Machining performance in the normal polarity EDM greatly differs from that in the reversed polarity one. The removal rate in the normal polarity EDM is high, though the electrode wear is large in EDM with graphite electrode. On the other hand, the removal rate in the reversed polarity EDM is relatively low, though the electrode wear is small, which leads to no electrode wear EDM under a proper machining condition. Therefore, it is considered that it might be more effective to mix both the normal polarity EDM and the reversed polarity one.
In this paper, the effect of polarity changing on the EDM performance with graphite electrode is experimentally investigated, using high speed polarity changing device in which the number and the polarity of discharge can be quickly changed at will, in order to attain higher removal rate and lower electrode wear. Main conclusions obtained are as follows:
(1) The removal rate takes a maximum when the reversed polarity discharge is a little mixed into the normal polarity EDM with kerosine type fluid. It is due to the removal of carbonized layer and/or heat resolved carbon of machining fluid from the workpiece surface.
(2) The removal rate, the electrode wear and the surface finish in EDM with kerosine type fluid depend on the ratio of normal polarity discharge to the reversed polarity one regardless of the number of each polarity discharge in a cycle.
(3) The removal rate in EDM with deionized water increases linearly with increasing the ratio of normal polarity discharge.
(4) The electrode wear and the surface finish in EDM with kerosine type fluid become larger with increasing the ratio of normal polarity discharge.
View full abstract