Performance evaluation of minimal cutting fluid application by vegetable oil based cutting fluid in hard turning of AISI4340 steel
Abstract.
Turning of hardened steel is normally carried out with copious supply of cutting fluid to improve the cutting performance. Most of the cutting fluids in regular use are petroleum based emulsions which create several environmental problems. In this context, pure dry milling is a logical alternative as it is free from the problems associated with the cutting fluid. On the other hand, achievable tool life and part finish are often affected while machining under completely dry condition. Under such situation, the concept of minimal cutting fluid application (MCFA) presents itself as a possible solution for hard turning. In this study an effort was made to study the effect of vegetable oil based cutting fluid with minimal cutting fluid application on hard turning of AISI4340 steel. Accordingly, soya bean oil based emulsion was prepared by adding important additives. A detailed analysis was performed using Taguchi technique to find out the effect of fluid application parameters on cutting force and surface roughness. The study showed promising results in terms of reduction in cutting force and improvement in surface finish.
Introduction
Cutting fluids play an important role during metal cutting. The cooling and lubrication effect provided by the cutting fluids help in reducing the heat developed due to high friction at the cutting zone. In the hard turning process, there is a high urge to reduce cutting temperature, cutting force and friction so as to reduce tool wear and to improve surface finish. Hence hard turning operation is normally performed under conventional wet cooling.
Besides providing technological benefits, conventional cutting fluids pose the following environmental problems (i) pollute the shop floor due to chemical break-down of the cutting fluid at high cutting temperature; (ii) creates biologically hazardous environment to operators due to bacterial growth; (iii) requires additional system for pumping, local storage, filtration, recycling and chilling (iv) water pollution and soil contamination during final disposal [1]. People exposed to large quantities of cutting fluids may have skin contact and they may inhale or swallow the mist particles of cutting fluid. The additives present in the petroleum based cutting fluids may cause dermatitis, problems in the respiratory and digestive systems and even cancer due to their toxicity [2].
Handling of cutting fluid may include the pre-treatment and treatment of cutting fluid wastes. The cost of fluid pre-treatment/treatment is sometimes higher than the purchase price of the cutting fluid itself [3]. Enormous usage of cutting fluid in the shop floor increases the presence of oil content in the air which should be kept within the prescribed regulations suggested by occupational safety and health administration [4]. It is found that Europe alone consumes approximately 320,000 tons per year of metal working fluids, out of which at least two thirds need to be disposed [5].
The problems associated with cutting fluids can be completely avoided by using dry machining. But it is very difficult to implement on the existing shop floor as it needs extremely rigid machine tools and ultra hard cutting tools [6]. In order to alleviate the above-mentioned negative effects of cutting fluids, techniques like Minimal Quantity Lubrication (MQL) and Minimal Cutting Fluid Application (MCFA) have been evolved. Minimum quantity lubrication technique reduces the consumption of cutting fluids to a larger extent. It facilitates drastic reduction in the tool chip interaction and leads to reduction in cutting force, improvement in surface finish and dimensional accuracy [7]. The main limitation of the MQL method is the application of cutting fluid in the form of mist which increases the exposure of hazardous aerosols in the shop floor [8].
In minimal cutting fluid application, extremely small quantities of cutting fluid is injected in the form of ultra fine droplets at very high velocity (about100 m/s) into the cutting zone which is also called as pseudo dry turning. For all practical purposes it resembles dry turning in achieving improved surface finish, lower tool wear by maintaining cutting forces and power at reasonable levels. Another advantage of this method is that the fluid application parameters can be strategically tuned [9].
The present work aims at a systematic investigation on the viability of using vegetable oil based cutting fluid with minimal cutting fluid application technique. It is expected that the use of vegetable oil based cutting fluid integrated with minimal cutting fluid application technique will give a better performance, reduce the harmful effects to the operators, reduce the cost and consumption of cutting fluids in manufacturing industries.
Lot of research work have been reported on cutting performance under flood cooling with mineral oil based cutting fluids. A few research works are reported under minimal cutting fluid application with mineral oil based cutting fluid but no work is reported under minimal cutting fluid application with vegetable oil based cutting fluids. Therefore vegetable oil has been selected in this present investigations because it is bio-degradable, environmental friendly and having good lubricity behaviour [10].
