Tribology Online

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Japanese Society of Tribologists http://www.tribology.jp/trol/

Vol. 13, No. 6 (2018) 340-350.

ISSN 1881-2198 DOI 10.2474/trol.13.340

Short Communication

Study and Comparison of Lubricity of Green and Commercial Cutting Fluid Using Tool-Chip Tribometer

Suvin Parayantayyathu Somarajan

^1)*

and Satish Vasu Kailas

²⁾

1) Centre for Product Design & Manufacturing, Indian Institute of Science, Bangalore 560012, India

2) Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India

*Corresponding author: Suvin Parayantayyathu Somarajan ([email protected])

Manuscript received 13 February 2018; accepted 26 September 2018; published 31 December 2018 Presented at the 6th World Tribology Congress 2017 Beijing, September 2017

Abstract

A lubricant is a substance introduced between surfaces in mutual contact to reduce friction, which ultimately reduces the heat generated when the surfaces move. Liquid lubricant, designed specifically for metalworking processes, such as turning, drilling, milling etc. is known as cutting fluids (CF). Cutting fluid is a blend or combination of oil, emulsifier and additives, mostly derived from chemicals or petroleum products. However, it has got many side effects as it is toxic and harmful to environment during its disposal. Hence, vegetable based cutting fluid or green cutting fluid (GCF) is being developed and gaining importance with time. Properties of these cutting fluids are dependent on the nature of the base-oil, nature of surfactants and the properties of water used to make the CF. These fluids act on the nascent surfaces generated during cutting to form a low friction boundary layer as they slide past the cutting tool. We propose a unique tool-chip tribometer (TCT) in which these boundary layers, formed by the action of lubricants on freshly cut surfaces, can be generated and their tribological properties alone studied in isolation. This equipment can be used to study the fundamental mechanisms of the lubrication using emulsions on nascent surfaces and can be used in optimizing the composition of such fluids. The scope of this work is to assess different cutting fluids in this tool-chip tribometer. The tribological performances of metal cutting on nascent surfaces are compared. Unlike in the conventional methods of assessing the lubricity of cutting fluids using cutting tests, here the friction has been evaluated separately from cutting forces.

Keywords

tribology, cutting fluids, coefficient of friction, tribometer, surface roughness

This article is distributed under the terms of the latest version of CC BY-NC-ND defined by the Creative Commons Attribution License.

1 Introduction

Cutting fluid can be applied to the machining area through mist (spraying), flushing, or keeping the tool and work piece submerged in cutting fluid during machining operation.

Cutting fluid is highly effective, being an amalgamation of oil and water in required proportion, which improves the tool life by reducing the tool and work piece temperature during machining. This is achieved by flushing the cutting fluid to the area of cutting or machining resulting in heat carried away by water molecules. It also acts as a lubricant, thereby diminishing friction between tool and work piece resulting in the machined product having an improved surface finish [1]. It is used in all metal working industries for machining purposes such as turning, drilling, milling, cutting, etc. Ultimately, the role of a cutting fluid is to lower manufacturing costs by lubricating and cooling the machining zone. Its cost is estimated to be as high as 16% of total metalworking manufacturing costs. It also has

an adverse impact on the environment and on social well-being due to its toxicity and non-bio-degradable nature [2].

Of late, the regulations for worker safety and environmental concern in manufacturing have resulted in reduction of usage of cutting fluid. Therefore, the cutting fluid formulation should not only aim at bringing down manufacturing costs but should also be environmental friendly and non-hazardous for the workers. Thus most essential change required is the reduction or even elimination of harmful additives [3,4]. This is achieved by the replacement of mineral-based cutting fluid with more biodegradable products such as vegetable oils and esters. It is documented in literature that performance evaluation of cutting fluid should be based on machining tests rather than using particular tribological set-ups (experimental set-ups simulating a particular aspect of actual set-ups).

Frictional set-ups like Rubbing tests and Chip generating Tests exist in practice. A unique feature of the tribology of metal cutting is that nascent surfaces get generated in the cutting

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operation and sliding occurs between the newly generated surface(s) against the cutting tool in the presence of a lubricant.

