Investigation of Depth-dependent Characteristics in Micro EDM Drilling Based on Direct Inter-electrode Area Observation

In the EDM process of drilling deep blind micro-hole with an aspect ratio larger than 10, machining speed decreases significantly in the last stage of machining due to frequent retreat of the micro tool. Up to now, reasons for the speed decrease were mostly thought to be gradual deterioration of the machining environment in the inter-electrode area, mainly caused by debris accumulation and concentration in the narrow gap area. However, it is difficult to confirm the state of the gap area in micro scale by direct observation. Therefore, an affirmed explanation for the speed decrease in micro EDM drilling has not been reported. In this research, in order to analyze the behaviors of bubble, debris and discharge characteristic in the inter-electrode area and the reason for speed decrease, direct observation of the gap area was carried out with an originally designed sandwich workpiece and a high-speed camera. The characteristics of discharge, bubble behavior and change of discharge area have been investigated and discussed in different hole depths.