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Machining Science and Technology, 10:87 131 Copyright # 2006 Taylor & Francis Group, LLC ISSN: 1091-0344 print/1532-2483 online DOI: 10.1080/10910340500534316 ENVIRONMENTALLY CONSCIOUS MACHINING AND GRINDING WITH CRYOGENIC COOLING S. Paul and A. B. Chattopadhyay & Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, West Bengal, India & High production machining and grinding is becoming essential to enhance productivity and profitability. But such machining technique inherently increases the machining and grinding zone temperature, which in turn enhances tool wear rate and impairs the product quality. Historically cutting compounds and grinding fluids are applied to control such high temperature. Not only are such cutting fluids not very effective, but most importantly they are being perceived as a major source of pollution from the machining and grinding industry. The drastic cooling action of liquid nitrogen jets does provide desirable temperature control along with other technological benefits. Several research groups and a few organisations in different parts of the world are investigating the effects of cryogenic cooling on the machinability and grindability of different work materials using different approaches. Keywords Environment, Machining, Grinding, Cryogenic Cooling INTRODUCTION In today s competitive manufacturing environment, enhancement of productivity with improved product quality quite often decides the sustainability of a manufacturing organisation. Along with enhancement of productivity, reduction in piece-cost as well as maximisation of profit rate is becoming the more relevant economic and management criterion due to globalisation and the cheaper labour market in the emerging industrial countries. Manufacturing can be very broadly divided into primary and secondary manufacturing. In primary manufacturing, the basic size and shape of the component is imparted whereas under secondary manufacturing the final size and shape is provided to the component with desired surface finish and tolerances. Within secondary manufacturing, material removal processes are the most important group, and dominate the economics of Address correspondence to S. Paul, Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, West Bengal 721 302, India. E-mail: spaul@mech.iitkgp.ernet.in
88 S. Paul and A. B. Chattopadhyay manufactured products. Conventional material removal processes, namely machining and grinding, also play substantial roles in the domain of overall manufacturing. The above economic objectives of minimisation of cost and maximisation of profit rate quite can often be achieved under a high production machining environment by using a combination of high cutting velocity and feed rate. High production machining is inherently characterized by generation of heat and high cutting temperature, which adversely affects tool life, dimensional accuracy and surface integrity of the product as well as the high machining temperatures are more prevalent in machining high strength and heat resistant materials. Reducing heat generation through optimum selection of machining parameters, proper cutting fluid application and using heat resistant tools are some of the current approaches to control such problems and achieve sustainable high production machining with acceptable product quality. The nature of contact at the chip-tool interface on the rake surface is elasto-plastic in nature. Under high production machining at high cutting velocity, the contact becomes almost plastic in nature (1). Conventionallyapplied cutting fluid fails to penetrate the chip-tool interface and only provides bulk cooling. Even the addition of extreme pressure additives to enhance the lubrication properties at chip-tool interface does not improve the situation to the desired extent. Thus, conventionally applied cutting fluids cannot effectively provide a reduction of cutting temperature and tool wear (2). However if a high pressure jet of water soluble cutting fluid is applied at the chip-tool interface, it can reduce friction and temperature to some extent and enable the reduction cutting forces and improve tool life, as the high momentum of the high pressure jet of the coolant can lift the chip and penetrate into the chip-tool interface to some extent (3). Cutting forces, temperature and tool wear can also be reduced significantly by improving the machinability characteristics of the work material by optimizing the process parameters and tool geometry (4 6). The appropriate selection and use of coated and special tools like cbn may also help in reducing cutting temperature and associated problems (2). Grinding, also a machining or material removal process, is widely used to provide the desired surface finish with dimensional and form tolerance along with acceptable surface integrity in a manufactured component. However, grinding is also being employed for bulk material removal as in the case of creep feed grinding, fast feed grinding (7) and high efficiency deep grinding (8). Grinding is carried out by the removal of work materials in the form of chips by the action of very small abrasive grits of uncertain geometry as opposed to the specified geometry of conventional cutting tools at one or two order of magnitude higher cutting speed. Thus grinding is characterised by very high specific energy requirement, which is again
Cryogenic Cooling in Machining and Grinding 89 one or two order higher than most other machining process (7, 9 14). Such high specific energy, in grinding steels of 20 J=mm 3 to 200 J=mm 3, in combination with rather low work speed, leads to grinding zone temperatures in excess of 1000 C. If the high grinding zone temperature is not properly controlled that would result in several problems like thermal damage to the ground surface in the form of surface oxidation or burning, redeposition of the grinding chips on the finished surface, changes in the metallurgical characteristics of the surface and sub-surface and the introduction of tensile residual stresses further leading to reduction in fatigue life, enhanced stress corrosion and even the formation of surface and sub-surface cracks. Thus, it may be inferred that a high grinding temperature provides poor surface integrity and thus poor product quality. Hence, control of grinding temperature is the essential requirement to enable maximisation of economic benefits and to achieve the desired product quality of ground components. In grinding, a copious amount of grinding fluid (typically soluble oil water emulsion in concentration of 1:20 to 1:100) is applied at low pressure for controlling the grinding zone temperature. However, such flood cooling cannot provide the necessary reduction in grinding temperature as most often the coolant stream fails to reach the grinding zone due to the formation of a difficult-to-penetrate air boundary layer around the grinding wheel. Further, even if the grinding fluid reaches the grinding zone, it may not participate in heat removal through local convective heat transfer if the grinding temperature is more than the film boiling temperature of the grinding fluid, which is typically around 120 to 150 C for soluble oils. Thus any strategy for control of grinding zone temperature would have to take into account the above two facts. The grinding temperature in plunge surface grinding, both pendulum and high efficiency deep grinding, can be estimated as (8): where h ¼ B gq T l 0:5 c pffiffiffiffiffiffiffiffiffi ð1þ kqc h ¼ grinding temperature B ¼ a factor depending on infeed, thermal properties of work material, contact length q T ¼ total heat flux on the contact length k ¼ thermal conductivity of the work piece C ¼ specific heat of the workpiece q ¼ density of the workpiece g ¼ apportionment coefficient i.e., percentage of total heat entering the work piece v w
90 S. Paul and A. B. Chattopadhyay l c ¼ contact length between the grinding wheel and workpiece v w ¼ work speed Equation (1) is not only useful in estimating the grinding zone temperature but also suggests strategies which may be adopted for effective control of grinding temperature. For example some strategies for temperature control are as follows (12):. lower wheel speed typically reduces the specific energy requirement and heat flux (q T ). use of softer wheel typically does not allow the rise in force due to wear flat development owing to continuous self-sharpening and thus lower heat flux (q T ). lower material removal rate leads to less heat flux (q T ). use of neat oil instead of grinding fluids using neat oil enhances the film boiling temperature of the grinding fluid (typically 350 C) and thus allows more heat to be removed by the fluid lowering thereby (g) apportionment coefficient i.e., percentage of total heat entering the workpiece. use of additives in grinding fluids it improves the ability of the grinding fluids to reach the grinding zone and thus in turn lowers (g) the apportionment coefficient. use of cbn wheels and monolayer cbn wheels being sharper and more wear resistant as compared to alumina or silicon carbide grits, provide less force and thus lower heat flux (q T ); further, the thermal conductivity of cbn is higher than that of conventional abrasive grits leading to sharing of heat from the grinding zone and in turn lowering (g) apportionment coefficient. the following two techniques improves the ability of the applied grinding fluid to reach the grinding zone by hindering the formation of a resistant air boundary layer and thus lower (g) apportionment coefficient. painting and card board scrapper technique. curved grooves on the faces of the wheel surface. the following techniques improves local heat transfer to the applied grinding fluid lowering (g) apportionment coefficient. randomly spaced holes parallel to the wheel axis in face grinding. intermittent grinding by slotted wheels. ZZ or through wheel coolant delivery method. high pressure jet infusion technique. on-line ultrasonic cleaning and high pressure cleaning of the wheel surface these techniques are essential in deep grinding where the contact length is large potentially leading to wheel loading. They lower the chance of wheel loading by hindering the formation of and aiding in
