Machining of Microchannels using Micro-milling Process Tej Pratap and Karali Patra* Department of Mechanical Engineering Indian Institute of Technology Patna, Patna-800013, INDIA Abstract The demand of micro parts has increased over last few years, especially in the field of microelectronics, micro-fluidic systems, biomedical, aerospace and many other fields. Micro-milling process is gaining rapid acceptability to fabricate these micro parts due to its numerous advantageous characteristics such as flexibility, cost-effectiveness, repeatability, and its ability to produce real 3D features. The present work experimentally investigates the machining of microchannels using micro-milling process. Experiments are carried out by micro tungsten carbide end mill on copper plate with varying process variables (feed rate and cutting speed). Micro-milling cutting forces are analyzed and a suitable cutting condition which gives lower cutting forces is selected to machine the microchannels. Accuracy of the micro-milling process is verified through measurements of dimensions of finished microchannels and found to be in good agreement. Keywords: Microchannels; Micro-milling; Accuracy; Process variables; Cutting forces.
1. INTRODUCTION Past few decades have observed a phenomenal growth of the applications of miniaturized components or components with micro features in various industries including electronics, biomedical, aerospace, etc [1, 2]. Some specific applications include micro channels for lab-on-chip, micro fuel cells, micro fluidic chemical reactors, micro-nozzles for high temperature jets, micro heat exchangers, etc [1]. Micro parts and micro systems become more important for enhancing product performance and industrial economic growth and to achieve these goals there is a growing need of a fast, reliable and repeatable approach for fabrication of highly accurate features and components on wide range of materials for different applications. Some other manufacturing processes, such as wet chemical etching, laser, electro chemical machining (ECM) and LIGA, are primarily applicable to silicon or silicon-like materials on planar geometries having low aspect ratio [3, 4, 5]. These methods associated with low material removal rate (MRR), lack of ability to produce high aspect ratio threedimensional components, complex setup, higher initial cost and larger processing time [6, 7]. Due to these limitations, researchers have turned to the miniaturization of mechanical manufacturing processes [8, 9]. Micro-milling is a mechanical machining process in which micro size milling tool is used to achieve the desired geometry of the workpiece by removing the materials in the form of chips [10]. It is a scaled down version of the conventional milling with specific characteristics such as size effect, cutting edge radius and minimum chip thickness which add complexity in the cutting process [7, 11]. The rapid acceptability of micro-milling process in the production of micro molds, dies, heat exchangers, etc., may be due to its advantageous characteristics such as flexibility, cost effectiveness, repeatability and ability to produce 3D high aspect ratio features with good surface quality [8].
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However, due to the miniaturized size of the cutting tool some challenges are associated with it that include breakage of cutting tool, formation of burr, etc. Additional process such as powder blasting may be required to remove burr formed during micro end milling [12]. For micro-milling process it is most important to analysis the cutting force variation and select a suitable cutting condition so that tool breakage does not occur [13]. Selection of optimal cutting condition for specific tool with certain geometry gives precision micro milling and good surface quality [14]. Major issues like tool run-out, elastic recovery, ploughing, minimum chip thickness and tool wear are responsible for dimensional deviation and poor surface quality of the microchannels. Application of coolant and small axial depth of cut per pass significantly improve surface finish and dimensional accuracy [15]. The present work deals with the micro-milling of microchannels on copper workpiece applied as a heat sink in electronics cooling. The applicability of the micro-milling process is verified through cutting forces and finished microchannels dimensional analysis. The effects of cutting conditions on cutting forces and material removal rate are experimentally investigated to select suitable cutting parameters for the proposed micro-milling of microchannels. 2. EXPERIMENTAL PROCEDURE An experimental setup for the machining of microchannels is shown in Fig.1. An air turbine-driven high-speed spindle which can provide a rotational speed of up to 60,000 rpm with a run out of less than 1 μm has been mounted to the Z-axis of the multi-purpose micro CNC machine (Model No. DT-110, Mikrotools Ltd.). The Experiments are performed by micro tungsten carbide end mill having tool diameter of 0.5 mm and tool run out of 0.005 mm with varying feed rate (f) and spindle speed (N) on a 45 mm x 40 mm x 3 mm copper workpiece. Constant Axial depth of cut of 0.05 mm/pass is maintained for all the experiments. Machining is carried out under dry condition. Cutting forces are acquired using the force dynamometer (Kistler Type 9256C2) having accuracy of 0.02N. The close up view in Fig. 1 shows the cutting force directions acquired through Kistler dynamometer.
