Method for producing threaded shaft for power steering device, and power steering device

A method for producing a threaded shaft for power steering device, the method for producing the threaded shaft for use with a power steering device equipped with:

• a ball screw shaft, which is a shaft member having a substantially circular cross-section and provided to steer steered road wheels in accordance with rotation of a steering wheel, the ball screw shaft having a spiral shaft-side ball screw groove formed in an outer peripheral surface in a predetermined axial range;
• a nut formed into an annular shape configured to surround the ball screw shaft and having a nut-side ball screw groove formed in an inner peripheral surface for constituting a spiral ball recirculation groove in conjunction with the shaft-side ball screw groove, the nut provided to be rotatable relative to the ball screw shaft;
• a plurality of balls interposed rollably in the ball recirculation groove;
• and
• an electric motor configured to apply a steering assist force to the ball screw shaft by rotationally driving the nut and converting rotation of the nut to axial movement of the ball screw shaft, and
• the method for producing the threaded shaft being a method for producing the ball screw shaft formed such that a groove radius, which is a distance from a rotation axis of the ball screw shaft corresponding to an axis of the ball screw shaft to the shaft-side ball screw groove, periodically changes in a circumferential direction of the rotation axis, the method comprising:
• a first step of causing the ball screw shaft to rotate about the axis of the ball screw shaft as the rotation axis;
• and
• a second step of grinding the ball screw shaft by causing a grindstone to rotate and move along the rotation axis of the ball screw shaft while performing the first step,
• wherein the first step and the second step are performed so as to satisfy an expression g/(5×p)<w<g/p, where w is a rotational speed of the ball screw shaft during grinding of the shaft-side ball screw groove, g is a rotational speed of the grindstone during grinding of the shaft-side ball screw groove, and p is a circumferential length of one circumference of the shaft-side ball screw groove around the rotation axis.