The principle of spark machining is usually that the workpiece is connected with the positive pole of the power supply. If negative polarity machining is adopted, it will lead to reverse machining. That is, the electrode loss is large. Reverse processing can only be used when processing the same material, which is often called iron striking. During EDM, the tool electrode and the workpiece are respectively connected to the two poles of the pulse power supply and immersed in the working fluid, or the working fluid is charged into the discharge gap. The automatic gap control system controls the tool electrode to feed to the workpiece. When the gap between the two electrodes reaches a certain distance, the pulse voltage applied on the two electrodes will break down the working fluid and produce spark discharge.

A large amount of heat energy is instantaneously concentrated in the micro channel of discharge, the temperature can be as high as more than 10000 ℃, and the pressure also changes sharply, so that a small amount of metal materials on the working surface immediately melt and gasify, and explosively splash into the working liquid, condense rapidly, form solid metal particles, and are taken away by the working liquid. At this time, a small pit trace is left on the surface of the workpiece, the discharge stops briefly, and the working fluid between the two electrodes returns to the insulating state.

Then, the next pulse voltage breaks down at another point relatively close to the two electrodes to produce spark discharge, and the above process is repeated. In this way, although the amount of metal etched by each pulse discharge is very small, because there are thousands of pulse discharges per second, more metal can be etched and has a certain productivity.