Improving Machining Performance for Deep Hole Drilling in the Electrical Discharge Machining Process Using a Step Cylindrical Electrode

: The performance of electrical discharge machining for drilling holes decreases with machining depth because the conventional ﬂushing and electrode cannot completely eliminate debris particles from the machining area. In this study, a modiﬁed electrode for self-ﬂushing in the electrical discharge machining process with a step cylindrical shape was designed to improve machining performance for deep hole drilling. The experimental results of the step cylindrical electrode showed that the material removal rate increased by approximately 215.7%, 203.8%, and 130.4%, and the electrode wear ratio decreased by approximately 47.2%, 63.1%, and 37.3%, when compared with a conventional electrode for the diameters of 6, 9, and 12 mm, respectively. In addition, the gap clearance and concavity of the side wall of the drilled hole was reduced with the step cylindrical electrode. The limited high ﬂank of the electrode led to an increase in the escape area of the debris that was partially removed from the machining area, and the limited secondary spark on the side wall of the electrode resulted in a reduction in machining time.


Introduction
Electrical discharge machining (EDM) is a modern processing machining method for materials that are difficult to use with conventional methods [1][2][3] as a result of high hardness, high wear resistance, and other special properties [3][4][5][6][7][8][9]. EDM is a machining process that removes some of the work material from the workpiece using the high temperature produced by spark discharges to erode the workpiece [3,8,10]. EDM is also known as spark machining, which involves spark eroding material from the conductive workpiece while it is submerged in a dielectric medium [11][12][13][14][15]. The principle of the EDM process is the controlled removal of material using an electrode as a cutting tool, which conducts electricity well and moves toward the workpiece by controlling the release of the electric current flowing through the sparking part under the machining gap [1,3]. EDM is suitable for non-contact workpieces as it sparks in a cycle of time (on-time) and stopping (off-time) during this period, called the duty factor [1,16]. EDM erodes the workpiece to produce the finished part to the desired shape and surface quality [17]. The EDM spark process is an eroding mechanism under a dielectric liquid substance that prevents short circuits in the discharge system, which prevents interruptions in the machining process. Using dielectric fluid to flush the system removes debris after discharge [18]. The spark discharge conducted in the dielectric fluid is suitable to machine most conductive materials, regardless of the hardness of the workpiece due to the non-contact of the cutting tool (electrode) and workpiece [19,20]. Therefore, the quality of the EDM post-processing is high and EDM can be used with conductive materials [3,20]. The obtained surface quality texture properties depend on the EDM parameters such as discharge current, discharge voltage, pulse duration, pulse interval, and flushing method [21,22].
The problems of spark discharge after the cycle has finished and debris removal by flushing dielectric fluid through the machining gap to remove debris material to maintain consistent electrical conditions are yet to be addressed [3,23]. The material removal rate is an important consideration; altering the machining gap can increase the material removal rate, as well as improve surface finish [24]. The EDM process has been widely used for manufacturing molds and dies, as well as in the aerospace and automotive industries [25][26][27][28][29][30]. However, problems with deep hole machining or deep hole drilling often lead to undesirable surface quality and geometry problems. The debris removal efficiency is hampered by the difficulty in debris elimination, and the residual debris that returns to the conductor during the second spark appears to be recast onto the wall of the hole, resulting in a poor roughness appearance of the internal wall [31][32][33], as well as an undesirable shape of the holes that are not well-drilled. Therefore, many researchers have studied how to improve the deep hole machining process. However, poor machining performance based on the removal of debris material, surface quality, and machining accuracy usually occurs in deep hole drilling due to the difficulties in gap cleaning and debris removal from the sparking area [34][35][36]. This leads to low production efficiency. Deep hole machining and small-sized holes in the EDM process are important for improving the performed result process, which is why it is necessary to study various parameters [37][38][39].
The improvement in machining performance in several distinct areas has been thoroughly researched. Wang et al. [20] researched the effects of electrode jump height and speed on the movement of debris and bubbles during electrical discharge machining. They showed that a large electrode jump height (mm) and jump speed (mm/min) can absorb cleaned oil (fresh dielectric fluid) into the bottom of the machining gap, resulting in more stable machining. Munz et al. [40] investigated the influence of flow rate for side flushing of dielectric fluid on the debris removal and process performance. They reported that the increase in the dielectric flow improved decontamination in the machining gap, flushing away gas bubbles and debris more rapidly, which improved the machining performance in the terms of material removal rate. Dwivedi et al. [41] improved machining performance of the EDM process using a rotating cylindrical electrode. They found that the rotating electrode increased the material removal rate by 41% and reduced surface roughness by 12% compared to using a stationary electrode. Teimouri and Baseri [42] used a rotational magnetic field and rotary electrode in the EDM process. They confirmed that the enhanced flushing of debris from the machining gap increases machining performance. Chuvaree et al. [34,43] investigated the dimension accuracy of an EDM deep hole using a multihole interior flushing electrode. They reported that the improved flushing technique is able to achieve a better closed gap tolerance and higher material removal rate compared with the conventional flushing method. Nadeem et al. [44] examined how to improve the performance of EDM through relief-angled tool designs on tungsten carbide material. The performance of the relief-angled electrodes was found to be significantly better than the performance of a conventional cylindrical tool. Chuvaree et al. [45] studied the effect of multi-aperture inner flushing on the characteristics of EDM deep hole. They showed that the multi-aperture inner flushing achieved a higher material removal rate, shorter machining time, and lower gap clearance than the side flushing of a conventional electrode. Moreover, various parameters affecting the material removal rate during EDM and the flushing method also play an important role, especially in the case of hole formation [44]. The melt debris accumulates in the internal walls around the drilled hole and forms a solidified recast layer [31,42]. The conditions of EDM machining depend on several factors, such as the current or duty factor and gap flushing efficiency [46][47][48]. Based on previous research, optimization of the machining conditions by adjusting the jump height is able to make machining more stable. However, a large electrode jump height is related to low sparking time, which results in a higher production time. In addition, modifying EDM equipment is a complicated process and has an increased machining cost. In this work, the design of the shape of the electrode was investigated to remove debris in the machining gap. The aim of this study was to examine the influence of a self-flushing electrode (cylindrical step shape) for EDM deep hole drilling on the improvement of machining performance, as well as machining time, material removal rate (MRR), electrode wear ratio (EWR), the quality of the machined surface (Ra), and the gap clearance of the drilled hole.