Lubricity is the measure of the reduction of friction and wear by a lubricant. The study of lubrication and wear mechanisms is called tribology. Friction measurement using a pin on disc or dynamometer techniques is well known. But the limitation or drawback of this approach is the fact that the estimated friction coefficient is obtained from cutting tests and not from friction tests [5]. So by interpreting or evaluating frictional force from other measured forces such as cutting force and feed force may not give the actual friction generated. It is very difficult to discern the role of primary shear zone, secondary shear zone, and rubbing zone in friction generation. As a consequence, the estimation of the friction coefficient is difficult as well. Moreover, it has been shown previously that such tests can only estimate an average friction coefficient on the rake face, whereas, the friction coefficient varies along the contact.

Contact conditions are very different along the secondary shear zone or along the rubbing zone. As a consequence, the macroscopic parameters analyzed are inadequate to provide local information about friction [6-8]. The sliding velocity is about zero close to the cutting edge and increases slowly until it reaches a maximum value at the end of the tool-chip contact area on the other side in the rubbing zone, the friction velocity is almost equal to the sliding velocity.

Moreover, during a cutting operation, the chip flows on the rake face and never comes back. Contrarily, in pin-on- disc systems, the pin always rubs on the same track. The main limitation of this approach is the fact that the estimated friction coefficient is obtained from cutting tests and not from friction

tests. Therefore the question arises whether the conventional methods of assessing the lubricity of cutting fluids are reliable and whether a more reliable system can be devised. These are the questions that keep popping up and hence, it necessitates the development of an accurate cutting process simulation to identify optimum conditions in terms of tool, material, etc. to sustain productivity and improving quality of machining operations.

2 Tool chip tribometer

The efficiency of the cutting fluid in rendering a low friction layer on the freshly cut surface will depend on the composition of the cutting fluid emulsion and on the speed, load, and temperature characteristics in the tribological system [9]. Evaluation of lubricity of metal cutting fluids necessitates a testing facility which tests their lubricity on freshly cut surfaces.

Methodology followed is shown in Fig. 1. A unique tribometer- Tool chip tribometer [TCT] has been used for experimental investigation [9]. The uniqueness is that it can execute friction testing on freshly cut surfaces. In this experimental facility, experiments are conducted by performing cutting operation of a disc inside a pool of lubricant and friction force is measured in-situ.

The schematic representation of tool-chip tribometer and set-up is shown in Figs. 2 and 3 respectively. The tank made of stainless steel has a capacity of 2.2 liters and the cutting fluid is taken in this tank. The surface (disc) that needs to be tested is mounted on a spindle rotating about a vertical axis, with the axis positioned such that it rotates inside the tank. There is a provision to raise or lower the position of the tank using

Fig. 1 Schematic representation of methodology followed to evaluate friction of newly machined surface using tool chip tribometer [9]

Fig. 2 Tool chip tribometer schematic [9]

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Tribology Online, Vol. 13, No. 6 (2018) /342 Japanese Society of Tribologists (http://www.tribology.jp/)

the rotating spindle so as to engage the tool against the disc. A single point metal cutting tool which can be loaded against the disc to cut and generate a new surface is fixed inside this tank.

When the tank is raised and the tool is engaged against the disc, it generates a circular track of 24 mm radius and 6 mm width.

The surface thus generated remains inside the pool of the cutting fluid emulsion without getting exposed to the ambient.

The friction between the cut surface, and the ball is measured, while the ball (pin with ball set-up is) is positioned at a radius same as that of the cut-track. The friction-ball pin is mounted on an arm which is pivoted on bearings and the tractional resistance experienced by the arm due to friction at the pin-disc interface is measured by a load cell. The friction arm is pivoted on the stage such that the ball and the tool can move in and out horizontally, and fine-adjustments of their relative positions can be made to ensure that the ball pin engages the track generated by the tool while pin slides against the disc surface. By suitably adding weights to the friction force (load cell connected) measuring arm the normal load on the ball-disc interface can be varied. The ratio of tangential force to the normal load applied will give the coefficient of friction [COF]. The location of the pin with ball set-up is diametrically opposite to that of the cutting tool and the time lag between the instant of cutting and the point of measurement will depend on the rpm of the disc.

3 Experimental plan

The work is comprised of measuring the coefficient of friction between a pin with a high speed steel (HSS) ball (3 mm diameter) attached and a nascent surface created by cutting metal using a wedge shaped single-point tool for orthogonal cutting. Test specimens were made of mild steel (En8) with Ra 1.5 µm and Aluminium 6061 T6 (Hardness 115 HV) and Ra 0.9

µm. The tool and the friction measuring ball were made of HSS (C-0.94%, Cr-4.1%, Mo-5%, W-6%, V-2%, Co-5%, and remaining iron (all % by weight)), Ra of the ball is 90 nm. Tool specification in American Standard Association (ASA) system is 8⁰-0⁰-10⁰-6⁰- 0⁰-0⁰-0.3 inch.

