Patent ID: 6357274
Filing Date: 2002-03-19
Classification: B21C,B21K,H01T

Abstract:
A method of cold forging a metallic shell for a spark plug, an outer periphery of said shell having a tapered stepped portion between a large diameter part and small diameter part, to provide a seal when attached to an engine, said method comprising the steps of:securing a columnar material in a stepped inner cavity of a first molding die; pressing a first punch against said material in an axial direction to transform a shape of said material into a first processed part having a tapered bearing surface between the large diameter part and small diameter part, said first processed part having a large diameter head part and large diameter hole at one end and a small diameter foot part positioned at an opposite end of said head part and said large diameter hole, the small diameter foot part having a smaller outer diameter than an outer diameter of the large diameter head part, said first processed part having a first tapered part formed at a boundary between said large diameter head part and said small diameter foot part; securing the first processed part in a stepped inner cavity of a second molding die, said second molding die having a stepped inner cavity, said second molding die having a tapered bearing surface with a greater tapering angle than a tapering angle of the first tapered part; Inserting a second punch into the larger diameter hole of the first processed part to form a tapered bearing surface at the boundary between the large diameter part and the small diameter part, said second punch having a larger outer diameter than that of the small diameter foot part of the first processed part, said second punch pressing in an axial direction and transforming a shape of said first tapered part to conform to the bearing surface of the second molding die to create a second processed part having a stepped columnar shape and a second tapered part with the second tapering angle; securing the second processed part in a stepped inner cavity of a third molding die, said third molding die having a stepped inner cavity that forms a tapered bearing surface at the boundary between the large diameter part and the small diameter part, the tapered bearing surface having a smaller tapering angle than the tapering angle in the second tapered part; and inserting and axially pressing a third punch into the large diameter hole in the second processed part to transform said second tapered part to conform to the bearing surface of the third die to form the third processed part into a stepped columnar shape and into a third tapered part with a tapered angle having a third tapering angle smaller than the second tapering angle of the second tapered part, said third punch having an outer diameter of a tip that is smaller than the small diameter foot part of the second processed part.