Experimental Materials
The schematic diagram of the experimental setup is shown in Figure 1a. The experimental testing was carried out on an electrical discharge machine (CNC EDM 430, Aristech, Taichung City, Taiwan). The work material was the grinding plate of plastic mold steel AISI P20 material, which is a material used for pre-quenching of the produced injection plastic mold, shown in Table 1. A workpiece setup in the form of a couple of steel plates with a thickness of 10 mm was mounted in the working tank of the machine by a vise. The alignment of the workpiece along the y-axis of the machine was checked by the dial test indicator. The electrode was mounted on the head of the machine, then the alignments in the x-axis and y-axis along the z-axis were controlled by ±0.005 mm for the moving distance in the z-axis of 50 mm. The drilling positions were located on the parting line of the workpiece which used an electrode touching on both sides of the edge of the workpiece then moving to the center of drilling hole. The depth of the drilling hole was limited to 50 mm as shown in Figure 1b. Distance and alignment were controlled by a Mitutoyo dial test indicator (Resolution 0.001 mm, Accuracy ±0.003 mm, Nitutoyp Corporation, Kanagawa, Japan). The machining time and depth of the drilling hole were directly recorded to calculate the MRR. The EWR was evaluated at the end of the machining tests. The quality of the machined surface was measured by athematic roughness (Ra) using a roughness tester (MarSurf PS1, MAHR, Göttingen, Germany), and the gap clearance along the depth of the drilled hole was measured using an optical microscope (STM6, OLYMPUS, Tokyo, Japan). a higher production time. In addition, modifying EDM equipment is a complicated process and has an increased machining cost. In this work, the design of the shape of the electrode was investigated to remove debris in the machining gap. The aim of this study was to examine the influence of a self-flushing electrode (cylindrical step shape) for EDM deep hole drilling on the improvement of machining performance, as well as machining time, material removal rate (MRR), electrode wear ratio (EWR), the quality of the machined surface (Ra), and the gap clearance of the drilled hole.

