CHAPTER 4 APPLICATION OF SEMISOLID LUBRICANTS FOR IMPROVING RAKE FACE LUBRICATION 4.1 INTRODUCTION During minimal fluid application, since only a

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1 CHAPTER 4 APPLICATION OF SEMISOLID LUBRICANTS FOR IMPROVING RAKE FACE LUBRICATION 4.1 INTRODUCTION During minimal fluid application, since only a very small quantity of cutting fluid is used for the dual purpose of cooling and lubrication, some additional system of lubrication if available, will further improve the cutting performance. It is reported that semi-solid lubricants can be effectively used during metal cutting to achieve better cutting performance and it forms an alternative to the conventional flood cooling techniques (Vamsi Krishna and Nageswara Rao, 2008). There is a great potential for enhancing cutting performance during minimal fluid application with the aid of solid lubricants. Hence, it was decided to explore whether the application of semisolid lubricant along with minimal fluid application can improve cutting performance. An attempt was made to investigate the effect of a semi solid lubricant such as grease in pure form as well as a mixture with 10% graphite on the cutting performance during hard turning of AISI 4340 steel with minimal fluid application and a comparison was made with wet and dry turning under similar cutting conditions. 4.2 SILICON GREASE AS A SEMI SOLID LUBRICANT Grease is a semi solid lubricant which is composed of calcium, sodium or lithium soap base emulsified with mineral or vegetable oils. It is used in high pressure applications and during metal cutting where liquid lubricants cannot be retained. Greases are shear-thinning or pseudo-plastic fluids, which undergo reduction in viscosity under shear. Greases are employed where heavy pressures exist, where oil drip is undesirable, and/or where the motions of the contacting surfaces are discontinuous so that it is difficult to maintain a separating lubricant film in the contact zone. Grease-lubricated bearings have greater frictional characteristics at the beginning of operation. Under shear, the viscosity drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. 78

2 In this research work, commercially available bearing grease having the specification LGWA 2 (DIN 51825) was used as a semisolid lubricant in its pure form and as a mixture with 10% graphite. It is a high load, wide temperature range bearing grease and being recommended for a wide range of industrial and automotive applications. Properties of LGWA 2 bearing grease are summarized below. Excellent lubrication at peak temperatures up to 220 C for short periods. Effective lubrication in wet conditions Good water and corrosion resistance Excellent lubrication under high loads and low speeds Graphite is widely used as a solid lubricant because of its low cost and excellent lubricating action on account of its layered structure. Density of graphite is g/cm 3 and its Mohs hardness ranges from 1.85 to Figure 4.1 shows the crystalline structures of graphite. The inter-planar spacing, i.e., the distance between the adjacent interlayer for graphite is 3.35A. In graphite, the inter layer bonding is very weak and one layer slides over the other under the application of shear loads. Figure 4.1 Crystalline structure of graphite 79

3 4.3 DEVELOPMENT OF SEMI SOLID LUBRICANT APPLICATOR A semi solid lubricant applicator was developed for applying silicon grease at specific locations. Figure 4.2 shows a line sketch of the applicator. It consists of a cylindrical container (C) with a piston (P) inside which can move forward against the force of a stabilizing spring (S). When compressed air enters in to the cylinder through the inlet (I), it forces a certain amount of semisolid lubricant through the outlet (O) on the lid (L) of the semisolid container (C). The semisolid lubricant coming out of the outlet (O) moves through the tube (T). A nozzle is fixed at the free end of the tube which can deliver grease at specific contact zones. The rate of delivery of the grease can be controlled by the control valve (V). A relief valve is installed in the circuit to protect the system from accidental overloads in the event of blocks in the nozzle. Fine adjustment of the rate of flow of the semisolid lubricant can be achieved by adjusting the spring tension. This is done by rotating the container lid in the proper direction. When the lid is rotated in the clockwise direction, the spring gets compressed offering more resistance to the motion of the piston (P) and thereby reducing the rate of flow of the semisolid lubricant. Likewise rotation of the lid in the anti-clock wise direction increases the rate of flow of the semisolid lubricant. Figure 4.3 presents a photograph of the semisolid lubricant applicator. Fixtures were designed to locate the semisolid lubricant applicator at three desired locations as shown in Figure EXPERIMENTATION Cutting experiments were carried out on a Kirloskar Turn master-35 lathe. AISI 4340 steel with hardness of 45 HRC was used as work material. Multicoated hard metal inserts with a specification of SNMG was used as cutting tool. A specially formulated cutting fluid (Varadarajan et al., 2002b) was used as the cutting fluid during minimal fluid application and was applied as a pulsing slug at the tool work interface. The pressure at minimal fluid applicator was kept at 80 bar and the frequency of pulsing was maintained as 300 pulses /min. Using the pneumatic semisolid lubricant applicator, semi solid lubricant was applied at the rate of 25 grams/min at the tool-chip interface, tool-work interface and at the top 80

