## 4 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.  
Figure 12: Analysis of alloy element mapping on the conventional electrode (CE). (**a**) Elemental mapping in the area of EDX 1; (**b**) elemental mapping in the area of EDX 2.  
Figure 13: Analysis of alloy element mapping on the step cylindrical electrode (SCE). (**a**) Elemental mapping in the area of EDX 3; (**b**) elemental mapping in the area of EDX 4.  
3. The machining accuracy in electrical discharge machining for deep hole drilling was improved with the step cylindrical electrode. The limited area of secondary sparking resulted in lower concavity and gap clearance on the wall of the drilled hole compared to the conventional electrode.
4. The roughness of the machined surface for the step cylindrical electrode was higher than that of the conventional electrode because the limited height of the flank of the electrode led to reduced secondary sparking area on the side wall of the machined surface, which in turn led to a higher peak height and valley depth on the wall of the drilled hole.  
The concept of step cylindrical electrode design is able to be applied for use in industry in order to improve machining performance. This is because the step cylindrical electrode can easily prepare and be used for drilling the depth hole without modified equipment and a machining process. However, the increase in the material removal rate is related to reduction in the quality of surface roughness. Therefore, the approximate parameters needed in order to improve quality of a machined surface is an interesting future research topic. In addition, the approximate ratio between electrode diameter and height of an electrode body for improved machining performance of a small electrode is also an interesting subject for future research.  
Conceptualization, [PERSON], [PERSON]; Formal analysis, [PERSON]; Funding acquisition, P.J.; Investigation, [PERSON], P.J.; Methodology, [PERSON], P.J.; Project administration, [PERSON]; Validation, [PERSON]; Writing--original draft, [PERSON]; Writing--review and editing, P.J. All authors have read and agreed to the published version of the manuscript.  
This research was financed in a part by the Research Development Institute of Rajamangala University of Technology Kungthep.  
Data Availability Statement: Data sharing not applicable.  
We would like to thank the Research Development Institute of Rajamangala University of Technology Kungthep, Thailand, for the administrative support.  
The authors declare no conflict of interest.