Crowning forming method and crowning forming apparatus

A crowning forming method in which a desired crowning is formed in a tooth shape formed on an outer circumference of an annular material by a cylindrical die, wherein

• the cylindrical die comprises a first tooth-shaped die part and a second tooth-shaped die part, a tooth shape of the first tooth-shaped die part being formed on an inner circumference to have a constant tooth width from an upper end part of the first tooth-shaped die part to a lower end part of the first tooth-shaped die part and to correspond to a tooth width of a part of the annular material in which the desired crowning has been produced, and a tooth shape of the second tooth-shaped die part being formed on an inner circumference to have a constant tooth width from an upper end part of the second tooth-shaped die part to a lower end part of the second tooth-shaped die part and to correspond to a tooth width of a central part of the annular material, which is a part of the annular material other than the part of the annular material in which the desired crowning has been produced, the method comprising:
• forming a tooth shape on an outer circumference of the annular material by the tooth shape of the inner circumference of the first tooth-shaped die part;
• and
• moving the annular material in which the tooth shape is formed into the second tooth-shaped die part, and pressing an inner circumferential surface of the annular material outwardly in such a way that a diameter of the central part of the annular material is greater than the diameter of each of upper and lower end parts of the annular material, thereby pressing the central part of the tooth shape of the annular material against the tooth shape of the inner circumference of the second tooth-shaped die part,
• wherein the tooth shape of the first tooth-shaped die part is formed to be smaller than the tooth shape of the second tooth-shaped die part.