Electro-Discharge Machining (EDM) employs the principle of thermal energy (melting and vaporization) to remove material from the workpiece. This thermal energy is produced by means of electric sparks. In die sinking EDM, the electrode and workpiece are connected with two terminals of a power source and a small gap is maintained between them. Suitable dielectric fluid (like kerosene) is applied in this inter-electrode gap (IEG), which in normal state behaves like an insulator. When sufficient potential difference is applied across this dielectric column, the dielectric breaks down to become conductive and thus spark is generated. Since flow of electrons from tool electrode to workpiece via the dielectric is mandatory, so EDM process can be applied to electrically conductive electrode and workpiece materials. Moreover, a profile on the electrode must be provided based on the desired profile. Therefore, die-sinking EDM electrode material 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 employed as EDM electrode. Among metallic electrode electrolyte copper, tellurium copper, silver tungsten and brass are commonly used. Non-metallic material graphite is also used as EDM electrode, especially with pulse generator. A graphite electrode is rigid and tough and can maintain its form for longer duration sustaining mechanical and thermal shocks. It has good machinability and thus detailed features can easily be incorporated on the electrode; however, sharp edges cannot be provided as it is very brittle. High porosity also imposes problem associated with moisture and dielectric entrapment. It is worth mentioning that sometimes graphite is impregnated with copper to form copper-graphite to take advantage of both copper and graphite. When used as EDM electrode, it offers many benefits over the standard materials. Various similarities and differences between copper electrode and graphite electrode are given below in table format.

Similarities between copper electrode and graphite electrode

• Copper and graphite both are electrically conductive. Conductivity is essential as the electrode needs to liberate electrons under the presence of potential difference for dielectric breakdown and also to generate spark.
• Irrespective of electrode material, the tool electrode is always given negative polarity (cathode) in 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.
• None of them leads to the production of burr, residual stress or other mechanical damages to the machined surface.

Differences between copper electrode and graphite electrode

Copper Electrode Graphite Electrode Copper electrode comes under the category of metallic EDM electrode. Graphite being a non-metallic material, graphite electrode is classified as non-metallic electrode. Being a ductile metal, it can be fabricated easily. Sharp corner and edges can also be provided without much problem. Being a brittle material, graphite depicts poor machinability. So die having sharp corners and edges is very difficult to fabricate. Density of copper is typically 8.96g/cm3. Thus, die made of copper is much heavier. Density of copper is typically 2.26g/cm3. For same size of die, graphite die is lighter than copper one. Melting point of pure copper is about 1085°C. Sometimes copper electrode tends to change its form due to extreme spark heating. Melting point of graphite is more than three times higher than that of copper. Thus it tends to retain its form even at high EDM temperature. Because of its low melting temperature, copper electrode is also affected by thermal shocks. Thus several mechanical properties may change undesirably. Graphite electrode is less vulnerable under thermal shocks. So it can retain its properties for a longer duration. Copper electrode cannot sustain high current density. So material removal rate (MRR) is limited. Graphite electrode can allow high current density and thus MRR can be enhanced easily. Copper electrode does not absorb dielectric as it is not porous. Being a porous material, graphite electrode automatically absorbs dielectric during machining. This alters spark characteristics and electrode life. When not in use, it does not absorb water vapour even if it is kept in open atmosphere. It can entrap moisture in the porous regions. This moisture should be removed before using it. Copper electrode is cheaper. It can also be recycled (has salvage value). Graphite electrode is comparatively costly and cannot be recycled.

References

• Nonconventional Machining by P. K. Mishra (Narosa Publishing House).
• Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
• Zeis et al (2017). Deformation of thin graphite electrodes with high aspect ratio during sinking electrical discharge machining. CIRP Annals. 66(1): 185-188.