minimum quantity lubrication in micromachining: a...

Proceedings of the 2016 International Conference on Industrial Engineering and Operations Management Kuala Lumpur, Malaysia, March 8-10, 2016

Minimum Quantity Lubrication in Micromachining: A Greener Approach

Ahmad Syahmi Arham Bin Zulkifli, Rubina Bahar* and Tasnim Firdaus Ariff

Department of Manufacturing and Materials Engineering, Faculty of Engineering International Islamic University Malaysia

Kuala Lumpur, Malaysia *Corresponding Author. Email: < [email protected]>

Abstract— Minimum Quantity Lubrication (MQL) uses minimum amount of lubricating fluid to reduce the friction between a cutting tool and the work piece. The conventional cutting fluid applied using flooding method causes high volume of the coolant wastage plus environmental damages due to disposal issues. MQL is suitable for machining operations including milling, turning, and drilling while surface modification processes are not very appropriate for MQL due to occurrence of particle sticking. For Micromachining, MQL has more opportunity as the heat generated in the small machining area can be smoothly transferred by MQL compared to flood cooling. Not much information are available about MQL performance and metal’s thermal conductivity. In this paper, study of Micro-milling using MQL is presented. Three different metals have been tested with same cutting parameters to observe the effect of MQL on metal’s thermal conductivity. Three different work metals are selected which are Copper, Aluminum alloy 1100 and Cast Iron with cutting parameters including depth of cut, feed rate, and spindle speed. Finally, surface roughness is measured to see the combined effect of thermal conductivity and MQL on the machined surface. It has been found that lower thermal conductivity metal is more suitable to employ MQL as the coolant method.

Keywords— Green Machining; Minimum Quantity Lubrication (MQL); Micro-machining; Micro-milling; Thermal Conductivity.

I. INTRODUCTION

Over the past few decades, machining processes are making rapid progress in moving toward micro-machining. The non-lithography-based micro manufacturing techniques have been developed including micro milling, micro drilling, and micro turning. As compared to micro electromechanical system (MEMS) micro manufacturing, the micro mechanical machining products have higher precision dimension ranging between 10 μm ~ few mm. Various materials can be machined by non-lithography-based micro machining method compared to MEMS which is limited mainly to silicon based materials [1],[2].

Whatever machining methods are used--macro or micro; coolant supply is needed most of the time to ensure proper cooling and lubrication to the machined surface. But nowadays, increased cost from flood coolant system due to amount of metal removal and longer cleaning time make conventional flood cooling method incompetent. Additionally, with stronger focus on green machining, waste disposal of the used coolant is an issue and the situation worsens due to release of huge quantity of cutting coolant during operation. Furthermore, inappropriate handling of the coolant will lead to serious environmental problems which may damage soil and water resources. It is reported by [3] and [4] that coolant fluid also imposes health risk upon human respiratory and digestive system as well as the skin. Besides, when it comes to machining parameters like higher feed rate and cutting speed, the coolant fluid cannot reach the cutting zone and thus the effective heat transfer rate deteriorates.

As the conventional flood cooling method involves major concerns to be addressed regarding human health and environment, the emergence of Minimum Quantity Lubrication (MQL) provides an alternative method for cooling and lubrication. It is supported by the studies in [4] and [5] which found MQL as an alternative method to conventional flood cooling about a decade ago. Their method used ten-thousandth of the quantity of lubrication used in flood-cooled machining and was defined as “Microlubrication” by [6] and “Near Dry Machining” (NDM) by [3], [4] and [7].

In this paper, the effect of MQL on the surface property of three different metals during micromachining has been studied. These metals with different thermal conductivities have been machined using micro-milling with variation in cutting speed, depth of cut and feed rate. The results showed that depending on the metal, the surface quality and roughness varied.

1782© IEOM Society International


II. METHODOLOGY

A. Description of The MQL System

Minimum Quantity Lubrication (MQL) is the machining method by using a small quantity of coolant to reduce cuttingtemperature by reducing the friction between a cutting tool and a work piece. The correct quantity “Minimum” varies depending on what process, tool and material will be done. According to German DIN specification, the minimum quantity is below 50mL/hour (1.7 oz./hour) of coolant and for special case is up to 150mL/hour (5 oz./hour). MQL method is ineffective on an abrasive processes and works very good on cutting processes including milling , turning and sawing [8].

Fig. 1. The MQL setup and Machining

As seen in Figure 1(a) the MQL set up from Bluebe with the blue colored coolant tank on the top. Figure 1(b) shows the MQL nozzle in action during machining. Bluebe FK type can deliver air and lubricant to the nozzles which can be adjusted separately with lubricant consumption 4-8 ml per hour / per nozzle. Accu-Lube LB-6000 was used as the lubricant. The lubricant base is non-toxic, renewable raw materials which is also biodegradable. Then, it is extracted and blended on the basis of vegetable oils. Therefore, it is environmentally friendly to be used with all metals without chances of discoloration or damage. B. Materials and Method

Three different materials were used as the workpiece to examine the effect of MQL on the machined surface. These areAluminum alloy 1100, Copper and Cast Iron.