Experimental Materials
The schematic diagram of the experimental setup is shown in Figure 1a. The experimental testing was carried out on an electrical discharge machine (CNC EDM 430, Aristech, Taiwan). The work material was the grinding plate of plastic mold steel AISI P20 material, which is a material used for pre-quenching of the produced injection plastic mold, shown in Table 1. A workpiece setup in the form of a couple of steel plates with a thickness of 10 mm was mounted in the working tank of the machine by a vise. The alignment of the workpiece along the y-axis of the machine was checked by the dial test indicator. The electrode was mounted on the head of the machine, then the alignments in the x-axis and y-axis along the z-axis were controlled by ±0.005 mm for the moving distance in the z-axis of 50 mm. The drilling positions were located on the parting line of the workpiece which used an electrode touching on both sides of the edge of the workpiece then moving to the center of drilling hole. The depth of the drilling hole was limited to 50 mm as shown in Figure 1b. Distance and alignment were controlled by a Mitutoyo dial test indicator (Resolution 0.001 mm, Accuracy ±0.003 mm, Japan). The machining time and depth of the drilling hole were directly recorded to calculate the MRR. The EWR was evaluated at the end of the machining tests. The quality of the machined surface was measured by athematic roughness (Ra) using a roughness tester (MarSurf PS1, MAHR, Germany), and the gap clearance along the depth of the drilled hole was measured using an optical microscope (STM6, OLYMPUS, Japan).

Electrode Design
In this work, the electrical discharge machining was performed with conventional flushing of the copper electrode; the properties of the electrode are shown in Table 2. The conventional electrode was cylindrical in shape (CE), as shown in Figure 2a. The newly design shape electrode was created in the flank area in the region close to the sparking area (Step Cylinder Electrode, SCE), which is vital for self-flushing in the electrical discharge machining process for deep hole drilling, as shown in Figure 2b. For this design, we expect that the step shape of the electrode will increase the escape area for bubble flows, and the debris removed from the sparking area will lead to improvements in machining performance. Both sets of electrodes were produced by a computer numerical control turning machine (PC TURN50, EMCOTRONICS, Austria). The electrode designed in this work can be compared to the conventional electrode consisting of a shank electrode and body electrode, which is shown in Figure 2a. The newly designed electrode consists of a shank electrode, neck electrode, and body electrode, as shown in Figure 2b. The diameter of the electrode in the experiment was varied 3, 6, 9, and 12 mm, and the total length of any electrode was 103 mm, which was controlled in the range of ±10 µm. A feature of the workpiece included the design of a splice by couple plate, which is shown in detail in Figure 2c. The machining test was used in order to evaluated machining performance for any electrode designed, this was repeated 3 times with the new electrode.

Electrode Design
In this work, the electrical discharge machining was performed with conventional flushing of the copper electrode; the properties of the electrode are shown in Table 2. The conventional electrode was cylindrical in shape (CE), as shown in Figure 2a. The newly design shape electrode was created in the flank area in the region close to the sparking area (Step Cylinder Electrode, SCE), which is vital for self-flushing in the electrical discharge machining process for deep hole drilling, as shown in Figure 2b. For this design, we expect that the step shape of the electrode will increase the escape area for bubble flows, and the debris removed from the sparking area will lead to improvements in machining performance. Both sets of electrodes were produced by a computer numerical control turning machine (PC TURN50, EMCOTRONICS, Austria). The electrode designed in this work can be compared to the conventional electrode consisting of a shank electrode and body electrode, which is shown in Figure 2a. The newly designed electrode consists of a shank electrode, neck electrode, and body electrode, as shown in Figure 2b. The diameter of the electrode in the experiment was varied 3, 6, 9, and 12 mm, and the total length of any electrode was 103 mm, which was controlled in the range of ±10 µm. A feature of the workpiece included the design of a splice by couple plate, which is shown in detail in Figure 2c. The machining test was used in order to evaluated machining performance for any electrode designed, this was repeated 3 times with the new electrode.

