## 3 Results and Discussion
### Effect of Electrode Design on Machining Time (MT)
In this way, the productivity of EDM could be substantially improved. The phenomenon behind this sound reduction in machining time is believed to be the sparking behavior in different tool designs. Since, the material removal during EDM is based on the successive sparks produced in front of the exposed surfaces of the electrodes. At the beginning of the machining feature (hole), the spark generation was initiated at the tool's footing face and the material erosion was caused from the top surface of the substrate. In the case of conventional design type (cylindrical electrode; \(\rm D_{CR0}\)), as the machined feature progressed and a certain depth was achieved, the tool progressively tended to penetrate longitudinally inside the machined depth. At this stage, the surface area available to cause the sparking was greater. In addition to the footing face of the tool, the side surface around the tool periphery also produced electric sparks with the side walls of the machined feature. The phenomenon may be termed as the side sparking or the peripheral sparking. The surface area at  
Figure 5: The effect of different design types on the machining time of EDM of WC. the bottom of the tool remained constant throughout the machined feature. However, as the machining depth was further achieved the surface area of the tool around the periphery further increased because of the tool's uniform cross-section. Resultantly, the amount of side sparking was enhanced as depicted in Figure 6a. The surface area available for the cumulative sparking (bottom sparking as well as side sparking) continuously went on increasing because of the uniform tool cross-section throughout the tool length. This endless increase in the surface area continued as long as the hole was formed. If the input spark energy remained constant and the surface area for electric discharges was increased, then the constant amount of energy needs to be distributed over the larger area (bottom surface plus the inner walls of the hole). In this way, the energy density per unit area reduced and consequently the machining rate slowed down. On the other end, the provision of the relief angle in the tool (D\({}_{\text{R}}\)) built the tapered walls of the tool instead of straight vertical walls. As a result, the gap between the hole's inner walls and the tool's side walls increased. The gap due to the relief angle was so enlarged that the voltage difference between the two walls became high and consequently it did not allow the spark to be initiated. In this way, the surface area available for sparking was the only tool's bottom faced-area and the chances of side or peripheral sparking were minimized as could be envisioned from the schematic shown in Figure 6b. Since, during EDM the erosion of tool also took place along with the workpiece, therefore the possible drawback of this design (D\({}_{\text{R}}\)) could be the reduction in tool diameter at the bottom side due to the longitudinal wear of the tool. Thus, another design type named as relief angle with land (D\({}_{\text{RL}}\)) was introduced in which the relief angle was provided at a height of 1 mm starting from the bottom end of the tool. The tool diameter remained the same within a 1 mm height and then reduced because of the relief angle. At the beginning of the machining, the sparking phenomenon during this design of the electrode was similar to the previous two designs. The sparking behavior changed as the machining depth progressed. Up to the machining depth of 1 mm, starting from the top most layer of the work surface, the surface area for sparking also increased (bottom surface and peripheral surface). This increase in collective surface area (bottom and peripheral) acquired a maximum value when the machining depth of 1 mm was achieved. As the depth further propagated the surface area for sparking remained constant since the sidewalls of the tool were straight in a 1 mm distance and after that the sides were tapered due to the provision of the relief angle.