Cryogenic Cooling in Machining and Grinding 91 the removal of local large scale adhesions of work material on the wheel surface and maintaining the wheel sharpness, low grinding force and thus low heat flux (q T ). ELID (electrolyte in-process dressing) grinding maintains wheel sharpness specially in finish grinding lowering and maintaining low heat flux (q T ) Most of the techniques discussed above to control temperature in grinding depend on the copious use of grinding fluid, either soluble oil or neat oil. Similarly, conventional cutting fluids are still used widely in the machining industry. In Germany alone, as cited by Dhar et al. (2), 650,000 tons of water-based emulsions and mineral oils were used as cutting fluid annually. But there are environmental and health problems associated with use of cutting fluids. The major problems with conventional cutting fluid application are (2):. environmental pollution due to chemical break-down of the cutting fluid at high cutting temperature. biological hazard to operator due to bacterial growth, dermatological problem and inhalation of toxins during chemical break down of the cutting fluid. requirements of additional system for pumping, local storage, filtration, recycling, chilling and large space.. water pollution and soil contamination during final disposal. Simply the cost of disposal of used coolant has increased substantially and in Germany in 1994, it was estimated to be one billion DM (2). Noting the environmental and economic issues involved with use and disposal of cutting compounds, Inasaki et al. (15) observed that in future cutting compounds and grinding fluids will be replaced with environmentally benign agents and gradually dry machining and grinding technology will be developed. They also noted that cryogenic cooling of the grinding zone with liquefied gases may develop as a potentially effective and non-polluting technology for controlling grinding zone temperature. Such alternative approaches towards temperature control in machining started in the late 1960s and early 1970s (16 19), where cooling of the work piece with liquid nitrogen was studied in turning and face milling. Liquid nitrogen was applied as a jet at the machining zone. Some machining of pre-cooled workpieces was also carried out. Though some benefits were reported these benefits were very material specific. Further systematic and detailed research into this technology only started in the mid-1980s and the effort has intensified since the 1990s. (9 14, 20 48).
92 S. Paul and A. B. Chattopadhyay Evans (37) attempted diamond turning of stainless steel for fabrication of metallic reflectors with cryogenic cooling and observed improved tool life with acceptable product quality specially form accuracy. Effects of cryogenic cooling in turning a Kevlar composite and its influence on tool wear and product quality were also studied (38). Hong and his group (33, 34) also initiated extensive research in the area of cryogenic cooling in machining of steel and titanium alloys. The turning of silicon nitride with cryogenic cooling was undertaken by Rajurkar and his group (35, 36) and reported a significant improvement in tool life attributed to cryogenic cooling. Wang and Rajurkar (46) reported that for machining of advanced ceramics, titanium alloys, Inconel alloys, and tantalum with liquid nitrogen (LN 2 ) cooled cutting tools, the temperature in the cutting zone reduced to a lower range and the temperature-dependent tool wear reduced significantly under all machining conditions. The surface roughness of all materials machined with LN 2 cooling was found to be much better than the surface roughness of materials machined without LN 2 cooling after the same length of cutting. Wang et al. (43) combined traditional turning with cryogenically enhanced machining and plasma enhanced machining. By joining these two non-traditional techniques with opposite effects on the cutting tool and the work piece, it has been found that the surface roughness reduced by 250%; the cutting forces decreased by approximately 30 50%; and the tool life was extended up to 170% over conventional machining. Hong et al. (44) reported that in cryogenic machining the tool life increased up to five times the state-of-the-art emulsion cooling, outperforming other machining approaches adopted for Ti-6Al-4V alloy. Hong et al. (45) reported that the cold strengthening of titanium material increased the cutting force in cryogenic machining of Ti-6Al-4V but lower friction reduced the feed force. It was found that the friction coefficient on the tool chip interface was considerably reduced in cryogenic machining. Increased shear angle and decreased thickness of the secondary deformation zone, findings from a chip microstructure study, offer further evidence that friction is reduced. Hong and Ding (47) observed that a small amount of liquid nitrogen applied locally to the cutting edge is superior to emulsion cutting in lowering the cutting temperature in turning Ti-6Al-4V alloy. Hong and Ding (48) also presented an environmentally safe approach of micro-manipulation of cutting temperatures in machining AISI=SAE 1008 low carbon steel, a material widely recognized as a difficult-to-machine material from the viewpoint of chip control. Liquid nitrogen was selectively applied to the chip and the tool rake face in well-controlled jet. Consequently, it expanded the
Cryogenic Cooling in Machining and Grinding 93 chip-breaking range of feed and cutting speed, with a reduced chip tool interface temperature and increased tool life. Researchers in Cranfield University, UK have studied grinding of titanium alloys with cryogenic cooling (32). Materials such as titanium are extremely difficult to grind due to their high reactivity, low thermal conductivity and low volume specific heat. These properties give rise to high grinding zone temperatures, which can result in low quality surfaces and poor mechanical performance. The use of liquid nitrogen as a coolant can reduce the grinding zone temperature and minimise environmental interactions. Ground surfaces on cryogenically ground titanium alloys have been shown to exhibit reduced levels of surface burn and oxidation, better surface finish, lower residual surface tensile stress, with less wheel loading and hence a reduced need for frequent wheel dressing. Recently, Hoffmeister and Maiz (39) have reported benefits due to the application of cryogenic cooling with liquid nitrogen on grinding temperature in high efficiency deep grinding. The effect of cryogenic cooling becomes more beneficial if co-applied with Minimum Quantity Lubrication (MQL). But systematic research in the area of grinding with cryogenic cooling, to the best of our knowledge, was initiated around mid-1980s by Chattopadhyay et al. (40), who observed some benefits of cryogenic cooling with liquid nitrogen with respect to the grinding forces and surface quality. But most importantly, a few companies have already tried to translate the research finding in the area of cryogenic cooling in machining from the laboratory to the actual shop floor (41). MACHINING OF STEELS WITH CRYOGENIC COOLING Experimental Procedure Nowadays, machining of steels is routinely carried out with cbn, ceramic, coated carbides and uncoated carbide inserts depending on product requirements and the availability of machine tools. Because of the development of toughened ceramic inserts, all machining is undertaken with flood cooling to provide some bulk cooling irrespective of the tool material and to enhance tool life. Frequently tool life is sacrificed when the cutting velocity is increased to enhance productivity and reduce cycle time, production cost and increase profit rate. Thus there is incentive to study the effect of cryogenic cooling in machining different steels with respect to different machinability indices vis-à-vis dry and conventional flood cooling. Detailed systematic experimental investigations (20 24, 26 28) were undertaken to study the effect of cryogenic cooling by liquid nitrogen jets in plain turning of five commonly used steels in the industry by integrated
