Stretch-forming process

Claim 1

A method of plastically forming an elongated hollow metal member having walls capable of containing a compressible fluid under pressure, the method comprising the steps of: providing internal support within the hollow interior of the member by exposing the hollow interior thereof to a compressible fluid medium to impose a outwardly directed force uniformly over the interior periphery thereof at a pressure insufficient to cause mechanical working of the walls of the member; while maintaining said internal support, mechanically gripping the opposite longitudinal ends of the member and forcibly pulling said opposite ends in opposite longitudinal directions to thereby apply to the member a longitudinal tension of sufficient magnitude to exceed the elastic limit of the member and initiate elongation through plastic deformation thereof; while continuing to maintain said internal support and longitudinal tension bending the member intermediate of its ends in a direction transversely of the direction of the longitudinal tension; and relaxing said longitudinal tension, said bending load, and said internal support.

Claim 2

A method of plastically forming an elongated hollow metal member having walls capable of containing a compressible fluid under pressure, the method comprising the steps of: providing internal support within the hollow interior of the member by exposing the hollow interior thereof to a compressible fluid medium to impose a outwardly directed force uniformly over the interior periphery thereof at a pressure insufficient to cause mechanical working of the walls of the member; while maintaining said internal support, mechanically gripping the opposite longitudinal ends of the member and forcibly pulling said opposite ends in opposite longitudinal directions to thereby apply to the member a longitudinal tension of sufficient magnitude to exceed the elastic limit of the member and initiate elongation through plastic deformation thereof; while continuing to maintain said internal support and longitudinal tension bending the member intermediate of its ends in a direction transversely of the direction of the longitudinal tension; and relaxing said longitudinal tension, said bending load, and said internal support.

Claim 6

A method of forming an elongated, hollow, open-ended metal member having walls capable of containing a compressible fluid under pressure, the method comprising the steps of: disposing sealing plug means in the open ends of the member to seal the hollow interior of the member from the ambient atmosphere; supplying compressible pressure medium to the sealed interior of the member via at least one of said plug means to impose on the member a circumferential tension load of a magnitude insufficient to result in mechanical working of the member yet sufficient to provide internal support for the walls of the member when the member is subjected to bending and stretching; engaging the opposite longitudinal ends of the member adjacent said plug means with gripping means and pulling said gripping means in opposite longitudinal directions to superimpose on said circumferential tension a uniform longitudinal tension load of sufficient magnitude to elongate the member beyond its yield point; simultaneously with said supplying and said pulling steps, moving said gripping means transversely of the direction of said pulling in a manner to move the member laterally into interfering contact with a forming die to impose on the member a transverse bending load of sufficient magnitude to laterally bend the member beyond its yield point; relaxing said uniform longitudinal tension load and said bending load; and subsequently releasing said compressible medium from the sealed interior of the hollow member to relax said circumferential tension load.

Claim 7

A method of forming an elongated, hollow, open-ended metal member having walls capable of containing a compressible fluid under pressure, the method comprising the steps of: disposing sealing plug means in the open ends of the member to seal the hollow interior of the member from the ambient atmosphere; supplying compressible pressure medium to the sealed interior of the member via at least one of said plug means to impose on the member a circumferential tension load of a magnitude insufficient to result in mechanical working of the member yet sufficient to provide internal support for the walls of the member when the member is subjected to bending and stretching; engaging the opposite longitudinal ends of the member adjacent said plug means with gripping means and pulling said gripping means in opposite longitudinal directions to superimpose on said circumferential tension a uniform longitudinal tension load of sufficient magnitude to elongate the member beyond its yield point; simultaneously with said supplying and said pulling steps, moving said gripping means transversely of the direction of said pulling in a manner to move the member laterally into interfering contact with a forming die to impose on the member a transverse bending load of sufficient magnitude to laterally bend the member beyond its yield point; relaxing said uniform longitudinal tension load and said bending load; and subsequently releasing said compressible medium from the sealed interior of the hollow member to relax said circumferential tension load.