Takashi Takagi

High Efficiency Electrical Discharge Machining with Graphite Electrode

High speed cutting of metal mold by machining center has recently been developed, and it has gradually taken the place of EDM, since the machining efficiency of EDM process is smaller than that of cutting one. From this point of view, higher efficiency EDM is greatly expected to be realized. However, EDMs ability has never been sufficiently displayed, since the machining condition is usually set to avoid the unstable machining. In general, excessive stagnation of debris between the electrode and workpiece leads to an occurrence of arc phenomenon and a generation of heat resolved carbon. Therefore, the exclusion of debris from the working gap is very important for stable machining. It has conventionally been carried out with absorbing or flushing of dielectric fluid through the holes drilled in the electrode or the workpiece, or jumping motion of the electrode. However, these methods have disadvantages such as the time necessary to machine holes or the low machining efficiency.In this study, a new excluding method of debris using the rapid flow due to evaporation of dielectric fluid is proposed, and the EDM characteristics of this method are experimentally discussed. As a result, it was made clear that higher efficiency EDM can be attained by this method and that debris can be smoothly removed when the discharge current is large, the gap is narrow and the pulse interval time is short as far as the machining is stable.

The Effect of High Speed Polarity Changing on EDM Performance

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.

Study on Electrical Discharge Machining of Copper Alloy MS46

Copper alloy MS46 has recently been developed as a metal mold material for shell mold core in place of cast iron FC which has conventionally been used. In this paper, the electrical discharge machining performances of copper alloy MS46 are experimentally investigated, analyzing the crater shape generated by a single pulse discharge, the metal removal rate, the electrode wear rate, the surface finish and so on. Then the surface integrity of MS46 for metal mold is evaluated. Main conclusions obtained in this study are as follows:(1) The metal removal rate in EDM of MS46 is the same as that in cast iron FC.(2) No electrode wear EDM of MS46 is difficult because of less material adhesion to the surface of electrode than that in case of cast iron.(3) EDMed surface of MS46 has some excellent properties for metal mold such as better surface finish than that of cast iron, no crack on the machined surface and harder recast layer than base material.