Wire and sinker type electrical discharge machining (EDM) extensively employs copper and its alloys as the electrode material. However, due to the process necessities, the requirements vary with sinker and wire EDM. Since the electrodes play an important role in the conformity of the final output to the design, it is essential to understand the requirements and characteristics of the electrode material to assess the limitations of copper and its alloys.
Requirements of an electrode material:
Apart from common requirements such as cost and electrical conductivity, the following requirements are expected, depending on the type of EDM.
The electrode material requires being a good conductor of heat along with being easily machinable. Given the temperatures at which the machining takes place, it is expected of the electrode material to endure high temperature without undergoing deformation. Although from the outset the EDM looks like a process without any physical stress, at a microscopic level, the electrodes are under a high amount of stress due to continuous sparks during the machining process. The ability to withstand this stress without wear is essential as an electrode.
The Electrode in wire EDM is required to be ductile, which eliminates an excellent sinker EDM electrode material, graphite. The wire EDM is prone to frequent flushing of the machined parts and therefore the electrode material is required to offer reasonable resistance to sparks.
Limitations of copper and its alloys as electrode material:
The most important limitation of copper as an electrode is its vulnerability towards the sparks generated during machining process. In comparison with graphite electrodes, copper electrodes last only half as much. Pure copper is highly malleable. This makes it difficult to be machined and it tends to clog the grinding wheel during manufacturing. The burrs on the outer surface of the copper electrodes also pose a challenge. Sometimes the deburring of an electrode takes longer than the manufacturing of electrode itself. To eliminate the machinability issues concerning 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. In addition, the scarcity of tellurium copper also affects the viability of its widespread use as an electrode material. On the other hand, brass, an easily available alloy of copper, suffers from high rate of wear, sometimes as high as 6 times the base metal. However, during finishing, the corner wear of brass lowers to 0.7 times the base metal volume. As a result of the high wear characteristic of brass electrodes, they are typically only used for drilling holes and cavity sinking. Similarly, copper is also limited to drilling holes and creating slots and not used in applications needing high accuracy and detail. Copper and copper alloys also face a disadvantage before they are put in use. As they come in rolled forms, they are susceptible to stress while being wire cut as an electrode.
As mentioned, graphite, a widely used electrode material in sinker EDM, is rendered useless in wire EDM due to their lack of malleability. Hence, copper and its alloys are preferred electrode material despite their shortcomings. Copper is marred by its inability to sustain stress during operations, which more often than not results in wire cuts. During operation this presents a challenge as the wire has to re-attached to the spool again and is bound to change the orientation of the of the base metal. Along with that, they are difficult to flush during off operation. Despite all these inadequacies, brass is widely used as wire EDM electrode and copper is preferred for finishing operations.