EFFECT OF TOOL POLARITY ON THE MACHINING CHARACTERISTICS IN ELECTRIC DISCHARGE MACHINING OF SILVER STEEL AND STATISTICAL MODELLING OF THE PROCESS

EFFECT OF TOOL POLARITY ON THE MACHINING CHARACTERISTICS IN ELECTRIC DISCHARGE MACHINING OF SILVER STEEL AND STATISTICAL MODELLING OF THE PROCESS DILSHAD AHMAD KHAN Department of Mechanical Engineering Krishna Institute of Engineering & Tech, Ghaziabad, UP, (INDIA) dilshad.limra@gmail.com MOHAMMAD HAMEEDULLAH Department of Mechanical Engineering Aligarh Muslim University, Aligarh, UP, (INDIA) hameedullah_amu@yahoo.com Abstract : Electric discharge machining (EDM) is a thermoelectric process in which electrical energy is converted into thermal energy and this thermal energy is used for the machining purpose. It is the common practice in EDM to make tool negative and work piece positive (direct polarity), but researches shows that reverse of it is also possible in which tool is positive and work piece is negative ( reverse polarity), but not much work has been carried out on the reverse polarity till now. This paper discusses the effect of tool polarity on the machining characteristics in electric discharge machining of silver steel. High metal removal rate, low relative electrode wear and good surface finish are conflicting goals, which cannot be achieved simultaneously with a particular combination of control settings. To achieve the best machining results, the goal has to be taken separately in different phases of work with different emphasis. A 3 2 factorial design has been used for planning of experimental conditions. Copper is used as tool material and Silver steel of 28 grade is selected as work piece material with positive and negative polarities. The effectiveness of EDM process with silver steel is evaluated in terms of metal removal rate (MRR), percent relative electrode wear (%REW) and the surface roughness (S.R) of the work piece produced at different current and pulse duration levels. In this experimental work spark erosion oil (trade name IPOL) is taken as a dielectric and experiments have been conducted at 50% duty factor. The study reveals that direct polarity is suitable for higher metal removal rate and lower relative electrode wear but reverse polarity gives better surface finish as compared to direct polarity. Direct polarity gives 4-11 times more MRR and 5 times less relative electrode wear as compared to reverse polarity, and reverse polarity gives 1.3-2.7 times better surface finish as compared to direct polarity. Second order regression model is also developed for output parameters. Keywords: polarity, electric discharge machining, relative electrode wear (REW), regression model. 1. Introduction Dilshad Ahmad Khan et al. / International Journal of Engineering Science and Technology (IJEST) Electrical discharge machining, commonly known as EDM, is a process that is used to remove metal through the action of an electrical discharge of short duration and high current density between the tool and workpiece. There are no physical cutting forces between the tool and workpiece. EDM has proved especially valuable in the machining of super tough, electrically conductive materials such as the new space-age alloys. These metals would have been difficult to machine by conventional methods, but EDM has made it relatively simple to machine intricate shapes that would be impossible to produce with conventional cutting tools. This machining process is continually finding further application in the metal machining industry. It is being used extensively in the plastics industry to produce cavities of almost any shape in the metal moulds. Although, the application of EDM is limited to the machining of electrically conductive workpiece materials, the process has the capability of cutting these materials regardless of their hardness or toughness. Among the several EDM process parameters polarity of the electrode is one of the important parameter. The polarity of the electrode can be either positive or negative. The current passing through the gap creates spark that produces high temperature causing material evaporation at both electrode spots. The plasma channel is composed of ion and electron flows. As the electron processes (mass smaller than anions) show quicker reaction, the anode material is worn out predominantly. This ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5001

