Abstract: Powder mixed near dry electrical discharge machining(PMND EDM) uses gas-liquid-powder mixture as the three phase dielectric medium. Problems such as high short circuit rate and low machining efficiency in dry electrical discharge machining(DEDM) can be solved effectively. Based on the breakdown mechanism of pure gas medium, theory analysis of breakdown mechanism for three phase dielectric medium is carried out. Function of liquid and solid dispersed phase is illustrated. Formula of electric field strength between electrodes for PMND EDM is established. Equation for discharge gap calculation in PMND EDM is given. It is theoretically proved that the discharge gap of PMND EDM is larger than that of DEDM due to the addition of dispersed phase. And discharge gap increase is considered to be one of the important reasons for machining efficiency improvement. Comparison experiments are conducted in compressed air and three phase dielectric medium, respectively. Discharge gap is obtained. And it is verified that three phase flow can increase discharge gap. Discharging voltage waves are obtained, and it is confirmed that short circuit rate is reduced and machining efficiency is improved due to the addition of dispersed phase to continuous phase.

Key words: Discharge gap, Electrical discharge machining, Machining stability, Powder mixed near dry, Three phase dielectric medium