International Journal of Machine Tools & Manufacture 42 (2002) 193–200Investigations on hard turning with minimal cutting fluid application (HTMF) and its comparison with dry and wet turning A.S. Varadarajan, P.K. Philip, B. Ramamoorthy * Manufacturing Engineering Section, Department of Mechanical Engineering, Indian Institute of Technology, Madras, Chennai 600036, India Received 23 August 2000; received in revised form 13 July 2001; accepted 24 July 2001 Abstract Environmental concerns call for the reduced use of cutting fluids in metal cutting practice. New cutting techniques are to be investigated to achieve this objective. Hard turning with Minimal Fluid application (HTMF) is one such technique, which can alleviate the pollution problems associated with cutting fluids. In the present work a specially formulated cutting fluid was applied as a high velocity, thin pulsed jet at the immediate cutting zones at an extremely low rate of 2 ml/min using a fluid application system developed for this purpose during turning of hardened steel. The performance of HTMF is studied in comparison with that of conventional hard turning in wet and dry form. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hard turning; Minimal; High velocity; Thin pulsed jet 1. Introduction Turning of hardened steel components may be carried out by adopting suitable machining strategies [1]. Hard dry turning is one such strategy. During hard, dry turning, the work can be turned to its final dimension in the hardened state. This is accomplished by selecting suitable cutting tools such as CBN or ceramic tools and machine tools of high rigidity [2]. This method eliminates the conventional process cycle consisting of initial turning, subsequent hardening by heat treatment and final finish grinding. Hard turning can save time, improve surface quality, reduce operations and decrease rejections [3,4]. But hard, dry turning involves heat dissipation without coolants and hence demands costly high performance cutting tools and extremely rigid machine tools. In many instances this cannot be implemented directly on the shop floor, as the existing machine tools lack the requisite rigidity [5]. Hard turning in accompaniment with a copious supply of cutting fluid is the normal practice on the shop floor, which is supposed to exploit the cooling, lubrication and chip removal * Corresponding author. Tel.: +91-44-445-8538; fax: +91-44-2350509. E-mail address:
[email protected] (B. Ramamoorthy). action of cutting fluids. However recent concepts and awareness of sustainable manufacture are often on a collision course with the use of cutting fluids. Used in large quantities they pose problems of procurement, storage, disposal and maintenance. In other words, apart from the cost, flood application is not environment or people friendly. This factor assumes considerable significance in the recent climate of strict work safety and environmental protection. According to the Occupational Safety and Health Administration (OSHA) regulations, the permissible exposure Level for mist within the plant (PEL) is 5 mg/m3 and is likely to be reduced to 0.5 mg/m3 [6]. A plausible solution to overcome these issues is an approach, which is intermediate between pure dry and conventional wet turning. The concept of pseudo dry turning or turning with minimal cutting fluid is very much relevant in this context [7]. In this method extremely small quantity of cutting fluid is used and it almost resembles dry turning. There are reports of attempts to introduce cutting fluid at the tool–chip interface through specially designed cutting tools with the objective of achieving better cutting performance. Such attempts brought forth better tool life [8] better surface finish [9], low cutting force [10] and better chip forms [11]. Research work carried out in our laboratory [12– 16] has indicated that good cutting performance could be achieved in terms of surface finish, tool wear and 0890-6955/02/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 0 - 6 9 5 5 ( 0 1 ) 0 0 1 1 9 - 5 4.1. 1(a) and a more localized one in Fig. The composition of the work material is given below 1. Selection of work material The work material was a through hardenable steel (AISI 4340) which was hardened to 46 HRC (460 HV) by heat treatment. thin pulsed jet during turning of round bars of through hardenable AISI 4340 steel of 46HRC using multicoated hardmetal inserts with sculptured rake face.1 mm/rev and the depth of cut at 1. high (600 pulses/min) pulsing rate and a collimated slug. The base was a commercially available mineral oil. Sweden were used in the present study. so that cutting fluid could be injected to more than one location or more than one machine tool at the same time. additives such as surfactant. The test rig facilitated the independent variation of the injection pressure (p) the frequency of injection (N) and the rate of injection (Q). The effect of fluid delivery parameters on cutting performance was evaluated and on that basis their optimum levels were identified.05 to 0. the mode of delivery and direction of delivery were optimized using Response Table methodology.S. wet turning and during minimal application in the optimized mode. Special fixtures were designed. No modifications need be done on the cutting holder or inserts. the frequency of pulsing.5 ml/min. biocide and a deodorizing agent. pressure of injection. 