effect causes minimum wear to the tool electrodes and becomes of importance under finishing operations with shorter on-times. However, while running longer discharges, the early electron process predominance changes to positron process (proportion of ion flow increases with pulse duration), resulting in high tool wear. In general, polarity is determined by experiments and is a matter of tool material, work material, current density and pulse length combinations [Ho and Newman (2003)]. Literature shows that not much work has been carried out directly on the polarity of the electrode till now, so this experimental work shows the effect of electrode polarity on the machining characteristics of the silver steel. Patel et al. (1989) conducted experiments using copper both as tool electrode and work piece, erosion of cathode was found to be about 2 3 times more than the erosion of anode and it increased with increase in pulse duration till it attained the maximum value at about 200µs. Xia et al.(1996a,b) shows the energy distribution in the anode and cathode. When copper was used for both anode and cathode under the same pulse conditions. The energy distribution to anode and cathode was about 40% and 25%, respectively. They reported that the energy distributed to anode is always greater than that to cathode, and is rarely affected by the discharge duration, in both single discharge and continuous pulse discharges. They investigated the influence of carbon adhesion to the anode on the removal amount under different machining conditions, such as discharge duration, polarity, and dielectric flushing flow rate. It was concluded that the difference in removal amount between anode and cathode is mainly caused by the phenomenon of carbon adhesion onto the anode surface, not by energy distribution, when hydrocarbon dielectrics are used and both anode and cathode are made of copper. Wong Y. S et al. (1998) performed experiments on AISI- O1 tool steel using powder mix dielectric(pmde-edm) and t his has been tested through all possible combinations of the pulse current, pulse duration, servo feed rate and electrode polarity. They found that there was a clear difference between surfaces produced by positive and negative electrode polarities. Although surfaces resulting from both are matt, a dull appearance was characteristic of machining with a positive electrode. Yan B.H et al. (1999) performed micro-hole machining on carbide and observed that Polarity is the most important parameter that affects the tool electrode wear rate and the expansion of machined micro-hole. Experiments show that the tool electrode wear in negative polarity machining is much larger than that in positive polarity machining. In positive- polarity machining, the tool electrode wear almost does not take place, which results in a better profile of the machined micro-hole. While carrying out parametric optimization of the process on D2 tool steel work pieces using 75/25 tungsten copper electrode with normal polarity, Marafona and Wykes (2000) found that a black layer of carbon was deposited on the tool when low current intensity and long pulse duration were used. This layer inhibited further tool wear and a higher current intensity could then be used to improve MRR without corresponding increase in TWR. It was likely that the carbon came from the dielectric medium. Uhlmann and Roehner (2008) conducted experiments with novel tool materials Boron doped CVD-diamond (B-CVD) and polycrystalline diamond (PCD) with micro EDM to reduce the tool wear. They found that apart from the specifications of the tool electrode material, the process behavior and especially the wear behavior of the tool electrode is influenced by the polarity.the removed material volume of a single discharge is bigger at the anode compared to the volume at the cathode due to the faster accelerating electrons in the plasma channel. This effect is known as the polarity effect. In experimental investigations B-CVD lead to a fifty times higher wear with positive polarity than with negative polarity. The machining time simultaneously increases by more than 200%, from 40 min with positive to 140 min at negative tool electrode polarity. Moreover tool electrode polarity significantly affects surface formation on the work piece. Janmanee & Muttamara (2010) performed experiments on tungsten carbide (WC-Co) with EDM process by using tools of graphite (Poco EDM -3), copper-graphite (Poco EDM-C3) and copper-tungsten (solid) at positive and negative tool electrode polarity and found that negative tool perform well. MRR was very high when tool polarity was negative. Lin et