94 S. Paul and A. B. Chattopadhyay chip breaker (of two different geometries) type uncoated composite carbide inserts. The different machinability indices studied were chip morphology, chip reduction coefficient, cutting forces, cutting temperature, surface finish, surface integrity, tool wear and tool life and dimensional accuracy. Table 1 provides the detailed experimental conditions. Continuous turning of steel bars of diameter 200 mm and 750 mm length was carried out on a heavy-duty lathe NH22 HMT Lathe, 11 kw under dry and wet machining environments and also under cryogenic cooling by liquid nitrogen jets. Two different geometries of integrated chip breaker cutting insert were chosen to study the effect of chip breaker design on the performance of cryogenic cooling. The cutting velocity and feed were chosen such that the lower bound corresponded to the industry standard for semi-finish machining and the higher levels especially that of the cutting velocity, lies beyond that used in routine industrial practice. The depth of cut was kept unchanged at 2 mm, as the effect of depth of cut on most of the machinability indices is very predictable and it is not expected to influence the performance of cryogenic cooling. The liquid nitrogen jets were applied at the cutting zone along the main and auxiliary cutting edge as shown in Figure 1 using two specially TABLE 1 Machine tool Work specimens Material Application Size Cutting tool inserts Grade Geometry Tool geometry Process parameters Cutting velocity Feed Depth of cut Environment Experimental Conditions for Cryogenic Machining of Steels NH22 HMT Lathe, 11 kw AISI 1040 low carbon steel AISI 1060 medium carbon steel AISI E4340C nickel-chromium alloy steel AISI 4140 molybdenum-chromium alloy steel AISI 4320 nickel-chromium-molybdenum alloy steel Power transmission shafts Soft and hard gears /200 750 mm and /160 600 mm Composite Carbide TTS WIDIA (ISO-P30 equivalent) 6 6 6 6 15 75 and 0.8 mm (Orthogonal Rake System=ISO-old) 60 150 m=min 0.12 0.24 mm=rev 2 mm and (1.5 mm for selective trials) Dry Conventional flood cooling wet (selective) Cryogenic cooling
Cryogenic Cooling in Machining and Grinding 95 FIGURE 1 Liquid nitrogen delivery system for cryogenic cooling. designed nozzles. The liquid nitrogen jet coming along the main cutting edge is designed mainly to protect the rake face and the principal flank surface whereas the jet directed along the auxiliary cutting edge is primarily to protect the auxiliary flank face and to hinder the growth of auxiliary notching. Liquid nitrogen is fed from a self-pressurised vacuum and super insulated dewar pressurised by allowing liquid nitrogen to evaporate via an auxiliary tapping line and to fed back thereby pressuring the vapour space above the liquid. The dewar pressure can be set on the economiser and it was set at 10 bar. A flexible vacuum and super insulated stainless steel
96 S. Paul and A. B. Chattopadhyay cryogen delivery tube was used to connect to the dewar via pressure regulator to the delivery nozzles as can be seen in Figure 1 so as to issue the liquid nitrogen jet at any chosen pressure. In the present work, liquid nitrogen jets were issued at a pressure of 2 bar. During machining the tool holder was mounted on the dynamometer to continuously monitor the cutting forces under all the machining environments. The Kistler 1 3 component piezoelectric dynamometer in turn was directly mounted on the cross slide after removing the compound slide. The cutting force signals from the piezoelectric dynamometer were fed to matching charge amplifiers. And finally the tangential and feed forces were continuously gathered on a PC based 12-bit data acquisition system at a sampling speed of 2000 samples per second per channel. During machining the chips were also collected and preserved for further analysis. The gross chip morphology and chip reduction coefficients were analysed. The chip reduction coefficients were determined as the ratio of the chip thickness to uncut chip thickness. The chip thicknesses were measured by digital vernier calliper with a least count of 0.02 mm. The cutting temperature was measured using a tool-work thermocouple technique, as shown in Figure 2, with due care to avoid generation of parasitic emf and electrical short circuit (22). The calibration of the work-tool thermocouple has been carried out by external flame heating technique. Figure 3 schematically shows the detail setup. FIGURE 2 Schematic representation of tool work thermocouple technique.
Cryogenic Cooling in Machining and Grinding 97 FIGURE 3 Schematic representation of calibration technique of tool work thermocouple. The work-tool thermocouple junction was constructed using a long continuous chip of the work-material obtained by machining and a tungsten carbide insert to be used in actual machining. To avoid generating a parasitic emf, a long carbide rod of same composition was used to extend the insert. A standard chromel alumel thermocouple was mounted at the site of tool-work junction of chip and insert junction. The oxyacetylene torch simulated the thermal phenomena in machining and raised the temperature at the chip-tool interface. Standard thermocouples directly monitored the junction temperature measured by a temperature controller and the emf generated by the hot junction of the chip-tool was monitored by a digital multimeter. In this manner calibration curves were generated for all five steels and two inserts. Typically linear relationships were obtained with typical multiple correlation coefficient of 0.95 0.99. Using this method the cutting temperature could be measured very reliably under dry machining conditions but the measurement proved difficult and not very reliable under cryogenic cooling. Thus the cutting zone temperature was evaluated numerically using finite element modelling as detailed elsewhere (22). The dimensional deviation was characterised by measuring the change in diameter of the job along the length after turning. Such deviations were measured by mounting a precise dial indicator of least count 2 mm on the cross slide and traversing it along the job-length of 400 mm. Machining was interrupted at regular intervals and the cutting tool inserts were withdrawn to study the pattern and extent of wear on the main and auxiliary flanks of the insert for all the trials. The average width of the
98 S. Paul and A. B. Chattopadhyay principal flank wear, V B and auxiliary flank wear, V S were measured using in inverted metallurgical microscope fitted with micrometer of least count 1 mm. The principal and auxiliary flank and the rake surface of the cutting tool inserts were also critically examined under a stereo zoom microscope and photographs taken whenever required. At the end of tool life or after the full cut, the cutting inserts were inspected under a scanning electron microscope. The surface roughness along the job-axis was monitored by a contact type computer aided surface roughness analysis system using a sampling length of 0.8 mm. RESULTS The chip samples collected while turning all the five steels by both the SNMG and SNMM inserts with different Vc-S 0 combinations under both dry and cryogenic cooling conditions have been visually examined and categorized with respect to their shape and colour. Characterisation of chip colour, oxide analysis and temperature evaluation has been reported earlier using image processing systems (24). The typical results of such a categorisation of the chips produced at different conditions and environments by AISI 1040 steel at lower feeds (0.12 and 0.16mm=rev) and higher feeds (0.20 and 0.24 mm=rev) are shown in Table 2a and 2b, respectively. The patterns of the chips formed in machining ductile metals generally depend upon the mechanical properties of the work material, tool geometry, particularly the rake angle, levels of V c and S 0, the nature of the chip-tool interaction and the cutting environment. In the absence of a chip breaker, thelengthanduniformityofchipsincreasewithincreaseinductilityand softness of the work material, tool rake angle and cutting velocity unless the chip-tool interaction is adverse, causing intense friction and built-up edge formation. Table 2a and 2b shows that AISI 1040 steel when machined by the SNMG inserts under dry conditions produced ribbon-type continuous chips at low feed (0.12 mm=rev)andmoreorlesshalf-turnchipsathigherfeeds. The geometry of the SNMG insert is such that the chips of this softer steel (AISI 1040 steel) first came out continuously, curled along the normal plane and then hit the principal flank of this insert, and then broke into pieces. Under cryogenic cooling, the form of these ductile chips did not change appreciably but their back surface appeared much brighter and smoother. This indicates that the reduction in temperature and the presence of inert nitrogen enabled a more favourable chip-tool interaction and elimination of built-up edge formation. The colour of the chips also became lighter, i.e., metallic or golden from blue or grey, depending upon V c and S 0 due to post-cooling by liquid nitrogen in addition to its effect in reducing the chip-tool and work-tool interface temperatures.
