Electrochemical material removal characteristics of a NdFeB permanent magnet

Electric motors are used in various fields with an ever-increasing demand. In the electromobility sector as an example, a growth of a few hundred thousand motors per year is stated. In many cases, the rotor magnets for electric drives are produced by sintering. Special drives for two-axes positioning with precise movements require individual geometries of the rotor magnets such as spherical shapes. In this case, post-processing of the usually cylindrical or rectangular block shaped bar magnets is necessary. One common technique for post-processing is electrical discharge machining (EDM), which allows the machinability of hard or brittle materials, but also causes thermal impacts, which influence the magnetic properties of the material in a negative way. To overcome this negative effect it is necessary to re-magnetize the rotor magnets after EDM. Additional measurements of the magnetic field strength are required to ensure correct values. Electrochemical machining (ECM) represents an alternative technique for post-processing of rotor magnets, since it offers the possibility to avoid re-magnetization after the shaping process, because machining temperatures are well below the Curie temperature of most materials. In this study, ECM of a NdFeB permanent-magnetic material is presented. The experiments were carried out according to DIN-SPEC 91399. The material-specific removal characteristics indicate an active dissolution with the chosen machining parameters. Comparative measurements of the magnetic field strength before and after ECM indicate that there is no negative influence of the removal process. The results suggest that ECM with pulsed current and oscillating tool cathode is an applicable technique for shaping rotor magnets.