Method of forming box-like frame members

Abstract

A box-like frame member is formed by compressing an internally-pressurized tubular blank within a die having a cavity conforming to the final box-like cross section desired for the product, and increasing the pressure to exceed the yield limit of the wall of the blank to expand the blank into conformity with the die cavity. The blank is selected so that the final product and the die cavity have a circumference preferably no more than about 5% larger than the circumference of the blank, to avoid weakening or cracking of the blank through excessive circumferential expansion. The internal pressure forces the blank evenly into the corners of the die on closing and allows the blank to be confined within the die without sections of the die pinching the blank on closing of the die.

I claim: 
     
       1. Method of forming a box section frame member of which at least an elongate portion is of uniform smoothly continuous cross sectional profile having at least two generally opposed and planar side faces .Iadd.and corners.Iaddend., comprising, .Iadd.providing a die defined by die sections having open and closed positions, .Iaddend.each having a channel section die cavity portion, a planar mating surface portion and the cavity portion having each channel side extending perpendicular to the mating surface portion, which die sections in the closed position have the mating surface of each section in mating engagement with the mating surface of each adjacent section and the cavity portions defining a die cavity up to about 5% larger in circumference than the circumference of .[.the.]. .Iadd.a .Iaddend.tubular blank and with a smoothly continuous box section cross section profile corresponding to the box section cross sectional profile of the desired final frame member, providing a tubular blank having a continuously smooth arcuate cross section; .Iadd.the circumference of which is such that forming of said blank to the shape of said die cavity will cause expansion of the circumference of said blank by no more than about 5%; .Iaddend.positioning the blank between open die sections.[.j.].applying internal hydraulic pressure to the blank at least sufficient to overcome frictional forces exerted on the blank by the die sections on closing of the die sections and tending to expel the wall of the blank laterally outwardly between adjacent mating surfaces of the die sections and less than the yield limit of the wall of the blank; closing the die sections after pressurizing the blank to deform the blank inwardly in the areas corresponding to the generally opposed planar side faces and to force the blank evenly into the corners of the box section .Iadd.profile.Iaddend.; expanding the blank circumferentially by increasing the internal hydraulic pressure within the blank above the yield limit of the wall until all exterior surfaces of the blank conform to the die cavity .Iadd.and thereby increasing the circumference of the tubular blank by no more than 5%.Iaddend.; separating the die sections; and removing the expanded blank from the die. 
     
     
       2. Method as claimed in claim 1 wherein the bottom of each channel .Iadd.section cavity .Iaddend.is planar. 
     
     
       3. Method as claimed in claim 1 wherein the die cavity is of uniform cross section throughout its length. 
     
     
       4. Method as claimed in claim 1 comprising bending the tube before placing it between .Iadd.the .Iaddend.die sections each having a cavity conforming to the bent shape of the tube. .Iadd..[. 
     
     
       5.  Method as claimed in claim 1 wherein the circumference of the die cavity is no more than about 5% larger than the circumference of the tubular blank..]..Iaddend. 
     
     
       6. Method as claimed in claim .[.6.]. .Iadd.1 .Iaddend.wherein the circumference of the die cavity is about 2 to about 4% larger than the circumference of the tubular blank. .Iadd. 
     
     
       7.  Method of forming a box section frame member of which at least an elongate portion is of uniform smoothly continuous cross-sectional profile having at least two generally opposed and planar side faces and corners, comprising, providing a die defined by die sections having open and closed positions, each having a die cavity portion having in cross section a corner, a planar mating surface portion, which die sections in the closed position have the mating surface of each section in mating engagement with the mating surface of each adjacent section and the cavity portions defining a die cavity up to about 5% larger in circumference than the circumference of a tubular blank and with a smoothly continuous box section cross section profile corresponding to the box section cross-sectional profile of the desired final frame member, providing a tubular blank having a continuously smooth arcuate cross section the circumference of which is such that forming of said blank to the shape of said die cavity will cause expansion of the circumference of said blank by no more than about 5%; positioning the blank between open die sections; applying internal hydraulic pressure to the blank at least sufficient to overcome frictional forces exerted on the blank by the die sections on closing of the die sections and tending to expel the wall of the blank laterally outwardly between adjacent mating surfaces of the die sections and less than the yield limit of the wall of the blank; closing the die sections after pressurizing the blank to deform the blank inwardly in the areas corresponding to the generally opposed planar side faces and to force the blank evenly into the corners of the box section profile; expanding the blank circumferentially by increasing the internal hydraulic pressure within the blank above the yield limit of the wall until all exterior surfaces of the blank conform to the die cavity and thereby increasing the circumference of the tubular blank by no more than 5%; separating the die sections; and removing the expanded blank from the die. .Iaddend. .Iadd. 
     
