ELECTRO DISCHARGE MACHINING OF AISI 304 USING SOLID AND BUNDLED ELECTRODES

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1 5 th International & 6 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 01) December 1 th 1 th, 01, IIT ELECTRO DISCHARGE MACHINING OF AISI 0 USING SOLID AND BUNDLED ELECTRODES Harshit K. Dave 1*, Sudhanshu Kumar, Nipul C. Rana, Harit K. Raval 1,,, Department of Mechanical Engineering, S.V.N.I.T, Surat, Gujarat *1 harshitkumar@yahoo.com, sudhanshuk7@gmail.com, ncr.ngpp@gmail.com, hkraval1@yahoo.co.in Abstract This paper presents the investigation on generation of square shape cavities with solid and bundled tool electrodes. The workpiece and tools material are AISI 0 and electrolytic copper respectively. Effect of peak current and pulse on time has been observed on machining performance in terms of material removal rate and tool wear rate. Three different levels of peak current and five levels of pulse on time have been selected for investigation. To remove the un-machined pinned shape structure of workpiece, 1mm orbital radius has been given to both the electrodes. Experiments have been designed using fractional factorial design. The experimental results indicate that solid tool yields 5% more material removal rate than bundled tool electrode and TWRfor both tool is decreasing with pulse on time. Keywords: Orbit radius, solid tool, bundled tool, factorial design 1 Introduction Electro discharge machining process is non contact type machining process that uses the electric spark to erode the electrical conductive materials. The electrically conductive tool electrode, which has the male shape of the die cavity, is prepared to machine the die cavity. The method is especially effective in machining hard die steels, complex cavities and small workpiece. Die casting, injection moulding, forging, extrusion, upset forging and powder compaction dies are manufactured using EDM technology (Rajurkar et al., 01). Electro discharge machining process is the unconventional machining process that is mostly used in die making industries. Die making industries require complicated geometry with high dimensional accuracy. The machining of these complex shapes is difficult with conventional machine tools (Ekmekci et al., 006). The possibilities of generating non-circular geometry with EDM technology is much more than any other machine tool system. The hardness, brittleness or melting points of the material do not affect the machining and tool does not need to be harder (Ferreira, J. C., 007). The application of EDM process is simple and economical than any other un-conventional machining process. EDM is basically electro-thermal process in which electric energy is transferred to the thermal energy. So, the application of thermal energy is controlled by the input of electrical parameters such as discharge current, open circuit voltage, pulse duration, gap voltage and duty cycle (Jain V.K., 00). The performance of EDM may be influenced with the tool material, design and manufacturing method of electrodes. Tool electrode design also affects on the cost of machining. Excessive tool wear during EDM process leads to unpredictable tool life and inconsistent component dimensions. In EDM, tool geometry is very important because electrode is employed to produce its replica on the workpiece. Several attempts have been made to find out the effect of tool electrode designs on machining of complex geometries. Mehra et al. (01) compared the performance of a hollow electrode with a solid electrode in blind hole drilling of ductile iron at 5-15A peak current. MRR was compared using Taguchi design. Higher MRR was obtained with hollow electrode compare to solid electrode at constant pulse on time and current. At constant flushing pressure authors found higher MRR with hollow electrode.gu et al. (01) compared the performance of bundled electrode with a solid die-sinking electrode.experiments were conducted at very high peak current (0-17A). Besides inner flushing with different flow rates, a 0.5 mm radius orbital motion was adopted. Orbital motion was not given to the solid electrode. It was obtained that bundled electrodes can endure a much higher peak current than solid electrode which results in a substantially higher MRR and a comparably lower TWR. The benefit of the bundled electrode makes it much more feasible for 96-1