Copper is among the most used metal in the industry after Iron and Aluminium. The metal is ductile with excellent thermal and electrical conductivity. Aluminium Alloy 1100 is also known as commercially pure aluminium because it contains at least of 99.00% aluminium of its composition. It is a good electrical as well as thermal conductor. Cast Iron contains 2.5–4.0% carbon and 1–3% silicon. Cast iron tends to be brittle although it has relatively low melting point with good fluidity, castability, and excellent machinability, resistance to deformation and wear resistance. The thermal conductivity of Cast Iron is lowest among the three metals chosen for machining in this investigation. Table 1 specifies the physical properties of the three different materials.

TABLE 1. PHYSICAL PROPERTIES OF COPPER, ALUMINUM AND CAST IRON [9].

Metal Thermal Conductivity

Density Coefficient of Thermal Expansion

Melting Point

Specific heat

Brinell Hardness Range

(J/s-mm°C) (g/cm3) °C-1 x 10-6 (°C) (Cal/g°C) Copper 0.4 8.97 17 1083 0.092 80~85Aluminum alloy 1100

0.22 2.7 24 660 0.21 50~100

Cast Iron 0.06 7.16 12.1 1257 0.11 125~250

Fig. 1(a).The Bluebe MQL Setup Fig. 1(b). The MQL micro-milling



The cutting tool used was a two flute tungsten carbide face milling cutter with 0.1 mm tool edge radius and 0.5 mm diameter.

The cutting parameters that involve in micro milling process are i) depth of cut ii) feed rate, and iii) spindle speed. Design Expert software with Three Factorial Level model is used to analyze the surface roughness. From this model, eight experiments need to be conducted with varied parameters and levels for each work piece. Table 2 shows the cutting parameters.

TABLE 2. PARAMETER CONFIGURATION OF MICRO MILLING PROCESS WITH MQL.

Runs Depth of cut, (mm)

Feed rate (mm/min)

Spindle speed (rpm)

Presence of MQL

1 0.05 1 2000 Yes2 0.15 1 2000 Yes3 0.05 2 2000 Yes4 0.15 2 2000 Yes5 0.05 1 2800 Yes6 0.15 1 2800 Yes7 0.05 2 2800 Yes8 0.15 2 2800 Yes

During micro milling machining process, an infrared thermometer was used to capture the temperature of the machined surface since thermal property is one of the main factor to influence the MQL.

III. RESULTSThe machining data provided the temperature recorded at the machined surface for different cutting parameters. After

completion of the experiment, the surfaces for different material under different cutting condition were examined using a VEECO surface roughness testing device.

A. Machining Temperature

Measuring the surface temperature for each experiment was done by an infrared camera. It is seen in Table 3 that thetemperature was slightly lower for the higher conductive material; which is expected because of quick removal of heat through the high thermally conductive metal. The lowest machined surface temperature during machining was obtained for Copper which was 23.44°C and highest temperature during machining was obtained on Cast Iron machined surface which was 26.42°C. With the same material removal rate and all other conditions remaining same, the machined surface temperature varied mainly because of higher thermal conductivity of the material.

TABLE 3. MACHINED SURFACE TEMPERATURES .

Runs Depth of

cut Feed Rate

Spindle Speed Copper Aluminum Cast Iron

mm mm rpm (°C) (°C) (°C)

1 0.05 1 2000 23.44 23.78 26.3

2 0.15 1 2000 23.47 23.98 25.97

3 0.05 2 2000 23.61 23.99 26.21

4 0.15 2 2000 23.77 24.03 25.38

5 0.05 1 2800 23.79 24.23 26.07

6 0.15 1 2800 23.81 24.24 26.4

7 0.05 2 2800 24.07 24.3 26.13

8 0.15 2 2800 24.35 24.6 26.42



B. Analysis

The heat generated during micro milling can be divided in two portions; one portion is absorbed by the work metal while the other part is absorbed by the MQL coolant system. Hence, the energy balance equation can be written as

Htotal=Hmetal+HMQL (1)

Where H is the heat energy. Htotal is the amount of heat dissipated during the machining process.

Hmetal is the energy absorbed by the metal which can be expressed as =kA[dT/dx]

Here, k=thermal conductivity (w/m°C), A=area,

dT/dx=temperature gradient over the metal thickness

While HMQL=the portion of the heat energy absorbed by the MQL mist=hMQLA(Tm-Ta)

Here hMQL=Heat transfer coefficient for the mist created during MQL,

Tm=Metal temperature on the machined surface(°C)

Ta=ambient temperature (°C)

Therefore, (1) can be re-written as

Htotal= kA[dT/dx]+ hMQLA(Tm-Ta) (2)

As seen from (2) if the heat carried by the metal is higher, the effect of MQL cooling becomes less dominant. So we may conclude about the energy transfer process that Copper, which has the highest thermal conductivity and lowest machined surface temperature will depend more on the energy absorbed by metal rather losing the heat through MQL coolant. The opposite phenomena will be observed for the least conductive metal Cast Iron: more MQL cooling and less cooling through the metal.