4 side of the chip as shown in Figures 4.4(a), 4.4(b) and 4.4 (c) respectively. A photograph of the experimental set up is shown in Figure.4.5. An 18 run experiment was designed to determine the effect of application of semi solid lubricants on cutting performance. During the experiment, cutting speed, feed and direction of application of semisolid lubricant were varied at three levels as in Table 4.1. Parameters which were kept constant during the experiment are shown in Table 4.2. Figure 4.2 Line sketch of semi sold lubricant applicator Figure 4.3 Photograph of semi sold lubricant applicator 81

5 Figure 4.4 Application of semi solid lubricant at three different locations, (a) toolchip interface (D1), (b) tool-work interface (D2), (c) top side of the chip (D3) Figure 4.5 Photograph of experimental set up for investigating the influence of semisolid lubricant on cutting performance 82

6 Table 4.1 Process variables and their values Factor Level 1 Level 2 Level 3 Cutting velocity (m/min) Feed (mm/rev) Mode of lubrication Direction of semisolid lubricant Minimal fluid application (L1) Tool-chip interface (D1) (Figure 4.4(a)) Minimal Fluid application with semisolid lubrication (L2) Tool-work interface (D2) (Figure 4.4(b)) Minimal Fluid application with graphite impregnated semisolid lubrication (L3) Back side of chip (D2) (Figure 4.4(c)) Table 4.2 List of parameters that were kept constant and their values Parameters Values Rate of fluid application 5 ml/min Frequency of pulsing 300 pulses/min Pressure at the fluid applicator 80 bar Composition of cutting fluid 10% concentrate Rate of semi solid lubricant application 25 grams/min Depth of cut 0.5mm The performance parameters such as surface roughness, main cutting force, cutting temperature and the average flank wear were measured during each trial. A stylus type perthometer was used for measuring surface roughness. The cutting force was measured using a Kistler type lathe tool dynamometer. The cutting 83

7 temperature was measured using an extrapolative prediction method (Varadarajan et al., 2000) and the average flank wear was measured using a tool maker s microscope. In order to ensure the reliability of the results, all experiments were repeated three times, and the average of these measurements was taken as the final value. Observations during the experiment are summarized in Table 4.3. The relative significance of the operating parameters was determined by response table methodology using Qualitek-4 Software. ANOVA analysis was carried out to assess the percentage influence of the individual parameters on cutting performance. 4.5 RESULTS AND DISCUSSION Figure 4.6 presents the relative significance of operating parameters on the main cutting force. Figure 4.7 presents the relative significance of operating parameters on cutting temperature. The relative significance of surface finish, tool wear and tool-chip contact length are presented in Figures 4.8, 4.9 and 4.10 respectively. Table 4.4 presents the summary of the analysis carried out using Qualitek-4. It presents a set of levels of operating parameters for achieving minimum cutting force, minimum surface roughness, minimum cutting temperature, minimum tool wear, minimum tool-chip contact length and maximum surface finish. From Figures 4.6 to 4.10, it is observed that the direction of application of semi solid lubricant forms the most significant parameter influencing the cutting performance in terms of main cutting force, cutting temperature, surface finish, tool wear and tool chip contact length. Among the type of semisolid lubricants, it is seen that the application of semisolid lubricant impregnated with graphite in accompaniment with minimal fluid application brought forth the least cutting force when compared to conventional minimal fluid application. It was also observed that application of semisolid lubricant at the tool chip interface in accompaniment with minimal fluid application corresponding to D1 brought forth lower cutting force when compared to the other two directions. 84

8 Trial No. Cutting velocity (m/min) Feed (mm/rev) Table 4.3 Observations during 18 run experiment Mode of Lubrication Direction of semisolid lubricant Cutting Force (N) Cutting Temp (ºC) Surface Finish (µm) Tool-chip contact length (mm) Tool wear (mm) L1 D L2 D L3 D L1 D L2 D L3 D L2 D L3 D L1 D L3 D L1 D L2 D L2 D L3 D L1 D L3 D L1 D L2 D

9 Figure 4.6 Relative significance of operating parameters on Cutting force Fig. 4.7 Relative significance of operating parameters on cutting temperature 86

