US Patent 7237424 Split die for forming grooved workpiece

A method of forming a radially expandable externally grooved tubular fastener from metal, comprising the steps of providing a suitable tubular blank (11) having a tubular wall (12) and squeezing the tubular wall between an internal member (17) with a surface which engages the internal tubular wall face (15) of the blank and a plurality of external members (18) provided with suitably shaped surfaces engaging the external tubular wall face (16) of the blank (11) thereby to form grooves (23) on the external tubular wall face (15) of the blank (11).