## 3 Results and Discussion
### Effect of Electrode Design on Radial Undercut at the Hole Entrance (RUC_ex)  
On the other side, the hole diameter at the exit plane seemed to be smaller than the actual tool diameter. The result of this difference in diameters was termed as the radial undercut at the hole exit, which was calculated, for all the tool designs, using Equation (4). The experimental results of EDM in terms of radial overcut against each of the ten designs are plotted as shown in Figure 9.  
The major cause of the undercut at the hole exit was the presence of taperness alongside the inner walls of the machined hole. That is why graphical trends in the case of undercut were matching to those trends noticed for hole taper angle. For example, in the case of relief angle design type (D\({}_{\text{R}}\)) as the size of the relief angle increased from 10 to 45\({}^{\circ}\), the undercut at the exit of the hole continuously increased. Tool wear was the main reason behind this progressive escalation in the undercut. As the tool worn  
Figure 8: The effect of different design types of the tool on the radial overcut at the hole entrance after the EDM of WC.  
Figure 9: The effect of different design types of the tool on the radial undercut at the hole exit after the EDM of WC.  
out from the bottom plane the length of the tool was reduced. At the reduced length the corresponding diameter of the electrode also reduced due to the presence of the relief angle. Thus, the diameter of the electrode available at the final cuts, to finish the hole, was comparatively smaller than the diameter available at the initial cuts. In this way, the undercut was produced. The higher the relief angle is, the smaller the diameter of the electrode available to finish the hole at the exit layers is. Therefore, the high amount of undercut was produced as can be observed in the case of \(\mathrm{D_{R45}}\), which produced the largest undercut (661.255 \(\mathrm{\SIUnitSymbolMicro m}\)) within the category of tools with a relief angle design (\(\mathrm{D_{R}}\)). The trend of the undercut caused by the design type \(\mathrm{D_{RL}}\) was again similar to the trend of \(\mathrm{D_{R}}\). Just in the case of the 45 degree relief the undercut was reduced. However, this reduction was so small (less than 5 \(\mathrm{\SIUnitSymbolMicro m}\)) that it could be ignored. For example the undercut with a 30 degree relief is 343 \(\mathrm{\SIUnitSymbolMicro m}\) whereas with a 45 degree relief it was 338.75 \(\mathrm{\SIUnitSymbolMicro m}\). Thus it can be said that the results pattern of \(\mathrm{D_{RL}}\) designs was similar to the case of \(\mathrm{D_{R}}\) designs. The design having a relief angle of 10\({}^{\circ}\) with 1 mm land (\(\mathrm{D_{RL10}}\)) produced the minimum of 92 \(\mathrm{\SIUnitSymbolMicro m}\) undercut within the category of \(\mathrm{D_{RL}}\) design. An extraordinary undercut was observed in the case of \(\mathrm{D_{RL90}}\) where the relief angle of 90 was provided. The exit diameter of the hole was found to be highly undersized as compared to the designed diameter of 8 mm. The land of 1 mm was found to be totally eroded, which indicates that the behavior of the current flow was highly erratic, which affected the electrode wear in an entirely different pattern as compared to the wear observed in all the other tools. Upon a comparison among the performance of different electrodes, it can be noticed that the minimum size of undercut at the hole exit was found to be 26.50 \(\mathrm{\SIUnitSymbolMicro m}\) when conventional cylindrical tool (\(\mathrm{D_{C}}\)) was employed during EDM of WC. All the remaining electrode designs produced the larger amounts of undercuts in comparison with the conventional design. Thus, the conventional cylindrical design (\(\mathrm{D_{C}}\)) was relatively more practical if the objective was to keep the machined hole with minimum undercut at the exit plane of the hole.