Process for forming axially-extendable bolt by upsetting

Abstract

An axially-extendable-bolt forming process by upsetting includes preparing a die having a through bore, and an internal upsetting cavity and a buffer space, a rod-shaped bolt precursor and a knock-out member, primarily pressing a leading end of the bolt precursor onto a leading end of the knock-out member, and secondarily pressing the bolt precursor onto the knock-out member. In the preparing step, the bolt precursor and knock-out member are fitted into the through bore of the die so that they can be brought into contact with each other. In the primarily-pressing step, the leading end of the bolt precursor is swollen finely and is brought into contact with an inner peripheral surface of the through bore of the die so that an excess material, being present adjacent to a leading-end surface of the bolt precursor, is flowed out into the buffer space. As a result, an axially-and-outwardly projecting flash is formed on a leading-end outer periphery of the bolt precursor so that irregularities, having been generated by shearing, are equalized. In the secondarily-pressing step, an intermediate portion of the bolt precursor is swollen in the internal upsetting cavity, and the bolt precursor is provided with an upset portion.