3. RESULTS AND DISCUSSION 3.1 Micro Milling Cutting Force Analysis Three cutting forces in the direction of X, Y, and Z and moment about X direction are analyzed in time domain for each cutting condition. The cutting forces and moment variation in time domain for spindle speed of 40000 rpm and feed of 20 mm/min are shown in Fig. 3.
Fig. 1. Micro-milling setup and cutting force directions
2.1 Cutting Conditions and Data Acquisition System The ranges of cutting conditions for analysis of cutting forces and material removal rate (MRR) are shown in Table 1. The cutting parameter (feed rate and spindle speed) have four levels at equal interval and the axial depth of cut in Z-direction is kept constant. Y-direction is the feed force direction. A cutting condition is selected on the basis of lower cutting forces to avoid early tool failure and that condition is used for fabricating all the microchannels. Table 1: Cutting condition for micro-milling process Sl. No. 1 2 3
Parameter Feed rate Spindle speed Axial depth of cut
Range 10 - 40 mm/min 10000 - 40000 rpm 0.05 mm/passes
Cutting force data acquisition system is shown in Fig. 2. The workpiece is clamped on the Kistler dynamometer on the feed table of the machining center. Low amplitude charge produced by dynamometer are amplified by charge amplifier and then acquired by the data acquisition system at a sampling frequency of 1000 Hz.
Fig. 3. Cutting forces and moment variation
Typical plot of the maximum values of cutting forces and moment at different spindle speed and feed are shown in Fig. 4.
(a)
Fig. 2. Cutting force data acquisition system
2.2 Width and Height Measurement of Microchannels Width and height of the microchannels are measured with the help of OLYMPUS autofocusing microscope (model MM6CAF). Widths of the microchannels are measured at top surface as well as at bottom surface along the longitudinal direction. Height of the microchannels are measured for all channels.
(b)
rate that might lead to failure of the milling tool and increase the production cost as well as affect the surface quality. Hence a cutting condition with spindle speed of 40000 rpm and feed rate of 20 mm/min is selected in this work to machine all the microchannels with width of 500µm and height of 900µm. 3.3 Width and Height Measurement (c)
The width at the top surface as well as bottom surface and height of the microchannels are analyzed along the length of the channels for all microchannels as shown in Fig. 6 (a) - (b). (a)
(b)
(d) Fig. 6. (a) Height measurement
(b) Width measurement
The variations of microchannels width along longitudinal positions at top and bottom surface are shown in Fig. 7 (a) - (b). Fig. 4. Cutting forces and moment variation with feed (a) Fx (b) Fy (c) Fz (d) Mx
From the cutting force analysis shown in Fig. 4 it is observed that cutting force Fx is increasing with increase in feed and forces Fy, Fz and moment Mx initially decrease and beyond the feed value of 20 mm/min these values increases. This may be due to increase of specific cutting force at low feed [10]. 3.2 Material Removal Rate (MRR) Analysis (a)
The material removal rate is analyzed at varying feed at constant speed for all the cutting speed and shown in Fig. 5.
(b)
Fig. 5. Material removal rate variation at different cutting condition
MRR analysis gives the increasing trend with increase in feed and small variation with increase in speed. On the above analysis it can be seen that cutting speed of 40000 rpm and feed of 20 mm/min resulted the minimum cutting forces as well as moderate MRR. Beyond this feed value MRR increased but at the same time cutting forces increased at higher
Fig. 7. (a) Width (Top surface) variation. (b) Width (Bottom Surface)
However the width variations along the longitudinal position for both top and bottom surfaces are small (maximum variation about mean for top surface is 1.3% and 1% for bottom surface). It is found that width at the top surface compared to that at bottom surface of the microchannels is more. The width variation from top to bottom surface is due to the large depth of cut/passes, progressive tool wear, difficulty of chip removal with increasing depth and tool deflection. Figure 8 shows the width variation of a microchannel along the depth. The angle of taper is 1.680. Maximum error associated with microchannels width is 9.33% and it occurs at the bottom surface of the microchannels.
