This paper describes the results of the detection and observation of arc column movement during inner-flushing EDM processes, which demonstrates that the arc columns, the dielectric fluid and the bubbles can interact with each other substantially. A hollow tool electrode was used to apply inner-flushing. Monopulse discharge and the resulting craters were observed. It is found that when the discharge current is constant, the length of the enlongated crater peaks at a certain flushing flow speed, which implies both the evaporation effect of arc columns to the fluid, and the cutting-off effect of the fluid against arc columns.
