wheel Gear shaping fw Workpiece Lifting direction Radial feed stroke fr Offset AV Out running flank + - In running flank Fig.17 Types of shaping wheels Shaft gear shaping wheel Bell gear shaping wheel Hollow gear shaping wheel . 10.16 Terms of workpiece and tool during shaping The bell design assures that the mounting nut does not collide with the workpiece clamping or the workpiece itself. Disk gear shaping wheel Fig. Shank gear shaping wheels are used for internal gears with small circular pitch diameters. often internally toothed shaping wheels are used (hollow shaping wheels) [Baus06]. Such tools can also be used to produce workpiece contours of embedded internal external gearings.Planing and Shaping 461 Lengh of stoke H 10. Alternately. 10. 10. caused by various gear geometries. The machining process in the case of gear shaping has some peculiarities. in which a hob tooth removes the same chip each time.18 shows the cutting part geometry on a shaping wheel tooth. The three-flank chips have the biggest influence on wear. The cross-section calculated with the help of a digital computer program is plotted over the uncoiled cutting edge every single stroke [Sulz73]. all chips are cut by one tooth in shaping.18 Tooth geometry of a shaping wheel For gear shaping. Figure 10. Figure 10. As opposed to hobbing.20. The flanks of a gear shaping wheel are involute-shaped. to achieve a high use of the tool height. The rake angle at the tooth tip γk and on the flanks has no essential effect on the tool’s service life and can thus be freely selected from the standpoint of wear. are shown in Fig. two-flank and even three-flank chips are produced during the machining process. The chip cross-sections remain in the case of spur gears practically constant across the width of the workpiece. profile deviations during shaping wheel re-sharpening are avoided. The tool orthogonal clearance at the circular pitch (cut A-A) is selected small. The cross-sections of undeformed chip are shown schematically in the upper part of the illustration and the flow . The chip forms of the tool life-relevant u-shaped chip cross-sections. the tool orthogonal clearance is produced by the relief grinding of the flanks and the tooth tip. The tool orthogonal clearance at the tip α K (cut B-B) can on the other hand not be freely selected but is calculated with the help of the tool orthogonal clearance on the circular pitch and the tool engagement angle [Bouz76]. In this way. HSS with hard material coatings and a tip rake angle between γk = 10◦ and γk = 0◦ is preferred.462 10 Processes with Translatory Primary Movement C B-B B A A-A d A αF αK γK B γF C Fig. As element C shows.19 shows the cross-sections of undeformed chip in the manufacture of a tooth gap of a common gear. so that one shaping wheel tooth produces one workpiece gap. Figure 10.19 shows that one-flank. The smallest cross-sections of undeformed chip arise on the wear-endangered location. 10. Figure 10. so that it is possible to increase performance compared to uncoated .3 Undeformed chip thickness h Risky position fw A E Out running flank A In running flank E Uncoiled cutting edge Fig. The upper part of the illustration shows the width of flank wear land along the uncoiled cutting edge for three different tool operating lives. 10. As a result. this chip is pressed onto the rake face by the top chip and strongly impeded from flowing.21 shows the typical development of wear on the tooth of an uncoated shaping wheel.Planing and Shaping 463 Nu mb er of st rok es 10. wear on the in-running flank is much smaller and evenly distributed. The photographs in the lower half show the associated chips. As opposed to that. The chip on the outgoing flank is very thin in comparison to the chip of the top and entry cutting edge.19 Cross-sections of undeformed chip during gear shaping of a tooth flank directions of the chip parts on the top and on the flanks are indicated by the plotted arrows. The wear maximum can clearly be recognized at the transition from the top to the outgoing flank. the wear-endangered location at the transition from the top to the outgoing flank exhibits the smallest crater centre distance and furthermore a shrinking of the crater edge. On the other hand. The second cross-section of undeformed chip in Fig.20 is the most often encountered in practice. The lower part of the illustration shows the measurement of the crater edge and photographs of the shaping wheel tooth at the end of the tool life. Wear protection from a hard material layer reduces the stress on the cutting part significantly. which prevents further use of the shaping wheel because of the subsequent rapid