Outline – Machine Structures• • Design Requirements Structural Elements – Materials • Design Considerations – Columns – Beds • Manufacturing Techniques – – – – – • • • Chapter 5 Cast IIron C Welded Steel Polymer Concrete Casting Granite-based Structures Carbon Fiber Composites Structural Damping p g Finite Element Analysis Elimination of Static Deformations ME 551 2 Design g Requirements q • Structure of the machine houses (and supports the operation of) all the vital (moving or stationary) elements of the machine. – It is i the th skeleton k l t off the th machine. hi – Without a good structure, the rest of the machine will be ineffective. • Some design requirements are – – – – – – Symmetrical (and Simple) Design Minimum Weight g High Static and Dynamic Stiffness High Structural Damping High Secular and Thermal Stability Independent Foundation Chapter 5 ME 551 3 Structural Elements • Structural elements can be classified as Flat Bed – Machine Beds • T-Bed, Slanted Bed, • Floor Plates, etc. – Columns • Open or Enclosed Design Slanted Bed – Portals/Bridges • Open or Enclosed Design Column Chapter 5 • Machine structures can be categorized into three classes: – Open Frame – Closed Frame – Truss-type Truss type (Enclosed) Structures ME 551 4 Open p Frame Structures1,2 • Most traditional machine tools employ this configuration. – Also known as C- or G frames • Provides easy y access to the workspace. • Not as stiff as the closed frames. • Employs stacked axes. axes • Prone to Abbe offset errors. Chapter 5 ME 551 5 Closed Frame Structures1. l tf ) – High thermal stability – Workspace is relatively small in proportion to the overall dimensions of the frame. • Symmetrical S t i l structure t t i is quite rigid. advanced frame geometries are deployed: – Cubic / Cuboid – Tetrahedron – Octahedron etc.and strong enclosed structures that are especially suitable for parallel mechanisms h i (lik (like Hexapod H d platforms). Structural Loop (“Force Flow”) Workpiece Chapter 5 ME 551 6 Truss-type yp Structures • In precision machine design design. • Main actuator must be located on the bridge.2 • Most precision machine tools utilize this architecture: Deformed Frame – Commonly referred to as O frames Axial Force Tool • Accessing the workspace is fairly easy. • Such (truss-type) geometries yield stable. Chapter 5 ME 551 7 . • The hexapod (Stewart platform concept originally developed for flight simulators) gives six limited degrees of freedom. d a ced co controller o e a architecture c ec u e a and d • Advanced algorithms make programming possible. – The tool angle is limited to about 20 degrees from the vertical. Chapter 5 ME 551 8 Structural Materials Ferrous metals Cast Iron Steel Invar Super Nilvar™ Nitralloy™ Chapter 5 Nonferrous metals Al. (6061-T651) Copper Brass (Cu Alloy) ME 551 Non-metals Composites Granite Zerodur™ Polymer concrete Portland concrete Carbon Fibers 9 . (Cast 201) Al.Example p .Octahedral Hexapod p • Machine is built by Ingersoll Company • It employs an octahedral geometry to support the hexapod “tool” platform. Column Structures1 Without Ribs With Ribs Chapter 5 ME 551 10 Stiffness Properties p of Columns1 Chapter 5 ME 551 11 . Bed Structures1 Chapter 5 ME 551 12 Bed Structures (Cont’d) ( ) Chapter 5 ME 551 13 . Chapter 5 ME 551 15 .Properties p of Bed Designs g 1 Chapter 5 ME 551 14 Cast Iron Structures2 • Widely used in machine construction • Stable with thermal anneal. relieve • Provides good damping and heat transfer • Low cost for moderate sizes • Integral features can be cast in place • Design and manufacturing rules are well-established. aging or vibration stress aging. – Special polymers are mixed with specially prepared/sized aggregate. – Unlike metal castings. Chapter 5 ME 551 17 . ates Chapter 5 ME 551 16 Polymer y Concrete Casting g2 • Polymer concrete (PC) is a relatively new material used in precision machine design. profiles. castings a PC will not develop hot spots while curing even in thick. and reactiveresin concrete castings all refer to the same technique. – Epoxy-granite-. a and dp plates. damping improved with shear dampers • Low cost • Integral features/parts can be welded in place • Structures can be made from tubes. p o es. uneven sections. mineral-.Welded Steel Structures2 • Often used for larger structures or small-lot small lot sizes • Stable with thermal anneal • Low damping. • For PC castings. the same rules for draft allowance apply as for metal castings if the mold is to be removed. hydraulic lines etc.PCC (Cont’d) ( ) • Instead of ribs. Chapter 5 ME 551 19 . • When bolting or grouting non-PC components to a PC bed. – They can have much greater damping.g. bimaterial effect must be considered. • PC does not diffuse heat as well as cast iron. • PC can accommodate