Patent ID: 8141404

Claim:
A method of making a generally tubular structural member having a variable wall thickness that is constant circumferentially but variable longitudinally, the method comprising the steps of: providing a tube having initial outer and inner diameters and an initial wall thickness; providing a stationary die having a die cavity for receiving said tube, said die cavity having a die first diameter and a die second diameter, said die second diameter being larger than the die first diameter, and a tapered portion extending between said die first diameter and die second diameter; providing a mandrel, said mandrel being adapted to be inserted into said tube and having a first end connected to a support rod and an opposite second end, said mandrel first end having a mandrel first diameter and said mandrel second end having a mandrel second diameter, the mandrel second diameter being less than the mandrel first diameter, wherein the mandrel includes a tapered portion extending between said first and second ends, and wherein said die first diameter is less than the mandrel first diameter but larger than the mandrel second diameter; inserting said tube through said die cavity and causing said tube and die to move with respect to each other along the longitudinal axis of said tube, wherein said tube is drawn through the die cavity while maintaining said die stationary; inserting said mandrel within said tube; and reciprocating the mandrel in and out of the die cavity while said tube is drawn through said die cavity whereby the tube wall thickness is reduced as the mandrel first end is advanced towards said die first diameter; the tube wall thickness being reduced from the initial wall thickness by an amount that increases as the mandrel first end is moved towards said die first diameter and decreases as the mandrel first end is moved away from said die first diameter to enable variable reductions in the tube wall thickness while said tube is drawn through said die cavity; wherein movement of the mandrel is effected by a control mechanism operably attached to the mandrel and movement of the tube is effected by a drawing machine operably attached to the tube, and wherein movement of the mandrel is independent of the movement of the tube and wherein said die is maintained stationary during the reciprocal movement of said mandrel; wherein the tube is provided with at least two longitudinally extending regions of constant reduced wall thickness at desired locations along its length, each of said at least two longitudinally extending regions of constant reduced wall thickness comprising a different reduced wall thickness; and wherein each longitudinally extending region of constant reduced wall thickness is adjacent to at least one tapered region of varying wall thickness.