Abstract: The problem to be solved by the present invention is to clear up an eccentric load onto a pin and a problem of deformation and cavity due to flowing material during extrusion when spring washer blind-holes in a piston for an automobile transmission are formed by extrusion with a pin. After an annular region along the peripheral edge portion on the back of an end plate of a secondary workpiece is flattened, the secondary workpiece is set on a lower die. Further, while the lower die is pressed with upper dies and cushion pressure is applied on the lower die, pins, for piercing the spring washer blind-holes are made to perform an extrusion operation onto the end plate of the secondary workpiece from the backside of the end plate. The extruded material is made to flow into recess holes of the upper die, and thus spring washer blind-holes are formed. Then, projections which have flowed and protrude on the front side are removed by turning with a lathe.

Abstract: The problem to be solved by the present invention is to clear up an eccentric load onto a pin and a problem of deformation and cavity due to flowing material during extrusion when spring washer blind-holes in a piston for an automobile transmission is formed by extrusion with a pin. After an annular region along the peripheral edge portion on the back of an end plate 3c of a secondary workpiece 3 is flattened, the secondary workpiece 3 is set on a lower die 11f. Further, while the lower die is pressed with upper dies 10f, 10g and cushion pressure is applied on the lower die 11f, pins 13, 13 . . . for piercing the spring washer blind-holes are made to perform an extrusion operation onto the end plate 3c of the secondary workpiece 3 from the backside of said end plate. The extruded material is made to flow into recess holes 10f1, 10f1 . . . of the upper die 10f and thus spring washer blind-holes 5, 5 . . . are formed.