Surface Magnetic Flux Density Patterning in EDM of Permanent Magnets

Traditional machining of permanent magnets is difficult because of magnetic forces and the brittleness of the materials. However, electrical discharge machining (EDM), a non-contact thermal machining method, has been successfully applied for the shape machining of magnetic materials. Magnetic materials are characterized by a Curie point, and their magnetic flux density decreases 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 our previous research, magnetic flux density was observed to vary with the EDM conditions [1]. This was confirmed when a cylindrical shape magnet was machined to reduce its height. We also found that surface magnetic flux density varied with internal temperature. Furthermore, in the machining of a bottomed hole in the center of a square magnet, patterning was performed using the magnetic flux density of the opposite surface. It was considered that both the change in shape and internal temperature exerted an influence. However, their relative contributions remained unclear. In this study, to clarify the relative degrees of influence of the change in shape and internal temperature in the machining of a bottomed hole in a square magnet, electro chemical machining (ECM) has also been applied to machine a bottomed hole with the same dimensions as that used in EDM. Because the internal temperature does not rise in ECM, only the influence of the shape change should appear in the magnetic pattern. The ECM equipment and machining results are presented herein. The machining results by ECM are compared with those by EDM, thereby allowing to individually study the influence of shape change and internal temperature.