APPLICATION OF ABRASIVE WATER JET MACHINING IN FABRICATING MICRO TOOLS FOR EDM FOR PRODUCING ARRAY OF SQUARE HOLES

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1 APPLICATION OF ABRASIVE WATER JET MACHINING IN FABRICATING MICRO TOOLS FOR EDM FOR PRODUCING ARRAY OF SQUARE HOLES Vijay Kumar Pal 1*, S.K. Choudhury 2 1* Ph.D. Scholar, Indian Institute of Technology Kanpur, Kanpur, , vijayp@iitk.ac.in 2 Professor, Indian Institute of Technology Kanpur, Kanpur, , choudhry@iitk.ac.in Abstract In the field of Abrasive Water Jet (AWJ) machining, the current focus is on the fabrication of complex threedimensional features. The current work is aimed at using this strategy to manufacture micro-tools for machining arrays of square holes using Electric Discharge Machining (EDM). Based on selected parameters of AWJ process(step over, traverse speed, path strategy),micro tools were fabricated on brass sheet of 6 mm thickness.the performance of such fabricatedtool was investigated by performing experiments on EDM machine to make arrays of square blind holes( texture ) on stainless steel and Ti-6Al-4V alloy sheet. The depth of texture was measured through 3D profilometer of FOV (2x) and objective (5x) embedded with vision 64 software.the textured square holes were obtained and its depth was achieved in the range of 10 to 60 microns and thecorresponding depth obtained of the texture is less on titanium. Discrepancy between the tool dimension (square) and work piece (textured) was found because of the tool wear on the electrode. Key words: AWJM, Texturing, EDM, Electrode (Tool) 1 Introduction Micro-machining and miniaturization of products has become the need of the industry. Currently in commercial applications micro-machining is mostly performed by non-conventional machining processes like lasers, electro-discharge machining (EDM) and chemical etching [1]. Now-a day s texturing on the surfaces is major interest of research because surface texturing is an effective means to improve the tribological properties [2]. It is mostly performed by Cutting, Burnishing, Laser machining, Electron beam writing, Lithographic methods, Thermal reflows, Focused ion beam machining etc. Tool design and mask fabrication are the major obstacles to perform texturing by non-conventional machining processes as EDM/ECM, lithography etc. because these masks and micro-tools are costly and very less sources/companies are available to fabricate them.abrasive Water Jet Machining (AWJM) is a state- of the- art non-traditional machining process making use of high pressure water converted to high velocity jet mixed with abrasives with an ability to cut various materials ranging from soft material like plastics, rubber, wood etc. to hard materials like titanium and inconel. Originally AWJM technique was only used for linear cutting and shape cutting of difficult to cut materials [4]. Now, researchers have also started experimenting on generating blind features using AWJM. The literature study reveals that the geometrical features, like channels (single slot) and pockets (closed loop path) can be fabricated using AWJ milling. In the present study, a novel path strategy is introduced to fabricate the micro-tool of different shapes (here, array of square spikes) using the concept of multi-pass linear traverse cutting. Here, the distance between the two parallel passes was kept more than the nozzle diameter [5]. The objective of the present work is to fabricate micro-tools (electrode) for EDM process on brass sheet by AWJM process. Primary experiments were carried out to find out suitable range of process parameters, based on these selected parameters, tools were fabricated. EDM is one of the advantageous methods to provide shape on very hard material.the performance of such fabricated tool by AWJM process wasshown by performing experiments on EDM machine to make texture (arrays of square holes) on stainless steel and Ti-6Al-4V sheet. 2 Methodology 2.1 Experiments to fabricate micro-tool Experiments were performed using commercially available abrasive water jet machine (OMAX Corp.). Table 1 shows the specifications of the machine.in AWJMprocess, highly pressurised water is mixed 535-1

