Motor yoke

Abstract

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.

What is claimed is:  
     
         1 . A method of manufacturing a cup shaped motor yoke having circumferential walls at given angular regions to which magnets are to be attached and circumferential walls at remaining angular regions which play roles as main magnetic paths, by drawing a flat sheet of metal plate, comprising steps of: 
 putting the metal plate between a die having a cavity and a holding plate having a through-hole which is axially aligned with the cavity so that the metal plate is held down therebetween at positions around outer circumferences of the cavity and the through-hole corresponding to the given angular regions and is placed with a clearance having a predetermined distance from one of the die and the holding plate at positions around the outer circumference of the cavity corresponding to the remaining angular regions, and    pressing down the punch against the metal plate through the through-hole into the cavity so that material of the metal plate extends axially into the cavity, while the material of the metal plate flows to move in the cavity circumferentially from each side of the given angular regions to sides of the remaining angular regions,    whereby the yoke, whose each thickness of the circumferential wall at the given angular regions is thinner than that at the remaining angular regions, is formed.    
     
     
         2 . A method of manufacturing a cup shaped motor yoke according to  claim 1 , wherein the given angular regions are located symmetrically with respect to an axial center of the cavity.  
     
     
         3 . A method of manufacturing a cup shaped motor yoke according to  claim 1 , wherein the holding plate is provided on a surface facing the die around the through-hole with projections so that the metal plate is held down between the die and projections and each height of the projections corresponds to the predetermined distance of the clearance.  
     
     
         4 . A method of manufacturing a cup shaped motor yoke according to  claim 1 , wherein the predetermined distance of the clearance falls within a range of 8 to 15 percents of the thickness of the metal plate.  
     
     
         5 . A method of manufacturing a cup shaped motor yoke according to  claim 1 , further comprising steps of: 
 holding down the yoke between another holding plate having a through-hole and another die with a cavity whose diameter is smaller than that of the die and whose periphery end on a side of facing the another holding plate has round corners so that each curvature of the round corners at positions corresponding to the given angular regions is larger than that at the remaining angular regions, and    pressing down another punch, whose diameter is smaller than that of the punch, against the yoke through the through-hole of the another holding plate so that each thickness of the circumferential walls of the yoke at the given angular regions become further thinner and each thickness of the circumferential walls thereof at the remaining angular regions become further thicker.    
     
     
         6 . A method of manufacturing a cup shaped motor yoke according to  claim 5 , wherein the curvatures of the round corners are continuously and gradually reduced from each middle point of the given angular regions toward middle points of the remaining angular regions oppositely adjacent thereto.  
     
     
         7 . A motor yoke having first portions to which magnets are to be attached and second portions which constitute main magnetic paths and being formed by drawing a flat sheet of a metal plate, comprising: 
 first circumferential walls constituting the first portions, and    second circumferential walls constituting the second portions whose each thickness is thicker than that of the first circumferential walls,    wherein not only axial material flow of the metal plate but also circumferential material flow of the metal plate from each side of the first circumferential walls to sides of the second circumferential walls oppositely adjacent thereto are traced in the motor yoke.    
     
     
         8 . A motor yoke according to  claim 7 , wherein the motor yoke is a partly flat shaped yoke having curved portions, which are the first circumferential walls, and having parallel flat portions, which are the second circumferential walls.  
     
     
         9 . A motor yoke according to  claim 7 , wherein the motor yoke is a nearly cylindrical yoke whose outer shape of the circumferential wall is oval in cross section and whose inner shape of the circumferential wall is circular in cross section and the first circumferential walls are portions on opposite end sides of the circumferential wall in a major axis of oval and the second circumferential walls are portions bridging between the first circumferential walls, and, further, wherein the thickness of the circumferential wall of the motor yoke is continuously and gradually increased from each circumferentially middle point of the first circumferential wall toward circumferentially middle points of the second circumferential walls oppositely adjacent thereto.