Cryogenic Cooling in Machining and Grinding 99 TABLE 2 Shape and Colour of Chips of AISI 1040 Steel Feed, S 0 (mm=rev) SNMG 120408 26 TTS SNMM 120408 TTS V c (m=min) Environment 0.12 0.16 0.12 0.16 Shape and Shape and Colour Colour Shape and Colour Shape and colour 66 Dry 4 Light blue 4 Bluish grey. Golden. Bluish grey Cryogenic Metallic 4 Metallic. Metallic Metallic 85 Dry Bluish 4 Blue. Light blue. Bluish grey Cryogenic Metallic 4 Metallic. Mettallic. Metallic 110 Dry Deep blue 4 Deep blue. Blue. Grey Cryogenic 4 Metallic 4 Metallic. Metallic. Metallic 144 Dry Burnt blue 4 Burnt grey. Deep blue Burnt blue Cryogenic. Metallic 4 Metallic. Metallic. Metallic V c (m=min) Environment Shape and Colour Feed, S 0 (mm=rev) 0.20 0.24 0.20 0.24 Shape and Shape and Colour Colour Shape and colour 66 Dry 4 Bluish grey 4 Grey and blue. Grey 4 Bluish grey Cryogenic 4 Golden 4 Golden. Metallic. Metallic 85 Dry 4 Bluish grey 4 Grey and blue. Grey 4 Bluish grey Cryogenic 4 Golden 4 Golden. Mettallic Metallic 110 Dry 4 Deep blue 4 Deep blue. Grey 4 Grey Cryogenic 4 Golden 4 Golden. Metallic. Metallic 144 Dry 4 Burnt blue 4 Deep Blue. Deep blue. Burnt blue Cryogenic 4 Golden 4 Golden. Metallic. Metallic Chip shape Group 4 Half-turn. Tubular helical Spiral Ribbon When examined under a microscope, no significant change in the microstructure was found at the under-surface of the chips when the machining environment was changed from dry to cryogenic cooling, but micro-hardness was found to change to some extent due to cryogenic cooling. The nature and extent of an increase in the micro-hardness of the chips of AISI 1040 steel due to cryogenic cooling under different conditions are typically shown in Table 3. Another important machinability index is the chip reduction coefficient f (ratio of chip thickness after and before a cut). For a given tool geometry and cutting conditions, the value of f depends upon the nature of the chip-tool interaction, chip contact length and chip form, all of which
100 S. Paul and A. B. Chattopadhyay TABLE 3 Micro-Hardness on the Underside of the Chip Cutting insert Cutting velocity (m=min) Environment l-hardness (HV) Feed, S o (mm=rev) 0.12 0.20 0.24 SNMG 120408-26TTS 110 Dry 155 180 191 Cryogenic 181 195 213 144 Dry 149 163 188 Cryogenic 175 187 218 SNMM 120408 TTS 110 Dry 140 168 192 Cryogenic 201 220 241 144 Dry 160 195 200 Cryogenic 195 208 231 are expected to be influenced by cryogenic cooling as well as by the levels of V c and S 0. Typical variations in the value of f with change in tool configuration, V c and S 0, as well as machining environment evaluated for AISI 1040 steel are shown in Figure 4. Figure 4 clearly shows that the value of f gradually decreased with the increase in V c, though in different degrees for the different tool-work combinations, under both dry and cryogenic conditions. The value of f usually decreases with the increase in V c, particularly at its lower range, owing to plasticization and shrinkage of the shear zone with a reduction in friction and built-up edge formation due to an increase in temperature and sliding velocity. In machining steels with tools like carbide, the built-up edge FIGURE 4 Variations in the chip reduction coefficient, f, with an increase in the cutting velocity at different feeds under dry and cryogenic environments for SNMG and SNMM inserts while machining AISI 1040 steel.
Cryogenic Cooling in Machining and Grinding 101 usually initially grows with increasing temperature due to the increase in V c and also S 0 and then decreases with a further increase in V c. Figure 4 also reveals that cryogenic cooling reduced f, particularly at lower values of V c and S 0, for both the inserts, implying a reduction in friction and built-up edge formation at the chip-tool interface and wear at the cutting edges, resulting from a reduction in cutting temperature and the presence of an inert nitrogen atmosphere. Similarly, Figure 5 and Figure 6 show that both P z and P x gradually decreased with an increase in V c for AISI 1040 steel machined with either inserts. This may be attributed mainly to reduction in f and reduction in the strength or more specifically the dynamic yield shear strength of the work material at higher shear plane temperature which is expected to result at higher cutting velocity. It is also important to note that though apparently P z and P x should increase proportionally with the increase in feed, S 0, actually the rate of increase of P z and P x with that of S 0 has been much less. This can be attributed to a reduction in f with the increase in S 0 due to an increase in the average effective rake angle of the tool and thus to an increase in the uncut chip thickness (Figure 7). Figure 5 shows that, as expected, P z decreased when AISI 1040 steel was machined under cryogenic conditions, particularly when V c and S 0 are low and the tool is SNMM, mainly through a reduction in f. However, Hong and his group (45) though reported reduction in feed force but did not observe reduction in main cutting force under cryogenic cooling. FIGURE 5 Variations in the main cutting force, P z, with an increase in the cutting velocity at different feeds under dry and cryogenic environments for (a) SNMG and (b) SNMM inserts while machining AISI 1040 steel.
102 S. Paul and A. B. Chattopadhyay FIGURE 6 Variation in the feed force, P x, with an increase in the cutting velocity at different feeds under dry and cryogenic environments for (a) SNMG and (b) SNMM inserts while machining AISI 1040 steel. FIGURE 7 Schematic view of machining with varying uncut chip thicknesses.