increase of the width of flank wear land. 10. The depth of the craters is basically the same at the three labelled locations of the cutting edge. A possible measure for machining time in the gear .464 10 1 3 2 E Processes with Translatory Primary Movement 4 A z x da [mm] z 1 mm Photos 1 mm E z x da [mm] 133. z: Number of teeth Fig. β = 0°. On the other hand.5 2 −68 1. Heat-treated steels with a tensile strength of Rm = 900−1100 N/mm2 are ideally machined with a cutting speed of vc = 30 − 40 m/min with a roughing process [Able04].1 138. heat-treated steels (especially in the case of hollow wheels) are used. in crane construction). The most practical cutting parameters for industrial applications do not stem from the largest tool life or maximum setting data but rather from an optimal combination of tool life and machining time.7 1 42 0 2 42 0 1 42 1. b = 25 mm 16MnCr5N Rm = 600 N/mm2 HS6−5−2−5.5 −204 1 mm m = 3 mm. α = 20°. and more rarely cast iron materials. γ = 5° K 1: Tool. Increasing the rolling feed reduces the tool operating life less than increasing the cutting speed.20 Influence of the cross-section of undeformed chip onto the chip flow during gear shaping tools in two ways: on the one hand.5 131.3 x da [mm] 1 42 0 2 −68 1 1 mm Gear shape: Material: Cutting tool material: Cross-section of undeformed chip z x da [mm] 1 42 0 2 42 1 133. Cutting speed selection also depends on the respective workpiece width.3 −204 A fW 133. 10. it is possible to increase the tool operating life by 1. construction steels are used as gearwheel materials (e.5–4 times while keeping the same cutting parameters. For finishing. Case-hardened steels are gear shaped in a technologically sensible way with a cutting speed of vc = 40 − 60 m/min using a roughing process.g.5 137. Eighty percent of the materials used for gearbox gear wheels are case-hardened steels. 2: Workpiece da: External diameter. it is possible to maintain the same tool operating life while selecting more productive cutting parameters. x: Profile offset. In special cases. Otherwise. the cutting speed can be increased to up to vc = 140 m/min if the rolling feeds are reduced. Planing and Shaping Width of flank wear land VB / mm 10.3 465 1.0 Position of maximum wear m = 3 mm b = 25 mm β = 0° z1 = 42 z2 = –63 16MnCr5G vc = 55 m/min fW = 0,63 mm/DH 0.8 0.6 0.4 K K A E 30 gaps 20 0.2 0 10 A K risky position uncoiled cutting edge Out running flank Head E In running flank 25 µm 500 µm 500 µm Fig. 10.21 Flank and rake wear on a cutting tooth shaping process is the rolling speed W, which is related to the machining speed and results from the double stroke number nDH and the rolling feed fW as follows [Kauv87]: W = nDH · fW [m/min] (10.2) The machining costs result from the tool price, tool life with a given cutting speed/feed combination as well as the overall auxiliary machine and personal costs [VDI3333]. The danger of a cutting wheel/workpiece collision in gear grinding is especially high when the rolling feeds are high. There is a risk that the cutting wheel can collide with the unmachined workpiece material during the return stroke. Due to the continuous rolling feed, penetration between the shaping wheel tooth and the workpiece during the return stroke would occur – as shown in Fig. 10.22 – if measures are not taken to prevent the collision. The largest penetration occurs in the upper face cross section of the workpiece. This is especially problematic when manufacturing internal gearings. The collision causes a “cut”, in which case the flank face of the shaping tooth takes over the role of the rake face. Severe damage to the cutting edge results from these cutting conditions as well as considerable degradation of the gear quality and heavy loading of the machine. 466 10 Processes with Translatory Primary Movement TDC Length of stroke BDC Interpenetration at the end of return stroke Working stroke 1 2 Workpiece BDC 3 Return stroke 4 Next working stroke Gear shaping wheel Direction of rotation Fig. 10.22 Cause of collision danger during gear shaping The main influencing variables that affect collision during the return stroke are, besides the rolling feed, the plunge depth, the geometry of the workpiece and shaping wheel, the amount of lift during the return stroke of the shaping wheel and the axis offset. The real variable to avoid collision is the amount of lift off and the axis offset. Since the lifting movement of the shaping spindle is not always sufficient, especially in the case of internal gears, the support of the gear shaping machine is laterally shifted by a certain amount AV, resulting in a slanted lifting motion. 