Study on quantitative estimation of bubble behavior in micro hole drilling with EDM

In drilling blind micro holes with electrical discharge machining (EDM), the machining environment gradually deteriorates with the increase of the hole depth due to debris accumulation and concentration in the narrow gap area. The bubble flushing in the gap area was thought to be the main driving force for the exchange of dielectric liquid and debris exhaust. However, due to the difficulty in observing the gap area directly, the quantitative evaluation of bubble behavior has not been reported to date. In this paper, the feasibility of observation and quantitative evaluation of bubble behavior in micro hole drilling was investigated. A set of solutions was put forward to solve the problems from observation to data processing. Two aspects of works were performed: First, the generation of bubbles and debris was observed and analyzed in a single discharge process using a pair of needle electrodes to avoid the visual blocking by the micro tool; Second, the bubble escaping from the micro hole has been observed and quantitatively estimated in micro hole drilling with EDM. The obtained results proved the feasibility of the method on quantitative estimation of the bubble behavior and showed with the increase of the hole depth, the size of single bubble exhibited the trend to increase, but the bubble escaping frequency decreased significantly. These characteristics of bubble behavior led to the low efficiency of dielectric exchange and debris exhaust and weakened the insulation strength of the gap area, eventually resulting in frequent abnormal discharges. •A method to quantitatively estimate the bubble behavior in micro hole drilling with EDM was proposed and the parameters used in observation were determined.•The single discharge was clearly observed by the high-speed camera with a pair of needle electrode, in which the characteristics of bubble and debris were analyzed with corresponding voltage waveform.•The depth-dependent behavior of bubble was estimated quantitatively, and its influence on the drilling process with EDM was discussed.