2 ELECTRO DISCHARGE MACHINING OF AISI 0 USING SOLID AND BUNDLED ELECTRODES application in large area rough machining.murugesan et al. (01) compared multi-hole electrode with a solid electrode in blind hole drilling of Al-15% SiC metal matrix composite. A 1 mm copper rod with an array of 8 holes of 1mm diameter drilled in it has been used as a multi-hole electrode. From experiments, it was found lower machining time in case multi-hole electrode compare to solid electrode. Forcing the dielectric through a number of small holes increases the velocity of the dielectric, which in turn increases the debris removal.ojha et al. (011) studied the effect of different tool angles on EDM machining performance. Three different electrodes of constant cross-section area of 50 mm and varying angles of 50, 90 and 10 degreewereused in the investigations. The significance of effective flushing was found on response characteristics. It was found that MRR increases with increase in tool angle owing to increase in current. After certain level, the MRR tends to decrease due to inefficient flushing. Yilmaz et al. (010) compared experimental investigation of EDM fast hole drilling of aerospace alloys (Inconel 718 and Ti-6Al-V). Different electrode type and material, viz. single and multichannel tubular electrodes made of brass and copper materials were selected. It was concluded that the single-channel electrode has comparatively better MRR and lower electrode wear ratio (EWR). Mohan et al. (00) studied the machining characteristics of SiC/605 Al composite using a tube electrode with rotation. MRR, TWR and Ra were adopted to evaluate the machinability at 5, 8 and 11A peak current. The effect of EDM drilling was found with the rotating tube electrode has produced higher MRR than the rotating solid electrode. The decrease in hole diameter has produced a better MRR, Ra and higher TWR. Dave et al. (01) studied the effect of different flushing conditions i.e. through flushing and jet flushing in drilling of deep holes. Higher range of peak current (1, 1 and 8A) was considered for investigation. The result indicates the higher MRR with through flushing (tubular) tool electrode. From the past research works, it has been observed that different tool designs have been used to improve the machining performance of EDM. and tubular or tool with multi holes are used for generation of cavities. Peak current and pulse duration are most affecting electrical parameters (Kansal et al., 006; Tasi et al., 007). Most of the work has been reported at higher range of peak current and pulse duration. In the present paper, square shape cavity has been generated using two different types of tool electrodes i.e. solid and bundled tool electrode.identical values of flushing pressure have been maintained for both types of tool. Effect of electrical parameters like peak current and pulse on time at relatively lower values have been investigated contrary to that reported in the literature to check its feasibility. Experimental Plan and Procedure Experimental investigations were carried out on die sinker EDM JOEMARS AZ50R. The workpiece material used for the experiments is AISI 0 (0.08% C, 18% Cr, % Mn, 8% Ni). The tool electrode was made of pure electrolytic copper. Two types of electrodes i.e., solid and bundled (group of copper tubes) have been used in this investigation. The dimension of solid tool is 8mm x 8mm (square) and the bundled electrode has been made of 9 tubular copper cell electrodes having inner and outer diameter of the individual tubes 1.77 mm and.66 mm respectively. The experimental set up has been shown in Fig 1. (a) (b) Figure 1 Experimental set up with (a) bundled tool (b) tool Commercially available dielectric fluid is used during the experiments. Thedielectric fluid being used in present study has flash point of 00 C and viscosity of 50SUS at 100 F. Two different types of flushing, jet flushing and through flushing, have been used..1 Selection of tool geometry Cylindrical tool shape is more common geometry that is being used in electro discharge machining. So circular shape was selected and trial experiments were performed to check the feasibility of the circular shape.in the case of bundled electrode some of the material in the form of pin shaped could not be removed from the cavity (Fig a). So, orbital motion (radius 1 mm) was adopted for bundled electrode to remove the material. But by doing this, proper shape was not generated though unwanted material removed from the 96-