10.3.3 Gear Planing The motion sequence in gear planing basically corresponds to the kinematics of the shaping process. Nonetheless, the term “gear planing” has become established for this process in industrial practice, and it will also be used in the following. The principle of gear planing is shown schematically in Fig. 10.23. The tool used in gear planing, the “planing rack”, consists of a gear rack segment with a relief-ground flank (Fig. 10.24). The cutting force is received by a support crest so that the tool can be exploited optimally, i.e. can be re-sharpened to a small residual thickness. Since the planing rack is derived from a gear rack, the rolling motion of the workpiece must be superimposed with a translatory motion in the longitudinal direction of the gear rack when manufacturing involute-shaped gears. Due to the finite length of the planing rack, it is necessary that the workpiece be moved back to the starting position after producing several gaps with a decoupled feed motion. This is referred to as “partial rolling” as opposed to continuous rolling methods such as hobbing, gear shaping and hob peeling. 10.3 Planing and Shaping 467 Streight toothed spur gear Schematic diagram Feed motion Primary motion Helical gear Helical rack with limited space beside the gears Planing rack back motion Fig. 10.23 Principle of gear planning Supporting rack (New) Planing rack Fig. 10.24 Planing rack (Source: Maag) Since for this reason gear planing is less economical than other rolling methods, it is only used in special cases, e.g. to produce externally toothed cylindrical gears with large dimensions and high strength. The advantage here is that tool change is easy and can be done without affecting quality during the manufacture of a workpiece. References [Abel93] Abel, R.: Hartbearbeitung mit Schneidkeramik und Bornitrid. VDI-Z Spezial Werkzeuge 8, 20–26 (1993) [Able04] Abler, J., et.al.: Verzahntechnik: Informationen für die Praxis. 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Berlin (1988) [DIN6583] DIN 6583: Begriffe der Zerspantechnik: Standbegriffe. Deutsches Institut für Normung (ed. Beuth. Beuth. Beuth. Beuth. Deutsches Institut für Normung (ed. Unterteilung. Unterteilung. Beuth. Berlin (1962) [DIN770b] DIN 770-2: Schaftquerschnitte für Dreh und Hobelmeißel. Begriffe. In: Institut für Normung (ed. Berlin (2003) [DIN8589g] DIN 8589-7: Fertigungsverfahren Spanen – Teil 7: Feilen. Deutsches Institut für Normung (ed. Unterteilung. Beuth. Energie-Arbeit.).). Berlin (2003) [DIN8589e] DIN 8589-5: Fertigungsverfahren Spanen – Teil 5: Räumen. Beuth.). Berlin (2005) [DINEN10052] DIN EN 10 052: Begriffe der Wärmebehandlung von Eisenwerkstoffen. Berlin (1985) [DIN6581] DIN 6581: Begriffe der Zerspantechnik: Bezugssysteme und Winkel am Schneidteil des Werkzeugs. am Schneidkeil und an der Schneide. Beuth. – Teil 1: Drehen Einordnung. Begriffe. Unterteilung. Berlin (2003) [DINEN10027a] DIN EN 10027: Bezeichnungssysteme für Stähle – Teil 1: Kurznamen.). Deutsches Institut für Normung (ed.). Berlin (2006) .).). Deutsches Institut für Normung (ed. Beuth. Unterteilung. Begriffe. Kräfte. Berlin (2003) [DIN8589i] DIN 8589-9: Fertigungsverfahren Spanen – Teil 9: Schaben.). Beuth. Deutsches Institut für Normung (ed.). Stoßen.).. Institut für Normung (ed. Einteilung. Berlin (2003) [DIN8589c] DIN 8589-3: Fertigungsverfahren Spanen – Teil 3: Fräsen Einordnung.).). Beuth. Schichttypen und Eigenschaften. Berlin (1998) [DINENISO4957] DIN EN ISO 4957: Werkzeugstähle.). Beuth. Beuth.).und Automatisierungstechnik (ed.). Beuth.). Bezeichnung der Hauptgruppen und Anwendungsgruppen. Richtwerte für das Schälen und Glattwalzen von Bohrungen. Zusammenstellung der Kenngrößen. VDIGesellschaft Produktionstechnik (ed. Deutsches Institut für Normung (ed. Berlin (1994) [VDI3324] VDI 3324: Leistendrehtest . Beuth. Definitionen und Kenngrößen der Oberflächenbeschaffenheit. (1993) Guidelines [BGR 143] BGR 143: Tätigkeiten mit Kühlschmierstoffen. power. Beuth. Berlin (1999) [VDI3210] VDI 3210 Blatt 1: Tiefbohrverfahren. Berlin (2001) [DINISO1832] DIN ISO 1832: Wendeschneidplatten für Zerspanwerkzeuge. Berlin (2006) [VDI3321] VDI 3321: Schnittwertoptimierung: Grundlagen und Anwendungen. energy. Berufsgenossenschaftliche Regeln für Sicherheit und Gesundheit bei der Arbeit. VDI-Gesellschaft Produktionstechnik (ed. VDI-Gesellschaft Produktionstechnik (ed. Deutsches Institut für Normung (ed. Second edition. Beuth.). Blatt2 : Tiefbohren. HVBG (ed. VDI-Gesellschaft Produktionstechnik (ed. VDI Gesellschaft Produktionstechnik (ADB). Ort und Lauf.und PVD-Verfahren.Geometrische Tolerierung – Tolerierung von Form. Blockmetalle und Gußstücke aus