Abstract: In a forging process step, a small diameter portion 12 is formed in the leading end portion of a cylindrical shaft material 30, and an installation depressed portion 11d is formed on a base end face of the shaft material 30 for installing a ball 15 therein to receive thrust force, by cold forging process using a forging mold 31. In a worm forming process after the forging process step, a worm 13 is formed in the small diameter portion 12 by rolled dies. The small diameter portion 12 and the installation depressed portion 11d are formed by cold forging process, so that an outer circumferential face of the small diameter portion 12 and an inner face of the installation depressed portion 11d become even surfaces with small surface roughness.

Abstract: A pitch radius of a worm tooth decreases toward an axial center of a worm. A first side flank of the tooth is defined such that a normal line to the first side flank is angled relative to an axial line in a longitudinal cross section of the worm. The normal line is normal to the first side flank at a contact point between a pitch line of the worm tooth and the first side flank. The axial line is parallel to a central axis of the worm and extends through the contact point. An angle between the normal line and the axial line at the first side flank is generally constant throughout an entire axial extent of the worm.

Abstract: An object of the present invention is to provide a method of manufacturing a rotational shaft having a worm, wherein a worm portion has a small bending amount. According to the method of the invention, a worm portion is formed at a worm forming portion by a rolling process with rolling dies, and a small diameter portion is formed at least at one axial end of the worm forming portion, wherein a diameter of the small diameter portion is smaller than a diameter of a bottom of a worm tooth to be formed at the worm forming portion, so that surplus material portion of the worm forming portion can flow toward the small diameter portion.

Abstract: An apparatus manufactures a die for rolling an hourglass worm in improved processing accuracy. The apparatus includes a swing table swung, a movement table mounted on the swing table, and a workpiece shaft arranged on the movement table. The axis of a disk-shaped workpiece is perpendicular to a swing axis of the swing table. The workpiece includes a peripheral surface with a cross-section extending along the workpiece axis and defining an arc substantially identical to the arc of the root surface of the hourglass worm. The movement table is arranged so that the swing axis of the swing table extends through a center of curvature of the arc defined on the peripheral surface of the workpiece. A synchronization mechanism swings the swing table in synchronism with the rotation of the workpiece shaft.

Abstract: In a forging process step, a small diameter portion 12 is formed in the leading end portion of a cylindrical shaft material 30, and an installation depressed portion 11d is formed on a base end face of the shaft material 30 for installing a ball 15 therein to receive thrust force, by cold forging process using a forging mold 31. In a worm forming process after the forging process step, a worm 13 is formed in the small diameter portion 12 by rolled dies. The small diameter portion 12 and the installation depressed portion 11d are formed by cold forging process, so that an outer circumferential face of the small diameter portion 12 and an inner face of the installation depressed portion 11d become even surfaces with small surface roughness.

Abstract: An apparatus manufactures a die for rolling an hourglass worm in improved processing accuracy. The apparatus includes a swing table swung, a movement table mounted on the swing table, and a workpiece shaft arranged on the movement table. The axis of a disk-shaped workpiece is perpendicular to a swing axis of the swing table. The workpiece includes a peripheral surface with a cross-section extending along the workpiece axis and defining an arc substantially identical to the arc of the root surface of the hourglass worm. The movement table is arranged so that the swing axis of the swing table extends through a center of curvature of the arc defined on the peripheral surface of the workpiece. A synchronization mechanism swings the swing table in synchronism with the rotation of the workpiece shaft.

Abstract: An object of the present invention is to provide a method of manufacturing a rotational shaft having a worm, wherein a worm portion has a small bending amount. According to the method of the invention, a worm portion is formed at a worm forming portion by a rolling process with rolling dies, and a small diameter portion is formed at least at one axial end of the worm forming portion, wherein a diameter of the small diameter portion is smaller than a diameter of a bottom of a worm tooth to be formed at the worm forming portion, so that surplus material portion of the worm forming portion can flow toward the small diameter portion.