1. Within the practical range investigated these were a low (2 ml/min) delivery rate. The formulation contained. The cutting velocity was varied from 40 to 120 m/min at five levels while the feed was kept at 0. 2.14 mm/rev at five levels with the cutting velocity maintained at 80 m/min and depth of cut at 1.1. To evaluate the performance during the variation of feed. Selection of levels of parameters of fluid application The optimization procedure carried out during the earlier work lead to the identification of the parameters of fluid delivery and fluid composition [16]. A stylus type perthometer was used for measuring surface roughness. In the present work. The tool holder used from the same source had the specification PSBNR 2525 M12. Cutting experiments Experiments were carried out on a heavy-duty VDF S500 lathe of Heidenreich and Harbeck. The parameters of fluid application viz. Varadarajan et al. By selecting proper settings. so that the injection nozzle could be located in any desired position without interfering with the tool or work during actual cutting. The cutting force was measured using a Kistler dynamometer. A high-speed electrical mixing chamber facilitated thorough emulsification. The investigation revealed that a coolant-rich (60%) lubricant fluid with minimal additives is the ideal formulation for HTMF.25 mm during dry. main cutting force. The system can deliver fluids through six outlets simultaneously. 1(b). emulsifier.194 A. Formulation of cutting fluid Since the quantity of cutting fluid used is extremely low in this new method.3. / International Journal of Machine Tools & Manufacture 42 (2002) 193–200 cutting force when a specially formulated cutting fluid was applied on critical locations such as tool–work interface or the tool–chip interface using a cutting fluid application system in the form of a high velocity. It is a general-purpose steel having a wide range of application in automobile and allied industries by virtue of its good hardenability enabling it to be used in fairly large sections. 1. stabiliser.2. cutting temperature and the average flank wear were measured during dry turning. It consisted of a P-6 Bosch fuel pump generally used for diesel fuel injection in truck engines coupled to a variable electric drive. 1. specially formulated cutting fluids with appropriate ingredients and properties were used in the present investigation. TiN and TiCN coatings from SANDVIK.25 mm. HTMF in its optimized mode is compared with conventional wet turning and dry turning under identical cutting conditions. wet and optimized minimal application. evaporator. experiments were conducted with the feed ranging from 0. Bars of 75 mm diameter and 320 mm length were used in the present investigation. 2. The cutting temperature was measured using tool–work thermocouple technique and the average flank wear was measured using a tool maker’s microscope. the rate of injection could be made as small as 0. The performance parameters such as surface roughness. rate of delivery. Experimental procedure 2. . in addition to coolant and lubricant. Fluid injection system An overall view of the special test rig developed for injecting the cutting fluid is presented in Fig. Hamburg. The fluid application system developed is relatively simple and can supply cutting fluid to six machine tools without necessitating any modifications on the existing system setup. composition of cutting fluid. Selection of cutting tool Based on their easy availability and widespread use multicoated hardmetal inserts with sculptured rake face geometry with the general specification.2. high (20 MPa) injection pressure. Germany. SNMG 120408 with a P30 or equivalent substrate with TiC. A. Varadarajan et al. wet and during minimal application in optimized condition. It is observed that cutting force is lower during minimal application when compared to dry turning and conventional wet turning. Tool life tests were carried out in the three modes at a constant feed of 0. (b) A more localized view of fluid application. 