These cutting operations are conducted inside a tank containing the cutting fluid and friction force is measured in- situ (Friction force is measured while cutting). The frictional behavior of each of the cutting fluid with oxidized surfaces (disc pre-cut in the presence of air and then dipped in the cutting fluid) and nascent surfaces (cut while submerged in cutting fluid and hence, unoxidized) are compared. Two different materials were used as the disc material specimens for the study. The test specimens (75 mm circular disc, 10 mm thick) were made of EN-8 steel and Aluminium, which were machined and used.

To generate the nascent surfaces, the disc was made to engage against the tool placed in a tank containing the fluid. Time lag between the instant of cutting and the point of measurement is contingent on the rpm of the disc. The disc surfaces all the time remained 2 mm below the liquid level in the tank as shown in Fig. 2. A 3 mm deep and 6 mm wide cut was made on the disc. Friction on cut surface have been measured in-situ. Two different groups of commercial cutting fluids were used along with in house developed eco-friendly Green cutting fluid (GCF).

GCF from renewable components was developed with non- toxic, biodegradable, ecofriendly base materials. Coconut oil used as base oil was obtained from oil mill as raw coconut oil without any chemical refining. The food grade emulsifiers EF- 1, EF-2 and EF-3 were obtained from emulsifier market with 99.98% of purity. Green additives (essential oils) A-1 and A-2 were obtained from Falex International Export and Import, Bangalore, with 99.98% of purity. GCF was prepared using the following components;(a) Coconut oil base (40%) (b) Emulsifiers (40%) - [EF-1(77%); EF-2(20%); EF-3 (3%)](c) Additives – [A-1 (3%

w/w), A-2 (2% w/w), A-3 (5% w/w), A-4 (10% w/w)]. The cutting oil was mixed with deionized water in 1:20 ratio for further testing and characterization [8].

Cutting fluid used here is oil in water emulsions. Oil in water emulsions are preferred lubricants for metal cutting because of their capacity to engross the heat due to presence of large fraction (about 95%) of water and due to the lubricating effect induced by the oil(s). The relative demand for cooling (as against lubrication) is operation-specific. For example, a lubricant for a gear cutting operation is expected to induce lubricity rather than acting as a coolant, whereas the heat removal aspect can be of importance in high speed metal cutting. The ratio of (cutting oil to water) that used in the present work is 1:20. Different cutting fluids [GCF, commercial cutting fluid 1 (CCF1), and commercial cutting fluid 2 (CCF2)]

were tested and compared. Properties of the cutting fluids used for the testing are given in Table 1.

Formulations were tested with different set of experiments, varying one parameter and keeping all other parameters constant such as cutting speed or material of work piece. High- Speed Steel (HSS) tool was used and experiments were carried out for 30 rpm, 50 rpm, and 70 rpm with a fixed load of 1 kg (10 N). Table 2 shows parameters used to evaluate machining performance of cutting fluids. In these experiments, frictional force required or consumed for a given set of experiments was measured using a 3 mm steel ball fixed to the pin. By changing the cutting fluid one by one from mineral-based to vegetable- based for each set of experiments, the fluid that gave better performance in terms of reduced frictional force and good Fig. 3 Tool chip tribometer setup

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surface finish was observed. The results obtained from the tests are not absolute values but relative to the performance efficiency of a reference fluid (CCF). Figure 4 shows a typical evolution of the coefficient of friction [COF] with time. The test consists of a ball sliding on an uncut surface followed by cutting an uncut surface incessantly, measuring the COF continuously before and after the tool engagement and disengagement.

When cutting is terminated by withdrawing the tool without interrupting the ball sliding against the surface, a smooth transition from a state of high COF to low COF is observed, after that it is steady during repeated sliding of pin.

4 Results and discussion

In this study frictional force or Coefficient of friction [COF], and Surface Roughness (Ra) of machined work piece (Disc) have been measured. Thus, the performance was evaluated with respect to frictional force and product quality (surface roughness). The surface morphology characterization of the cut mild carbon steel (MS) disc was examined using 3D profilometer, Veeco, WYKO NT1100, optical preforming system, USA.