al. (2001a,b) did experiments on hybrid machining process combining EDM and ultrasonic machining (USM) to achieve surface modification of an aluminium alloy with copper electrode and silicon carbide abrasive particles of 20µm diameter.from the experimental results they found that significantly, the roughness of the surface machined by negative polarity was coarser than the surface machined by positive polarity. A study on dry EDM by Kunieda and Yoshida (1997) with copper as tool electrode and steel as work piece revealed that in case of EDM in air, the tool electrode wear ratio was much lower and MRR much higher when tool electrode was negative. Han F. et al. (2006) conducted experiments on sub-micrometer order size machining using micro EDM with both the polarities. Material removal rate with reversed polarity was considerably lower than that of straight polarity. The machining time was about eight times longer than straight polarity. The machining surfaces obtained using straight polarity and reversed polarity was also observed in the experiments. The surface obtained by reversed polarity was found to be smoother than that using straight polarity. Muttamara, et. al. (2009) compared the result of negative and positive tool polarity in electric discharge machining of alumina. Copper, graphite (Poco EDM-3) and copper-infiltrated-graphite (PocoEDM-C3) electrodes were used. They found that positive electrode have more MRR and less electrode wear ratio but better surface finished were achieved with the negative tool electrode. They concluded that the surface roughness can be reduced by 12% compared with negative work piece polarity. Liu, et al. (2008) conducted electric discharge milling of silicon carbide ceramic and the effects of tool polarity, peak voltage, pulse on-time, pulse off-time, and peak current on process ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5002

performance have been investigated. They found that under the same conditions the MRR in positive tool polarity was 3.5 times that with negative tool polarity, wear ratio in negative tool polarity was 2 times that with positive tool polarity and SR in negative tool polarity was five times that with positive tool polarity. Jilani, et al. (1984) have presented some result on direct and reverse polarity in their experiments on machining of low carbon steel workpiece at low current densities. Tests were conducted with brass (±) and copper (±) tools with distilled, tape and a mixture of distilled and tape water as dielectric. They concluded that for brass (+) tools effective machining could not be achieved altogether and the work electrode at the end of the test run was found to be coated with a thin layer of tool material. Copper (+) tool resulted in high electrode wear ratio in the range of 45-140%. They found zero electrode wear when copper tools with negative polarity were used. They presented result on metal removal rate for both the polarity for copper tools. The result shows that metal removal was high in case of positive tool when dielectric was distilled water. Lee & Li (2001) investigated the results of electric discharge machining of tungsten carbide at varying pulse duration and current at direct and reverse polarity settings, the tool materials was graphite (Gr), copper (Cu) and copper tungsten (CuW) and tungsten carbide as the workspace material. The results show that in machining tungsten carbide, the use of negative electrode polarity is more desirable this is because the material removal rate is higher, and the relative wear ration is lower (15-20%) than using a positive electrode (17-34%). The material removal rate was 5-20% higher than for machining with a positive tool. Negative polarity produced a fairly constant relative wear ratio, where as positive polarity gave a generally decreasing trend. From the observations, it is evident that negative tools performs better than positive tools in terms of material removal rate, relative wear ratio, dimensional accuracy and surface finish of work piece produced. From the above literature review it is evident that effect of polarity on EDM performance is conflicting and to draw a general conclusion about the effect of polarity on electric discharge machining performance is very difficult, so in this paper polarity effect of polarity on machining performance is observed. 