2(a. 1. / International Journal of Machine Tools & Manufacture 42 (2002) 193–200 195 Fig. Main cutting force Fig.1 mm/rev.b) shows the variation of cutting force with feed and cutting velocity respectively under specified conditions. (a) Overall view of the fluid application system. Presumably minute capillaries exist at the tool–chip interface especially if the seizure and sublayer plastic flow at the tool–chip contact zone are not total. 2. 3. So capillaries can also exist in the body of the chip as extensions of outer surface serrations. .S. Results and discussion 3.1. as was found in this case by an examination of the chip underside. Variation of cutting force during dry. Penetration of the cutting fluid with EP addi- Fig. In the present investigation the tool–chip contact length was the least during minimal application followed by wet turning and dry turning over the entire cutting range as observed in Fig. Environmental factors can affect the mobility of near surface dislocation on the chip surface and this chemomechanical effect is known as the Rebinder effect [21]. This high velocity (of the order of 100 m/s for a pressure of 20 MPa) facilitates better penetration of the cutting fluid on impact to the root as well as the underside of the chip facilitating its passage to the tool–chip interface resulting in the reduction of friction. The penetration of fluid vapor as described earlier leads to the contamination of the rake face. This relieves a part of the compressive stress and promotes chip curl that reduces tool chip contact length. Promotion of plastic flow at the backside of the chip due to Rebinder effect and 3. It can be seen that all the three mechanisms are active during fluid injection. The velocity varies as a function of the square root of the injection pressure [18]. 1. Contamination of tool rake face. wet and during minimal application in optimized condition. These particles. Overall reduction of cutting temperature. 2.196 A. Tool–chip contact length Reduced contact length and improved cutting ratio are indicative of favorable frictional conditions at the tool– chip interface. (a.b) respectively. 3. During minimal application the cutting fluid is applied at the tool–work interface and there is a possibility of some tiny fluid particles penetrating the work surface near the cutting edge. 3. Varadarajan et al. Such a condition is not possible in conventional wet turning where no such fragmentation is taking place and the kinetic energy of the fluid jet is in no way comparable to that during fluid injection. Through high-pressure injection the fluid is fragmented into tiny droplets the size of which is inversely proportional to the pressure of injection [17].2. which will form the top of the chip in the next revolution [22]. (a.b) Variation of tool–chip contact length during dry. This prevents adhesion of the virgin chip surface on to the tool surface and shifts the condition from seizure and sublayer plastic flow to one of sliding and as a consequence there is a considerable reduction in the contact length. . In conven- Fig. wet and during minimal application in optimized condition.S. / International Journal of Machine Tools & Manufacture 42 (2002) 193–200 Fig. Reduction of tool–chip contact length is expected to occur due to the following [20]. So also penetration of the fluid through the mass of the chip can influence chip curl and the primary deformation process. A quantitative analysis of the former is given in [19]. owing to their high velocity and smaller physical size can penetrate and firmly adhere to the work surface resulting in the promotion of plastic flow on the backside of the chip due to Rebinder effect. tives in to the interface can reduce the frictional contribution to cutting force. 4. 3(a.b) Variation of cutting ratio during dry. depth of cut=1. 5(a–c) respectively. In Fig. m is the mass of the evaporation fluid and L is the evaporation enthalpy. 4 (a. A higher cutting ratio implies better lubrication at the tool–chip interface and formation of chips of thinner sections. The specific heat capacity Cp for water is 4. (a–c) Chip section micrograph during dry turning wet turning and HTMF in optimized condition (cutting velocity=80 m/min. low cutting temperature reduces adhesion tendency and promotes contact area restriction.25 mm). 