Camshaft-manufacturing method and gear shaft-manufacturing method

A gear shaft-manufacturing method, comprising:

• preparing a plurality of split dies including recessed gear accommodation portions provided on side surfaces and rib accommodation portions formed in the recessed gear accommodation portions, configuring a split die group by butting side surfaces of the split dies, and forming a plurality of gear accommodation portions including the recessed gear accommodation portion between the split dies, in order to prepare the split die group;
• preparing a gear including a through-hole, a gear body which is smaller than each of the gear accommodation portions, and ribs which are provided on both sides in a thickness direction of the gear body and configured to be fitted into the rib accommodation portions, and a hollow shaft, accommodating the gear body in the gear accommodation portion and fitting the ribs in the rib accommodation portion, and inserting the hollow shaft into the through-hole of the gear, in order to prepare processing;
• and
• introducing pressurized fluid into a hollow portion of the hollow shaft and performing hydroform processing, and fixing the gear to the hollow shaft by expanding the hollow shaft, in order to perform the processing.