Cryogenic Cooling in Machining and Grinding 103 FIGURE 8 Correlation between measured and predicted cutting temperatures. Figure 8 shows the correlation between the measured average chip-tool interface temperature and that predicted or estimated by finite element analysis. For validating the proposed finite element model, the experimental data for both inserts, namely the groove type SNMM and pattern type SNMG under dry machining conditions have been used. The estimated temperatures are found to be greater than the measured temperatures and the deviations are greater, the lower the temperature. Thus it may be inferred that at higher cutting speed-feed combination the deviation is reduced. The average deviation between the estimated and measured temperature throughout the domain is around 6%, which is acceptable from an engineering viewpoint. Figure 9 shows representative computed temperature distributions for AISI 1040 steel under dry and cryogenic machining conditions. As expected the maximum temperature is found to occur at around the middle of the chip-tool contact length. The average shear plane temperature under dry machining conditions appeared to be around 300 C, which is much less than that at the chip-tool interface. Application of a liquid nitrogen jet substantially lowered the maximum level of temperature at the chiptool interface and at the wear land. Similar observations were also reported by Hong and his group (44, 47). But the overall temperature distribution pattern is similar. Such observations are valid throughout the simulation domain. The present method of application of liquid nitrogen however has not appreciably changed the shear plane temperature. With cryogenic cooling, the underside of the chip was cooled leading to closer chip curling
104 S. Paul and A. B. Chattopadhyay FIGURE 9 Typical distribution cutting temperature while turning AISI 1040 steel in dry and cryogenic environments. and reduction in chip-tool contact length. The integrated chip breaker type inserts under cryogenic cooling provided the benefits of restricted contact length and hence reduced chip contact length. Though chip contact length was reduced under cryogenic cooling, the maximum temperature still occurred at the mid section of the chip-tool interface. Thus there was no concentration of heat at the tip of the insert, which would have been detrimental. Figure 10 shows the variation in average chip-tool interface temperature with increase in speed and feed when AISI 1040 steel is turned by the pattern type SNMG and groove type SNMM insert under both cryogenic and dry machining conditions. The machining temperature significantly increased with increase in cutting velocity and feed, though in differing degrees, under all the conditions where increased energy input was expected. Cryogenic cooling has enabled a significant reduction in machining temperature which may reasonably be attributed to a reduction in chip-tool contact length, a reduction in forces due to restricted contact cutting effect and also by enhanced heat transfer under cryogenic cooling. The benefit of cryogenic cooling was greater at lower cutting velocity. This result is expected because at lower velocity a large portion of the chip-tool contact remains elastic in nature unlike elasto-plastic at higher cutting velocity, which is likely to allow a more effective penetration of cryogen at the interface. Productivity and economy of manufacturing by machining are significantly influenced by the life of the cutting tools. Cutting tools may fail by
Cryogenic Cooling in Machining and Grinding 105 FIGURE 10 Variation in average chip-tool interface temperature (machining temperature) with cutting velocity at different feeds under dry and cryogenic environments while turning AISI 1040 steel. brittle fracture, plastic deformation or wear. Turning carbide inserts having enough strength, toughness and hot hardness generally fail by gradual wear. As machining progresses the tools develop crater wear at the rake surface and flank wear at the clearance surfaces, (as shown schematically in Figure 11) due to the continuous interaction and rubbing of the chips and work surfaces, respectively. Among the aforesaid wear mechanisms, the principal flank wear is the most important because it raises the cutting forces. The life of carbide tools, which mostly fail by wear, is assessed by the actual machining time at which the average value (V B ) of its principal flank wear reaches a preset limiting value, like 0.3 mm. Therefore, attempts should be made to reduce the rate of growth of flank wear (V B ) in all
106 S. Paul and A. B. Chattopadhyay FIGURE 11 Geometry and pattern of tool wear. FIGURE 12 Growth of average flank wear under dry, wet and cryogenic environments with machining time of AISI 1060 steel.
Cryogenic Cooling in Machining and Grinding 107 possible ways without sacrificing MRR. It has already been mentioned that wear of cutting tools are generally quantitatively assessed by the magnitudes of V B, V S, K T, etc. shown in Figure 11, out of which V B is considered to be the most significant parameter at least in R&D work. The beneficial effects of cryogenic cooling on growth of flank wear are shown in Figure 12 while machining AISI 1060 steel. After 30 min of machining the SNMG insert sustained around 350 mm average flank wear in both dry and wet environments whereas cryogenic cooling reduced the average flank wear to 200 mm. For the SNMM insert a similar reduction in flank wear is also shown. A similar beneficial effect of cryogenic cooling on the machining of AISI E4340C steel can also be noted in Figure 13. SEM pictures of worn cutting tool inserts in Figure 14, clearly depict the beneficial role of cryogenic cooling in reducing tool wear. Similar observations were also reported by other researchers (35 36, 44, 46). Effective temperature control has reduced both flank and crater wear and thus enhanced the tool life. Under both dry and wet machining there is severe notch wear on the main cutting edge due to oxidation and chemical wear and on the auxiliary cutting edge due to abrasive wear caused by the rubbing of unmachined ridges against the auxiliary cutting edge. Cryogenic cooling has also substantially reduced the growth of notch wear on both the main and auxiliary cutting edges. Figure 15 and Figure 16 show the beneficial effect of cryogenic cooling on surface finish and dimension accuracy respectively while machining another steel AISI E4340C. Again this can be attributed to temperature control and a reduction in tool wear. FIGURE 13 Growth of average flank wear under dry and cryogenic environments with machining time of AISI E4340C steel.
108 S. Paul and A. B. Chattopadhyay FIGURE 14 Control of tool wear under cryogenic cooling vis-à-vis dry and wet machining of AISI1060 steel. INFERENCE Based on the results of the detailed and systematic experimental investigation described, the following can be inferred:. Application of cryogenic cooling by liquid nitrogen jets is not only environmentally-friendly but also led to substantial technological benefits as has been observed in machining some steels by carbide tools.. Cryogenic cooling by liquid nitrogen jets significantly reduces the cutting forces in machining of all the five steels using uncoated carbide inserts without affecting the working environment.
Cryogenic Cooling in Machining and Grinding 109 FIGURE 15 Variation in surface roughness with machining time while turning AISI E4340C steel under dry and cryogenic environments.. The present cryogenic cooling systems enabled a reduction in average chip-tool interface temperature upto 34% depending upon the work materials, tool geometry and cutting conditions and even such a small reduction enabled a significant improvement in the major machinability indices.. The most significant contribution of the application of liquid nitrogen jets in machining these steels has been the dramatic reduction in flank wear, which has led to a remarkable improvement in tool life. FIGURE 16 Variation in dimensional deviation along length of cut while turning AISI E4340C steel under dry and cryogenic environments.