This work presented is an analysis of cutting fluid performance in tool-chip tribometer operation. Figure 5 shows the COF variation with time when only water is used as cutting fluid. It has been observed that the COF values are in the range of 0.6 to 1.2 for 30 rpm and 70 rpm respectively. This may be due to the high friction between ball and newly generated surface as there is no oil constituent mixed with the water. The ball was ploughs into the aluminium material when repeated sliding is done. Coefficient of friction started increasing with fluctuations. After few minutes ball fell keeping slip and drag signs on the disc. Dry sliding was also tested but the ball could not withstand the friction and the sound of rubbing between ball surface and the workpiece kept increasing. As the ball is harder it may be plough into the aluminium workpiece with repeated rubbing. Debris were like thin broken hair, few were continuous and curled.

Figures 6, 7, 8 and 9 show the variation of COF with time for different cutting fluids [GCF, commercial cutting fluid 1 (CCF1), and commercial cutting fluid 2 (CCF2)]. Metallic surfaces are usually covered with their oxides. The chemical nature of the surfaces covered with oxides is different from that of nascent metallic surfaces, that is the reason we observe that friction or coefficient of friction values increase when the pin with ball slides on the newly cut surface inside the pool containing cutting fluids compared to uncut or oxidized disc surface. For 30 rpm, the coefficient of friction values for all the cutting fluids investigated for MS work piece as disc material were in the range of 0.19-0.23 for repeated sliding on uncut surface, 0.23-0.28

1

Sl NO

Property Value

GCF CCF1 CCF2

1 pH value 7.5 8.6 9.0

2 Viscosity 1.41mPa.s 1.08 mPa.s 1.06 mPa.s 3 Stability Stable Stable Stable 4 Colour Whitish Milky white Whitish

Table 1 Properties of cutting fluid samples

2

Levels Speed

(RPM) Cutting velocity (m/sec)

-1 [Low] 30 0.07

0 [Med] 50 0.13

1 [High] 70 0.18

Pin (Ball) radius : 3 mm Cut- track width: 6 mm Tank to hold the lubricant (cutting fluid) :

2.2 liter capacity HSS tool Disc : MS and AL

Table 2 Parameters used to evaluate machining performance of cutting fluids

Fig. 4 COF variation with time [9]

Fig. 5 COF variation with time using only water as the cutting fluid for MS disc

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sliding on cut surface (nascent surface) and 0.2-0.23 repeated sliding on after cut surface respectively. The coefficient of friction values for all the cutting fluids investigated for Al work piece as disc material were in the range of 0.19-0.23 for repeated sliding on uncut surface, 0.28-0.35 sliding on cut surface (nascent surface) and 0.19-0.25 repeated sliding on after cut surface respectively.

Nascent surfaces are highly chemically active. The higher level of friction for the nascent surface is expected as the newly generated surface will be cleaner due to continual cutting.

Friction force was found to decline with sliding time (distance) and the transition from a surface freshly formed to a continually slided one was found to result a smooth transition. This difference in coefficient of friction is attributed to the formation of a boundary film during sliding [2,7]. Carboxylate-type structures (C=O), which gets generated due to the tribological action under repeated sliding conditions in the presence of water will improve the lubricity [9]. Fourier transform infrared (FTIR) spectroscopy of the disc after sliding in cutting fluid also shown presence of C=O as shown in Fig. 10. Which confirms that caboxylate formed in repeated sliding in cutting fluid.

The coefficients of friction of all the three surfaces on which the ball slides are different, with the nascent surface showing the highest level of friction. In the ‘rubbing’ test the oxide layer on the ball fixed to pin and that generated during the cleaning of the disc, after it has been cut (disc preparation), are efficient in combining with the cutting fluid during the rubbing of the

‘oxidised’ surface to yield organic compounds which form an antifriction film at the interface. The same mechanism operates, but much less efficiently, when the rubbing happens on a nascent surface. Therefore, a true description of the friction Fig. 6 COF variation with time compared using GCF, CCF1,

and CCF2 as the cutting fluids for MS and Al disc with 30 rpm speed

Fig. 8 COF variation with time compared using GCF, CCF1, and CCF2 as the cutting fluid for MS and Al disc with 70 rpm speed

Fig. 7 COF variation with time compared using GCF, CCF1, and CCF2 as the cutting fluid for MS and Al disc with 50 rpm speed

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generated by cutting can only be done when the tribofilm generated during cutting is tested for friction and not when it got altered by exposure to the environment [9,11].