Magnesiumlegierungen.). Beuth. Berlin (2006) [DINENISO4287] DIN EN ISO 4287: Geometrische Produktspezifikationen (GPS) – Oberflächenbeschaffenheit: Tastschnittverfahren – Benennungen.494 References [DINEN1753] DIN EN 1753: Magnesium und Magnesiumlegierungen.und ähnliche Verfahren). VDI-Gesellschaft Produktionstechnik (ed. Beuth. Berlin (2005) [ISO3002d] ISO 3002/4: Basic quantities in cutting and grinding – Part 4: Forces. VDI/VDE-Gesellschaft Mess. Deutsches Institut für Normung (ed. Beuth. VDI-Gesellschaft Produktionstechnik (ed. Berlin (1997) [DINENISO1101] DIN EN ISO 1101: Geometrische Produktspezifikation (GPS) . Berlin (1994) [VDI3208] VDI 3208: Richtwerte für das Tiefbohren mit Einlippenbohrern. (1984) [ISO3685] ISO 3685: Tool-life testing with single-point turning tools.).).). Beuth. Beuth.). Deutsches Institut für Normung (ed. Köln (2006) [VDI2601] VDI 2601: Anforderungen an die Oberflächengestalt zur Sicherung der Funktionstauglichkeit spanend hergestellter Flächen. Berlin (1999) . Berlin (2003) [VDI3206] VDI 3206: Auslegung von Drehprozessen.). Beuth. Deutsches Institut für Normung (ed. Carl Heymanns. Beuth.). Berlin (2005) [VDI3198] VDI 3198: Beschichten von Werkzeugen der Kaltmassivumformung CVD. Bezeichnung. Berlin (1996) [VDI3209] VDI 3209: Blatt 1: Tiefbohren mit äußerer Zuführung des Kühlschmierstoffes (BTA.). VDIGesellschaft Produktionstechnik (ed. Berlin (2005) [DINISO513] DIN ISO 513: Klassifizierung und Anwendung von harten Schneidstoffen für die Metallzerspanung mit geometrisch bestimmten Schneiden.).). Richtung. Deutsches Institut für Normung (ed. Beuth. Beuth. Berlin (1991) [VDI2840] VDI 2840: Kohlenstoffschichten – Grundlagen. First edition.Prüfverfahren zur Beurteilung des Bruchverhaltens und der Einsatzsicherheit von Schneiden aus Hartmetall beim Drehen. Qualitätssicherung bei der PVD. Beuth.). Berlin (2002) [VDMA2002] Kühlschmierstoffe – Frische Luft am Arbeitsplatz. VDIGesellschaft Produktionstechnik (ed.und CVDHartstoffbeschichtungen.). Berlin (1977) [VDI3397] VDI 3397: Blatt 1–3. VDI (ed. Beuth.References 495 [VDI3333] VDI 3333: Wälzfräsen von Stirnrädern mit Evolventenprofil. www. Ein Leitfaden für die Praxis.de. Arbeitsgruppe Aerosole (ed. VDI-Gesellschaft Werkstofftechnik (ed. VDMA. VDMA. Berlin (2005) [VDI3824] VDI 3824: Blatt 1–4. Kühlschmierstoffe für spanende Fertigungsverfahren. 5 April 2011 (2006) . Fachverband Allgemeine Lufttechnik. Beuth. Frankfurt am Main (2002) Web Pages [Meta06] Wärmebehandlungsarten bei Stahl.metallograf.).). 229. 127–128. 138. 321. 451–454 with linear tool movement. 24–25 Blisk. 74–76. 121. 32. 74. 403. 295. 283. 99. 313–316 Aluminium alloys hardenable. 151. 155. 250. 256–257. 281. 258. 441 tubular. 260–262. 448–449 external cylindrical. 170. 458 Austenite. 125. 118. 445–452. 71–72. 289. 313. 293. 334. 167. 72. 37 Bearing ratio. 193. 129. 272. diagonal cut. 156. 282. 343. 77 497 . 120. 45–47 BRAGG’s condition. 82. 349 Average mechanical strain. 289–290 Auxiliary process time. 274. 291 non hardenable. 97. 226. 64. 250–254. 96–98. 269–270. 71–78. 266–267. 108–109. 263–264. 84. 61–62 Adhesion. 287. 193. 383. 232. 451–454 gearwheel. 319 Bound. 454 tooth graduation. 293–294. 113. 289. 313. 292. 280. 450 Build-up edge. 120–121. 450 form. 263 Active energy. 32. 291–292. 318. 397. 286. 147–149. 277. 73. 265–267. 450 circular. 103–104. 153. 36 Breakage hypothesis. 35 Boron nitride. 260–262. 445 tooth pitch. 454 face. 336–337 Adhesive wear. 447. 244 B Bainite. 324. 270–271. 418. 251–253. 445–446. 451–452 tool. 234–236. 256. 346–347. 289–290 Adiabatic shear band. 366. 299. 192. 449. 449 pot-. 269–270. 289 Band sawing. 131. 61–62 Active power. 101. 263–268. 251–253. 445. 211. 455 external. 452 Broaching bushing. 75. 209 Alloying elements. 173. 297–299. 118. 293 intermetallic phases. 127–128. 76. 329. 1. 302. 321. 265. 312. 460 cutting length. 181. 452–454 with rotary tool movement. 275–276. 7. 302. 152–153. 266. 295. 285. 277–279. 152–153. 121. 142. 263. 248. 270. 252. 78. 292. 445. 293 Annealing on certain properties.Subject Index A Abrasion. 70. 95. 271. 180–184. 235. 245. 439–442 BARKHAUSEN noise. 277. 254–256. 155. 248–249. 448. 450 profile. 449 internal. 223. 117. 334 Abrasive wear. 450 pre-. 282–283. 304–305 Borehole methode. 249–250. 291 ADI. 158–161. 291. 445–449 finish. 445–449 cutting disc. 287–288. 445. 259 Application groups. 394. 267–268. 277. 169. 449–451 rotation external cylindrical. 95. 303 Acid-proof steel. 292–293. Nb)C-Co. 88. 66. 111–112 manufacturing. 442–444 Clearance angle. 253–254. 274. 116. 234–235. 276 Crater wear. 313 Corner radius. 425. 165–166. 182. 265. 129–133. 267. 219. 289 chilled. 255. 277. 169–170 Cutting edge chipping. 