Patent ID: 7152446

Claim:
A method for forming an internal gear in which a cylindrical raw material is externally inserted onto a forming die with an outer gear part formed thereon and fixed to said forming die, a forming roll is relatively moved in an axial direction of said forming die with respect to said raw material with said forming roll press-contacted with an outer peripheral surface of said raw material and relatively revolved about the axis of said forming die such that an inner peripheral surface of said raw material is pressed against said outer gear part of said forming die, thereby forming an internal gear part on the inner peripheral surface of said raw material, wherein each tooth of said outer gear part of said forming die is formed in a helical tooth, an annular weir part having an inside diameter equal to or less than a tooth tip circle diameter of said internal gear part is formed on the inner peripheral surface of said raw material adjacent to a second end part of said outer gear part on a rear side in a moving direction of said forming roll, and said forming die is rotated such that a first end part of said outer gear part on a front side in the moving direction of said forming roll is moved ahead of the second end part of said outer gear part on the weir part side when said forming roll is fixed and said forming die is rotated.