METHOD FOR MANUFACTURING STEERING SHAFT OF STEERING DEVICE
A method for manufacturing a steering shaft of a steering device includes: a reduced-diameter section forming process in which a second-phase member is formed from a first-phase member by forming a reduced-diameter section in the first-phase member; and a ball screw groove forming process in which a third-phase member is formed from the second-phase member by forming a ball screw groove in the second-phase member by rolling.
1 . A method for manufacturing a steering shaft of a steering device, the method comprising:
a reduced-diameter section forming process in which a second-phase member is formed from a first-phase member being a metal rod, by forming a reduced-diameter section in the first-phase member; and
a ball screw groove forming process in which a third-phase member is formed from the second-phase member by forming a ball screw groove in the second-phase member,
wherein:
the reduced-diameter section has a radius measured from a reference axis which is smaller than a material radius, wherein the reference axis is an axis that is parallel with a longitudinal direction of the first-phase member and extends through a center of the first-phase member at a cross section perpendicular to the longitudinal direction of the first-phase member, and wherein the material radius is a radius of an outer circumference of the first-phase member which is measured from the reference axis;
in the reduced-diameter section forming process, the reduced-diameter section is formed in a first region that is a predetermined region including a first end of the first-phase member, wherein the first-phase member includes the first end and a second end in the longitudinal direction of the first-phase member; and
in the ball screw groove forming process, the ball screw groove is formed in a second region of the second-phase member by rolling, wherein the second region is a predetermined region outside of the first region in a direction of the reference axis.
2 . The method as claimed in claim 1 , wherein in the ball screw groove forming process, the rolling of the ball screw groove is started from a state that rolling dies are pressed on the second-phase member such that each of the rolling dies overlaps with both of the first region and the second region in the direction of the reference axis.
3 . The method as claimed in claim 1 , wherein:
the steering device includes a steering shaft housing;
the steering shaft housing includes a body and a steering shaft supporter;
the body has a tubular shape and is structured to contain the steering shaft; and
the steering shaft supporter is formed in an inner periphery of the body, and is structured to contact with the ball screw groove so as to regulate displacement of the steering shaft in a warp direction of the steering shaft.
4 . The method as claimed in claim 1 , wherein:
the second-phase member includes a third region between the first region and the second region in the direction of the reference axis; and
the second-phase member includes in the third region a taper portion increasing gradually in radius measured from the reference axis, as followed in a direction from the first region to the second region.
5 . The method as claimed in claim 1 , wherein the third-phase member includes an incomplete screw portion in a third region between the first region and the second region in the direction of the reference axis.
6 . The method as claimed in claim 1 , the method further comprising:
an internal screw forming process in which an internal screw is formed to open at the first end,
wherein the internal screw forming process is implemented after the ball screw groove forming process.
7 . The method as claimed in claim 6 , wherein the internal screw extends to overlap with the ball screw groove in the direction of the reference axis.