10 Figure 4.8 Relative significance of operating parameters on surface finish Figure 4.9 Relative significance of operating parameters on tool wear 87

11 Figure 4.10 Relative significance of operating parameters on tool -chip contact length Table 4.4 Levels of operating parameters for optimum performance Desired Outcome Direction of Cutting Feed Mode of application velocity (mm/rev) lubrication of semisolid (m/min) lubricant Low Cutting Force L3 D1 Low Cutting Temperature L3 D1 Better Surface Finish L3 D1 Minimum Tool Wear L3 D1 Min.Tool Chip Contact Length L3 D1 The cutting fluid particles entering at the tool-work interface can reach the tool-chip interface through the micro cracks that exist on the work near the tool tip (as explained earlier). But extreme thermal conditions that prevail at the tool-chip interface can adversely affect the lubricating ability of the cutting fluid. But when 88

12 the solid lubricant was applied at the tool-chip interface, it takes the latent heat of fusion from the tool-chip interface.this reduces the severity of the thermal conditions that prevail at the tool chip interface and prevents the complete degradation of the lubricating properties of the cutting fluid particles present at the tool-chip interface. The graphite particles present in the semisolid lubricant further reduces friction at the tool-chip interface. Moreover a mixture comprising of cutting fluid particles, molten semi solid lubricant and the graphite particles act as a dielectric that prevents intermolecular and inter atomic interaction between the chip and the tool surfaces. This prevents adhesion of the chip to the tool surface and changes the conditions prevailing at the tool-chip interface from sticking to one of sliding leading to drastic reduction in cutting force and reduces tool-chip contact length which further reduces the cutting force. Reduction in frictional forces brought about by better rake face lubrication can bring forth reduction in cutting temperature, reduction in tool wear and improvement in surface finish. When the cutting fluid was applied at the tool work interface, some quantity of the cutting falls on the uncut work surface which forms the top side of the chip during the next rotation (Figure 4.11). The top side of the chip is characterized by myriads of micro cracks with nascent crack tips. In normal case, the micro cracks can coalesce due to intense surface interaction. When they coalesce, the chip becomes stronger and shows a tendency to bend towards the rake surface which leads to increased tool-chip contact length and associated increase in the main cutting force, tool wear, and surface roughness. But when tiny droplets of cutting fluid get adsorbed on the top side of the chip owing to their small size and high velocity, they dope the nascent surfaces generated and prevent the coalesce of crack tips. This leads to the weakening of the top side of the chip and the chip tends to bend away from the tool rake face resulting in reduction of tool-chip contact length and associated benefits such as lower cutting force, lower tool wear, and lower cutting temperature. 89

13 Figure 4.11 Presence of fluid particles on the uncut surface forms the top side of the chip When semisolid lubricant was applied at the tool-chip interface it takes the latent heat of fusion from the tool chip interface and reduces the severity of the thermal conditions that prevail there and prevents the complete degradation of the lubricating properties of the cutting fluid particles presents at the tool chip interface. The effectiveness of heat transfer on the tool rake face depends on the duration for which the agency that removes heat remains in contact with the surface. More the time of contact, more will be the effectiveness of heat transfer. Since the semisolid lubricant can stick on the contact surface it remains there for a longer duration than is possible for a droplet of cutting fluid and extracts more heat from the rake face and preserve the lubricating capabilities of the fluid particles that reach the tool-chip interface via the capillaries on the work surface near the tool tip. This enhanced lubricity on the rake face reduces cutting force, cutting temperature and hence tool wear. Moreover, the mixture consisting of fluid particles and traces of molten semisolid lubricant can act as a dielectric preventing surface interaction as explained in the previous section. Reduction of surface interaction between the surfaces of the tool and the back side of the chip can further reduce the tool-chip contact length and improve the cutting performance (Figure 4.12 (a) and (b)). 90

14 Tool-chip contact length (a) Tool-chip contact length (b) Figure 4.12 Tool chip contact length (a) in the absence of semisolid lubricant, (b) in the presence of semi solid lubricant, L2<L1 91