cast in place components such as bolt inserts. • Highly Hi hl loaded l d d machine hi substructures b t t (e. bearing rails. carriages) are made from cast iron or steel. conduit. steel Chapter 5 ME 551 18 PCC (Cont’d) ( ) • PC structures can have the stiffness of cast iron structures. ribs PC structures use internal foam cores to maximize their stiffness-to-weight ratio. – Attention must be paid to the isolation off heat sources to prevent the formation of hot spots. Granite-Structures • Used exclusively in precision instruments and CMMs.6 [g/cm3] Elasticity modulus: 40 [GPa] Tensile strength: 16 [MPa] Th Thermal l exp. it will distort! • This very hard (and brittle) material is very stable: – – – – • Chapter 5 Density: 2. Otherwise. t th – Mechanical properties can be tightly controlled t ll d – Joining process can be complicated – Quite expensive • The application of this new material to this field is still in its early stages. Chapter 5 ME 551 21 . concrete the granite is hydrophilic: – It must be sealed off properly to avoid absorbtion of water. coefficient: ffi i t 7. g ME 551 20 3 Carbon Fiber Composites p • Fibre reinforced composites have very high hi h values l off a specific ifi modulus of elasticity and specific strength. – Serves as reference planes/surfaces – Quite costly • Just like concrete.35×10 7 35 10-66 [1/K] Not all grades are suitable for precision machine design. Slanted Bed Designs g for Lathe1 Adhesive Joint Adhesive Surface Cast iron guideway plate on cast concrete Location of Cast-ribs Lower Section of Bed Mounting for Hydraulics Steel Insert for Machine Foot Oil Chamber Conduit Transfer Tap Chapter 5 Turning Fixture ME 551 22 Structural Damping p g • Damping is needed to absorb energy from the process: – T To preventt chatter h tt and d damage d to t the th surface f – To absorb energy from structural modes excited by the servos and other sources • Damping can be obtained by internal means: – Material damping – Damping by micro-slip at joints • Damping can be obtained by external means: – Constrained layer dampers (or shear dampers) – Vibration absorbers – Active dampers • Velocity control loops in servo systems • Actively controlled masses attached to the structure Chapter 5 ME 551 23 . slides) of the various components via micro micro-slip slip.e. • Visco-elastic layer damps motion between structure and constraining layer (from bending or torsion) by di i ti dissipating ki ti energy into kinetic i t heat. Chapter 5 ME 551 25 . d i • One alternative method to increase the damping of the structure is to employ shear dampers. Chapter 5 ME 551 24 Shear Dampers p 4 • Steel structures are known to h have littl internal little i t l damping. bolted joints.Combined Damping p g Effect6 • Major part of the damping for a machine system can be generated at the mating surfaces (i. Application pp . – Coated inner tube is inserted and gap filled with epoxy.Shear Dampers p 4 • For this case. Chapter 5 ME 551 26 Application pp ((Cont’d)) Chapter 5 ME 551 27 . case the structural damping of a round tube is considered. – Constraining layer is wrapped with damping material. – Inner tube serves as constraining layer. • Commercial products like 3M 434/435/436™ constitute a visco-elastic polymer coated on a soft Aluminum constraining layer. Cross-section C ti off the th shear h damped test beam Chapter 7 ME 551 28 Vibration Damping p g Tapes p • Vibration damp(en)ing tape/foil is a band aid option to increase structural “band-aid” damping.Precision Grinder7 T-bed of the precision grinder includes 4 (visco-elastic (visco elastic material covered) steel inserts.Example p . • Very useful in dampening the vibrations off metal t l plates l t and d composite it panels. Square inserts also allow the circulation of cooling fluid. Chapter 5 ME 551 29 . l • Somewhat sensitive to high temperatures: – Nominal operating temperature range (for the tape) p ) is -60 to 20oC. Ma agnification function Vibration Absorbers1 Chapter 5 ME 551 30 Absorber Designs g 2 ConstrainedLayer Beam Adjustable Position TMD Mass Anchor St t Structure Chapter 5 ME 551 31 . Finite Element Method • FEM is an indispensible engineering analysis tool to find approximate solutions to technical problems defined by partial differential equations.) are routinely utilized to design/analyze/optimize structural members. Abaqus™. • FEA Packages (ANSYS™. stresses.or shell model) ■ Choice of element order (linear. MARC/Mentat™. parabolic) Abstraction for the FE-model Creation Presentation of the Results Abstraction of Guides and Drives as Springs Compilation of the Overall Model ■ Definition of boundary conditions Program’s Internal Processes (force.or plastic region) Heat Transfer Mechanical Vibrations Electromagnetic Fields (Maxwell™) Fluid