Method of making a tubular member

Abstract

In a method of making a metallic shell for a glow plug, a columnar blank is extruded to have an enlarged recess on the upper end surface of the blank so as to make an upper tubular portions, while extruded to have an enlarged recess on the lower end surface to make a first tubular end circular in section, an outer diameter of which is smaller than that of the upper tubular portion. The blank is transferred without inverting it, and the blank is reduced at its upper tubular portion to make a second tubular end hexagonal in section, an outer diameter of which is smaller than that of a middle portion of the blank, but greater than that of the first tubular end. Then, the blank is transferred without inverting it, and punched to communicate the first tubular end with the second tubular end by means of an axial bore. In other method, a mandrel is forced into the upper tubular portion to form a second tubular end hexagonal in section, an outer diameter of which is smaller than that of a middle portion of the blank, but greater than that of the first tubular end. In the forcing process, a cooling liquid oil is supplied between the upper tubular portion of the blank and the mandrel to alleviate friction heat, the cooling liquid oil flowing down between the upper tubular portion of the blank and the mandrel is led to the axial bore as an escape path of the liquid oil.