110 S. Paul and A. B. Chattopadhyay. This reduction in tool wear might have been responsible for the retardation of abrasion and notching, as well as the decrease or prevention of adhesion and diffusion type thermal sensitive wear at the flanks as well as reduction of built-up edge formation which accelerates wear at the cutting edges by chipping and flaking. Deep notching and grooving, which are very detrimental and may cause premature and catastrophic failure of the cutting tools, are significantly reduced by cryogenic cooling.. Dimensional accuracy and surface finish are also substantially improved mainly due to the significant reduction of wear and damage at the tool tip by the application of liquid nitrogen.. Cryogenic cooling, if properly employed, can provide, besides environmental friendliness, a significant improvement in both productivity and product quality and hence with overall machining economy even after recovering the additional cost of the cryogenic cooling system and the cryogen. MACHINING OF Ti-6Al-4V WITH CRYOGENIC COOLING Experimental Investigation Rapid tool wear is inherent to the machining of titanium and its alloys and has been a persistent deterrent to wide use of these alloys in applications like aerospace, process industries marine application, etc. where their use is most desired due to their unique properties of high strength to weight ratio and excellent corrosion resistance. Tool wear in the machining of these alloys is a consequence of several detrimental effects arising from following characteristics:. Titanium alloys have poor thermal conductivity. Heat, generated by the cutting action, does not dissipate quickly from the cutting zone, namely the shear plane and the chip-tool interface. Therefore, most of the heat is concentrated on the cutting edge and the tool face leading to high cutting temperature.. Titanium alloys have strong alloying tendency or chemical reactivity with materials in the cutting tools at machining temperatures. This causes galling, welding, and smearing along with rapid tool wear due to adhesive wear.. Titanium alloys have a relatively low modulus of elasticity and thus have more springiness than steel. Hence the work has a tendency to move away from the cutting tool unless heavy cuts are maintained or proper backup is employed. Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing, and tolerance problems. Thus rigidity of
Cryogenic Cooling in Machining and Grinding 111 the entire system is consequently very important, as is the use of sharp, properly shaped cutting tools.. Titanium alloys work-hardening characteristics are such that they demonstrate a complete absence of built-up edge. Because of the lack of a stationary mass of metal (built-up edge) ahead of the cutting tool, a high shear angle develops. This causes a thin chip to contact a relatively small area on the cutting tool face and results in high bearing loads per unit area. This high bearing force, combined with the friction developed by the chip as it passes over the bearing area, results in a great increase in temperature on a more localized region of the cutting tool. Furthermore, the combination of high bearing forces and temperature produces a cratering action close to the cutting edge, resulting in rapid tool wear.. Segmented chip formation, one of the most important characteristics of chip formation while machining titanium alloys, as can be seen in Figure 17, may also induce chatter due to fluctuation of the cutting forces under high cutting speed-feed rate combinations resulting in an undulating surface finish and tool wear, especially in the case of coated tools. Though ultrahard tool materials like cbn and PCD perform better than other tool materials, the high cost of these cutting tools compels the machining industries to look for other options. Among the remaining available tool materials, ISO K20 grade straight tungsten carbide (WC=Co) cutting tools perform best in machining the titanium alloys. High cutting temperatures, one of the main reasons for the rapid tool wear and hence poor machinability of titanium alloys, calls for an efficient cooling strategy to reduce the prevailing temperature in the tool-chip=workpiece contact zones. In this regard, cryogenic cooling with liquid nitrogen as a cooling medium is a potentially good choice, both for its excellent cooling ability and for its environmental friendliness. FIGURE 17 Segmented chip morphology in machining of Ti-6Al-4V alloy.
112 S. Paul and A. B. Chattopadhyay This section presents results of experimental investigations (25, 29 31) on the role of cryogenic cooling with liquid nitrogen as the cooling medium on the mechanism of tool wear and tool life in machining Ti-6Al-4V alloy using uncoated microcrystalline K20 tungsten carbide inserts compared to dry and wet (soluble oil) machining. Experimental Procedure Plain turning experiments were carried out on a Ti-6Al-4V alloy bar on an 11 kw centre lathe by ISO K20 SNMA 120408 type uncoated microcrystalline grade carbide inserts of geometry 6 6 6 6 15 75 0.8 (mm) with an edge radius in the range of 33 mm under dry, wet and cryogenic cooling (in the form of liquid nitrogen jets) environments. Liquid nitrogen jets impinged on the tool rake and flank surfaces using a specially designed nozzle. Figure 18 shows the liquid nitrogen delivery nozzle. Machining tests were carried out at cutting speeds of 70, 100 and 117 m=min. The feed and depth of cut employed were 0.20 mm=rev and 2.0 mm, respectively. Machining was interrupted at regular intervals to measure and study the growth of flank wear with machining time under all the machining environments. Flank wear and edge depression were measured using an inverted metallurgical microscope fitted with a measuring micrometer of 1 mm. The cutting inserts were also examined under Scanning Electron Microscope (SEM). RESULTS AND DISCUSSION Tool wear in machining titanium alloys is reportedly due to adhesiondissolution diffusion of tool material into the flowing chip at the tool-chip interface. The temperature at the cutting zone, even at moderate cutting velocities, is generally around 900 C. At such high temperature, titanium FIGURE 18 Liquid nitrogen delivery nozzle for machining Ti-6Al-4V.
Cryogenic Cooling in Machining and Grinding 113 chip maintains intimate contact with the tool on the rake face and flank surface through an interfacial layer that is evidenced by heavy secondary deformation of the under side of the chip as seen in Figure 17. The high temperatures prevailing in the cutting zone enhances the chemical reactivity of titanium with the tool material and causes transport of tool material into the adherent part of the layer of chip through adhesion-dissolution-diffusion. At low cutting velocities, the adherent layer might protect the tool, as the tool-chip interface temperature might not reach the levels that increase the chemical reactivity of titanium. However, at high cutting velocities the work piece-tool relationship at the cutting zone will significantly alter and prolonged machining at high cutting velocity will lead to increase in thickness of the interface layer on the rake face on account of increased chemical reactivity of titanium. At some point, this layer of adherent material is torn from the tool and transported by the flowing chip underside. This inevitably leads to pulling out of the grains from tool resulting in aggressive crater wear. It can be seen from the SEM pictures of the inserts in Figure 19 that the width of the crater is narrow unlike the crater wear observed when machining of steels (Figure 14). Also in machining of steels, the crater starts at a short distance from the cutting edge, but as can be seen in the SEM pictures, in machining of Ti-6Al-4V alloy with an uncoated carbide tool, the crater wear almost meets the flank wear at the tool cutting edge (Figure 20). It is also worth noting that the crater also extends up to the auxiliary cutting edge. This narrow crater width can be attributed to the short chip-tool contact length inherent to the machining of titanium and its alloys as a result of which, the cutting heat and temperature are distributed over a localised area along the cutting edge. This invariably confines the chemical wear phenomenon previously explained, close to the cutting edge. And as a result, the crater wear rapidly increases in the vicinity of the cutting edge. At the same time, the work piece-tool interaction at the tool flank surface is more of an intimate bulk rubbing of the surfaces leading to attrition-abrasion of the tool flank. FIGURE 19 SEM picture of worn out tool while turning Ti-6Al-4V at a cutting speed of 70 m=min.
114 S. Paul and A. B. Chattopadhyay FIGURE 20 Crater profile on the insert showing the crater extending up to the cutting edge in machining of Ti-6Al-4V alloy. Apart from the previously discussed nature of wear, micro or macro fracture of the principal and=or auxiliary cutting edge and tool nose can also occur due to the highly dynamic cutting forces arising from segmented chip formation, an inherent characteristic of machining titanium and its alloys. Further, there has been plastic deformation of the cutting edge along with crater wear leading to edge depression. It can be seen from the SEM views of the worn tool inserts that, as expected, both the rake and the flank wear increased with increase in cutting velocity under all the three machining environments. Adherent deposits of chip material can be found on the rake surface in all cases, an indication of the intimate adhesion between the chip and the tool rake surface. However, the wear on the tool flank as can be seen is mainly due to abrasion and physical attrition. Further, note that the nose of the tools underwent substantial wear in all cases. Another interesting feature that can be noted is the flaking of the tool material on the outer edge of the crater at approximately half the depth of cut from the tool nose. While this phenomenon occurred in all the machining trials (Figures 19, 21 and 22) under both wet and cryogenic cooling, it is noticeably absent under dry machining at 70 m=min. The probable cause appears to be thermal shocks under wet and cryogenic machining. With increase in cutting velocity, flaking occurred even under FIGURE 21 SEM picture of worn out tool while turning Ti-6Al-4V at a cutting speed of 100 m=min.