Frictional force and roughness of GCF measured were found to be comparable with that of commercial cutting fluids.

From the analysis of cutting fluid performance in tool chip tribometer operation, frictional force and roughness measured was found to be comparable with commercial cutting fluid.

The part (dimensional) accuracy was in some cases better than mineral-based cutting fluid CCF1 and CCF2. For example, roughness optical profilometer image for cutting in MS and Al work piece for CCF1 and GCF as shown in Tables 3, 4 and 5.

By using cutting fluid, we can reduce the tool-work interface temperature, mitigate the friction between workpiece and the tool and in turn ameliorate the surface finish [7]. It is difficult for cutting fluid to penetrate the tool-chip interface resulting in rise in coefficient of friction with increase in sliding speed. The

difference in results observed could be due to the influence of tribofilm formed between tool and work piece by cutting fluids during the experiments.

The consolidated roughness optical profilometer result for 30 rpm, 50 rpm and 70 rpm with GCF is better compared to CCF1 and CCF2 for cutting in Mild steel and Aluminium work piece as shown in Table 6. Wear scar measured from SEM is shown in Tables 7 and 8. Thus, it revealed that eco-friendly vegetable cutting fluid is comparable with the commercial or mineral-based cutting fluid in terms of surface finish. By using GCF, we can reduce the friction between the workpiece and the tool which in turn improves the surface finish. The influence of tribo-film formed by oil between the tool and the work formed by each of these cutting oils, could have brought the difference in results.

GCF gave comparable machining performance with respect to CCF. GCF promotes healthy work environment and deters Fig. 9 Consolidated MS results showing coefficient of friction values of GCF giving comparable results with CCF

Fig. 10 FTIR result of Cut track after cutting in TCT

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3

Sample Surface Roughness of cut track in Aluminium Disc- 30rpm GCF

CCF1

4

Sample Surface Roughness of cut track in Aluminium Disc- 70rpm GCF

CCF1

Table 3 Optical profiler image of Aluminium sample machined at 30 rpm cutting fluids

Table 4 Optical profiler image of Aluminium sample machined at 70 rpm using cutting fluids

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workplace ailments as opposed to the side effects caused due to petroleum-based cutting fluids. To make perfect choice of fluids, it is important to consider not only the performance characteristics of the fluids but also other factors such as waste treatment, disposal cost, fluid life, resistance to microbial attack, etc. [10,11]. Tool-chip tribometer is a promising machine for cutting fluid or emulsion related study.

5 Conclusion

On the basis of the experiments carried out on various samples of cutting fluids, the following conclusions could be inferred. The uniqueness of the Tool chip tribometer machine is

useful in the tribological studies of cutting fluids or emulsions on nascent surfaces, while the use of a conventional sliding equipment can assess the lubricity on oxidized surfaces. The lubricity of metal cutting lubricants is sensitive to the nature of the substrate – whether it is nascent or oxidized. The performance of GCF is stable and efficient with the variation of the cutting speed and feed rates, which is an impetus for further study, analysis, and improvement.

Acknowledgement

This work was supported by the grant Provision (2A) Tenth Plan (191/MCB) from the director of the Indian Institute of

5

Sample Surface Roughness of cut track in Mild steel Disc- 70rpm

GCF

CCF1

Table 5 Optical profiler image of MS sample machined at 70 rpm using cutting fluids

Table 6 Roughness measured using optical profiler

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7 Sample Wear mark on HSS ball after sliding with Aluminium Disc- 70rpm

GCF

CCF1

Table 7 Wear mark on HSS ball after sliding with Aluminium Disc- 70 rpm measured using SEM

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8 Sample Wear mark on HSS ball after sliding with Mild steel Disc- 70rpm

GCF

CCF1

Table 8 Wear mark on HSS ball after sliding with Mild steel Disc- 70 rpm measured using SEM

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Science, Bangalore, India, and the Department of Biotechnology (DBT DBT0/370/SVK-DC), Life Science Research Board (LSRB 0008) and DBT-IISc partnership program for advanced research in biological sciences and bioengineering to DC Infrastructure support from ICMR (Center for Advanced Study in Molecular Medicine), DST (FIST), and UGC (special assistance) is acknowledged.

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