The selection of EDM parameters for graphite and copper materials is quite different. EDM parameters mainly include current, pulse width, pulse gap, and polarity. Below we will introduce you to the rationale for the proper use of these key parameters.
The current density of graphite electrodes is generally 10~12 A/cm2. Therefore, within the range of the current size allowed by the corresponding area, the larger the current selection, the faster the graphite discharge machining speed, the smaller the electrode loss, but the larger the surface roughness.
The larger the pulse width is, the lower the electrode loss. However, the larger pulse width will deteriorate the processing stability, and at the same time make the processing speed slower and the surface rougher. During rough machining, in order to ensure low electrode loss, the larger pulse width is usually used, and its value is 100-300 us, which can effectively achieve low-loss machining of graphite electrodes. When finishing, in order to obtain a fine surface and a stable discharge effect, a smaller pulse width should be selected. Generally, the pulse width of graphite electrodes is reduced by about 40% compared with that of the copper electrode.
The pulse gap mainly affects the EDM speed and machining stability. The larger the value of the pulse gap, the better the processing stability, which is helpful to obtain better surface uniformity, but the processing speed will be lowered. In graphite electrode EDM, the pulse gap and pulse width are usually set at 1:1. In the case of stable graphite processing, the ratio of pulse gap to pulse width can be adjusted to 2:3. When the pulse gap is small, it is beneficial to form a covering layer on the surface of the electrode, which is helpful to reduce the loss of the electrode.
The selection of polarity in graphite electrode EDM is basically the same as that of the copper electrode. According to the polarity effect of EDM, positive polarity processing is usually selected when processing dies steel. And that means the electrode is connected to the positive pole of the power supply, and the workpiece is connected to the negative pole of the power supply. A larger current and pulse width and select positive polarity processing can achieve a very low electrode loss effect. If the polarity is wrong, the electrode loss will become very large. Only when the surface requires fine processing less than VDI18 (Ra0.8μm), and very small pulse width is used, negative polarity processing is selected to obtain better surface quality, but the electrode loss is large.