To improve strength of a screw shaft and make it easy to control a length dimension when forming the screw shaft for a ball screw by bulging work.
A tube p is inserted into a cylindrical bulging mold 21 having a screw forming surface 21b on an inner circumferential surface. While compressing the tube p with a jig for pushing shaft 22 in an axial direction, a screw shaft is pressurized by sending fluid into the tube p with pressurizing mechanism 23. The screw shaft is produced by expanding a diameter of the tube p to push the inner circumferential surface of the bulging mold 21 and to form screw grooves on an outer peripheral surface. When forming, as the tube p is compressed in the axial direction, even internal pressure is pressed to the tube p, the tube p is not expanded in the axial direction. The produced screw shaft is prevented from reduction in thickness and the strength is guaranteed. Since the compressed volume in the axial direction is fixed, it becomes easy to control the length dimension of the screw shaft.
MATSUNAGA KOJI
Takayoshi Tagawa
Naoya Kamata
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