Experimental Conditions
The experimental conditions for the investigated influence of the new electrode design on the improvement of machining performance are summarized in Table 3. The experimental tests were performed in the oil dielectric fluid (DIEL MS 7000, TOTAL) and

Experimental Conditions
The experimental conditions for the investigated influence of the new electrode design on the improvement of machining performance are summarized in Table 3. The experimental tests were performed in the oil dielectric fluid (DIEL MS 7000, TOTAL) and side flushing was supplied at a specific pressure through a nozzle to ensure continuous recirculation of dielectric fluid in the EDM tank. According to the optimized parameters with regard to the highest material removal rate [39,49] and characteristic surface appearance, the discharge current was fixed at 0.25 Amp/mm 2 for electrodes of any size. For the positive polarity electrode, pulse-on and pulse-off were controlled at 150 µs and 2 µs, respectively.

Results and Discussion
This study focused on the improvement in machining performance for a deep hole drilling tool composed of AISI P20 in the electrical discharge machining process using a step cylindrical electrode. Therefore, we examined the influence of a modified electrode on the improvement of machining performance, which was evaluated using the MRR and EWR. The quality of the machined hole was examined with respected to the surface roughness (Ra) and gap clearance. 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.

Machining Time and Material Removal Rate
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.    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  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.      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 for both conventional electrodes (CE) and step cylindrical electrodes (SCE) increased with the diameter of electrode. This was due to the debris becoming difficult to remove from the sparking area for the smaller electrode due to the incredibly small gap clearance and some debris accumulated on the machining surface due to the action of gravity [51]. Therefore, the sparking process was impeded, extending the machining time, which led to the decrease in material removal rate for the small electrode. The decreased area of the secondary spark on the side electrode and elimination of debris particles in the sparking area caused by the modified shape of electrode improved the material removal rate. The machining performance of the step cylindrical electrode clearly increased by approximately 215.7%, 203.8%, and 130.4% compared with the conventional electrode for electrode diameters of 6, 9, and 12 mm, respectively. However, the material removal rate of the step cylindrical electrode with a diameter of 3 mm was slightly higher than that for the conventional electrode. This can be explained by the height of the flank electrode (height at the end of the electrode) for small electrodes affecting the ability of debris particles and bubbles to flow out from the sparking area: where the MRR is the material removal rate (mm 3 /min); M w1 and M w2 are the workpiece weight (g) before and after machining, respectively; ρ w is the density of workpiece (7.78 g/cm 3 ); and t is the machining time (min).
Appl. Sci. 2021, 11, 2084 7 of 15 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 for both conventional electrodes (CE) and step cylindrical electrodes (SCE) increased with the diameter of electrode. This was due to the debris becoming difficult to remove from the sparking area for the smaller electrode due to the incredibly small gap clearance and some debris accumulated on the machining surface due to the action of gravity [51]. Therefore, the sparking process was impeded, extending the machining time, which led to the decrease in material removal rate for the small electrode. The decreased area of the secondary spark on the side electrode and elimination of debris particles in the sparking area caused by the modified shape of electrode improved the material removal rate. The machining performance of the step cylindrical electrode clearly increased by approximately 215.7%, 203.8%, and 130.4% compared with the conventional electrode for electrode diameters of 6, 9, and 12 mm, respectively. However, the material removal rate of the step cylindrical electrode with a diameter of 3 mm was slightly higher than that for the conventional electrode. This can be explained by the height of the flank electrode (height at the end of the electrode) for small electrodes affecting the ability of debris particles and bubbles to flow out from the sparking area:

Electrode Wear Ratio (EWR)
The electrode wear ratio (EWR) is defined by the removal volume of the electrode due to erosion wear to the removal volume of the workpiece in the machining process, as defined in Equation (2) [50]. The obtained results found that the electrode wear ratio of the step cylindrical electrode (SCE) was lower than the ratio for the conventional electrode