5.A. where Q is heat quantity in kcal. Cutting temperature Cutting temperature is a dominant factor in tool wear as all the wear mechanisms are temperature-dependant. (a. where the adherent film of lubricant retards the heat transfer.b) it is observed that higher cutting ratios are possible during minimal application than during dry turning and conventional wet turning. During conventional wet turning the quantity of heat (Q) extracted by convective heat transfer is given by Q MCp T (1) Fig. / International Journal of Machine Tools & Manufacture 42 (2002) 193–200 197 is minimum during minimal application followed by wet turning and dry turning. the evaporation enthalpy is 2260 kJ/kg and in the case of mineral oil it is about 210 kJ/kg. 6. The cutting fluid droplets by virtue of their high velocity can puncture the blanket of vapor formed and reach the hot interfaces facilitating more efficient heat transfer than is possible in conventional wet turning.10 mm/rev. In the case of water. Apart from reducing wear. Chip section micrographs during dry turning. Cp is the specific heat capacity and T is temperature reduction brought about. However. M is the mass of cutting fluid. Cutting ratio The cutting ratio (t/tc) is an index of the frictional conditions existing at the tool–chip interface. The lower cutting temperatures recorded in a cutting velocity range of 40–120 m/min and that in a feed rate range of Fig. as the fluid particles cannot have penetrant contact and consequently the chip curl due to Rebinder effect and the associated reduction in contact length is less pronounced. feed=0.4.b) Variation of cutting temperature during dry. Varadarajan et al. It is also observed that the deformation on chips formed during minimal application is less when compared to the other two. during minimal application. The evaporative heat transfer is facilitated by the increase in surface area caused by atomization and the quantity of heat removed (Q1) is given by Q1 MCp T mL (2) tional wet turning rake face lubrication is not as effective as in fluid injection. 3. evaporation of even a very small quantity of water is sufficient to create significant cooling. 3.2 kJ/kg K and that for mineral oil is 1. wet and during minimal application in optimized condition. It is observed that the chip thickness Where M. conventional wet turning and during minimal application are presented in Fig.S. .9 kJ/kg K.3. Since the evaporation enthalpy of water is very high. Cp and T have the same notation as before. cooling occurs due to both convective and evaporative heat transfer. Photographs of flank wear after a cutting time of 120 s during dry turning wet turning and during minimal application are shown in Fig. wet and minimal application at different feeds.b) are direct consequences of the above factors. 7(a.14 mm/rev during minimal application. According to Kaldor et al. (1) acceptable chips and (2) unacceptable chips. wet and during minimal application in optimized condition. 8(a) represents a comparison of the variation of surface roughness as a function of time. as is seen from Fig.b) Variation of surface roughness during dry. (a.3 mm after 150 s when the surface roughness became Ra=1 µm.7. wet and during minimal application in optimized condition (cutting velocity=80 m/min. 9(b) correspond Fig. Entangling chips can harm the surface finish and even lead to unexpected tool failure. there are two groups of chip forms. (Work=AISI 4340 steel 46 HRC.1 mm/rev and depth of cut=1. lowering of cutting temperature. 3.b) Variation of surface roughness and average flank wear with time of cut during dry. shortening of tool–chip contact length and increase of shear angle during minimal application should bring forth better surface integrity and improved tool life than otherwise. Surface finish The reduction in cutting force. Acceptable chips do not interfere with the work or the machine tool and do not cause problems of disposal. 0. 8(c). [23]. / International Journal of Machine Tools & Manufacture 42 (2002) 193–200 Fig. Fig 9(a) presents a comparison of chip forms obtained during dry. The results are self-explanatory. 3. (c) Photograph showing flank wear during dry turning wet turning and HTMF respectively.10 mm/rev.198 A. based on the convenience of handling. cutting velocity=80/min.b). (a.25 mm). Varadarajan et al. Fig. tool=multicoated hardmetal. 6(a. 7. The chip sampled shown in Fig. Unacceptable chips interrupt regular manufacturing operation. depth of cut=1.5. as they tend to tangle around the tool and work piece and pose safety problems to operators. It is observed that during dry turning the average flank wear was 0. feed=0.05–0.6. 8.25 mm). 3. Cutting fluid injection thus provides effective heat transfer leading to lower cutting temperatures than is possible during conventional wet turning. 8(b) represents the comparison of average flank wear with time. Comparison of chip forms The form of chip produced is one of the major parameters influencing productivity in metal cutting industry. feed=0. It is observed that a surface roughness within Ra=1 µm could be maintained for 360 s during minimal application where as it is 150 s in dry turning and 210 s in wet turning. Fig. Tool life Fig.S. . This technique can form a viable alternative to conventional wet turning. [4] T. 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