Influence of external magnetic field on permanent magnet by EDM

Permanent magnets are used in industrial devices. In general, a permanent magnet is made using a powder metallurgical process and magnetized in the final process. The shape of the magnet depends on the mold. When a special shape is required, it is created by machining before the magnetization process. It is difficult to machine after magnetization because the magnets are brittle and the magnetic force affects the machining tool. Electrical discharge machining (EDM) is a non-contact thermal machining method. Therefore, EDM can easily machine a magnet constructed from electrical conducting material. However, temperatures can exceed the melting point at the discharge point of EDM. A permanent magnet has a Curie point and temperature dependence. Past studies have concluded that magnetic flux density of permanent magnets changes when machining conditions change. The effect of heat on magnetic flux density was reduced when EDM was performed under finishing conditions. In addition, the behavior of magnetic flux density is affected by powerful external magnetic fields. This study investigated influences on magnetic flux density when the magnetic field changes during EDM of a permanent magnet. In particular, the magnetic field generated by the magnet changed by pressing a plate with high permeability on the surface of the permanent magnet. Internal temperature and surface magnetic flux density measurements on an opposite surface during machining working were performed to examine influences on the magnet when the magnetic field changed.