Dynamics (Fluent™) Chapter 5 ME 551 32 FEM Analysis y 1 CAD Model of the Machine “Specs” of the Simulation ■ Determination of the simulation objectives (deformations. members • Large number of engineering analysis can be packages: g conducted byy FEA p – – – – – Stress/Strain (in elastic. etc. temperature) and constraints ■ List of basic stiffness matrices ■ Structure the global stiffness matrix ■ Consideration of the boundary conditions ■ Solution of the resulting linear system ■ Derivation of stress from deformation values Chapter 5 ME 551 33 . Nastran/Patran™. natural modes) ■ Determination of modeling strategy (volume. Illustration1 Chapter 5 ME 551 34 Example – Portal Frame1 Chapter 5 ME 551 35 .FEM Analysis y . Vibration Analysis y .4 Hz Chapter 5 Natural Frequency: 73.Illustration1 Chapter 5 ME 551 36 Natural Frequencies q & Modes1 Natural Frequency: 42.4 Hz ME 551 Natural Frequency: 102 Hz 37 . Preloaded support c. Counter-weight C t i ht Chapter 5 ME 551 39 . Compensating C ti curve b.Vibration Analysis y 1 ((Cont’d)) Frequency [Hz] Chapter 5 ME 551 38 Elimination of Static Deformations5 • There are three methods to compensate the elastic deformations of the machine structure under the action of quasi quasi-static static loads: a. ti motor t + ballb ll screw inertia usually dominates. the servo-motor system needs to support the deadweight of that axis. • For F dynamic d i motions. floats. • Dead weight can be supported by an external system (i. Chapter 5 ME 551 40 Counter-weight g Systems y • For vertical axis. hydraulic systems. Chapter 5 ME 551 41 . this method can be very effective. ( counter weights. etc).Compensating p g Curve1 • The bearing rails are finished (grounded) so that they deform to the desired shape when the machine axes move. – Grinding process is expensive but it saves structural costs.e. • When the primary weight is that of the machine axis (not the workpiece!). – Energy gy wasted due to IR losses of machine. – M May need d to t choose h overrated t d servosystem. – Cables are elastic compared p to chains and hence they y should only be used for quasi-statically (i. components keep the proportions of the golden rectangle (Height/Width = 1. • Utilize symmetry wherever possible.618) in mind. – When needed. • Minimize the structural loop and use closed sections p whenever possible. Chapter 5 ME 551 43 . braided bands. • Large plate sections should be stiffened with ribs and other o e means ea s to o keep eep them e vibrating b a g like ed drumheads u eads.Counter-weight g ((Cont’d)) • Counterweight system contributes dynamics of a precision machinery to the – Cogging effect. • A cable and a smooth running pulley will give the least variation in force. • Cable Cable. mechanisms the sprocket sprocket's s pitch diameter varies slightly as it rotates producing a small cogging effect. elastic effect etc. machines Chapter 5 ME 551 42 Some Design g Rules2 • When sizing components. bands and chains are frequently utilized to carry the counterweight. • Maximize thermal diffusivityy of the machine and minimize heat input. use active damping systems. – In chain mechanisms. – Pulley friction and friction in the counterweight's bearings should be scrutinized for precision machines.e slow) moving elements. • A. H.” Precision Engineering. Vogel-Verlag. SME Press. 4. Chapter 5 ME 551 45 . Bamberg ME EN 7960 Course Notes.. Precision Machine Design. Verlag. SpringerVerlag 2005. Springer-Verlag. H. Vogel Werkzeugmaschinen. 18. E Bamberg. vol. pp. Weck. – Especially.Design g Rules ((Cont’d)) • Locate the work volume at the center of mass and in plane of support.075 Course Notes. • Use as many design tools as possible in design stage. Werkzeugmaschinen (Band 2). Precision Engineering Course Notes. 103-109. Löwenfeld. Notes University of Utah. “Zweites Forschungs und Konstruktionskolloquium Werkzeugmaschinen.” p. Verlag. 2009. L. p y. g Chapter 5 ME 551 44 References 1. (e g translational and rotational) close together. 1955.g. bearing. R. deLacalle. 6. and actuator limitations to help size structural components. ME 2. MIT. M. 1998. Marsh. support • Start at the tool tip (or workpiece) with estimates on cutting forces and acceleration – Then work backward through the structural system. Slocum. Solid Geometric Modeling g and FEM Packages. 2005 A. E. Brecher. – This will minimize cross coupling between modes. A. L. A. 2. H. Lamikiz. K.N. 7. • Try to make the natural frequencies of the various vibration modes (e. Coburg. 117. E. “An integrated Approach to Structural Damping. 3. Cranfield Institute of Technology (UK). Machine Tools for High Performance Machining. 1992. Slocum. 4 5. 1996. 2001. Utah 2006. Slocum. – Use guesstimates for sensor. C. 2006 Cranfield Unit for Precision Engineering (CUPE).