Electrode Wear Ratio (EWR)
The electrode wear ratio (EWR) is defined by the removal volume of the electrode due to erosion wear to the removal volume of the workpiece in the machining process, as defined in Equation (2) [50]. The obtained results found that the electrode wear ratio of the step cylindrical electrode (SCE) was lower than the ratio for the conventional electrode (CE) for electrodes of any size as shown in Figure 7. We found that both SCE and CE showed the nonlinear electrode wear ratio for the electrode diameter of 3 mm. This is because the poor removal of machined debris with the flushing system led to debris blending in the dielectric [52] and deposited on the machining surface under the action of gravity and formed a protective film [51]. In addition, the electrode wear ratio for the step cylindrical electrode decreased by approximately 35.0%, 47.2%, 63.1%, and 37.3% compared to the conventional electrode for the diameters of 3, 6, 9, and 12 mm, respectively. This is because suitable flushing as a result of the electrode design led to improved flushing performance to eliminate debris particles and resulted in a shorter machining time due to the reduction in secondary sparks, which led to a high energy concentration in the sparking area. From these results, we concluded that electrode wear was improved by self-flushing in the machining process with the step cylindrical electrode: where the EWR is the electrode wear ratio (%); M t1 and M t2 are the electrode weight (M w1 and M w2 are the workpiece weight (g) before and after machining); and ρ t is the density of the copper electrode (8.96 g/cm 3 ).

Electrode Wear Ratio (EWR)
The electrode wear ratio (EWR) is defined by the removal volume of the electrode due to erosion wear to the removal volume of the workpiece in the machining process, as defined in Equation (2) [50]. The obtained results found that the electrode wear ratio of the step cylindrical electrode (SCE) was lower than the ratio for the conventional electrode (CE) for electrodes of any size as shown in Figure 7. We found that both SCE and CE showed the nonlinear electrode wear ratio for the electrode diameter of 3 mm. This is because the poor removal of machined debris with the flushing system led to debris blending in the dielectric [52] and deposited on the machining surface under the action of gravity and formed a protective film [51]. In addition, the electrode wear ratio for the step cylindrical electrode decreased by approximately 35.0%, 47.2%, 63.1%, and 37.3% compared to the conventional electrode for the diameters of 3, 6, 9, and 12 mm, respectively. This is because suitable flushing as a result of the electrode design led to improved flushing performance to eliminate debris particles and resulted in a shorter machining time due to the reduction in secondary sparks, which led to a high energy concentration in the sparking area. From these results, we concluded that electrode wear was improved by self-flushing in the machining process with the step cylindrical electrode: where the EWR is the electrode wear ratio (%); Mt1 and Mt2 are the electrode weight (Mw1 and Mw2 are the workpiece weight (g) before and after machining); and is the density of the copper electrode (8.96 g/cm 3 ).   Figure 7. Electrode wear ratio of holes drilled with the conventional electrode (CE) and the step cylindrical electrode (SCE).