15 Table 4.5 Comparison of performance during dry turning, wet turning and turning with minimal fluid application Turning MFA with with silicon Dry Wet Minimum grease Desired Outcome Turning Turning Fluid impregnated Application with 10% (MFA) graphite Cutting Force (N) Cutting Temperature(ºC) Surface Finish(µm) Tool Wear(mm) Tool Chip Contact Length (mm) (V=80 m/min, F=0.07 mm/rev, DOC=0.5 mm, Pressure of pulsing=80 bar, Frequency of pulsing=300 pulses/min, Rate of application of silicon grease impregnated with 10% graphite =25 grams/min) Hence, the application of semisolid lubricant at the tool-work interface does not bring forth improvement in cutting performance. Similarly when the semisolid lubricant was applied at the back side of the chip, it facilitates cooling at the back side of the chip and promotes the chip curl leading to reduction in tool-chip contact length. But the presence of semisolid lubricant on the top side of the chip does not contribute much to the reduction of friction at the tool-chip interface. But when the semi solid lubricant application was done at the tool-chip interface and the minimal fluid application at the tool-chip interface (Figure 4.4(a)), the mechanism responsible for reduction of friction at the tool chip interface and the mechanism that is responsible for bending of the chip away from the tool rake face become active simultaneously as explained earlier. The cumulative impact of the two mechanisms can bring forth improvement in cutting performance. 92

16 Cutting Temperature ( o C) Cutting Force (N) Feed=0.1mm/rev, DOC=1.25mm Cutting Velocity = Variable Cutting Velocity (m/min) Dry turning Wet turning Conventional minimal fluid application Semis solid lubricant application Figure 4.13 (a) Variation of cutting force with cutting velocity 600 Feed rate=0.1mm/rev DOC = 1.25mm Cutting Velocity =Variable Cutting Veocity(m/min) Dry turning Wet turning Conventional minimal fluid application Semis solid lubricant application Figure 4.13 (b) Variation of cutting temperature with cutting velocity 93

17 Cutting Temperature ( o C) Cutting Force (N) Cutting Velocity =80m/min DOC = 1.25mm Feed rate = Variable Feed rate(mm/rev) Dry turning Wet turning Conventional minimal fluid application Semis solid lubricant application Figure 4.14 (a) Variation of cutting force with feed rate Cutting Velocity = 80m/min DOC = 1.25mm Feed rate = Variable Feed rate (mm/rev) Dry turning Wet turning Conventional minimal fluid application Semis solid lubricant application Figure 4.14 (b) Variation of cutting temperature with feed rate Comparison of the cutting performance during dry turning, conventional wet turning and hard turning with minimal fluid application in the presence of semisolid lubricant impregnated with graphite is available in Table 4.5. Cutting performance during hard turning with minimal fluid application in the presence of silicon grease impregnated with graphite was compared with dry, wet and 94

18 conventional minimal fluid application by conducting variable speed and variable feed tests at the optimal cutting condition and the results are presented in Figures 4.13 (a), 4.13 (b), 4.14 (a) and 4.14 (b). Further improvement in cutting performance was noticed (in Figure (a) to (d)) when the minimal fluid application was carried out along with application of silicon grease impregnated with graphite. The improvement in cutting performance is attributed to the enhanced lubricity at the tool chip interface offered by the graphite on account of its structure as described in section 4.2. (a) Dry Turning (b) Turning with MFA (c) MFA with Grease (d) MFA with Grease mixed with 10% Graphite Figure 4.15 SEM photograph of worn out inserts during (a) dry turning, (b) conventional turning with minimal fluid application, (c) turning with minimal fluid application in the presence of silicon grease applied at the tool-chip interface and turning with minimal fluid application with silicon grease impregnated with 10% graphite applied at the tool-chip interface under identical cutting conditions (V=80 m/min, f=0.07 mm/rev and DOC=0.5 mm) 95

19 Figures 4.15 (a), 4.15 (b), 4.15 (c) and 4.15 (d) present the SEM photograph of worn inserts during pure dry turning, conventional minimal fluid application and minimal fluid application in the presence of silicon grease and silicon grease impregnated with graphite. It was observed that damage on the tool was minimum during turning with minimal fluid application in the presence of silicon grease impregnated with 10% graphite applied at the tool-chip interface. 4.6 SUMMARY 1. It was observed that the introduction of silicon grease at the rate of 25 grams/min at the tool-chip interface improved cutting performance during hard turning with minimal fluid application. There was 14 % reduction in cutting force, 14% reduction in surface roughness and 3 % decrease in cutting temperature when compared to conventional minimal fluid application. 2. When silicon grease was impregnated with 10% graphite, there was further improvement in cutting performance. There was 20% reduction in cutting force, 49% reduction in tool wear, 23% reduction in surface roughness and 4% decrease in cutting temperature when compared to conventional minimal fluid application. 3. The present study illustrates the technique of application of semi solid lubricants in accompaniment with minimal fluid application as a potential performance enhancer for hard turning with minimal fluid application. 96