Ball screw and manufacturing method of nut for ball screw

A manufacturing method of a ball screw, the ball screw including a screw shaft having on an outer circumferential surface a spiral screw groove, a nut having on its first inner circumferential surface a screw groove facing the screw groove of the screw shaft, a plurality of balls rollably loaded in a spiral ball rolling passage formed by the both screw grooves, and a ball circulating passage to return the balls from an end point to a start point of the ball rolling passage for circulating the balls, the manufacturing method comprising:

• forming a concaved groove to be the ball circulating passage as by concaving a groove on a part of the cylindrical first inner circumferential surface of the nut;
• and
• integrally forming an outer circumferential formation on an outer circumferential surface of the nut within a range in an axial direction where the concaved groove is formed, wherein
• the concaved groove is formed on the first inner circumferential surface of a cylindrical nut blank by pressing the first inner circumferential surface of the nut blank with a convex, by a press method using a die which includes a cam mechanism, the cam mechanism comprising a cam driver internally inserted into the nut blank and moving in the axial direction, a cam slider disposed between the nut blank and the cam driver, and having the convex corresponding to the concaved groove, the convex moving in a radial direction of the nut with a movement of the cam driver, and a restraining member for restraining both end faces in the axial direction and the outer circumferential surface of the nut blank, and having a first recess on a second inner circumferential surface that receives the outer circumferential surface of the nut blank, and
• the outer circumferential formation is formed by projecting a periphery of the nut blank into the first recess of the restraining member.