The main components in EDM are the electric power supply, the dielectric medium, the work piece, the tool, and a servo control unit. The work piece and the tool are electrically connected to a DC electric power supply. The current density in the discharge of the channel is of the order of 10000 A / Square centimeter, and power density is nearly 500 MW / Square centimeter. The work piece is connected to the positive terminal of the electric source, so that it becomes the anode and the tool is the cathode.
A gap, known as SPARK GAP in the range, from 0.005 mm to 0.05 mm is maintained between the work piece and the tool, and suitable dielectric slurry, which is non conductor of electricity, is forced through this gap at a pressure of 2 kgf/square centimeter or lesser. When a suitable voltage in the range of 50 to 450 V is applied, the dielectric breaks down and electrons are emitted from the cathode and the gap is ionized. When more electrons collect in the gap, the resistance drops causing electric spark to jump between the work piece surface and the tool. Each electric discharge or spark causes a focused stream of electrons to move with a very high velocity and acceleration from the cathode towards the anode, and ultimately creates compression shock waves on both the electrode surface, particularly at high spots on the work piece surface, which are closest to the tool. The generation of compression shock waves develops a local rise in temperature.
The whole sequence of operation occurs within a few microseconds. The temperature of the spot is hit by the electrons is of the order of 10000 degree centigrade. This temperature is sufficient to melt a part of the metals. The forces of electric and magnetic fields caused by the spark produce a tensile force and tear off particles of molten and softened metal from this spot in the work piece. Thus, the metal is removed in this way from the work piece. The volume of the material removed per spark discharge is typically in the range of (1/1000000) to (1/10000) cubic millimeter. Tolerance value of + or – 0.05 mm could be easily achieved by EDM in normal production. The best surface finish that can be economically achieved on steel is 0.40 micron.
A variation of EDM is wire EDM. Wire EDM is otherwise called as Electrical Discharge Wire Cutting. In this process, a slowly moving wire travels along a prescribed path, cutting the work piece. This process is used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals. It can also cut intricate components for the electronics industry. A schematic sketch of wire cut EDM is shown below in Figure-2.