Lead-wire type electrical discharge machining (EDM) widely employs copper and its alloys as the electrode material. However, due to process needs, requirements vary with lead and wire EDM. Since electrodes play an important role in the conformance of the final result to the design, it is essential to understand the requirements and characteristics of the electrode material to evaluate the limitations of copper and its alloys.
Requirements for an electrode material:
In addition to common requirements, such Sodick EDM wear parts as cost and electrical conductivity, the following requirements are expected, depending on the type of EDM.
Electro-erosion by immersion:
The electrode material needs to be a good conductor of heat in addition to being easily machinable. Given the temperatures at which machining takes place, the electrode material is expected to withstand high temperatures without undergoing deformation. Although EDM appears to be a stress-free process from the outset, on a microscopic level the electrodes are subjected to a great deal of stress due to continuous sparks during the machining process. The ability to withstand this voltage without wear is essential as an electrode.
The electrode in wire EDM must be ductile, which eliminates an excellent penetration EDM electrode material, graphite. Wire EDM is prone to frequent washing of machined parts and therefore requires the electrode material to offer reasonable resistance to sparks. Limitations of copper and its alloys as an electrode material:
Electro-erosion by immersion:
The most important limitation of copper as an electrode is its vulnerability to sparks generated during the machining process. Compared to graphite electrodes, copper electrodes last only half the life time. Pure copper is very malleable. This makes machining difficult and tends to clog the grinding wheel during manufacture. Burrs on the outer surface of copper electrodes also pose a challenge. Sometimes deburring an electrode takes more time than manufacturing the electrode itself. To eliminate machinability problems related to copper, it is mixed with tellurium. The tellurium copper thus obtained is relatively more machinable than pure copper, although it has lower performance characteristics, as evidenced by increased wear and decreased metal removal rate. Furthermore, the shortage of tellurium-copper also affects the feasibility of its widespread use as an electrode material. On the other hand, brass, a readily available copper alloy, suffers a high rate of wear, sometimes up to 6 times higher than the base metal. However, during finishing, the wear on the corners of the brass is reduced to 0.7 times the volume of the base metal. As a result of the high wear characteristic of brass electrodes, they are generally only used for drilling holes and sinking cavities. Similarly, copper is also limited to drilling holes and creating grooves and is not used in applications that require high precision and detail. Copper and copper alloys also face a disadvantage before they are put to use. As they come in rolled forms, they are susceptible to stresses while being cut with wire as an electrode.