## 3 Results and Discussion
### Machining Time and Material Removal Rate  
Figure 3a shows the obtained results of machining time versus depth of the drilled hole for the conventional electrode (CE). We observed that the slope of the testing curve slightly increased at shallow depths, but the slope had a steeper incline with the machining depth for any size of electrode. In addition, a steeper slope was obtained for the smaller electrode. This can be explained as the increase in the depth of the drilled hole resulted in more difficulties with eliminating debris particles from the sparking area by conventional flushing (side flushing) due to the lower flow of dielectric fluid at a higher depth and through a smaller area. This can be confirmed by the photograph of the accumulated particles on the bottom of the machined hole in the experimental test as shown in Figure 4. An electrode diameter of 3 mm was limited to drilling a hole 7.292 mm deep for the conventional electrode (CE) and 9.358 mm deep for the step cylindrical electrode (SCE) due to the accumulation of debris particles in the dielectric fluid leading to the generation of a secondary spark and concavity in the hole walls [39; 49]. In the case of an electrode diameter of 6 mm, debris particles accumulated and increased the thickness of the recast layer, leading to electrode moving back for the depth of drilled hole of 33.893 mm. \begin{table}
\begin{tabular}{c c} \hline \hline
**Condition** & **Value** \\ \hline Electrode diameter (mm) & 3, 6, 9, 12 \\ Discharge current (Amp/mm\({}^{2}\)) & 0.25 \\ Discharge current (Amp) & 1.77, 7.07, 15.91, 28.29 \\ Electrode polarity & + \\ Pulse-on (us) & 150 \\ Pulse-off (us) & 2 \\ Voltage (V) & 150 \\ Gap spark (\(\upmu\)m) & 10 \\ Jump high (mm) & 2.0 \\ Jump down (s) & 2.0 \\ Side-flushing flow (L/min) & 1.0 \\ Setup of machining depth (mm) & 50.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental conditions. Figure 3b shows the experimental results for the step cylindrical electrode (SCE). Machining time proportionally increased with machining depth for electrode diameters of 6, 9, and 12 mm. This is because the step cylindrical electrode (large diameter and short length at the end electrode) pressed dielectric fluid into the sparking area when the electrode jumped down. The debris particles and bubbles produced during sparking were eliminated from the sparking area when the electrode jumped up, as the debris particles and bubbles were able to float throughout the sparking area due to a greater escape area at the side wall between the electrode and hole (the neck electrode is smaller than the end electrode). This led to the generation of a cycle of self-flushing and the ability to maintain stable conditions throughout the sparking process. This can be confirmed by the photograph of the side spark (secondary spark) on the electrode shown in Figure 5. The damage on the side surface of the electrode occurred along the conventional electrode (CE). The secondary spark on the step cylindrical electrode (SCE) can be found on the flank of electrode only (body electrode). In addition, the element on the surface of the electrode is analyzed and discussed in Section 3.4. The material removal rate (MRR) was calculated using the volume of work material removed and machining time, as described in Equation (1) [50]. The experimental results of the material removal rate are shown in Figure 6. We found that the material removal rate  
Figure 4: Photographs of each drilled hole and the actual depth of the drilled hole. (**a**) Drilled hole with conventional electrode (CE); (**b**) drilled hole with step cylindrical electrode (SCE). Figure 3: Machining time versus machining depth of drilled holes. (**a**) Results of conventional electrode (CE); (**b**) results of step cylindrical electrode (SCE). for both conventional electrodes (CE) and step cylindrical electrodes (SCE) increased with the diameter of electrode.