2. Experimental Setup 2.1. Machine Tool TOOL CRAFT E.D.M, model G-45, is a die-sinking machine manufactured by tool craft peenya industrial area Bangalore. It is energised by a 25 A pulse generator. A hydrocarbon spark erosion oil trade name IPOLE is used as the dielectric during the experiments. The surface roughness of the machined work piece is measured by portable surface roughness measuring instrument HANDYSURF (E-35A). 2.2. Experimental Details The properties of the electrode and workpiece materials are as follows: Tool - Copper Composition=99.9%copper, density=8.96gm/cm 3, melting point=1083 0 C, electrical resistivity=9µωcm, Hardness=100BHN. Workpiece-Silver steel Composition=1.13%C;0.22%Si;0.37%Mn;0.014%P;0.018%S;0.43%Cr, density=7.83gm/cm 3, melting point=1370 0 C, hardness=270bhn. Experiments has been conducted at 20,100 and 500 µs pulse durations and at 6.25, 14.0625, 21.875A current levels. Duty factor is taken as 50%, test duration was 20min. Machining is done at both the polarities. The tools are 60 mm in length and 12.5 mm in diameter. For the centre flushing system a cylindrical hole of 3 mm diameter is provided throughout the length of the tool, the work piece is of 60 mm in length and 10 mm in diameter. 2.3. Design of Experiments. It is difficult to formulate a mathematical model for the complex production system, such as EDM, by the existing methods of science and engineering. However accurately recorded data is abundant and can be obtained for most variables of importance which control productivity of the process. A systematic and quantitative analysis of the observed data could lead us to the equation governing the system performance, which forms its mathematical model. The EDM process may be modeled as a cybernetic black box system where investigations into the effect of dielectric and other operating parameters on the output quantities can be based on the model shown in above Fig 3, [Jilani and Pandey (1984)], in this figure Xi are the factors, whose effect on the process have to be investigated, ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5003

Fig. 1 cybernetic black box system Zm are the factors held constant during the test and Yu are the measured values of the responses. In the present study Xi are identified as: pulse on period and pulse current ; factor Zm are identified as : open circuit voltage ( 45 V ), machined material ( silver steel ), dielectric fluid (trade name Ipol), pulse duty factor (50%), tool electrode materials and its polarity (copper ±), and test duration (20 min) ; and factors Yu are identified as : metal removal rate ( mg/min), relative electrode wear (%), and surface roughness (hmax, µm). the experimental work done to study the factorial effect is planned in accordance with the statistical techniques of the experimental design. With a well designed experiment it is possible to determine accurately with a much reduced effort, the effect of change in any one variable on the process output (also known as response or yield) and the interaction effect between the different factors if any. If all the investigated factors are quantitative in nature, then it is possible to approximate the response Yu as a polynomial (equation 1). k k 2 Y u = b 0 + ( ) + (. ) + (.. ) bi xi bii xi b x. (1) ij i j i= 1 i= 1 i< j Where x i ( i = 1,2,. K ) are coded levels of k quantitative variables and b 0, b 1, etc are the least square estimates of the regression coefficient. The polynomial in equation (1) is known as regression function and the first term under the summation sign pertains to linear effect, the second term under the summation sign pertains to quadratic effects, and the third term pertains to interactions effects of the investigated parameters. For estimation of the regression coefficients in the equation (1) effect of each of the variables x i on yield Y u must be studied at least three different levels.this suggest the use of a 3 2 factorial experimental design. If the three levels of any factor x i are coded as -1, 0, 1, the second order response equation is easy to derive. Variables Table 2. Scheme of experimentation Trial No. 1 2 3 4 5 6 7 8 9 X 1-1 -1-1 0 0 0 +1 +1 +1 X 2-1 0 +1-1 0 +1-1 0 +1 x Table 3. Range of machining parameters selected for experimentation Levels in coded form Variables -1 0 +1 Pulse current I(A) 6.25 14.0625 21.875 Pulse duration t i (µs) 20 100 500 Where: X 1 = current, X 2 =pulse duration, The three levels for variables X 1, X 2 are given in Table 3. The coded values for the variables used in Table 2 and 3 have been obtained from the following transformation equations: ( ) I 21.875 X 1 = 2. + 1 21.875 6.25 (2) ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5004