C. Surface Finish after MQL machining

The results of experiments are seen in Figure 2. Figure 2 shows the surface roughness data for all the experimental runs as stated in Table 2. Run 8 with depth of cut 0.15 mm, feed rate 2 mm and spindle speed of 2800 rpm provided the highest surface roughness for all metals.

As seen from the figure, the metal with least thermal conductivity (Cast Iron) had better surface roughness condition compared to two other metals of higher thermal conductivity.

Figure 3 shows the surface condition of the machined metal under Veeco surface profile tester for Run no 7 with depth of cut=0.05 mm, feed rate 2 mm and spindle speed 2800 rpm. . Figure 3(a) shows the machined surface profile of Cast Iron while Figure 3(b) and Figure 3(c) shows the surface profile for Aluminum and Copper respectively. It is seen that the machined

Fig. 2. Surface Roughness of three different machined metals using MQL.



surface of Copper showed the most irregularities while the surface of the Cast Iron showed a better surface finish. It may be concluded that thermal conductivity was one of the main reasons as this property varied greatly among the three metals (Table 1). Other factors may include the Brinell hardness, which will be investigated in the next stage of experiments and not included here.

Fig. 3. Surface Profiles of the Metals.

IV. CONCLUSION

Minimum Quantity Lubrication (MQL) method has been attempted for micro machining as an approach toward greener machining practice. Feasibility of MQL has been investigated based on metal’s thermal conductivity. Three different thermally conductive metal have been micro-machined under variable cutting parameters using MQL as the only coolant system. It has been seen that the material with highest thermal conductivity performed poor compared to less thermally conductive metals. Copper, Aluminum and Cast iron were micro-milled using face milling process. The surface profile and the surface roughness test showed that Cast Iron had least value of surface roughness of 0.35 μm at 2000 rpm spindle speed while copper showed the surface roughness of 0.73 μm for a spindle speed of 2800 rpm. Effect of the other material properties on the machined surface will be investigated in later stages.

REFERENCES

[1] D. Cheng, K. and Hou, Micro-Cutting: Fundamentals and Application. John Wiley & Sons Inc, 2013.[2] J. Chae, S. S. Park, and T. Freiheit, “Investigation of micro-cutting operations,” Int. J. Mach. Tools Manuf., vol. 46, no. 3–4, pp.

313–332, 2006.[3] A. R. Machado and J. Wallbank, “The effect of extremely low lubricant volumes in machining,” Wear, vol. 210, no. 1–2, pp. 76–

82, 1997.[4] F. Klocke and G. Eisenblatter, “Dry cutting,” CIRP Ann. - Manuf. Technol., vol. 46, no. 2, pp. 519–526, 1997.[5] U. Heisel, M. L. Stuttgart, D. Spath, R. a Wassmer, U. W. Karlsmhe, M. Lutz, D. Spath, R. a Wassmer, U. Walter, M. L. Stuttgart,

and U. W. Karlsmhe, “Application of minimum quantity cooling lubrication technology in cutting processes,” Prod. Eng., vol. II,no. 1, pp. 49–54, 1994.

[6] T. F. Mcclure, R. Adams, and M. D. Gugger, Comparison of Flood vs . Microlubrication on Machining Performance ( Part I ).2007.

[7] M. Rahman, a. Senthil Kumar, and M. U. Salam, “Experimental evaluation on the effect of minimal quantities of lubricant inmilling,” Int. J. Mach. Tools Manuf., vol. 42, no. 5, pp. 539–547, 2002.

[8] T. Walker, The HANDBOOK A guide to machining with Minimum Quantity Lubrication. Unist, Inc., 2013.

Fig. 3(a) Machined Surface Profile for Cast Iron. Fig. 3(b) Machined Surface Profile for Aluminum.

Fig. 3(c) Machined Surface Profile for Copper.



[9] M. Groover, Fundamentals of modern manufacturing 4th edition. 2010.

BIOGRAPHY

Dr. Rubina Bahar is an Assistant Professor in Materials and Manufacturing Engineering Department, International Islamic University Malaysia. She obtained her BSc in Mechanical Engineering from Bangladesh University of Engineering and Technology (BUET) in 2002. After graduation she worked with ABB in Bangladesh. Later she obtained M.Engg (Mechanical) from National University of Singapore (NUS) in 2006 and PhD also from NUS in 2011. Throughout her post graduate studies she was awarded with the research scholarship from NUS. Her main area of research during her stay in NUS was desalination (conversion of saline water to freshwater) using Vapour Compression Distillation process and Membrane Distillation process. She joined International Islamic University Malaysia (IIUM) in 2013. She is currently project leader of two ongoing research projects to address the problem of freshwater supply system in the Malaysian households. Her interest also covers thermal management of machining process and energy harvesting from waste/renewable energy resources.


minimum quantity lubrication in micromachining: a...

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