Electrical discharge machining or EDM is a process used for cutting intricate shapes, cavities and contours in pre-hardened steel. There is no heat treatment required. Compared to other machining methods, EDM involves less processes as there is no need to soften and then re-harden the steel. One of the key components for EDM are the electrodes. Rapid recurring current discharges between two electrodes remove the material from the work piece.
There are different types of electrodes used for EDM. Graphite electrodes is one of the most widely used. There are several reasons why graphite is preferred by many. It is fast and easy to produce. It is also resistant to thermal shock. Thermal shock happens when different parts of an object to expand by different amounts. This differential expansion can causes stress on the material. At some point, this stress can exceed the strength of the material, causing a crack to form.
Graphite electrode does not melt. It goes directly from solid to gaseous state. Graphite removes material better than copper or copper-tungsten while wearing slower. The wear rate tends to diminish as the discharge increases, unlike copper, whose wear increases at higher currents. This makes graphite more suited for machining of large electrodes since working with a high current intensity provides decreased roughing time.
Although graphite is prone to abnormal discharge, this can be eliminated through quality flushing, and lowering the intensity of discharge during negative polarity machining. However, since graphite is a ceramic, it is sensitive to mechanical shock. It must be handled and machined with care.
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