Advantages of Graphite as an EDM Electrode Material
Graphite has many advantages, making it the most widely used material for EDM electrodes.
●It is easy to process.
●It is very resistant to thermal shock.
●Its low thermal expansion coefficient (3 times smaller than copper) ensures the stability of the electrode geometry during EDM.
●It is provided in chunks.
●It does not melt, but directly changes from solid to gaseous at 3,400 ° C, reducing wear.
●Its density is five times that of copper, so the electrode is lighter.
●Compared to copper, it has a higher metal removal rate and less wear.
●It has unique characteristics, that is, as the peak current increases, the wear rate tends to decrease.
More about graphite
Graphite for EDM processing is an anisotropic material with a grain size ranging from a few microns to about 20 microns. In the 1970s, improvements made by graphite manufacturers (isotropic, stable quality, large blank size), coupled with the advent of EDM machines equipped with iso-plus generators, made graphite the most commonly used material for EDM processing electrodes.
Three independent graphite groups can be defined:
1. Low-density (1.76 g / cm3) large-grain graphite (about 20 µm)
2. High frequency (1.82 g / cm3) fine-grained graphite (~ 10 µm)
3. High-quality grain graphite with a mass greater than 1.86 g / dm3 (~ 4 µm)
More large-grain graphite is used in rough machining mode, while fine-grain graphite can provide the best surface finish for finishing. As the price of graphite becomes cheaper and cheaper, EDM processing shops usually stock two or even three types or grades of graphite. Cheaper large-grain graphite for roughing, then coarse-grained graphite for finishing, or a combination of roughing and finishing properties; and expensive premium grains that may be used for expensive finishing and precision operations graphite.
Why choose graphite?
Graphite has several advantages over other materials. It is resistant to thermal shock. It is the only material whose mechanical properties increase with temperature. It has a lower CTE to achieve geometric stability. It is easy to process. It does not melt, but it sublimes at very high temperatures (3,400ºC). Finally, it has a lower density (five times lower than copper), which means that the electrodes are lighter. When graphite wears slowly, its material removal performance is better than copper or tungsten copper. The wear rate tends to decrease with increasing discharge, which is different from copper, where copper wear increases at higher currents. Therefore, graphite is suitable for the processing of large electrodes because working at high current strength can reduce the roughening time.
Although graphite is prone to abnormal discharge, this phenomenon can be eliminated by high-quality flushing and reducing the release intensity during negative polarity processing. However, as a result of this compromise, the processing of tungsten carbide is more complicated than the processing of copper tungsten electrodes. In addition, since graphite is ceramic, it is sensitive to mechanical shock, so it must be handled and processed with care.
Graphite grade comparison
It is not recommended to compare one grade of graphite with another grade of graphite just by looking at physical properties, and not to perform performance tests on graphite in actual EDM operations. However, some physical properties of graphite are listed below, which have some impact on the performance of EDM operation. These physical properties include: apparent density, average grain size, resistivity, strength, hardness, density, etc.
Generally, as the density of graphite increases, the wear rate decreases and the surface finish improves. Dense graphite is used to machine parts where geometry is critical. However, the use of less dense graphite usually results in better material removal.
With very fine grain graphite, wear is reduced and surface finish is improved. On the other hand, less material is removed, block size is reduced, and prices are rising.