In die-sinking EDM, sensing a state of machining is important for both preventing surface damage and improving material removal rate. This paper describes how to sense a state of machining using voltage waveform between a tool electrode and a workpiece. By examining 8 characteristics of the waveform, it is found that the average and the variance of the discharge voltage change acccording to the state of machining. It is also shown that discharge pulses are discriminated into two groups with these two characteristics. In a very bad state of machining, the variance is found to be better sensing parameter than the average. For practical purpose, sensing with high frequency component of the discharge voltage is examined and it is found to have the same capability as sensing with the variance. With the high-frequency method, every pulse is classified into effective or non-effective pulse in realtime. It is shown that the accumulation of effective pulses follows the penetration depth. This proves that the proposed method is practical for sensing a state of machining.
