Today we’d like to briefly introduce the graphite electrode for EDM（electric discharge machining）. EDM is a kind of precision machining technology, which is widely used in graphite mold processing industry.

The EDM device consists of a pulse generator, a workpiece and a graphite tool electrode. The processed workpiece and the graphite tool electrode are immersed in kerosene or other emulsion. The voltage on the capacitor is gradually increased after charging. When the voltage continues to rise until a spark discharge occurs between the gap of graphite tool electrode and the workpiece, most of the energy on the capacitor is released in an instant. The discharge channel has a small cross-section, and its current density can reach up to 105~106A/cm². Because of the small cross section of the channel and the short discharge time (microsecond level), the electrode surface temperature of the spark channel reaches 5000 ~ 10000℃. Such high temperature causes partial melting or even vaporization of the metal workpiece. The melting and gasification of this metal have explosive power. Such an explosive force can throw the molten metal into the fluid. The molten metal is solidified into tiny metal beads and removed from the workpiece due to the surface tension.

In EDM processing, the surfaces of the anode and the cathode are respectively eroded by the effect of bombardment of electrons and ions and the effect of the transiently high temperature. As the energy obtained by the anode and the cathode surface is different, the erosion amount is different.

During the discharging process of short pulse, the mass of electron is small. And the electron is easy to accelerate. But the mass of positive ion is big. So it’s not easy to accelerate. Therefore, the speed of anodic erosion of short pulse EDM is higher than that of cathode, when the workpiece is connected to the anode and the graphite tool electrode is connected to the cathode.

Positive ion of large mass has enough time to accelerate when the wide pulse machining is used. The damage effect of bombardment on cathode is greater than that of electron with small mass on anode. Meanwhile, the graphite tool electrode is connected to the anode, and the workpiece is connected to the cathode.

Roughing generally pursues a high processing speed. Therefore, during rough machining, we can select a higher peak current and a large pulse width, connect the workpiece to the negative electrode of the pulse power supply, and connect the graphite tool electrode to the positive electrode.

In the fine machining with short pulse, the surface roughness is the main index. At this time, the graphite tool electrode is connected to the negative electrode of the pulse electrode, and the workpiece is connected to the positive electrode. The machining speed is not high, and the erosion of graphite tool electrode is small, but the surface roughness of the workpiece is good.

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