Structure and lubrication properties of vegetable oils
Vegetable oils comprise of primarily triglycerides, which are tri-esters of long chain fatty acids combined with glycerol. These oils normally contain 4 to 12 different fatty acids. The proportion of each fatty acids in a cutting fluid depend not only on the type of the plant but also on the geo-climate and the weather condition. The physico-chemical properties of various vegetable oils are presented in table 1.
Table 1
Physico-chemical properties of vegetable oils
Soya bean High oleic soya bean Sun flower Rapeseed Jatropha Curcass Neem Castor Coconut
Kinematic viscosity at 40??C [cSt] 32.93 41.34 40.05 45.60 47.48 68.03 220.6 36.2
Kinematic viscosity at 100??C [cSt] 08.08 9.02 8.65 10.07 8.04 10.14 19.72 6.76
Viscosity index 219 – 206 216 208 135 220 130
Saponification value
[mgKOHg -1] 189 – – 180 196.8 166 180 248-265
Total acid value
[(mgKOHg -1] 0.61 0.12 – 1.4 3.2 23 1.4 –
Iodine value [mg g-1] 144 85.90 – 104 97 66 87 6-8
Pour Point [0??C] -9.00 – -12 -12 0.00 9 -27 20
Flash point [0??C] 240 – 252 240 240 – 250 240
The triglyceride structure of the vegetable oils provides desirable qualities such as high natural viscosity, high viscosity index and structural stability over reasonably high operating temperature ranges. The flash point of vegetable oils is high which relates to a very low vapour pressure and volatility, thereby eliminating potential hazards during use [11]. The level of unsaturated products present in a cutting fluid decides the oxidation stability of vegetable oils. The lower the level of unsaturation, better the oxidative stability [12]. The long and polar fatty acid chains produce oriented molecular films which strongly interact with the metallic surfaces, reduce both friction and wear. In general, vegetable oils have a poor oxidative and thermal stability when compared to mineral oils. This issue can be set by methods such as reformulation of additives, chemical modification of vegetable oils and genetic modification of oil seed crop [13].
Composition of soya bean oil based cutting fluid
The cutting fluid developed (soya bean emulsion) consisted of soya bean oil with ethylene glycol, oleic acid, triethanol amine and other additives which helped the cutting fluid to withstand its properties when subjected to high temperatures of cutting and provided special binding properties.
Selection of workpiece and tool
AISI4340 steel of hardness 45 HRC with 375 mm length and 70 mm diameter was selected as a work material for this investigation which is widely used in die making, automobile and allied industries. It is a through hardenable low alloy steel which is known for its toughness fatigue strength and tensile strength. The cutting tool inserts and the tool holder were selected as per the recommendations of M/s TaeguTec India (P) Limited who are extending their technical/material support for this research work. The tool insert used for the present experimentation was SNMG 120408 and tool holder used was PSBNR 2525.
Selection of parameters
The selection of parameters for this experimentation was done based on the earlier work reported in the area of machining with minimal cutting fluid application [9]. The selected input parameters were varied at 3 levels. The input parameters are (1) Pressure at the fluid injector (50, 75, 100 bar), (2) Frequency of pulsing (250, 500, 750 pulses/min), (3) Quantity (rate) of application (3, 6, 9 ml/min), (4) Composition of cutting fluid (10%, 20%, 30% of oil in water) and (5) Direction of application (Tool-work interface and back side of the chip together, tool-work interface, backside of the chip). The selected output parameters were surface roughness and cutting force. Based on the earlier work reported [14], cutting parameters and standoff distance (distance between exit of the nozzle and cutting edge) were kept constant during experimentation. Accordingly, cutting velocity, feed rate, depth of cut and standoff distance were maintained at 80 m/min, 0.1 mm/rev, 1.25 mm 40 to 60 mm respectively.
Experimental Set up
Fig. 1 Experimental set up contains lathe and Minimal Fluid application set up.
Fig. 1 shows the experimental set up which consisted of a medium duty Kirloskar lathe which was modified with DC motors to provide variable speed and feed using variable controllers. Cutting force was measured using a Kistler dynamometer of type 9257B. Surface roughness was measured using Mitutoyo (SJ-210) portable surface roughness tester. The minimal cutting fluid setup facilitated the independent variation of pressure at fluid injector, frequency of pulsing and the quantity (rate) of fluid application.
Design of experiment
An 18 run experiment was designed based on Taguchi’s technique [15]. The selected five parameters were varied at three different levels as shown in the table 2.
