Change in Surface Magnetic Flux Density in EDM of Permanent Magnets-Influence of Internal Temperature and Shape on Machined Magnets

Traditional machining of a permanent magnet is difficult because of magnetic forces and brittleness of materials. However, electrical discharge machining (EDM), which is a non-contact thermal machining method, has carried out for shape machining of magnetic materials. Magnetic materials have a Curie point. As their magnetic flux density reduces when they are heated to a high temperature. Because EDM is a thermal process, it has the potential to control the magnetic flux density of a machined surface.In this study, to clarify the relationship between magnetic flux density and temperature distributions in depth direction of permanent magnet by EDM, internal temperatures of magnets were investigated using a K type thermocouple during EDM. Neodymium magnets were used as work-pieces. The magnetic flux density of a machined neodymium magnet was measured. In addition, the effects of duty factor (D.F.) and in Diamagnetic Field were also examined. The results showed that the average temperature inside of the magnet is determined by the input energy, depending on the discharge conditions. A decrease of surface magnetic flux density after EDM is affected by the magnitude of the area and the amount of decrease is due to the increase of the internal temperature of the magnet. In a diamagnetic field EDM, the reduction in magnetic flux density is large compared with a regular magnetic field. However, there is no difference in internal temperature each machining. Therefore, it isn’t determined simply by the magnitude of the input energy. It can be said that combination of heat history and machined magnet shape determine the magnetic flux density.