Ball screw device, power transfer mechanism, electric power steering apparatus, and method for manufacturing ball screw device

A method for manufacturing a ball screw device in which

• the ball screw device comprises a threaded shaft with a groove formed on an outer peripheral surface of the shaft, and a cylindrical nut whose screw-thread mates with the groove on the threaded shaft via a plurality of balls,
• the nut comprises a pair of mounting holes in each of which a deflector is installed, and a communication groove that allows the mounting holes to communicate with each other, the communication groove extending in a direction parallel to an axis of the nut and connects the mounting holes together, and
• an opening of the communication groove is formed on the outer peripheral surface of the nut, and the balls are able to roll through the communication groove,
• the method comprising:
• inserting a cutting edge portion of a ball end mill into a first mounting hole of the pair of mounting holes, and
• moving the cutting edge portion of the ball end mill toward a second mounting hole of the pair of mounting holes to form the communication groove in the nut.