141–143 Cold work hardening. 314. 58–59. 96. 217. 386. 143–144 PVD. 397. 120–122 uncoated. 70. 339. 147–149 magnetron sputtering. 294. 341 formation. 266–269. 129–134 PA-CVD. 305 formation. 330. 217. 266. 49. 422. 285. 131. 131. 53. 301. 288–290 with vermicular graphite. 284–286 malleable. 317 Cutting ceramics mixed. 82–83. 141–142. 446 with lamellar graphite. 391 Circular sawing. 462 Chip root. 84–87. 175. 443. 96–98. 129–132. 412. 99–101 Comb cracks. 256. 276. 128–141 evaporation. 214. 209. 112–113. 439. 285. 273.498 C Calculation of cutting parameters. 136. 192. 254. 219. 286. 144–145 vacuum evaporation. 395. 76. 270. 194–195. 134. 70. 464 Cast iron ADI. 450 Chromium. 69–71 micro geometry. 436–437. 339. 175–176. 121–123. 44–47. 45. 312 steels. 334. 299. 268. 59. 116–119 function gradient. 171–173 oxide. 437. 340 Cutting energy. 163. 59. 280–282 with spheroidal graphite. 111. 167–169. 118. 313. 283–290. 172 whisker-reinforced. 288–289. 117. 259. 321–322 Corrosion-resistant steels. 428. 285–287. 71. 111–112. 228. 280–283 Cemented carbides classification. 339–341. 308–309. 277–279. 257. 269. 194. 69. 163–165. 336. 341. 123–126 historical development. 420 Coarse-grain annealing. 115. 110–112. 122 components. 383. 166. 408 Chip forms. 120–123. 119–120. 173. 261. 326. 407. 56. 266. 296. 91–92 Cutting fluids non water-miscible. 395 Changes to the structure. 59. 280. 141. 292. 235. 329–330. 80. 221 . 179. 329. 311. 123. 125. 126 crystalline structure. 302. 211. 232. 68. 184. 61–62. 123 TiC/TiN-Co. 171 non-oxide. 117–123 coated. 76. 117. 240. 426 Calculation of shear angle. 40. 400. 423. 263. 278. 447. 2. 248. 121. 94. 238. 302–303. 395. 125–161. 268. 176. 141–145 HT-CVD. 163–164. 159. 78. 310. 91–92 Case-hardened steels. 112. 86. 277. 306. 168. 118. 137. 250. 41. 151. 262. 240. 126. 193. 274–277 Circle of THALES. 219–221 water based. 73–74. 111. 122 Cermets. 330. 33. 293. 282. 163. 111–112. 337. 141–149 sputtering. 74. 214–215. 86. 145–146 MT-CVD. 286–288 white. 335 inclination. 168. 146. Ni. 248. 221. 446–447. 131 fine. 184. 259. 395. 114–117. 462 Coating procedures CVD. 296. 325 rounding. 111–112. 280–283. 251. Ta. 395 grey. 115. 114–116 conventional. 95. 306. 305 Chip space. 160. 284. 329.and submicron grain. 194–195. 33. 289. 126 WC-(Ti. 439. 314. 51. 82. 286. 78–80. 169. 109. 223–224. 90–91 Circular-arc lead. 307–308 Continuous chip. 268–269 Subject Index Coating. 271. 179. 207–209. 367. 49. 152–153. 287. 165. 128. 423 Crystalline structure. 221–222 water-miscible. 206. 324. 162–163. 136 procedures with arc. 136 ion platting. 305–306 Circular milling. 290. 171–172. 273. 212. 318–319. 125. 369 D Deburring. 255. 393. 398 finishing face milling cutter. 101. 263. 156. 404 Free cutting steel. 91. diagonal cut. 161. 197–198. 391. 208 separation criterion. 306. 276–277. 173. 383. 209 Determination of optimal-time cutting speed. 309 Formation of cutting edge. 158. 198 First cut strategies. 308–309. 40. 55. 418–439 Dry ice. 86–87 Down milling. 178 natural diamond. 243. 272. 63. 104. 279. 168. 280–284. 416–417 rotary. 207. 326 Drilling profile counterboring. 190 Form milling. 184. 174. 189–190. 118. 13–14 inspection. 334 Finite element method (FEM) discretization. 323. 179. 248. 229–230. 88 Fringe projection. 13–14 H Hacksawing. 43 499 End milling. 121 Cutting power. 78. 209 remeshing routine. 173–174 monocrystalline. 401–404 F Face milling corner milling. 224. 184. 75. 265 Free. 168. 208–209 EULERian approach. 396. 261–267. 240. 199–200 implicit methods. 235–236. 400. 288–290. 94. 13–14 limit. 252. 326. 446–450. 70. 214 Cutting temperatures. 4. 424–425. 199–200 LAGRANGEian approach. 407 Dry milling. 155 Dimensions of wear crater centre distance. 301. 205. 232–233. 391. 348–350 Diamond application areas. 68. 330. 292. 304–306. 216. 29–30 G Gauges form. 61 Ferrite. 340 Effective cutting speed angle. 304. 300 Fine grain cemented carbide. 323–326. 61 force. 295. 422. 239. 149. 205. 61. 329 Diffusion annealing. 87 displacement of the cutting edge. 274–277. 304. 427 Cutting simulation. 400–401 Drag grinding. 57–59. 309 Dry machining. 123. 324. 232. 417 Deformation localization. 313–314. 57–60. 173–179 synthetical. 167. 78. 330. 175–176. 427. 159–161. 306 Flood lubrication. 173–176. 207. 151. 40. 237–238. 217. 50. 269–270. 184 . 264 wear. 87 crater depth. 424–425. 206. 399 Feed direction angle. 296. 199 explicit methods. 396–397. 427 power. 193. 154. 138. 466 Cutting portion. 308 Duplex steel. 55. 87 crater ratio. 288. 178–179. 265. 250–256. 45–46. 159. 177–179 classification. 286. 173–174 polycrystalline. 391. 80. 243. 396 energy. 216–217. 173–175 Diamond coatings. 199. 442 Hard coatings aluminium chromium nitride-coatings (AlCrN). 