Table 2
Design matrix for eighteen-run, three-level experiment with five factors
Standard order COLUMN
2 3 4 5 6
1 1 1 1 1 1
2 1 2 2 2 2
3 1 3 3 3 3
4 2 1 1 2 2
5 2 2 2 3 3
6 2 3 3 1 1
7 3 1 2 1 3
8 3 2 3 2 1
9 3 3 1 3 2
10 1 1 3 3 2
11 1 2 1 1 3
12 1 3 2 2 1
13 2 1 2 3 1
14 2 2 3 1 2
15 2 3 1 2 3
16 3 1 3 2 3
17 3 2 1 3 1
18 3 3 2 1 2
In the experimental phase, preliminary experiments were conducted through trial runs. Trial runs helped in fixing the range of parameters. In the second phase, experiments were carried out using Taguchi’s L18 orthogonal array.
Results and discussion
Fig. 2 presents the relative significance of fluid application parameters on attainable surface finish.
Fig. 2 Relative significance of fluid application parameters on surface roughness
(P ‘ pressure at the injector, f ‘ frequency of pulsing, q ‘ quantity of cutting fluid,
C ‘ composition of cutting fluid and D ‘ Direction of application)
ANOVA analysis was also carried out using Qualitek-4 software to find out the percentage influence of individual parameters on surface roughness and cutting force. It was found that the interaction effects were not significant.
Table 3 presents the percentage contributions of the input parameters on surface roughness.
Table 3
ANOVA summary of the input parameters on surface roughness
Col#/ Factor DOF (f) Sum Of Sqrs. (S) Variance (V) F-Ratio (F) Pure Sum (S’) Percent (%)
Pressure [bar] 2 0.125 0.062 55.533 0.122 52.231
Frequency [pulses/min] 2 0.022 0.011 10.054 0.02 8.672
Quantity [ml/min] 2 0.005 0.002 2.65 0.003 1.58
Composition [%] 2 0.012 0.006 5.339 0.009 4.156
Direction 2 0.061 0.03 27.329 0.059 25.218
Error 7 0.009 0.001 8.143%
Total 17 0.007 100.00%
From the ANOVA results, it was evident that pressure at the fluid injector forms the most significant parameter influencing the surface roughness. From Fig. 2, it was seen that pressure at the injector at level – 3 (100 bar), frequency of pulsing at level – 2 (500 pulses/min), quantity of application of cutting fluid at level – 3 (9 ml/min), composition of cutting fluid at level ‘ 2 (20% oil) and direction of application at level ‘ 1(Tool-work interface and back side of the chip together) contributed more on the reduction of surface roughness. The percentage significance of the pressure at the fluid injector on surface roughness was 52.231%.
Fig. 3 presents the relative significance of fluid application parameters on cutting force.
Fig. 3 Relative significance of fluid application parameters on cutting force
(P ‘ Pressure at the injector, f ‘ frequency of pulsing, q ‘ quantity of cutting fluid,
C ‘ composition of cutting fluid and D ‘ Direction of application)
From Fig. 3 it is seen that pressure at the fluid injector at level – 3 (100 bar), frequency of pulsing at level – 2 (500 pulses/min), quantity of application of cutting fluid at level – 3 (9 ml/min), composition of cutting fluid at level ‘ 2 (20% oil) and direction at level-1(Tool-work interface and back side of the chip together) contributed more on the reduction of cutting force.
Table 4 shows the ANOVA summary on percentage contributions of the input parameters on cutting force. From the ANOVA results, it was evident that pressure at the fluid injector forms the most significant parameter influencing the cutting force. The percentage significance of the pressure at the fluid injector on cutting force was 51.579%.
Table 4
ANOVA summary of the input parameters on cutting force
Col#/ Factor DOF (f) Sum Of Sqrs. (S) Variance (V) F-Ratio (F) Pure Sum (S’) Percent (%)
Pressure [bar] 2 13157.785 6578.892 30.582 12727.547 51.579
Frequency [pulses/min] 2 1745.467 872.733 4.056 1315.228 5.33
Quantity [ml/min] 2 811.872 405.936 1.887 381.634 1.546
Composition [%] 2 1013.185 506.592 2.354 582.947 2.362
Direction 2 6441.454 3220.727 14.971 6011.216 24.36
Error 7 1505.834 215.119 14.823
Total 17 24675.6 100.00%
The results of the analysis led to a set of levels of fluid application parameters to minimize surface roughness and cutting force are summarized in the table 5.
