Electric discharge machining (EDM) uses the principle of thermal energy (melting and vaporization) to remove material from a workpiece. This thermal energy is generated by electric sparking. In die-cutting EDM, the electrode and the workpiece are connected by two terminals of the power supply and a small gap is maintained between them. A suitable dielectric fluid (e.g. kerosene) is applied in this inter-electrode gap (IEG), which in its normal state behaves like an insulator. When a sufficient potential difference is applied to the dielectric column, the dielectric breakdown becomes electrically conductive and therefore generates a spark. Since the flow of electrons through the dielectric from the tool electrode to the workpiece is mandatory, the EDM process can be applied to both the conductive electrode and the workpiece material. Furthermore, the profile on the electrode must be provided according to the desired profile. Therefore, EDM electrode materials should have high electrical and thermal conductivity, good formability and machinability, high melting point and low cost.

Several metallic and non-metallic materials can be used as EDM electrodes. Among the metallic electrodes electrolyte copper, commonly used are tellurium copper, silver tungsten and brass. The non-metallic material graphite is also used as an EDM electrode, especially in conjunction with pulse generators. Graphite electrodes are rigid and tough, and can retain their shape for longer periods of time to withstand mechanical and thermal shock. It has good machinability, so detailed features can be easily added to the electrode; however, it cannot provide sharp edges because it is very brittle. The high porosity also poses problems associated with moisture and dielectric retention. It is worth mentioning that sometimes graphite is impregnated with copper to form copper-graphite to utilize both copper and graphite. When used as an EDM electrode, it offers many advantages over standard materials. The various similarities and differences between copper and graphite electrodes are given below in tabular form.

Similarities between copper and graphite electrodes

● Both copper and graphite are electrically conductive. Conductivity is essential because the electrode needs to release electrons in the presence of a potential difference in order to perform dielectric breakdown and generate a spark.

● Regardless of the electrode material, the tool electrode is always given a negative polarity (cathode) in an EDM circuit.

● For die sinking EDM, the electrode should be given

● Irrespective of material, the tool electrode undergoes gradual wear at a slow rate (tool wear rate, TWR). It is worth mentioning that, unlike ECM, the EDM tool is subjected to erosion along with the workpiece.

● Neither of them causes burrs, residual stresses or other mechanical damage to the machined surface.

Differences between copper electrode and graphite electrode