3 5th International & 6th All India Manufacturing Technology, Design and Research Conference (AIMTDR 01) December 1th 1th, 01, IIT machined cavity (Fig b). To improve this shape, more orbital radius (.5 mm) was given to same size circular tool. The cavity generated was much improved shape than the previous shape but was not perfectly circular (Fig c). So it was not possible to select circular shape tool for the experiments. Then square shape bundled electrodes were prepared and trial experiments were performed on it. The observed cavity was perfect square shape with the bundled electrode and so square shape tool was selected (Fig d) kept constant throughout the experiments. In similar type of work (Dave et al., 011), MRR is reported to be maximum at duty factor of 0.7. Hence, in present work duty factor is kept constant at 0.7. All cavities have been generated by providing orbital motion to the tool electrodes.both tool electrodes have been given 1mm orbital radius in order to get identical shape of square cavity. Table 1 Parameters and their levels Parameter Peak current (Ip) Unit A Pulse on time (ton) (a) (b) (c) (d) Figure Machined cavity (a) without orbital (bundled circular electrode) (b) with orbital radius 1 mm (bundled circular electrode) (c) with orbital radius.5 mm (bundled circular electrode) (d) with orbital motion (bundled square electrode). Selection of parameters In the present work current and pulse on time has been selected as input machining parameters because they can potentially affect MRR and TWR during EDM operation (Kansal et al., 006; Tasi et al., 007).Experiments have been carried out on Z Numerically Controlled (ZNC) die-sinking Electro Discharge Machine (EDM). In this machine minimum current 0.A and maximum current 50A can be set.some trials experiments have been conducted with both types of tool electrodes at higher and lower range of peak current. The machining depth was decided as mm. After trial experiments, it has been observed that at higher peak current, bundled electrode bear heavy wear and failed to achieve the required depth. Hence, lower range of peak current (, and A) has been selected for investigation. Pulse on time is from lower to medium range i.e. 1 to 155 µs. The machining conditions and number of levels of the parameters are selected as given in table 1.The other parameters are Values,, µs 1, 61, 9, 11, 155 Duty factor 0.7 Open circuit voltage V 170 Working gap voltage V 6 Polarity + Flushing pressure Kg/cm 0. In this investigation, three levels of peak current (Ip) and five levels of pulse on time (ton) has been taken. Using the factorial design method, total 15 trials have been decided. Each experiment has been replicated twice and average of both results has been used for analysis. The experimental design is such that each level of every factor comes at equal number of times. Table Experimental table and observation Exp. No. Ip ton MRR MRR TWR TWR (Vws) (Vwb) (Vts) (Vtb)

4 ELECTRO DISCHARGE MACHINING OF AISI 0 USING SOLID AND BUNDLED ELECTRODES Results and Discussions All experiments have been carried out using factorial design as mentioned in table 1. The final depth of cavity is fixed to mm for each experiment. As mentioned earlier, solid and bundled types tool electrode have been used. Therefore, two types of flushing methods have been applied i.e. jet and through flushing for solid and bundled respectively. Machining has been done with 1 mm orbit radius to avoid the unmachined pin shape with bundled electrode. Machining time has been carefully recorded for each experiment. The response parameters MRR and TWR have been calculated using weight difference method as shown in equations 1 and. Wwb W wa MRR= (1) ρw t Wtb W ta TWR= () ρt t where, W wb =weight of workpiece before machining, W wa = weight of workpiece after machining, W tb = weight of tool before machining, W ta = weight of tool after machining, ρ = density of AISI0 (8gm/mm ), ρ = w density of Copper (8.9gm/mm ), t = machining time.1 Effect of peak current on MRR and TWR Effect of peak current (Ip) on MRR (mm /min) has been plotted using average of all MRR at particular peak current. MRR (mm /min) t Figure Effect of peak current on MRR with both tools From Fig.,MRR can be observed increasing with peak current. Highest MRR has been calculated with A peak current for both type of tools. With increase in peak current, the thermal energy at the machining area increases due to this, higher MRR can be expected.in comparison with bundled tool electrode, solid tool electrode results higher MRR. Highest MRR obtained with solid tool electrode is 5% more than that of bundled tool electrode. The effective area of machining is larger in solid tool than that of bundled tool electrode. This may leads to the more stable machining in case of solid tool, results higher MRR. The effective area of both tool electrodes has been shown in Fig.. (a) (b) Figure Effective area (a) solid tool (b) bundled tool Effective area of solid tool = side side () = 6 mm π Effective area of bundled tool = (Outer dia.-inner dia.) () π = ( ) = mm As seen in Fig. 5, TWR for both types of tool increases with increase in peak current. This is due to the fact that an increase in dischargecurrent increases the pulse energy that leads to an increasein heat energy rate, which is subjected to both of theelectrodes, and in the rate of melting and evaporation. Peak current (A) 96-