ln ln 500 ( ) ti X 2 = 2. + 1 ln 500 ln 20 (3) 3. Results And Discussion 3.1 Experimental Results. In this study the machining characteristic factors valuated were material removal rate (mg/min), relative electrode wear (%) and machined workpiece surface roughness R a (µm). Material removal rate represents the average mass of material removed from the workpiece per unit time. In an EDM process, both the tool and workpiece are eroded during discharge. The relative electrode wear is calculated as the volume of material eroded from the tool electrode per unit time divided by the volume of the material eroded from the work piece in the same time. The parameter used for the surface roughness is Ra, which is the arithmetic mean of the departure of the roughness profile from the mean line. 3.1.1 Effect of polarity on metal removal rate The results of metal removal rate at direct and reverse polarity are plotted in fig. 2 at different current and pulse duration levels. From the results of metal removal rate with copper tool at direct and reverse polarity we see that with the increase of pulse duration metal removal rate increases up to 100 µs pulse duration and at higher pulse duration metal removal rate decreases. Fig 2. MRR vs. pulse duration at different current levels with direct and reverse polarity Metal removal rate depends on spark energy; spark energy (E s ) is a function of discharge current (I d ), gap voltage (V g ) and pulse duration (T on ). Spark energy follows the relation as E s =V g I d T on. As the pulse duration increases spark energy increases which in turn increase the metal removal rate but at higher pulse duration spark becomes unstable and conversely it decreases the metal removal rate. So at the lower pulse duration metal removal rate increases but at higher pulse duration it decreases. Form fig 2 it is seen that the highest metal removal rate is achieved at 100 µs pulse duration. It is known that spark energy depends on pulse current, so as the pulse current increases it increases the spark energy which in turn increases the metal removal rate. From fig. 2 it is noticed that as the current increases metal removal rate also increases and so at higher current metal removal rate is also higher. From fig 2 it is seen that the results of metal removal rate follow the same trend in reverse polarity as in case of direct polarity and it is seen that at all the current levels and at all the pulse duration the direct polarity gives the higher metal removal rate as compared to reverse polarity. Xia et al.(1996b) has been reported that the energy distribution to anode and cathode is about 40% and 25% respectively and it is never affected by the pulse duration,so that in the case of direct polarity 40% energy is applied at the work piece and 25% energy is applied at the tool and condition is reverse in the case of reverse polarity. So that higher metal removal rate is achieved with the direct polarity and this result agrees with Lee & Li (2001). From the experimental results it is seen that direct polarity has 4-11 times more metal removal rate as compared to reverse polarity. ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5005

3.1.2. Effect of polarity on relative electrode wear. The results of relative electrode wear at direct and reverse polarity are plotted in fig.3 at different current and pulse duration levels. From fig.3 it is seen that all the curves of relative electrode wear (REW) for direct and reverse polarity decreases as the pulse duration increases. Fig.3 Relative electrode wear (REW) vs. pulse duration at different current levels for copper tool at direct and reverse polarity. Marafona and Wykes (2000) has reported that at low current and at higher pulse duration hydrocarbon dielectric decomposes and fee carbon stick with the tip of tool, this carbon layer prevents the further tool wear. From the experimental work it is found that as the pulse duration increases relative electrode wear decreases and it could be because of adhesion of carbon layer to the tip of tool which reduces the tool wear and in turn relative electrode wear, so that at lower pulse duration the relative electrode wear is more and at higher pulse duration it decreases. From figure it is also seen that as the current increases from low to medium level the relative electrode wear decreases. At the higher current hydrocarbon dielectric decomposes and free carbon adhere to the tool tip and prevent its further wear and as