399 wide finishing face milling cutters. 45–46 Free. 395. 194. 309 E Edge rounding. 87 width of flank wear land. orthogonal cut. 308–309. 207–209 types of elements. 290. 439. 361.Subject Index Cutting force. 84–85. 339–340. 69–70. 258–259. 7. 203–205. 349. 91–92. 111–112. 154. 209–210. 406. 12 position deviations. 140–142. 407 titanium carbide coatings (TiC). 175–176. 339. 149. 105–109. 128. 302 amorphous carbon coatings. 293–296 Magnesium machining. 99. 12–13 structural deviations. 109. 280–282. 295–296 Main process time. 155–158 diamond coatings. 317 sulfidic. 263. 103. 254. 297 treatment. 306–307 High speed steels areas of application. 292–293. 271–273 Hard turning. 62 sources. 220. 264 Inconel. 386. 347. 159–161. 128–131. 86–88. 270. 451. 167. 212. 204. 298–300. 450 Manganese. 62 -treatable steel. 171. 396. 406 thread number of hob. 271––274. 276–277. 184. 98. 406–407 up. 295. 126. 184. 137–138. 128–131. 312–313. 162. 173–174. 126. 309–310. 423–424 I Imputed amortizations. 320–322. 149. 343. 169. 349 Major cutting edge. 318. 131. 6. 303. 149. 241–242. 274 high precision. 182–183. 152. 180. 247. 146–147. 285. 282. 259. 406–407 running gears. 19. 149. 184. 340. 149. 263. 419–422. 149. 163. 169–170. 158 titanium aluminium nitride coatings ((Ti. 293–294. 254–256. 114. 102–103. 349. 179. 161. 439. 264–267. 405–406 inserted blade hobs. 18. 299. 263. 56–57. 142. 267–268. 73–76. 192 positive. 126. 230. 175–176. 718 Indexable insert negative. 128–131. 422 . 106–109. 282. 404. 103–104 Subject Index classification. 350 Imputed interest. 70. 329. 270. 411 High pressure. 251. 82. 411–412. 266–269. 306–307. 299. 107.) aluminium oxide coatings (Al2 O3 ). 415. 102–103 manufacturing. 111. 394–395. 154–155. 234. 264–265. 298. 223. 151–152. 181. 10–12 Magnesium alloys. 155. 101. 266–268. N)). 339 Macro. 271–272. 123–124. 251. 124. 163–174. 212. 29 Lead. 43–44. 273. 329–330. 248. 411. 315–317. 284–289. 446. 96. 126. 275–277. 128–129. 78.and microgeometry form deviations. 233. 180–181. 237–337. 426 Length testing devices. 284. 406–407 solid steel hobs. 320 titanium carbon nitride coatings (Ti(C. 251. 259–261. 266–268. 41. 137–141. 96. 217. 449. 4. 323. 101. 6. 464 Hardening. 141. 96. 117. 259–261. 404–407 shifting. 33–34. 409. 100–101. 50–52.500 Hard coatings (cont. 98. 254–256. 163–165. 154. 13. 116–117. 150–154. 111. 256. 394–395. 312. 291. 111. 302–303. 452. 133–137. 149. 135. 192 K KIENZLE Equation. 454 Hard machining. 60. 105. 312. 109. 274 Heat balance. 152. 119. 400. 105. 120. 172–173. 405–406 HSS-tools. 13–21 M Machinability. 425 L Lamellar chip formation. Al)N). 321 Laser interferometer. 278. 165. 109. 406–407 dry. 164 titanium nitride coatings (TiN). 411 gash. 350–351 Inclusions. 334 self-lubricating coatings. 168. 287. 446 Hardened steel. 326–327. 147. 323. 320. 82–83. 280. 250–251. 345. 350–351 Machining standard values. 265. 255–264.and down cut. 184. 152–154. 46. 145–146. 291. 289. 33. 293. 120. 316–318. 203. 443. 292. 424–425. 316. 154–155. 293. 433 Machine hour-rate. 265. 136. 257. 145. 404. 401. 133–140. 113–117. 321 cooling. 104–107 Hobbing cutters with indexable inserts. 304–305. 86. 55–56. 162. 290–293 copper alloys. 27–30. 290 Material laws. 276. 252. 82. 5. 272. 284–286. 264. 178.Subject Index Manufacturing costs. 334 fibre-reinforced plastics. 267. 395 polymer matrix. 324. 162–167. 442 PCBN. 17 mechanical callipers. 342–346. 248. 304. 280–282. 69–71. 15 with mechanical converters. 78. 275–277. 314–315 titanium alloys. 171–172. 194. 99. 288. 18–19 with pneumatic converters. 115. 272. 385–386. 3–4 systematic occurring. 271. 8–24. 285 Normalizing. 400–401 Perlite. 287. 178–179. 400–401 Minimum undeformed chip thickness. 260–262. 369 Nitriding steel. 92 Molybdenum. 302. 165. 452 Model representation of shear plane. 322. 16. 395. 21 with electrical converters. 46. 17–18 electronic. 276. 315. 280. 251. 277. 96. 329 Passive force. 250. 400–401 resultant force. 85–86. 107. 65–66 Optimal-cost cutting speed. 180. 289 Phosphor. 245. 348 Optimal value function. 119. 293 nickel alloys. 272. 165. 15–17 micrometers. 328. 296. 255. 175. 184. 156. 322. 355. 331–337. 66–67. 330. 293–294. 255. 276. 269. 56. 302 Peripheral milling. 107. 271–272. 3–8 randomly occurring. 172. 124. 169. 396. 343 Manufacturing disturbances. 267. 274–276. 267. 311–315 magnesium alloys. 279. 13. 269. 108–109. 423 N Nickel. 431. 119. 318 PCD tools. 117. 332. 327–331. 184. 391–394. 87–88 MOHR’s slip theory. 211. 300. 158. 95. 132. 311. 103. 152–154. 269–270. 21 dial gauges. 112. 296–311. 391. 155–156. 393. 110–111. 126. 299–300. 266–267. 179. 362. 288–289. 173. 267. 371 Measuring. 322. 256. 169. 359–363. 