Home > News

EDM characteristics of graphite materials(on)

Pulished on Dec. 13, 2019

In the 20th century, copper was widely used as an electrode material in EDM. At that time, graphite materials became an alternative consideration only when large electrodes were made and the mold processing requirements were not high or rough.

(1) EDM speed

Graphite is a non-metallic material with a very high melting point of 3650 , while the melting point of copper is 1083 , so graphite electrodes can withstand greater current setting conditions. When the discharge area and the size of the electrode are enlarged, the superiority of high-efficiency rough machining of graphite materials becomes more significant. The thermal conductivity of graphite is 1/3 of that of copper. The thermal energy generated during the discharge process can be more effectively used to remove metal materials. Therefore, its processing efficiency is also higher than that of copper electrodes in medium and fine processing. According to processing experience, under the correct conditions of use, the discharge processing speed of graphite electrodes is 1.5 to 2 times faster than the overall copper electrode.

(2) Electrode loss

Graphite electrodes have the characteristics of being able to withstand high current conditions. In addition, under appropriate conditions for roughing, carbon in steel workpieces during machining and carbon in decomposition products generated at high temperatures during machining Particles, under the effect of polarity, part of the erosion and carbon particles will adhere to the electrode surface to form a protective layer, which guarantees that graphite electrodes have minimal loss during rough processing, even "zero loss". The main electrode loss in EDM comes from rough machining. Although the set rate for finishing is high, the part has less machining allowance, that is, less machining erosion, and the overall loss is less. . In general, graphite electrodes lose less than copper electrodes in rough machining at high currents, and may lose slightly more than copper electrodes in finishing. The electrode losses of the two are comparable.

Graphite Electrode

Graphite Electrode

(3) Surface quality

The particle diameter of graphite material directly affects the surface roughness of EDM. The smaller the diameter, the lower the surface roughness value. A few years ago using graphite materials with a particle diameter of φ5 μm, the best surface for EDM can only reach VDI18 (Ra0.8 μm). Today, the particle diameter of graphite materials has reached within φ3 μm. Stable to VDI12 (Ra0.4 μm) or finer grades, but graphite electrodes cannot be mirror-edged.

Copper material has low resistivity, dense structure, stable machining state is easily obtained by EDM finishing, and stable processing under difficult conditions, surface roughness can be less than Ra0.1 μm, and mirror EDM can be performed. It can be seen that if the discharge machining pursues extremely fine surfaces, it is more suitable to use copper materials as electrodes, which is the main advantage of copper electrodes over graphite electrodes.

However, under the condition of high current setting of copper electrode, the electrode surface is easy to become rough and even crack, while graphite material does not have this problem. The cavity with a surface roughness of VDI26 (Ra2.0 μm) is required Processing, using a UHP graphite electrode can complete the processing from rough to fine, to achieve a uniform texture effect, the surface will not be defective. In addition, due to the difference in the structure of graphite and copper, the corrosion point of the graphite electrode surface discharge is more regular than that of the copper electrode. Therefore, when processing the same surface roughness of VDI20 and above, the surface particle size of the workpiece processed with the graphite electrode is more distinct. The seed surface effect is better than the discharge surface effect of the copper electrode.