Ball screw device, machine component manufacturing method, machine manufacturing method, vehicle manufacturing method, machine component, machine, vehicle, hydraulic forming method, and hydraulic forming die

A hydraulic forming method comprising:

• preparing a tubular forming die with a laminate body having a plurality of plates laminated in an axial direction, the laminate body including an inner peripheral surface for a forming;
• disposing a process material on the inside of the tubular forming die;
• and
• plastically deforming the process material toward the inner peripheral surface of the tubular forming die by applying a hydraulic pressure to an inner surface of the process material,
• the inner peripheral surface of the laminate body includes a plurality of steps, each of the steps being based on a difference in inner surface height of two adjacent plates,
• a contour of an outer surface of the plastically deformed process material has a curved shape, and
• a curved line connecting a plurality of corners of the steps has a shape substantially matching the curved shape of the plastically deformed process material.