Cryogenic Cooling in Machining and Grinding 115 FIGURE 22 SEM picture of worn out tool while turning Ti-6Al-4V at a cutting speed of 117 m=min. dry machining conditions possibly due to strong adhesion between the chip and the tool rake as a result of the high temperature generated and subsequent tearing of the tool material by the chip underside. When the present Ti-6Al-4V alloy was turned at low cutting velocity of 70 m=min, the use of soluble oil could reduce the rate of tool wear to some extent but cryogenic machining reduced tool wear remarkably as can be seen in Figure 19. When the cutting velocity was raised to 100 m=min and 117 m=min (Figure 21 and Figure 22), not only did the tool wore out much faster but also the favourable effect of wet and cryogenic cooling decreased. The cooling rate possibly could not cope with the rapid generation of heat at the more intimate contact zone during the high speed machining. An alternative explanation is that the bulk plastic contact at the chip-tool interface may have prevented the coolant from reaching the chip-tool interface. It is clear from the SEM views of the worn-out tools that the beneficial effect of cryogenic cooling has been more effective in mitigating flank wear rather than crater wear. Figure 23 shows the growth of different flank wear features namely, average flank wear, V B, maximum flank wear, V M, average nose wear, V S and edge depression, E with machining time under different machining environments. For a set tool life criteria of average flank wear V B ¼ 300 mm, at 70 m=min the tool life obtained under dry machining is around 7 minutes against 14 minutes obtained under wet machining. But cryogenic cooling offered substantial improvement in tool life with 24 minutes of machining. Such a reduction in wear is likely due to the reduction in the extent of the temperature sensitive wear phenomena of diffusion and adhesion enabled by the direct and indirect cooling of the liquid nitrogen jets. As the cutting velocity was increased to 100 m=min and 117 m=min, the benefits of cryogenic cooling also decreased (Refer to Table 4). At higher cutting velocity of 117 m=min, the wear grew much faster due to higher temperature and faster plastic deformation and grain pull-out but even under such conditions, tool wear was substantially retarded by cryogenic
116 S. Paul and A. B. Chattopadhyay FIGURE 23 Growth of flank wear while turning Ti-6Al-4V under V C ¼ 70 m=min and S O ¼ 0.20 mm=rev under the three cooling environments. cooling (Figure 24). Reduction in the cutting temperature and the tool wear while turning Ti-6Al-4V has also been reported by other researchers (44, 46 47). TABLE 4 Tool Life Under Different Machining Environment While Turning Ti-6Al-4V Tool life in min Cutting velocity (m=min) Dry Wet Cryogenic cooling 70 6.5 13.6 23.5 100 3.0 3.0 5.0 117 1.5 1.5 3.4
Cryogenic Cooling in Machining and Grinding 117 FIGURE 24 Crater depth profiles after turning Ti-6Al-4V alloy at a feed of 0.20 mm=rev under dry wet and cryogenic cooling environments. INFERENCE Irrespective of the machining environments, the following were observed, which are characteristics of machining of Ti-6Al-4V alloy.. The chip-tool contact length is very small compared to machining of steels. Such small contact length increases machining temperature and enhances the chances of adhesion-dissolution-diffusion wear on the tool. There has been flaking of the rake surface just at the end of the crater wear region, especially under wet and cryogenic machining condition. This is attributed to the higher thermal gradient at the end of crater contact under wet and cryogenic cooling. The cutting edges underwent micro and macro fracturing along with plastic deformation. Such wear of cutting edge modified the effective tool geometry due to the edge depression of the cutting edge. Adherent chip material was visible throughout the crater surface indicating severe adhesion between the chip and the rake surface of the tool. The appearance of the crater surface is smooth but the extent of crater wear (crater depth) is high indicating a adhesion-dissolution-diffusion mechanism for crater wear. The flank wear region was not smooth, rather abrasive wear marks were visible. Thus the major tool wear mechanisms in turning Ti-6Al-4V alloy with uncoated carbide are adhesion-dissolution-diffusion wear at the crater along with abrasive and chemical attrition wear at the flank. Moreover there is also micro and macro fracture along the cutting edges along with plastic deformation leading to depression of the cutting edge.
118 S. Paul and A. B. Chattopadhyay. Cryogenic cooling by liquid nitrogen hindered the growth of tool wear very effectively at a moderate cutting velocity of 70 m=min enhancing the tool life from 7 minutes under dry machining and 14 minutes under wet machining to 24 minutes. However the benefit (of cryogenic cooling) decreased under high velocities of 100 and 117 m=min possibly due to improper penetration of liquid nitrogen in the chip-tool interface. GRINDING OF STEELS WITH CRYOGENIC COOLING Experimental Investigation Grinding has already been identified as a process with high specific energy requirement, which leads to high grinding zone temperature quite often in excess of 1000 C. Such a high grinding temperature, if not controlled properly, leads to poor surface integrity and product quality. Grinding is typically a finishing process. Thus a lot of value addition has already occurred on the component concerned. Thus, if the component is rejected and scraped, due to poor surface finish or an inappropriate residual stress introduced during grinding then the profitability of the manufacturing industry would severely affected. Thus development of appropriate temperature control strategy in grinding is very essential. Detailed systematic experimental investigations (9 14, 42) were undertaken to study the effect of cryogenic cooling by liquid nitrogen jets in plunge surface grinding of five commonly used steels in industry by aluminium oxide grinding wheel vis-à-vis dry and wet (conventionally applied soluble oil in water 1:20 dilution). The different grindability indices studied were:. chip morphology. grinding force and grinding specific energy. grinding temperature. residual stress. surface finish and surface topography. grinding wheel wear and wheel loading The jet of nitrogen was delivered at the grinding zone, as shown in Figure 25, through a specially designed nozzle of nominal diameter 4.5 mm at a pressure of 3.5 bar, which impinges on the surface of the job from a distance of 40 mm and at an angle of 25 from the plane of grinding. An externally pressurised liquid nitrogen dewar was used for application of the cryogen. Table 5 provides the detailed experimental conditions.
