Analysis of various graphite types for electrical discharge finishing

Nowadays graphite and copper are the most used tool electrode materials for electrical discharge machining (EDM). Especially for roughing operations to machine mould inserts made of steel, graphite allows high material removal rates, with high discharge energies and reduced relative tool wear. In addition, graphite is characterized by better machinability compared to copper, which leads to a cost-effective production of complex tool electrode geometries. To take advantage of the machinability and flexibility in producing complex tool electrode geometries, graphite is also used for EDM finishing. However, in EDM finishing, graphite shows still higher material removal rates but is also characterised by significantly higher relative tool wear compared to copper. This relative tool wear is fundamentally dependent on the material properties and the used process parameters. Therefore, this paper presents an approach for using graphite in EDM finishing operations. A parameter study was carried out using five different types of graphite to machine steel components. Within the investigations, different types of graphite with grain sizes in the range of 1.0 μm < Dg < 10.0 μm were applied. Furthermore, the characteristic EDM parameters discharge current, discharge duration, pause interval time, open circuit voltage as well as servo voltage were varied. The experimental results showed three sets of parameters that lead to a relative tool wear of ν ≤ 1.4 % and a surface roughness of Ra ≤ 1.2 μm. Using the default parameters of the machine tool for a surface roughness Ra ≤ 0.8 μm the relative tool wear was ν > 11.0 %. Therefore, the experiments showed a great improvement regarding the relative tool wear for graphite. Ten additional graphite types will be used for further investigations to gain a deeper understanding of the tool wear mechanisms. This will enable the reliable and efficient application of graphite as a tool electrode material for EDM finishing.