FORMING METHOD FOR BOSSED MEMBER

PURPOSE:To form the boss member having specified thickness and height by pushing a punch after forming the circumference of a prepd. hole in a truncated conical shape by approaching the punch with pressurizing the prepd. hole between die and plate presser by piercing it on a plate-like blank stock. CONSTITUTION:The plate-like blank stock B piercing a prepd. hole at the center part is set between a die 3 and plate presser 2. It is then pressurized by a hydraulic cylinder 6 toward the center from the plural directions at the outside of the blank stock. A plastic deformation is then started by the lift of the stress near the prepd. hole, propagated in the outside direction of the blank stock B in order and the inner part stress becomes in a balanced state. When a punch 1 is approached to the die 3, it is deformed in the truncated conical shape with the corner round part of the punch 1 and die 3 as a contact point. At the initial stage when this trucated conical part is formed the prepd. hole diameter is changed to a contracting trend and the material of sufficient volume is fed from the circumference to this truncated conical part. Thereafter, with further push-in of the punch 1 the punch 1 is slided at the inner side of the trucated conical part, the spreading formation of the prepd. hole is executed and the boss having specified thickness and height is formed.