9–10. 168. 126. 424. 58–59. 349 equation. 235. 201–204 Material measures. 11. 330. 283. 288. 253. 309 Non-ferrous metal aluminium alloys. 114. 331–333 thermosetting plastics. 300 Perlite-ferrite structure. 394 Particle jet erosion. 180. 304–306. 169–170. 282–291. 267. 172. 137. 344–349 Optimal-cost tool life. 391. 279. 279. 127–128. 177–181. 325 Non-metal aramid fibres. 37–38. 433 501 Nitrogen. 100–101. 149. 110. 117. 302. 70. 175. 393. 332 Non-metallic inclusions. 449 Mixed ceramics. 363. 308. 297. 300. 395. 411 down. 391–392. 254. 395. 412 secondary cutting edge. 365. 226. 64. 391 Minor cutting edge. 15–16. 316 P Parallel cracks. 286. 9. 299. 251. 184. 286. 435. 19–21 Microcinematography. 327. 320–323 O One-chisel method. 267. 401 . 21 indicating. 98. 398. 224. 396–397. 85–88. 347–348 Optoelectronic path measurement systems. 337 carbon fibres. 67. 302. 156. 53 Milling contact conditions. 158. 166. 78. 330 up. 307. 181–182. 250–256. 187–188. 9. 268. 293–296 magnesium wrought alloys. 174 with optical converters. 318. 283. 84–85. 36–37. 182. 258–263. 243. 169. 256. 19 Measurement errors. 220–223. 258. 75. 395 tool materials. 170. 158–159. 78–79. 10 Martensite. 395 graphite. 101. 99. 342. 369–370 Measuring instruments dial comparator. 282–283. 120. 5. 41. 171. 152–155. 122. 86. 312–327. 332. 3. 141. 108–109. 21 Oxidation. 127. 125. 320. 16. 261 Notch wear. 399. 14–21 measuring microscopes. 263–264 Plain milling. 308–309. 389. 459–461. 157. 404 gang milling cutter. 13–14. 336. 285. 267. 73. 48–49. 272. 56–58. 223. 424–425 components. 289. 254. 79. 329. 154. 111. 279. 39–41. 467 Plastic deformation. 267–269 . 458–460. 168. 184–185. 328. 210–211. 412. 49. 245. 93–94. 104.502 Planing face. 415 Slotting chip flow. 31. 447. 459–466 wheel. 456–457 pitch circle. 362. 466 cross-section of undeformed chip. 50–51. 24 height of the highest profile point. 65–67. 249. 3. 51–52. 49. 456–457 gear. 211 Shear bands. 341 Roughness parameters depth of the deepest profile valley. 456–457 axis intersection angle. 367. 292. 466–467 supporting rack. 458–459 gear shaping. 411 reference profile III. 292. 287. 223. 466 idling stroke. 209–212. 245. 289. 313. 324–325 Pneumatic measurement. 159. 310 formation. 209–210 Side and face milling. 147. 267. 61. 369–370 Pyrometry. 150–152. 454. 295. 466 Soft annealing. 445. 84–85. 133–136. 80. 425–426. 288–289. 395 Skive hobbing centring. 384 tool-in-use system. 458–460. 290 Set. 282. 457 soft. 49. 261. 272 Residual stresses. 466 planing feed stroke. 24 Subject Index mean roughness value. 224. 61. 384. 7–8. 454–456 diagonal angle. 454–455 parallel. 272–273. 199–202. 214. 411 Slab milling. 98. 45. 300–303. 463 face shaping. 175–179. 328–329. 273. 366. 78. 330. 251. 463. 198–199. 458. 33–35. 381. 334 Precision hard turning. 212. 141. 295–296. 292. 445–467 working stroke. 289–290. 76–77. 240. 319. 322. 293. 194. 259–260 Reference systems tool-in-hand system. 454 tangential. 411 protuberance. 55–56. 285. 340. 304. 391. 396–397. 314 Resultant force. 457–467 return stroke. 311. 91. 52. 88–94. 329. 437 Shaving axis intersection. 247–248. 80. 458 internal gear. 47–49. 58–59. 464 collision. 455 Plastomechanical processes. 463–464 development of wear. 460. 86. 94. 115. 131. 457 wheel. 269–270. 58–59. 55. 84–88. 306. 277. 455–456 plunge. 243–245. 465–466 lifting movement. 313–314. 294. 379–380 Segmented chip. 24 total height of the profile. 251. 278–279. 458. 24 greatest height of the profile. 451. 286. 424 Ribbon chip. 406–408. 256–259. 273 Scaling. 20–21 Polycrystalline diamond. 57 Residual austenite transformation. 459. 458–459 gear. 464 tools. 57. 306–307. 305–306. 273–274. 238. 467 rack. 48 Plunge milling. 313. 456–457 diagonal. 454–457 Shear angle. 404 Pyrometer. 200. 322–323 Recrystallization annealing. 42–44. 263. 292. 395. 274 Profile milling form cutters. 366. 194–195. 465–466 danger. 33–37. 209–210. 42–43. 51. 56. 62–64. 420. 272. 439–440 tooth chip. 314. 90–91. 367. 339. 75. 76. 24 S Saw bands. 69–70. 460–462. 248. 173. 201. 66 R Rake face. 397. 355. 411 heat treatment distortion. 352. 465 Ramping. 465–466 shaping tool. 401. 321. 254 cutting edge angle. 188–189. 240. 236. 44–45. 330 orthogonal rake angle. 438–439 Titanium alloys. 25. 96. 296–311. 312–314 Surface damage. 464 cold-working. 464 hardened. 185–186 holder. 397. 276 heat-treatable. 248. 119. 235. 51. 54. 27–29 Subjective surface inspection. 298 Stylus instrument. 238–242. 446. 187. 68–69. 369 Thread milling. 57 Tool(s) approach angle. 394 low-alloyed. 369–370 Temperature tool life rotation test. 281 Testing objective. 336. 269 construction. 443–444. 185. 292. 333. 66. 269–270. 61. 33. 50. 78. 33–37 Surface roughness. 266–267. 