5 5 th International & 6 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 01) December 1 th 1 th, 01, IIT TWR(mm /min) Peak current (A) Figure5 Effect of peak current on TWR with bothtools From Fig 5, one can observe that highest tool wear rate occurs at A peak current and approx. same for both types of tool. At the lower peak current (A), bundled tool electrode bears higher wear rate than solid tool electrode. However, at A peak current lower electrode wear has been observed in bundled tool than solid tool electrode.. Effect of pulse on time (ton) on MRR and TWR Pulse on-time is the time during which current flows in the electrodes. Since all the work is done during on-time, the duration of these pulses and the number of cycles per second (frequency) are important. Metal removal is directly proportional to the amount of energy applied during the on-time (Singh et al., 005). This energy is controlled by the peak amperage and the length of the on-time. With longer pulse duration, more work piecematerial will be melted away. The produced crater will be broader and deeper than a crater produced by shorter pulse duration. But extra large pulse duration can reduce material removal rate. This is because of expanding of plasma channel that distributes the energy concentration (Dave et al, 01). But if the interval is too short, the ejected work piece material will not be swept away by the flow of the dielectric and the fluid will not be de-ionized. This will cause the next spark to be unstable. This slows the machining rate. The effect of pulse on time on MRR has been shown in Fig. 6. The MRR is found decreasing with increase in pulse on time for both solid as well as bundled tool electrode. In case of solid tool, the MRR decreases linearly with pulse on time. This is due to the fact that higher pulse on time expends the plasma channel and energy got distributed which results lower MRR (Dave et al., 01). However in case of bundled electrode, MRR first increases then decreases with pulse on time. This may be because of short pulse duration causes less vaporization whereas long pulse on-time durationcauses the plasma channel to expand, resulting in lessenergy density on workpiece, which is insufficient to meltand/or vaporize the workpiece material(wang and Yan, 000). Thus MRR exhibits increasing (with bundled tool) trend in beginning but further increase in pulse on time decreases the MRR. Highest MRR has been observed at 1µs pulse duration (lowest ton) with solid electrode while highest MRR has been obtained at 61µs pulse duration with bundled tool electrode. Highest MRR with solid tool is 6% more than that with bundled tool electrode. The MRR with solid tool is higher than bundled tool for everypulse on time setting. This may be due to the fact that solid tool has more effective area than bundled tool as discussed in equations () and (). So, due to large effective area more sparks can be generated during pulse duration. These many sparks may be responsible for the higher MRR with solid tool than bundled tool electrode. MRR (mm /min) Pulse on time(µs) Figure6 Effect of pulse duration on MRR with both tools The effect of pulse on time on TWR has been studied through Fig. 7. TWR for both of the tools (solid and bundled) has been decreasing with increase in pulse on time. Highest wear rate has been obtained at 1µs and lowest TWR at 155µs pulse on time. In comparison with bundled tool, solid tool electrode showsless wear rate except at 1µs pulse on time. Lower TWR of solid tool electrode may because of larger effective area of solid tool electrode. Tool having larger effective area will conduct heat energy more rapidly and large portion of heat at tool face is conducted to the dielectric fluid present around the tool surface. Hence, solid tool electrode has been reported with lower TWR. This result is an agreement with researchers (Sohani et al., 009), they have reported decrease in TWR with increase in surface area of tool electrode. 96-5