Fig. 8. Width variation along depth of the microchannel
The variations of height of the microchannels with respect to channel number are shown in Fig. 9. Maximum error associated with microchannels height is 1.25%.
Fig. 9. Height variation
4. CONCLUSIONS This work contributes to the selection of suitable cutting condition for machining of micro-channels on copper workpiece using micro-milling process. A cutting condition (spindle speed and feed) which gives lower cutting force and adequate metal removal rate has been selected for fabrication of all the microchannels. Analysis of fabricated microchannel dimensions showed very small deviation in width and height of microchannels with respect to its length and channel number, respectively. However width of microchannels decreases with
depth may be due to large depth of cut/passes, progressive tool wear, difficulty of chip removal with increasing depth and tool deflection. But angle of taper along the depth of microchannel is only 1.68 0. The experimental results established the suitability of the micro milling process for fabrication of microchannels for heat sink. 5. REFERENCES [1] Chae J., Park S. S. and Freiheit T., Investigation of micro cutting operations, International Journal of Machine Tools and Manufacture, 46 (2006) 313-332. [2] Camara M. A., Rubio J. C. C., Abrao A. M. and Davim J. P., State of the Art on Micro-milling of Materials, A Review, Journal of Materials Science & Technology, 28 (2012) 673-685. [3] Liu C. W., Gau C., Ko H. S., Yang C. S. and Dai B. T., Fabrication challenges for a complicated micro-flow channel system at low temperature process, Sensors and Actuators A, 130–131 (2006) 575–582. [4] Malecha K., Golonka L. J., Microchannel fabrication process in LTCC ceramics, Journal of Microelectronics Reliability, 48 (2008) 866–871. [5] Ashman S., and Kandlikar S. G., A review of manufacturing processes for micro channel heat exchanger fabrication, Proceedings of Fourth International Conference on Nanochannels, Microchannels and Minichannels 2006, Limerick, Ireland, June 2006. [6] Liu X., DeVor R. E., Kapoor S. G. and Ehmann K. F., The mechanics of machining at the micro-scale: assessment of the current state of the science, Journal of Manufacturing Science & Engineering, 126 (2004) 666-678. [7] Gietzelt T. and Eichhorn L., Mechanical Micromachining by Drilling, Milling and Slotting, Micromachining Techniques for Fabrication of Micro and Nano Structures, Kahrizi M., Shanghai, China, (2012) 159-182. [8] Dornfeld D., Min S. and Takeuchi Y., Recent advances in mechanical micromachining, CIRP Annals - Manuf. Technol., 55 (2006) 745-768. [9] Bruno F., Friedrich C. and Warrington R. O., The miniaturization technologies: past, present and future, IEEE Transactions on Industrial Electronics, 42 (1995) 423 – 430. [10] Sooraj V. S., and Mathew J., An experimental investigation on the machining characteristics of microscale end milling, Int. Journal of Adv. Manufacturing Technology, 56 (2011) 951-958. [11] Cheng Z. J., Kang I. S., Park J. H., Jang S. H., and Kim J. S., The characteristics of cutting forces in the micro-milling of AISI D2 steel, Journal of Mechanical Science and Technology, 23 (2009) 2823-2829. [12] Yun D. J., Seo T. I., and Park D. S., Fabrication of biochips with micro fluidic channels by micro end-milling and powder blasting, Sensors, 8 (2008) 1308-1320. [13] Rahman M., Kumar A. S., and Prakash J. R. S., Micro milling of pure copper, Journal of Materials Processing Technology, 116 (2001) 39-43. [14] Afazov S. M., Zdebski D., Ratchev S. M., Segal J., and Liu S., Effects of micro-milling conditions on the cutting forces and process stability, Journal of Materials Processing Technology, 213 (2013) 671-684. [15] Karla P. M. V., Attanasio A., Ceretti E., Siller H. R., Nicolas J. H. T., and Giardini C., Evaluation of superficial and dimensional quality features in metallic micro-channels manufactured by micro-end-milling, Materials, 6 (2013) 14341451.