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 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. 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 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 (Rpm) and mean depth of valleys (Rvm) [53] in the  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 pm ) and mean depth of valleys (R 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.
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. 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. The influence of dimensional electrode on the quality of the machined surface could be explained by the gap clearance increasing with the size of electrode. This is due to more blending debris in the dielectric induced secondary spark on the side wall of the electrode and workpiece. When the step cylindrical electrode was employed in the machining test, the gap clearance decreased because the neck electrode led to debris easily escaping from the machining area. However, the limited body of the electrode (high 3 mm) induced the high sparking energy in the machining area and secondary spark on the side wall of the drilled hole which is related to the increased material removal rate and roughness of the machined surface [54]. The influence of dimensional electrode on the quality of the machined surface could be explained by the gap clearance increasing with the size of electrode. This is due to more blending debris in the dielectric induced secondary spark on the side wall of the electrode and workpiece. When the step cylindrical electrode was employed in the machining test, the gap clearance decreased because the neck electrode led to debris easily escaping from the machining area. However, the limited body of the electrode (high 3 mm) induced the high sparking energy in the machining area and secondary spark on the side wall of the drilled hole which is related to the increased material removal rate and roughness of the machined surface [54]. Figures 12 and 13 show the results of the analysis of the alloy elements mapped on the surface of the copper electrode and the average volume contents of copper, carbon, and iron in the region marked in Figure 5 for the conventional and step cylindrical electrodes, respectively. The weight percentage (%wt) of alloy elements was averaged by EDX, analyzing at least three positions in the different regions of the electrode surface, as shown in the SEM images. These positions included an area where the surface is bright and clear, an opaque surface, and the surface over the boundary joint area. Figure 12a shows the elemental mapping in the regions close to the end of electrode (EDX1 in Figure  5a). Carbon (C) of approximately 52.99%wt and ferrous (Fe) of approximately 35.01%wt accumulated more in the region that further away from the end of electrode (EDX2 in Figure 5a) as shown in Figure 12b which results element of carbon by approximately 44.57%wt and ferrous by approximately 27.81%wt. This is evidence of the severity of repeated sparks occurring in the process of joining the wall debris around the wall within the drill hole, leading to higher carbon and iron accumulation on the surface of the electrode. However, the content of copper (Cu) of approximately 26.29%wt on the step cylindrical electrode (EDX3 in Figure 5b), as shown in Figure 13a, was higher than the copper content of approximately 12.00%wt on the conventional electrode at the same region (Figure 12a). This can be explained by the reduction in secondary sparking occurring on the side surface of the electrode due to the removal of debris and bubbles during machining, which easily escape from the sparking area. In addition, the increased escape area with the increased gap distance in the machining process when using the step cylindrical electrode led to the elimination of secondary sparking, resulting in low damage and alloy accumulation on the surface of the electrode (Fe ≈ 0.09%wt, C ≈ 40.30%wt, and  Figures 12 and 13 show the results of the analysis of the alloy elements mapped on the surface of the copper electrode and the average volume contents of copper, carbon, and iron in the region marked in Figure 5 for the conventional and step cylindrical electrodes, respectively. The weight percentage (%wt) of alloy elements was averaged by EDX, analyzing at least three positions in the different regions of the electrode surface, as shown in the SEM images. These positions included an area where the surface is bright and clear, an opaque surface, and the surface over the boundary joint area. Figure 12a shows the elemental mapping in the regions close to the end of electrode (EDX1 in Figure 5a). Carbon (C) of approximately 52.99%wt and ferrous (Fe) of approximately 35.01%wt accumulated more in the region that further away from the end of electrode (EDX2 in Figure 5a) as shown in Figure 12b which results element of carbon by approximately 44.57%wt and ferrous by approximately 27.81%wt. This is evidence of the severity of repeated sparks occurring in the process of joining the wall debris around the wall within the drill hole, leading to higher carbon and iron accumulation on the surface of the electrode. However, the content of copper (Cu) of approximately 26.29%wt on the step cylindrical electrode (EDX3 in Figure 5b), as shown in Figure 13a, was higher than the copper content of approximately 12.00%wt on the conventional electrode at the same region (Figure 12a). This can be explained by the reduction in secondary sparking occurring on the side surface of the electrode due to the removal of debris and bubbles during machining, which easily escape from the sparking area. In addition, the increased escape area with the increased gap distance in the machining process when using the step cylindrical electrode led to the elimination of secondary sparking, resulting in low damage and alloy accumulation on the surface of the electrode (Fe ≈ 0.09%wt, C ≈ 40.30%wt, and Cu ≈ 57.61%wt), as shown in Figure 13b. These results confirmed that the machining performance of the electrical discharge machining process for deep hole drilling can be improved by using a step cylindrical electrode.

Conclusions
In this work, the modified electrode used for the self-flushing process in electrical discharge machining to investigate the improvement of machining performance for deep hole drilling can be described as follows: 1. The machining time increased with the machining depth for any type of electrode.
However, the slope of the step cylindrical electrode was lower than that of the conventional electrode because the neck design of the electrode increased the area for particle debris and bubbles to escape from the machining area while improving dielectric flushing in the sparking area. These effects led to a material removal rate improvement of approximately 13.6%, 215.7%, 203.8%, and 130.4% for electrode diameters of 3, 6, 9, and 12 mm, respectively. 2. The electrode wear ratio of the step cylindrical electrode was lower than that of the conventional electrode because the neck design of the electrode reduced secondary sparking on the side wall of the workpiece which concentrated and limited sparking areas to the bottom and flank face of the electrode. This led to a decrease in tool wear due to the reduction in machining time.