Abstract: It is an object of the present invention to provide a method of manufacturing a rotor for an electric motor, in which displacement of the center of gravity is prevented. According to a feature of the present invention, a rotor for an electric motor comprises an inner core fixed to a rotating shaft of the rotor and multiple coil units respectively fixed to the inner core, wherein the coil unit has an outer core, a bobbin and a winding wound on the bobbin, and wherein weight of outer cores as well as winding units (the bobbin and the winding wound thereon) is respectively measured and stratified into several groups, necessary number of the outer cores and winding units are respectively picked out from the same stratified group and then such outer cores and winding units are assembled to the inner core.

Abstract: There is provided a method of forming grooves in a resin bobbin (90), and the resin bobbin (90) has a cylindrical portion around which a coil is wound and a plurality of grooves are formed on a surface of the cylindrical portion in the winding direction of the coil. While a forming die (2) for grooving is being pushed onto the surface of the cylindrical portion, the forming die (2) for grooving is vibrated, and the forming die (2) for grooving is separated from the surface of the cylindrical portion after the completion of grooving of the cylindrical portion.

Abstract: It is an object of the present invention to provide a method of manufacturing a rotor for an electric motor, in which displacement of the center of gravity is prevented. According to a feature of the present invention, a rotor for an electric motor comprises an inner core fixed to a rotating shaft of the rotor and multiple coil units respectively fixed to the inner core, wherein the coil unit has an outer core, a bobbin and a winding wound on the bobbin, and wherein weight of outer cores as well as winding units (the bobbin and the winding wound thereon) is respectively measured and stratified into several groups, necessary number of the outer cores and winding units are respectively picked out from the same stratified group and then such outer cores and winding units are assembled to the inner core.