Motor yoke and method of manufacturing same

摘要

In a method of manufacturing a partly flat yoke having curved circumferential walls to which magnets are to be attached and having parallel flat circumferential walls which constitute main magnetic paths and whose each radial thickness is larger than that of the curved circumferential walls, by drawing a sheet of metal plate, the metal plate is held down between a die and projections of a wrinkle preventing plate provided at given angular positions around an outer circumference of a cavity of the die so as to correspond to the curved circumferential walls. Then, a punch presses down the metal plate into the cavity so that material of the metal plate extends axially into the cavity, while the material of the metal plate flows circumferentially from each side of the curved circumferential walls to sides of the parallel flat circumferential walls.