Table 5
Summary of fluid application parameters for optimum performance
Output parameter Pressure
[bar] Frequency [pulses/min] Quantity [ml/min] Composition [%] Direction
Surface roughness (Ra) 100 500 9 20 Both
Cutting force (N) 100 500 9 20 Both
Fig. 4 compares the results of surface roughness during hard turning with minimal cutting fluid application using soya bean oil based cutting fluid, hard turning under flood cooling using soya bean oil based cutting fluid, hard turning under flood cooling using mineral oil based cutting fluid and hard turning under dry condition. The investigation clearly showed that soya bean oil based cutting fluid with minimal fluid application yielded better surface finish when compared with the hard turning under cutting conditions.
Fig. 4 Comparison of Surface roughness under different cutting conditions
Fig. 5 shows similar comparison made on cutting force under different cutting conditions. The investigation clearly showed that soya bean oil based cutting fluid with minimal fluid application yielded less cutting force when compared with hard turning under cutting conditions.
Fig. 5 Comparison of Cutting force under different cutting conditions
The following parameter-wise discussion is based on the results available in table 5.
Pressure at the Fluid Injector: The pressure at the fluid injector kept at level ‘ 3 (100 bar) facilitated better surface finish and lower cutting force. The penetration power of the fluid droplet comes out of the nozzle is directly proportional to the exit velocity (approximately 70 m/s) [9] and the velocity of fluid droplet varies as a function of the square root of the injection pressure [16]. The cutting force spent for machining can be directly related to the friction at the tool-chip interface on the rake face [7]. The attempts taken for reducing friction on the rake face brought forth better surface finish, lower cutting force and lower energy consumption. When the pressure at the fluid injector was at level -3, better penetration would be facilitated which led to better lubrication at the contact surfaces and reduced the cutting force and improved the surface finish.
Frequency of Pulsing: It was observed that frequency of pulsing at level ‘ 2 (500 pulses/min) facilitated better cutting performance. It is reported that the frictional forces between two sliding surfaces can be reduced considerably by rapidly fluctuating the width of the lubricant filled gap separating them [17]. For any fixed rate of fluid application, when the frequency of pulsing (f) was 750 pulses/min, the quantity of fluid delivered per pulse becomes very less when compared with the quantity of cutting fluid delivered at the frequency of pulsing at 500 pulses/min. At level-1 (250 pulses/min, the frequency of pulsing might not have sufficient to provide the required fluctuation effect at the lubricant filled gap.
Quantity of cutting fluid: It was observed that the rate of fluid application at level ‘ 3 (9 ml/min) was advantageous in terms of better surface finish and lower cutting force. During minimal fluid application, cooling occurs due to both convective and evaporative heat transfer. The evaporative heat transfer is facilitated during minimal cutting fluid application by the increase in surface area caused by atomization [14].
Composition of cutting fluid: It was seen that composition of cutting fluid comprising of 20% oil and the rest water corresponding to level-2 offered improved cutting performance when compared to the composition with 10% oil. During minimal fluid application, very small quantity of cutting fluid is expected to perform both cooling and lubrication. It appears that a composition consisting of 10% oil is too lean to provide effective rake face lubrication at the tool-chip and tool-work interface. At level ‘ 3 (30% of oil) the viscosity of cutting fluid slightly increases which may affect the fluid flow through the injector nozzle.
Direction of fluid application: When cutting fluid is applied using two nozzles, it improves the performance through reduction of tool-chip contact length and better rake face lubrication whereas when a single nozzle is employed the mechanism namely reduction in tool-chip contact length is in operation [6]. Hence a twin jet configuration offered better cutting performance compared to single jet configuration due to the dual mechanisms of chip curl and rake face lubrication.
Conclusion
The results obtained from the investigation clearly shows that minimal cutting fluid application with vegetable oil based cutting fluid improved the cutting performance in terms of better surface finish and reduction in cutting force. It also produced promising results when compared to dry turning and wet turning and reduced the quantity of cutting fluid to a greater extent.
The influence of individual fluid application parameters on surface finish and cutting force was analyzed in detail and found that the fluid application parameters have direct influence on the cutting performance.
Hence it is clear that turning with minimal cutting fluid application with soya bean oil based cutting fluid can be used as an alternate considering the advantages on cutting performance. It also promoted green environment in the shop floor, minimized the industrial hazard and usage of large quantity of cutting fluid.
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