the pulse duration increases this carbon layer becomes thicker and relative electrode wear decreases as the pulse duration increases. The curve of high current lie in between curves of low and medium currents up to 100µs pulse duration and at 100µs pulse duration the value of relative electrode wear for medium and high current is almost same but at 500µs the value of relative electrode wear decreases up to 2.91%. When current increase the spark energy increase which in turn increases the relative electrode wear so that the graph of higher current is above the graph of medium current up to 100µs pulse duration and because at higher pulse duration carbon layer becomes thicker and it prevents the wear of tool so that at 500µs pulse duration the value of relative electrode wear decreases as compared to the value at medium current level. From fig.3 it is also seen that at 20µs pulse duration as the current increases relative electrode wear increases with reverse polarity and it is because at higher current spark energy is higher which increases the relative electrode wear. It is seen that in the case of direct polarity the carbon adhesion phenomenon exist which decrease the relative electrode wear as current increases, but in reverse polarity it is not so and it could be because in reverse polarity more hear is generated at the tool and at low pulse duration there is not sufficient time available to adhere the carbon layer to the tip of tool and before the adhesion of carbon layer to the tool tip tool eroded. So that with the increase of current at low pulse duration level relative electrode wear (REW) increases. From figure it is seen that with the reverse polarity at medium and high pulse duration relative electrode wear (REW) decreases with the increase of current, it is because experiments has been conducted at 50% duty factor so that at higher pulse duration higher pulse off time is present which is sufficient to adhere the carbon layer to the tool tip which decreases the tool wear. From the figures it is seen that all the graphs of relative electrode wear (REW) for direct and reverse polarity follow the same decreasing trend and the explanation of this nature of graph has been presented before. From the figures it is seen that lower relative electrode wear is achieved with the direct polarity at all current and pulse duration levels. As from the literature survey it is known that Xia, et al. (2004b) has reported that 40% heat is generated at anode and 25% heat is generated at cathode. They also reported that the energy distributed to anode is always greater than that to cathode, and is rarely affected by the discharge duration. So as in the case of direct polarity where wok piece is made anode and tool is made cathode 40% heat is generated at work piece and 25% heat is generated at tool and its reverse is true for reverse polarity means 25 % heat is generated at work piece and 40% heat is generated at tool. So that with direct polarity metal removal rate is higher and tool wear is lower as compared to reverse polarity. From fig.3 it is seen that direct polarity gives lower relative electrode wear as compared to reverse polarity, so that with the direct polarity per 100 volume erosion of work piece low volume of tool is eroded as ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5006

compared to reverse polarity and these results are supported by Lee & Li (2001). From the figures it is seen that lowest relative electrode wear is achieved at high current at high pulse duration with direct polarity and its value is 2.91%. During the experiments ratio of relative electrode wear ( REW at reverse polarity/ REW at direct polarity) ranges from 2.14-8.28 on an average reverse polarity gives 5 times more relative electrode wear as compared to direct polarity. So from the above discussion we see that direct polarity performs well with the concern of relative electrode wear. 3.1.3 Effect of polarity on surface roughness The results of surface roughness at direct and reverse polarity are plotted in fig.4 at different current and pulse duration levels. From the figure it is seen that the graph of surface roughness at low current decreases with the increase of pulse duration and this trend is opposite to the general trend of increasing surface roughness with the increase of pulse duration and this behaviour of the graph may be because of carbon adhesion to the tool and work piece it is needs to be further explored. From the curves of medium and high currents it