When a micro hole is machined by EDM with a cylindrical electrode, the hole diameter is different at the inlet and the outlet of the micro hole. The taper shape of the micro hole is caused by not only wear of the electrode but the eroded particles. The eroded particles cause secondary discharge during machining the micro hole. As a result, the diameter of the inlet becomes larger than that of the outlet. In this paper, a new method is proposed to reduce the difference in diameter between the inlet and the outlet of the hole. Observed was that the feed depth and machining time affect the formation of taper shape. On this experimental basis, ultrasonic vibration was applied to reduce machining time, and capacitance was changed during machining to use the difference in discharging energy of different capacitances. Using the proposed method, a straight micro-hole was fabricated.
