Effect of Electrode Shape on High Aspect Ratio Deep Hole Drilling by EDM

Abstract

In general, as the machining depth increases, the machining speed gradually slows down by electric discharge machining (EDM). In particular, when the aspect ratio exceeds 3 (L/D=3), the machining speed becomes significantly stagnant. This is because the machining debris and coalesced bubbles by EDM, which intervene between the working gaps, have a significant effect. The higher the aspect ratio (L/D>5), the longer the distance from the bottom to the top of the hole. Therefore, it is difficult to effectively discharge the machining debris and coalesced bubbles generated at the bottom of the hole. If they can be removed, deeper hole drilling can be performed. In this study, we investigated the effect of the tool electrode shape on deep-hole drilling using EDM. We developed a stirrer-shaped electrode. Compared with the round bar electrode, the stirrer-shaped electrode was able to machine approximately 3.5 times deeper. We have also developed an observation system for deep-hole drilling using the EDM method. Machining debris and coalesced bubbles were observed from a lateral view using the observation system. As a result, it was confirmed that the dielectric working fluid between the working gap turned black when the round bar electrode was used. This is because the machining debris was not removed from the top of the hole and remained at the side between the working gap. Therefore, the electrode and workpiece were frequently short-circuited, and the Z-axis was raised. Further, we made a dumbbell-shaped electrode and used it for EDM. By using this electrode, the bubbles generated by EDM were removed. As a result, deep hole drilling with a high aspect ratio was achieved.