2 APPLICATION OF ABRASIVE WATER JET MACHINING IN FABRICATING MICRO TOOLS FOR EDM FOR PRODUCING ARRAY OF SQUARE HOLES with abrasive particles in a mixing tube (Fig. 1). The abrasive water jet is focused through a focusing tube before making an impact on the selected area on the work material. Table 1 Machine specification Maximum traverse speed 4572mm/min Jet impingement angle 90 0 Orifice diameter 0.33 mm Abrasive flow rate kg/min Mixing tube diameter mm Mixing tube length mm Maximum working 45 kpsi pressure A novel path strategy is introduced to fabricate micro-pillars by AWJM process. The resulting machined surface is to be used as a tool in the EDM process. The path strategy is derived from the work of (Pal and Chaudhury, 2014) and briefly explained with the help of Fig.3. Here machining was carried by keeping the distance between two successive passes (also known as step-over (SO)) more than the diameter of jet such that there was no superimposition of the parallel passes of the jet. By this manner some material in the form of strip was retained between two consecutive passes during machining, as shown in the first and second raster paths (Fig. 3).. Both the raster paths cross each other perpendicularly. It can be clearly seen that, the two crossed raster paths generate thearrays of square on the same area. The cross motions of nozzle along these raster paths leave some material in between their consecutive passes which results in the square pillars. It is implicit that the height of these square lands is essentially the depth of the slots created due to metal removal during the AWJ passes. Figure 1 AWJM process (nozzle and mixing chamber) In this work brass sheet of 6 mmm thickness is considered as material. Test sampless were grinded before subjecting to AWJM in order to improve the flatness of the resulting tool. Fig. 2 shows the surface roughness profile of work piece. Figure3 Path strategy to fabricate square pins) tool (Array of Based on the present strategy, four different tools were fabricated with different geometric configurations by varying process parameters associated with the AWJM. The quantitative features of these tools are listed in Table 2.and the tools are shown in Fig. 4. If one changes the step over distance, the number density of the resulting features changes. Similarly by varying the traverse speed, the depth of cut (hence height of the features) can be varied. Figure 2 Surface roughness profile of grinded brass sheet 2.2 Path strategy used to fabricate (square pattern) shaped tool 535-2

3 Figure 4 Fabricated tools by AWJM Table 2 Parameters considered for AWJM Step over (mm) Traverse speed (mm/min) T 1 T 2 T 3 T Area (mm 2 ) 15x25 15x25 15x25 25x25 3 Performance of Textured Tool in Electric Discharge Machining The surfaces machined using the AWJM processes were used as tool (electrode) in the Electric Discharge Machine (EDM). Since these surfaces could not be directly fitted into an EDM, they were brazed at one end of the copper cylinders so that they could be inserted into the tool holder. This arrangement is pictorially presented in Fig.5.The performance of tools thus fabricated was investigated in an EDM setup. The process parameters for this machining were taken based on micro features machining and presented in Table 3. Figure5 EDM set up for texturing Table 3 EDM parameters in present work S.No. Parameter Value 1 Current 5 Amps 2 Voltage 75 volts 3 Pulse on time 150 micro sec 4 Duty Factor 72% The experiments were conducted on Ti-6Al-4V alloy and stainless steel sheets of 1 mm thickness. This work aims to achieve textured arrays of square holes on sheets. All the four tools developed in the previous section were used here to machine these materials. Figure 6 Textured images by various tools The responses of texturing tools were characterized by the depth and geometry of the texture obtained on these sheets. The images of the surfaces machined using the four tools enlisted in Table 2 are shown in Fig.6. The numbering is in accordance with the tool numbers in the Table 2. For the first two images (Fig

