The principles of EDM (charmilles wire edm filters)
The principle of EDM is to use the eroding effect of controlled electric spark discharges on the electrodes. It is thus a thermal erosion process. The sparks are created in a dielectric liquid, generally water or oil, between the workpiece and an electrode, which can be considered as the cutting tool. There is no mechanical contact between the electrodes during the whole process. Since erosion is produced by electrical discharges, both electrode and workpiece have to be electrically conductive. Thus, the machining process consists in successively removing small volumes of workpiece material, molten or vaporized during a discharge. The volume removed by a single spark is small, in the range of 10¡6¡10¡4 mm3, but this basic process is repeated typically 10’000 times per second. Figure 1.2 gives a simple explanation of the erosion process due to a single EDM discharge. First, voltage is applied between the electrodes. This ignition voltage is typically 200 V. The breakdown of the dielectric is initiated by moving the electrode towards the
workpiece. This will increase the electric field in the gap, until it reaches the necessary value for breakdown. The location of breakdown is generally between the closest points of the electrode and of the workpiece, but it will also depend on particles present in the gap [3]. When the breakdown occurs, the voltage falls and a current rises abruptly. The presence of a current is possible at this stage, because the dielectric has been ionized and a plasma channel has been created between the electrodes. The discharge current is then maintained, assuring a continuous bombardment of ions
and electrons on the electrodes. This will cause strong heating of the workpiece material (but also of the electrode material), rapidly creating a small molten metal pool at the surface. A small quantity of metal can even be directly vaporized due to the heating. During the discharge, the plasma channel expands. Therefore, the radius of the molten metal pool increases with time. The distance between the electrode and the workpiece during a discharge is an important parameter. It is estimated to be around 10 to 100 ¹m (increasing gap with increasing discharge current). At the end of the discharge, current and voltage are shut down. The plasma implodes under the pressure imposed by the surrounding dielectric. Consequently, the molten metal pool is strongly sucked up into the dielectric, leaving a small crater at the workpiece surface (typically 1¡500 ¹m in diameter, depending on the current). charmilles wire edm filters