184. 120. 233. 431. 42–44.Subject Index Specific cutting force. 54 Surface quality. 274. 233. 47 orthogonal clearance angle. 346. 238. 442–443. 243. 136. 259–261. 65–66. 310 nitriding. 344. 58. 195. 341. 99–110. 72. 399–400. 25. 237–240. 44–47. 263–269. 458 Stick-slip. 13. 299–300. 149. 67. 274. 267. 325 alumnides. 175. 64–67. 30. 91. 310. 416. 361. 232. 37. 196. 292. 304–305. 248. 99. 263. 263. 272. 154. 56–57. 21. 193. 434. 26–27 Submicron grain cemented carbide. 58. 265. 229. 265–266. 365. 266. 436. 8. 424. 259. 105–106. 446. 107. 158. 397. 286. 282. 45. 258. 278. 279. 79 Temperature measurement. 101. 270. 254–255. 411. 339. 191–192. 413–414. 185 included angle. 82. 251–255. 310 Tool-in-hand system. 237–337. 269–271. 294 Stress-relief heat treatment. 289. 149. 42–43. 66. 85. 192–193. 62. 27. 74. 263–266. 351 conditions. 21. 269–270. 268 Surface measurement mechanical measuring methods. 184–193 solid tools. 43–47. 31–33. 433 non-alloyed. 283. 242. 268. 385. 240 Tempering. 248. 285. 434–436. 230 Thermography. 68 Temperature fields. 313–314. 408–409. 223. 78. 427 Steel acid-proof. 411. 419 . 9–10. 290. 423 bit. 259. 392. 416. 336–337. 287. 105. 119 Substrate pretreatment. 269. 346 normal rake angle. 245–247. 256. 428. 187. 264 Sulphur. 60. 454–455 503 Surface rim zone. 95–97. 80. 384 Tool-in-use system. 27–29 optical measuring methods. 317. 171. 220–221. 449. 266–269. 339. 21–30 Surface layer. 181. 329. 54–55. 394–395. 274–279 tool. 340. 247–248. 452 heat-resistant. 440. 44. 346 straight lines. 263–264. 22–26. 367. 341. 59 designs. 273. 45. 9 Thermal energy. 59. 339. 260–261. 437–438 Thread moulding. 384. 59. 210. 384. 398 inclination. 322. 174. 167. 9 subjective. 42. 263 case-hardened. 266. 210. 100. 333–334. 277 non-rusting. 66–67. 267–268 high-alloyed. 57. 340 life behaviour. 252. 367. 263–264. 274. 178–179. 249 high-speed. 136 Thermocouple. 215–216. 69–71. 313. 76. 302. 216. 257. 161 Sulfidic inclusions. 249 machining. 249–250. 88. 321 Surface inspection. 386. 235. 320. 251. 168. 248 criterion. 109. 320. 263. 257. 329–331. 176. 213. 341 T Technical surfaces. 30–31 Temperature change cracks. 29–30 Surface parameters. 271–274. 182–183. 31. 395. 69–70. 423 function. 79–80 Temperature distribution. 62–63. 69–70. 252. 384. 104. 301–303. 26 W Wear abrasion. 122. 78. 66 Two-colour pyrometer. 74–85 diffusion. 388 undeformed chip thickness. 312. 49. 392. 169. 302. 384 non-circular. 386–390 finish planing. 267–268. 85. 209–212. 58–61. 286. 320–322 plastic deformation. 111. 101–103. 413 Two-chisel method. 123–124. 80. 301. 80. 4. 111. 103–104. 436. 78 built-up edges. 85–87 mechanisms. 401. 384. 101–102. 169. 387–388 internal. 385–391. 129. 285. 321 segmented. 299. 365. 30–37 X X-ray and neutron diffraction. 99–110. 391–392. 44–45. 440. 70. 413 orthogonal. 384. 292. 119–120. 79–80 thermoshock cracks. 434. 85. 425–426. 76 causes of. 76. 70. 193. 341 lamellar. 44. 389 nominal cross-section of undeformed chip. 193. 306. 56. 240–243 tribooxidation. 141. 94. 383. 73. 29–30 Working plane. 384. 362. 390 copy. 289–290. 78. 76. 268. 383–384. 423 kinematic form. 286. 396. 310 U Undisturbed manufacturing process. 458 Toroidal core method. 71–72. 56. 81–83. 49–52. 240. 387–388 thread die cutting. 37. 394. 285. 273. 385–386 feed. 413. 193–196 steels. 254. 101. 463 width of undeformed chip. 207. 192 Wet machining. 413 facet formation. 122. 263. 308 White-light interferometer. 451 transverse face. 36–37 Z Zementite. 389–390 profile die. 443 Subject Index Turn milling axially parallel. 157. 87. 247. 56. 75–76 adhesion. 296. 389–390 helical. 341 V Vanadium. 74 temperature change cracks. 62–63. 339. 169 form. 94. 423 Visual inspection. 396 of reference.) preparation. 279. 385 cylindrical profile grooving. 385 transverse profile. 252. 39. 104. 184. 181. 254. 74 forms. 445. 235. 175. 46. 388–389 rough. 113–117. 389 cross-section of undeformed chip. 131. 56 Workpiece rim. 101. 254 tools. 42. 42. 248. 296. 58–60. 321 Trochoid milling. 269–270. 73–75. 387–388 profile. 68–69. 244–245. 288. 279. 73 breaking. 88. 294. 386 chaser. 423 Turning centreless rough. 80. 96–98. 255. 390–391. 385 transverse parting off. 440. 43–45. 184–187. 388 transverse profile grooving. 51. 79–80 tool life rotation test. 35 Total active energy.504 Tool(s) (cont. 51–52. 442–443. 388 depth of cut. 387–388 circular. 49. 80 surface damage. 386 face. 303. 61 Tribooxidation. 387–388 tool approach angle. 446. 386 thread. 69–70. 49. 95–97. 73–74 Wedge angle. 387–388 thread chasing. 289. 384. 85. 328–329. 282 . 386 cylindrical face. 313 discontinuous. 167. 71–74 notches. 67 Types of chip continuous. 324 Tungsten. 207–209. 101. 463–464 cylindrical circular.