4 APPLICATION OF ABRASIVE WATER JET MACHINING IN FABRICATING MICRO TOOLS FOR EDM FOR PRODUCING ARRAY OF SQUARE HOLES 6), the work piece is stainless steel while for the later two titanium alloy work piece was used. Tool 1 (T1) had a smaller step over distance than tool 2 (T2). This resulted in smaller fins on the tool and subsequently the features produced during EDM. This is apparent from the Fig.6. Similarly, tool 3 had smaller step over than tool 4, thus producing smaller features on the corresponding work piece. It can also be observed from these figures that the shapes produced on these sheets correspond to the texturing on the tool. Figure 7 Enlarge view of texure obtained by Tool (T4) Enlarged image of one of the surfaces machined (corresponding to tool 4) is shown in the Fig. 7. The measurement shows that the produced features are of an approximate dimension of 700 µm x 700 µm. The image of the corresponding tool is shown in Fig. 8.The area of the top surface of each square fin in 600 µm x 600 µm. This discrepancy between the tool dimension and work piece can be explained as follows: As shown in Fig. 8 (a), the tool produced by AWJM has taper along the height of the fins. During EDM, the tool also undergoes some wear. Because of these, the tool area responsible for ED machining increases in time and produces features larger than the initial tool dimensions. Figure 8 Images of fresh and wear tool This image of the tool wear is shown in Fig 8. Fig 8 (a) shows a new tool and Fig 8(b) shows the corresponding worn out tool. Because of this taperness in the tool, the resulting feature is also expected to be tapered along the depth direction. This is confirmed by the optical image of the machined work piece surface as shown in Fig. 9.The depth of texture was measured through 3D optical profilometer of FOV (2x) and objective (5x) embedded with vision 64 software. Figure 9 (3D) Optical image of the square hole texture Since titanium alloy is difficult to machine compared to stainless steel surface, therefore corresponding depth obtained after machining (texturing) was found less on titanium. This fact is also reflected by larger tool wear while machining the titanium alloy surface. These comparisons can be justified since the same machining parameters were used while working on all these surfaces and tools. Based on these arguments, the following trends are expected. Tool wear should be more while machining titanium surface (compared to stainless steel surface). Because of larger tool wear, the ratio of obtained textured profile area to the tool surface area should be larger on titanium. Depth of features should be less on titanium. For same operating parameters and machined surfaces, fractional wear should decrease with tool cross section. Quantitatively, machining of stainless steel using tool cross sections 200x200 µm and 400x400 µm give features of dimensions 290x290 µm and 480x480 µm respectively. Similarly, tools of dimensions 400x400 µm and 600x600 µm produced features of cross sections 450x450 µm and 700x700 µm, accordingly, while machining titanium surface. Depth of cut on titanium was found in the range µm which was quite less than stainless steel machining (50-60 µm). This proves that stainless steel is easier to machine

5 Thus, all the expected trends are also obtained experimentally. 4 Conclusions From the advancement of technology, the capability of AWJM process can be extended to fabricating micro-tool (electrode) for EDM. Present work shows the path strategy to fabricate tool (arrays of square pin) on brass sheet. Performance of tools were analysed by conducting experiments on stainless steel and Ti-6Al-4V sheet. Based on the observations of present investigation following conclusions can be drawn: sliding in water, Applied Surface Science, 253(3): Pal, V. K. and Choudhury, S.K.(2014), Fabrication and analysis of micro pillars by using abrasive water jet machining, ICMPC, Hyderabad, March 7-9. Smaller step-over produces the smaller fin (top area) e.g Tool 1 (T1) has a smaller step over distance than tool 2 (T2). This result in smaller fins on the tool and subsequently the features produced during EDM. This discrepancy between the tool dimension and work piece was found because of taperness of tool. The tool produced by AWJM has taper along the height of the fins. Because of taperness in the tool, the resulting textured feature on the surface was also found tapered along the depth direction. Titanium alloy is difficult to machine compared to stainless steel surface therefore, corresponding depth (texture) obtained of the features is less on titanium References Kagaya, K., Oishi, Y. and Yada, K. (1986), Microelectro discharge machining using water as working fluid, micro-hole drilling, Precision engineering 8(3): Hashish, M. (1994),Controlled-depth Milling Techniques Using Abrasive-Water jets. In:Allen N G (ed) 1994 Jet Cutting Technology, Mechanical Engineering Publication Ltd, London Masuzawa,T. (1985), An approach to micromachining through machine tool technology, Annals of the CIRP : 34(1): Masuzawa, T., Fujino, M., Kobayashi, K., Suzuki, T.(1985), Wire Electric discharge grinding for micromachining, Annals of the CIRP 34(1): Masuzawa, T., Yamamota, M., Fujino, M.(1986),A micro punching system using wire-edm, Proceedings of the international symposium forelectro machining (ISEM-9): Momber, A.W., Eusch, I., Kovacevic, R.(1996), Machining refractory ceramics with abrasive water jet, Journal of Material Science. 31: Wang, X., Adachi, K., Otsuka, K., Kato, K, (2006) Optimization of the surface texture for silicon carbide 535-5