Abstract: Since it is configured that an female screw groove 2a of a nut 2 extends round an axis of the nut 2 through 360 degrees or less and a sectional shape thereof taken at right angles to the groove contains a circular arc portion 2c of 90 degrees or less and a rectilinear portion 2d which connects to the circular arc portion at a bottom of the groove and extends in parallel with the axis, after molding, a lower mold LM and an upper mold UM can simply be removed from a barrel mold BM in the axial direction, and consequently, the necessity of provision of a parting plane which passes through the thread groove 2a can be obviated, thereby making it possible to easily form the thread groove 2a which is free from a difference in level or a stepped portion.

Abstract: Since it is configured that an female screw groove 2a of a nut 2 extends round an axis of the nut 2 through 360 degrees or less and a sectional shape thereof taken at right angles to the groove contains a circular arc portion 2c of 90 degrees or less and a rectilinear portion 2d which connects to the circular arc portion at a bottom of the groove and extends in parallel with the axis, after molding, a lower mold LM and an upper mold UM can simply be removed from a barrel mold BM in the axial direction, and consequently, the necessity of provision of a parting plane which passes through the thread groove 2a can be obviated, thereby making it possible to easily form the thread groove 2a which is free from a difference in level or a stepped portion.

Abstract: The outside diameter ? of a land portion 1c is set to be larger than a BCD. Thus, when a ball 3 goes into a deep portion (a portion close to an axis line O of a screw shaft 1), the land portion 1c of the screw shaft 1 guides the ball. Consequently, the circulation of the ball 3 can smoothly be performed.