Cryogenic Cooling in Machining and Grinding 119 FIGURE 25 Schematic representation of liquid nitrogen delivery system. During plunge surface grinding, the work pieces were accommodated on a specially designed work holding device, which was mounted on a Kistler 1 3 axis piezoelectric dynamometer. The grinding forces in the tangential and normal direction were continuously monitored by recording TABLE 5 Machine tool Grinding wheel Spindle speed Wheel speed Table speed Infeed Environment Dresser Dressing depth Dressing lead Dressing speed Work material Work size Experimental Conditions for Grinding of Steels with Cryogenic Cooling Jung horizontal spindle surface grinder, 2.2 kw A60K5V, (/ 150 13 32 mm) 3000 rpm 23.5 m=s 8 m=min 10 mm 40 mm in steps of 10 mm Dry Flood cooling with soluble oil (1:20) Wet Cryogenic cooling liquid nitrogen jet 1 carat single point diamond dresser 5 mm and 10 mm 80 mm and 160 mm 3000 rpm Low carbon steel AISI 1020 MS High carbon steel AISI 1080 Rc 32 HCS Cold die steel D2 Rc 43 CDS Hot die steel H11 Rc 53 HDS High speed steel M2 Rc 64 HSS 50 mm 8 mm (9 mm for HSS) 25 mm
120 S. Paul and A. B. Chattopadhyay FIGURE 26 Grinding chips obtained while grinding (a) MS, (b) HCS, (c) CDS, (d) HDS and (e) HSS under dry, wet and cryogenic environments. them on a multi-channel recorder. Grinding chips were also collected during grinding under all the three environments. Figure 26 depicts the chip morphology obtained while grinding all the five steels under dry, wet and cryogenic grinding environment. Typically four different types of chips are obtained in grinding, namely, the sheared chips (which almost looks like machined chips under microscope), ploughed chips of leafy and irregular shape, spherical chips and small fragmented chips mainly produced during grinding hard and brittle materials. Figure 26 reveals all the different types of grinding chips. On application of cryogenic cooling, the numbers of spherical chips were significantly reduced indicating possibly a reduced grinding zone temperature and post-cooling of the chips by the liquid nitrogen jet. Further, more sheared chips were obtained upon cryogenic cooling. As the hardness of the work material increased and cryogenic cooling has been employed, significantly smaller fragmented chips have been obtained indicating change in material removal mechanism from shearing and ploughing to more fragmentation of the work material ahead of the abrasive grits. Typically the grinding reached steady state after 5 to 6 passes. Under steady state grinding, the actual infeed approaches the given infeed for all practical considerations resulting in a steady level of grinding forces. Once steady state has been reached, all grindability indices are measured, estimated and evaluated. Grinding forces are recorded as the average force
Cryogenic Cooling in Machining and Grinding 121 FIGURE 27 Variation in forces and specific energy with infeed while grinding CDS under dry, wet and cryogenic environment with fine and coarse dressed alumina wheel. over 5 passes after achieving steady grinding state. Figure 27 depicts the typical variation in tangential and normal grinding forces along with the grinding specific energy plotted against infeed while grinding cold die steel under all three environments with fine and coarsely dressed alumina wheels. As expected the fine dressed wheels resulted in higher forces and grinding specific energy irrespective of environment as compared to coarse dressed wheels because fine dressing introduces some wear flats initially to the wheels. Further, the grinding specific energy reduced with increase in infeed as the percentage contribution from ploughing, primary and secondary rubbing and wear flat rubbing diminishes with increase in infeed. Application of cryogenic cooling significantly reduced the grinding forces and specific
122 S. Paul and A. B. Chattopadhyay energy requirement as cryogenic cooling retained the sharpness of the grits due to the desirable temperature conditions yielding reduced grinding forces. Similar observations have also been recorded for other steels. The grinding temperature has been already identified as one of most important grindability indices. Grinding temperature has been measured with the help of an in situ dynamic thermocouple technique as schematically illustrated in Figure 28 and described in detail in Paul and Chattopadhyay (12). Thermal modelling of grinding has also been carried out using numerical approaches (12). Figure 29 depicts the typical variation in temperature with time while grinding cold die steel under all three environments as measured by the dynamic thermocouple as well as estimated by finite element modelling {solid line measured temperature and chain-dotted line FEM estimation}. These temperature measurements and numerical estimations were carried for all other combinations of process parameters, work materials and environments. Comparison between the measurement and numerical estimation of temperature indicates acceptable agreement, though frequently the estimated grinding temperature was higher than the measured value. Both measured and estimated temperature curves show the classic rise and fall FIGURE 28 Dynamic in situ thermocouple technique for grinding temperature measurement.
Cryogenic Cooling in Machining and Grinding 123 FIGURE 29 Variation in temperature with time while grinding CDS (solid line measured temperature and chain-dotted line FEM estimation).
124 S. Paul and A. B. Chattopadhyay of temperature along the length of contact irrespective of grinding environment. However, the maximum grinding temperature was reduced substantially on application of liquid nitrogen jet, which can be attributed to the drastic cooling of the grinding zone. Grinding temperatures have been determined from graphs similar to Figure 29 for all the possible combination of experimental parameters and the variation in grinding temperature with infeed for all the five work materials and three environments have been depicted in Figure 30. As expected, the grinding temperature increases with increase in infeed as the total heat and heat flux entering the work piece increase. Application of conventional soluble oil seems to reduce the grinding temperature to some extent for all the work materials undertaken due to bulk convective cooling effect but cannot provide the control of grinding temperature desired. However, application of liquid nitrogen in the form of directed jet at the grinding zone substantially reduces the grinding temperature primarily due to the drastic cooling action of liquid nitrogen jet with a bulk temperature of 196 C. Further cryogenic cooling also reduced the grinding forces to some extent that again helps in reducing temperature by controlling heat flux entering the workpiece. Such a reduction in grinding temperature is expected to provide significant benefits with respect to surface integrity. Surface integrity is evaluated with respect to different characteristics of ground workpiece. Some of them are metallurgical changes to the ground component, ground surface topography and most importantly by residual stresses induced upon grinding. Residual stresses were measured using X-ray diffraction technique using the two inclination method (11) and they were also estimated using elasto-plastic formulation of finite element methodology (11). Figure 31 shows the variation in surface residual stresses with infeed ground under all the environments. An increase in infeed has already been observed to provide an increase in grinding temperature, which in turn predominantly determines the level and nature of residual stress on the ground surface, though the mechanical grinding action of the abrasive grits and austenite-martensite transformation during cooling of the workpiece after grinding also affects the state of residual stress but to much smaller extent. Thus the level of tensile residual stress increased substantially with increase in infeed in all the 5 work materials under all environments. Wet grinding, though providing some temperature reduction, failed to provide significant benefit with respect to residual stress. The application of cryogenic cooling, on the other hand, effectively controlled the tensile residual stress throughout the experimental domain, a result which can be attributed to the desirable grinding temperature control.
Cryogenic Cooling in Machining and Grinding 125 FIGURE 30 Variation in grinding temperature with infeed for different steels under dry, wet and cryogenic cooling environments.
126 S. Paul and A. B. Chattopadhyay FIGURE 31 Variation in residual stress of ground components of different steels with infeed under dry, wet and cryogenic cooling environments.
Cryogenic Cooling in Machining and Grinding 127 The ground surface topography was studied under scanning electron microscope and the SEM pictures are shown in Figure 32. As the work material hardness increased the ground surface experienced less plastic deformation and redeposition, two characteristics that are significant when grinding low carbon steel (MS) and high carbon steel (HCS) under dry and FIGURE 32 Ground surface topography under dry, wet and cryogenic cooling environments as revealed by scanning electron microscope.