Patent ID: 7325434

Claim:
A method for manufacturing a ring-shaped magnet, the method comprising: arranging, in a penetrating hole formed in a die, a mandrel comprising a cylinder tip portion of a diameter d 1 , a cylinder base end portion of a diameter d 2 , and a taper portion of a taper angle θ 2 positioned between the cylinder tip portion and the cylinder base end portion, wherein said penetrating hole comprises a first penetrating hole portion of a diameter D 1 , a second penetrating hole portion of a diameter D 2 , provided D 1 <D 2 , and a tapered hole portion of a taper angle θ 1 positioned between the first penetrating hole portion and the second penetrating hole portion; loading the cylinder tip portion with a preform from which the ring-shaped magnet is to be made, the preform being a circular ring column shaped body having an inner diameter of d 1 ; and plastic-working the preform in a gap formed by the penetrating hole and the mandrel, by pressing the preform with a pressing punch having an inner diameter of d 1 and an outer diameter of D 1 ; wherein D 1 , D 2 , d 1 and d 2 have values that satisfy: d 1 <d 2 <D 2 , 0<(1 −D 1 /D 2 )×100≦70, and 30≦(1−( D 2 2 −d 2 2 )/( D 1 2 −d 1 2 ))×100≦94.