## 3 Results and Discussion
### Quality of the Drilled Hole  
In this work, the quality of the drilled hole was evaluated based on machining accuracy by means of gap clearance and the surface roughness of the machined hole. Figure 8 shows the results of the gap clearance between the wall of the electrode and the machined hole along the machining depth. A larger gap clearance was found when bigger electrodes were used. This is because the higher material removal rate induced more debris particles to be blended into the dielectric fluid, leading to an accelerated secondary spark on the side wall of the electrode; this was particularly evident in the case of the conventional electrode (CE) due to the low area for debris escape. Besides, there is the potential for the higher discharge current used for larger electrodes to generate violent secondary sparking on the wall of the drilled hole. The results for the modified electrode with a step cylindrical design showed that the gap clearance was reduced compared to the conventional electrode. The average gap clearance decreased approximately 44%, 30%, and 29% for electrode diameters of 6, 9, and 12 mm, respectively. The concavity of the wall of  
Figure 7: Electrode wear ratio of holes drilled with the conventional electrode (CE) and the step cylindrical electrode (SCE). the hole drilled with the step cylindrical electrode (SCE) also decreased. This is because the neck design of the electrode increased the escape area for the debris particles to easily flow out from the machining area. As a result, fewer secondary sparks occurred on the wall of the machined hole. The concavity of the drilled wall for the electrode diameter of 3 mm was non uniform because the poor removal of machined debris for small electrode induces a high concentration of debris particles which are electrostatically polarized, resulting in the secondary spark on the side of electrode and workpiece [52]. In addition, the poor debris particles escaping from the sparking area could occur when the deep holed drilling with the conventional electrode diameter of 6 mm increased more than 25 mm and could result in the irregular gap clearance at the end of the drilled hole. Figure 9 shows the obtained results for surface roughness. We found that the roughness of the machined surface increased with the size of the electrode due to the higher discharge currents employed for the operation of larger-diameter electrodes. However, the step cylinder electrode results for averaged roughness of the machined surface were higher than those for the conventional electrode by approximately 54.35%, 67.95%, and 42.43% for electrode diameters of 6, 9, and 12 mm, respectively. This can be explained by the limited height of the flank electrode leading to a decreased secondary spark area. The secondary spark on the side wall caused more damage to the machined surface. Large areas of secondary spark resulted in lower damage to the machined surface because the concentration of energy in the sparking area was reduced. This finding can be confirmed by the profile of the machined surface along the drilled hole as shown in Figures 10 and 11 for the conventional electrode (CE) and step cylindrical electrode (SCE), respectively. The mean height of peaks (R\({}_{\text{pm}}\)) and mean depth of valleys (R\({}_{\text{vm}}\)) [53] in the profile of the surface machined with the step cylindrical electrode are higher than those for the conventional electrode, and the roughness of the machined surface decreased with the decreasing diameter of the electrode. This finding can be explained by the high sparking current for the large electrode, leading to higher damage on the machined surface. The high efficiency of particle removal and the reduction in the sparking area with the step cylindrical electrode led to a high energy concentration in the sparking area and resulted in higher roughness of the machined surface. Figure 8: Gap clearance of the drilled hole with electrodes of various diameters.