is seen that as the pulse duration increases the roughness of the machined surface increases. Fig.4 Surface roughness vs. pulse duration at different current levels for copper tool at direct and reverse polarity From the figure it is seen that at high current level the surface roughness is lower than the surface roughness at medium current level up to 100µs pulse duration and at 500µs it increases this effect may be because of adhesion of carbon layer on the tip of tool which make a smooth surface on the tip of tool which replicates on the work piece but at higher pulse duration spark energy is more with remove the carbon layer and surface roughness again increases. From the curves of surface roughness at reverse polarity it is seen that as the pulse duration and current increases the surface roughness increases and this is the general trend of surface roughness with the increase of pulse duration and current spark energy increases which produces the wide and deeper crater at the machined surface which produces the matt and rough surface at the machined surface. From the figures it is seen that reverse polarity provides the lower surface roughness (better surface finish) then the direct polarity and this observation is supported by Han, et al. (2006) and Lin et al. (2001a,b). The curves of surface roughness at 14.0625A and 21.875 A with direct and reverse polarity follow the same trend, increase of surface roughness with the pulse duration. From figures it is seen that better surface finish is achieved with reverse polarity at low current and low pulse duration level. From the experimental results it is seen that direct polarity gives almost 1.3-2.7 times higher surface roughness as compared to reverse polarity. 3.2 Statistical Modelling From the experiments it is seen that higher MRR and lower relative electrode wear is achieved with direct polarity but the lower surface roughness is achieved with reverse polarity, so that mathematical model for MRR and relative electrode wear is prepared for direct polarity and for surface roughness it is prepared for reverse polarity. In all the model equations: X 1 = current, X 2 =pulse duration, Y= response or yield (MRR, S.R, REW). ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5007

3.2.1. Second order regression model for MRR at direct polarity. The regression equation Y = 1.37 + 0.145 X 1 + 0.0413 X 2 + 0.0480 (X 1 ) 2-0.000078 (X 2 ) 2 + 0.000011 (X 1.X 2 ) Analysis of Variance Source DF SS MS F P Regression 5 848.47 169.69 96.9 0.002 Residual error 3 5.25 1.75 Total 8 853.73 S = 1.32337 R-Sq = 99.4% R-Sq (adj) = 98.4% For the above model P value is 0.002 (< 0.05) and R-Sq is 99.4 % (>80%) so the above regression model for MRR is statistically adequate. Fig. 5 shows the curves of experimental and predicted value of MRR both the curves almost coincides. Fig. 5 experimental and predicted value of MRR for 9 different runs 3.2(b) Second order regression model for relative electrode wear at reverse polarity. The regression equation Y = 24.1-1.18 X 1-0.122 X 2 + 0.0341 (X 1 ) 2 + 0.000191 (X 2 ) 2 + 0.000179 (X 1.X 2 ) Analysis of Variance Source DF SS MS F P Regression 5 175.342 35.068 34.18 0.008 Residual error 3 3.078 1.026 Total 8 178.421 S = 1.01294 R-Sq = 98.3% R-Sq (adj) = 95.4% For the above model P value is 0.008 (< 0.05) and R-Sq is 98.3% (>80%) so the above regression model for relative electrode wear is statistically adequate. Fig. 6 shows the curves of experimental and predicted value of relative electrode wear, both the curves almost coincides. ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5008

Fig. 6 experimental and predicted value of relative electrode wear for 9 different runs 3.2.3 Second order regression model for surface roughness at reverse polarity. The regression equation Y = 2.31-0.0876 X 1 + 0.00705 X 2 + 0.00710 (X 1 ) 2-0.000012 (X 2 ) 2 + 0.000063 (X 1.X 2 ) Analysis of Variance: Source DF SS MS F P Regression 5 7.2099 1.442 136.56 0.001 Residual error 3 0.0317 0.0106 Total 8 7.2416 S = 0.102760 R-Sq = 99.6% R-Sq (adj) = 98.8% For the above model P value is 0.001 (< 0.05) and R-Sq is 99.6% (> 80%) so the above regression model for surface roughness is statistically adequate. Fig. 7 shows the experimental and predicted values of surface roughness of the machined part produced the curve of predicted values completely overlap the curve of experimental value, so that the model well presented the experimental values of surface roughness at reverse polarity. 