6 ELECTRO DISCHARGE MACHINING OF AISI 0 USING SOLID AND BUNDLED ELECTRODES TWR (mm /min) Pulse on time(µs) Figure7 Effect of pulse duration on TWR with both tools Conclusions An attempt has been made to study the effect of peak current and pulse on time on MRR and TWR in generation of square shaped cavity with solid and bundled tool electrodes. Experiments have been performed according to fractional factorial design. The following conclusions have been drawn through this investigation: 1. Square shaped cavity has been successfully generated with both types of tool electrodes i.e. solid and bundled tool.. MRR and TWR for both of tools have been increasing with increase in peak current.. tool gives 5% higher MRR than bundled tool electrode at A peak current.. MRR with solid tool has been observed linearly decreasing with increase in pulse on time but MRR with bundled tool electrode shows first increasing than decreasing trend with pulse on time. 5. At lower pulse on time (1µs), TWR for solid tool is more than bundled tool and is decreasing with increasing in pulse on time but at highest pulse on time (155µs) both tool electrodes show approximately same wear rate. References Dave, H.K., Desai, K.P. and Raval, H.K. (011), Effect of the orbital tool movement on the material removal rate during electro discharge machining, Proceedings of International Conference on Advances & Trends in Engineering Materials and their Applications, Montrel, Canada, pp Dave, H.K., Desai, K.P. and Raval, H.K. (01), Experimental investigations on orbital electro discharge Machining of Inconel 718 using Taguchi technique, International Journal of Modern Manufacturing Technologies, Vol. (1), pp Dave, H.K., Sudhanshu, Kumar and Raval, H.K. (01), Electro discharge drilling of through holes under different flushing conditions, Proceedings of th International and 5th All India Manufacturing Technology, Design and Research Conference, December 1 16, 010, Kolkata. Ekmekci, B., Tekkaya, E. and Erden, A. (006), A semi-empirical approach for residual stresses in electric Ferreira, J.C. (007), A study of die helical thread cavity surface finish made by Cu-W electrodes with planetary EDM, International Journal of Advanced Manufacturing Technology, Vol.1, pp Gu, L., Li, L., Zhao, W. and Rajurkar, K.P. (01), Electrical discharge machining of Ti6AlV with a bundled electrode, International Journal of Machine Tools & Manufacture, Vol. 5, pp Jain, V.K. (00),Advance machining Process, Allied Publishers Pvt. Limited, New Delhi. Kansal, H.K., Singh, S., Kumar,P. (007), Effect of Silicon Powder Mixed EDM on Machining Rate of AISI D Die Steel, Journal of Manufacturing Processes, Vol. 9, pp.1-. Mehra, R., Sharma, S.C. and Kalra, C.S. (01), Comparison of material removal rate of ductile cast iron using hollow and solid cylindrical electrical discharge machining electrodes, Journal of Academic Industrial Research, Vol. 1(5), pp Mohan, B., Rajadurai, A. and Satyanarayana, K.G. (00), Electric discharge machining of Al SiC metal matrix composites using rotary tube electrode, Journal of Materials Processing Technology, Vol , pp Murugesan, S., Balamurugan, K., Narayanan, C.S. and Venkatakrishnan, P.G. (01), Study on EDM of Al- 15% Sic MMC using solid and multihole Electrodes - a Taguchi approach, European Journal of Scientific Research, Vol.68, pp Ojha, K., Garg, R.K. and Singh, K.K. (011), Parametric optimization of PMEDM process using chromium powder mixed dielectric and triangular shape electrodes, Journal of Minerals & Materials Characterization & Engineering, Vol. 10, pp Rajurkar, K.P., Sundaram, M.M. and Malshe, A.P. (01), Review of Electrochemical and Electrodischarge Machining, Procedia CIRP, Vol. 6, pp Singh, S., Kansal, H.K., Kumar, P. (005),Parametric optimization of powder mixed Electrical discharge machining by response surface methodology, Journal of 96-6

7 5 th International & 6 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 01) December 1 th 1 th, 01, IIT Materials Processing Technology, Vol. 169 (), pp.7-6. Sohani, M.S., Gaitonde, V.N., Siddeswarappa, B. and Deshpande, A.S. (009), Investigations into the effect of tool shapes with size factor consideration in sink electrical discharge machining (EDM) process, International Journal of Advanced Engineering Technology, Vol. 5, pp Tsai, Y. Y., Lu, C. T. (007), Influence of current impulse on machining characteristics in EDM, Journal of Mechanical Science and Technology, Vol. 1, pp Wang, C.C. and Yan, B.H. (000), Blind-hole drilling of Al O /6061Al composite using rotary electrodischarge machining, Journal of Material Processing Technology, Vol.10, pp Yilmaz, O. and Okka, M.A. (010),Effect of single and multi-channel electrodes application on EDM fast hole drilling performance, International Journal of Advanced Manufacturing Technology, Vol. 51, pp