Method of, and apparatus for manufacturing a gear with a central through hole

A method of manufacturing a gear with a central through hole, said method comprising:

• A) a step wherein a hollow material having an outer surface and an inner peripheral surface defining a central through hole for the gear is prepared;
• B) a step wherein a hardened surface layer is formed in the region of said inner peripheral surface and said outer surface of said hollow material;
• C) a step wherein said hollow material having said hardened surface layer is hot-forged into a hollow gear-like forged body by a forging die having a first die element with a piercing punch entering into said central through hole of the hollow material from one side thereof and having a second die element with a piercing punch entering into said central through hole of the hollow material from another side thereof;
• wherein
• i) said forging die forms teeth of the gear on said outer side of said hollow material while producing an annular flash in said central through hole of the hollow material between said piercing punches;
• and
• ii) a distance between said piercing punches in a closed condition of said forging die defines a thickness of said annular flash which is 0.3 mm or more and no greater than 1.3 times (130%) of a depth of said hardened surface layer;
• D) a step wherein the annular flash is removed from the hollow gear-like forged body after the hot forging;
• and
• E) a step wherein thermal refining is implemented to the hollow gear-like forged body.