Straight hole micro EDM with a cylindrical tool using a variable capacitance method accompanied by ultrasonic vibration

A micro hole machined by EDM with a cylindrical electrode has different hole entrance and exit diameters. This taper shape of the micro hole is caused both by tool wear and by eroded particles that induce secondary discharge during hole machining. In this paper, a novel method is proposed to reduce the difference between the entrance and exit diameters of the holes. The effects of feeding depth, applied capacitance and machining time on the formation of the tapered shape were observed experimentally. Based on these experiments, ultrasonic vibration was applied to reduce the secondary discharge and electrode wear, and capacitance was varied when the tool was approaching the exit of the hole. Using the proposed method, a straight micro hole was successfully fabricated.