     
       8.  Method as claimed in claim 7 wherein the die cavity is of uniform cross section throughout its length. .Iaddend. .Iadd.9. Method as claimed in claim 7 comprising bending the tube before placing it between the die sections each having a cavity conforming to the bent shape of the tube. .Iaddend. .Iadd.10. Method as claimed in claim 7 wherein the circumference of the die cavity is about 2 to about 4% larger than the circumference of the tubular blank. .Iaddend. .Iadd.11. Method as claimed in claim 7 wherein the die consists of two die sections. .Iaddend. .Iadd.12. Method as claimed in claim 11 wherein each die section has a channel section cavity. .Iaddend. .Iadd.13. Method as claimed in claim 12 wherein the bottom of each channel section cavity is planar. .Iaddend. .Iadd.14. Method as claimed in claim 13 wherein said channel section cavities define sides, and the bottom of each channel section cavity is substantially 
     
     
        perpendicular to its sides. .Iaddend. .Iadd.15.  Method of forming a box section frame member of which at least an elongate portion is of uniform smoothly continuous cross-sectional profile having at least two generally opposed and planar side faces and corners, comprising, providing a die defined by die sections having open and closed positions, each having a die cavity portion having in cross section a corner, a planar mating surface portion, which die sections in the closed position have the mating surface of each section in mating engagement with the mating surface of each adjacent section and the cavity portions defining a die cavity up to about 5% larger in circumference than the circumference of a tubular blank and with a smoothly continuous box section cross section profile corresponding to the box section cross-sectional profile of the desired final frame member, providing a tubular blank having a continuously smooth arcuate cross section the circumference of which is such that forming of said blank to the shape of said die cavity will cause expansion of the circumference of said blank by no more than about 5%, positioning the blank between open die sections; applying internal hydraulic pressure to the blank less than the yield limit of the wall of the blank, such that as the blank is compressed, the internal pressure acting on the wall of the blank is sufficient to force the wall of the blank evenly into each of the corners defined by said die sections, thereby, maintaining the blank within the envelope defined by the die cavity; closing the die sections after pressurizing the blank to deform the blank inwardly in the areas corresponding to the generally opposed planar side faces and to force the blank evenly into the corners of the box section profile; expanding the blank circumferentially by increasing the internal hydraulic pressure within the blank above the yield limit of the wall until all exterior surfaces of the blank conform to the die cavity and thereby increasing the circumference of the tubular blank by no more than 5%; separating the dies sections; and removing the expanded blank from the die. .Iaddend. 
     
     
        .Iadd.     The method as claimed in claim 15 comprising bending the tube before placing it between the die sections and further providing a die which defines a cavity conforming to the bent shape of the tube. .Iaddend. .Iadd.17. Method as claimed in claim 15 wherein the die consists of two die sections. .Iaddend. .Iadd.18. Method as claimed in claim 15 wherein each die section has a channel section cavity. .Iaddend. .Iadd.19. Method as claimed in claim 18 wherein bottom of each channel section cavity is planar. .Iaddend. .Iadd.20. Method as claimed in claim 19 wherein the channel section cavities define sides, and, the bottom of each channel section cavity is substantially perpendicular to its sides. .Iaddend. 
     
     
        .Iadd.21.  The method as claimed in claim 18 wherein said channel section die cavity portion of each die section has each channel side extending perpendicular to the mating surface portion. .Iaddend. .Iadd.22. Method as claimed in claim 15 wherein the circumference of the blank is such that forming of said blank to the shape of said elongate portion will result in expansion of the circumference of said blanks by no more than from about 2% to 4%. .Iaddend.