Patent ID: 7823432

Claim:
A method of forming spring washer blind-holes in a piston for an automobile transmission, comprising: flattening an annular region of a metallic sheet blank to form a secondary workpiece, the annular region to be located along a peripheral edge portion of an end plate of a piston body, said flattening including: punching out a center hole in the center of the metallic sheet blank while simultaneously deep-drawing the metallic sheet blank in such a manner as to form a preliminary workpiece having a circumferential sidewall rising at an oblique incline with respect to an original plane of the metallic sheet blank; forming the preliminary workpiece into a secondary workpiece by forming the circumferential sidewall of the preliminary workpiece to be upright with respect to the original plane of the metallic sheet blank; increasing a thickness of the peripheral edge portion at a rear of the end plate by flattening part of the end plate located inside the circumferential sidewall; and further thickening the peripheral edge portion at the rear of the end plate and making a rear surface of the peripheral edge portion of the end plate perpendicular to a radially outer surface of the peripheral edge portion of the end plate by drawing an external periphery of the circumferential sidewall; forming a plurality of spring washer blind-holes along the peripheral edge portion at the rear of the end plate of the secondary workpiece, said forming of the plurality of spring washer blind-holes including performing an extrusion process comprising projecting a plurality of pins into the peripheral edge portion at the rear of the end plate to form the spring washer blind-holes, a quantity and location of the pins corresponding to a desired quantity and location of the spring washer blind-holes; and performing said extrusion process so that all of the material extruded by the extrusion process due to the projecting of the pins flows so as to project out of a front surface of the end plate to form front surface projections.