4. Conclusion Fig. 7 experimental and predicted value of surface roughness for 9 different runs From the experimental results it is seen that the higher metal removal rate and lower relative electrode wear (%) is achieved with the direct polarity but better surface finish is achieved with the reverse polarity. it is seen that direct polarity gives 4-11 times more metal removal rate and 5 times less relative electrode wear as compared to reverse polarity in EDM. Direct polarity gives almost 1.3-2.7 times higher surface roughness as compared to reverse polarity, so for the better surface finish reverse polarity is desirable. Presented model equations are statistically adequate and give almost the same values as from the experiments. ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5009

Acknowledgement The authors would like to express their deep gratitude to the Department of mechanical engineering of Aligarh Muslim University (AMU), for providing the laboratory facilities and financial support. References [1] Han F, Yamada Y, Kawakami T, Kunieda M.(2006): Experimental attempts of sub- micrometer order size machining using micro-edm. Precision Engineering 30, pp. 123 131. [2] Ho, K.H., Newman, S.T., (2003): State of the art electrical discharge machining. International Journal of Machine Tools & Manufacture 43, 1287 1300. [3] Janmanee P., Muttamara A.(2010): Performance of Difference Electrode Materials in Electrical Discharge Machining of Tungsten Carbide. Energy Research Journal 1 (2): 87-90, 2010. ISSN 1949-0151. [4] Jilani,S.T., Panday,P.C.,(1984): Experimental investigations into the performance of water as dielectric in EDM. international journal of machine tool design and research 24,31-43, no.1. [5] Khan, D.A., Hameedullah.M., (2010): Effect of Tool Polarity on the Electric Discharge Machining Performance of Silver Steel, published in Proceedings of IEEE International Conference on emerging Trends in Engineering & Technology (IETET-2010), Kurukshetra, Hariyana, INDIA, pp: 191-196, ISBN:978-93-80697-22-2. [6] Kunieda, M., Yoshida, M., (1997):Electric discharge machining in gas. Annals of the CIRP 46 (1), 143 146. [7] Lee S.H, Li X.P.(2001): Study of the effect of machining parameters on the machining characteristics in electrical discharge machining of tungsten carbide. Journal of materials processing technology 115 (2001) 344-358. [8] Lin, Y.C., Yan, B.H., Huang, F.Y., (2001a): Surface improvement using a combination of electrical discharge machining with ball burnish machining based on the Taguchi method. International Journal of Advanced Manufacturing Technology 18, 673 682. [9] Lin, Y.C., Yan, B.H., Huang, F.Y.,(2001b): Surface modification of Al Zn Mg aluminium alloy using the combined process of EDM with USM. Journal of Materials Processing Technology 115, 359 366. [10] Liu Y, Ji R., Li Q., Yu L., Li X.,(2008): Electric discharge milling of silicon carbide ceramic with high electrical resistivity. International Journal of Machine Tools & Manufacture 48 (2008) 1504 1508. [11] Marafona, J., Wykes, C.,(2000): A new method of optimizing material removal rate using EDM with copper tungsten electrodes. International Journal of Machine Tools & Manufacture 40 (2), 153 164. [12] Muttamara A., Fukuzawa F., Mohri N., Tani T., (2009): Effect of electrode material on electrical discharge machining of alumina. Journal of materials processing technology 2 0 9 (2009 ) 2545 2552. [13] Patel, M.R., Barrufet, M.A., Eubank, P.T., (1989): Theoretical model of the electrical discharge machining process II, the anode erosion model. Journal of Applied Physics 66 (9), 4104 4111. [14] Uhlmann.E., Roehner.M.,(2008): Investigations on reduction of tool electrode wear in micro-edm using novel electrode materials. CIRP Journal of Manufacturing Science and Technology 1 (2008) 92 96. [15] Wong, Y.S., Lim, L.C., Rahuman, I., Tee, W.M., (1998): Near-mirror-finish phenomenon in EDM using powder-mixed dielectric. J. Mater. Process. Technol. 79 (1 3), 30 40. [16] Xia H., Hashimoto, H., Kunieda M., Nishiwaki N., (1996a): Measurement of Energy Distribution in Continuous EDM Process, J. of JSPE, 62, 8, 1141-1145 (in Japanese). [17] Xia H, Kunieda M, Nishiwaki N, (1996b): Removal amount difference between anode and cathode in EDM process. Int J Electrical Machining,1, pp. 45 52. [16] Yan B.H; Huwang F. Y; Chow H.M; Tsai J.Y,(1999): Micro-hole machining of carbide by electric discharge machining. Journal of Materials Processing Technology,87, pp. 139 145. ISSN : 0975-5462 Vol. 3 No. 6 June 2011 5010