This paper describes a calculation of the debris movement caused by the electrostatic force in the narrow gap filled with the dielectric liquid in EDM process. The debris are conductive particles which are formed when the materials are removed from electrode surfaces by the thermal energy from the arc plasma and the impulsive pressure caused by the expansion of the dielectric liquid. The debris behavior in the gap influences the discharge ignition greatly. The debris are moved due to the electrostatic force when the voltage is applied to the gap. It is observed experimentally that the debris go to one electrode and return back to the other electrode repeatedly, or gather to form columns in the gap. And the discharge occurs on the position where there is a column made of debris.
In the calculation an axisymmetric 3 dimensional model is used and the calculation is carried out in the case that there is only one debris in the gap. The debris contacts with the cathode at first, and then moves to the anode. The distribution of electric potential in the gap is calculated with the finite-difference method at first, and then the distribution of electric field on the debris surface is obtained from the electric potential. Due to the electric field, the electric charge is induced on the debris surface, and as a result the electrostatic force is applied to the debris. It is found from the calculated results that the electrostatic force becomes greater and greater when the debris moves from the cathode to the anode. The equation of motion for the debris movement is solved with the applied mathematical calculation software. The calculated result is compared with the experimental result of debris movement observed by the high speed camera. It is found that the calculated result is close to the measured one, on the assumption that the debris is not fully charged.