METHOD FOR PINCH FREE TUBE FORMING

摘要

A method of forming an elongated box shape tubular frame member from a round tube includes the steps of: providing a lower die member having a cavity submerged substantially below the top surface of the lower die and having a cavity width equal to the width of the desired box shape frame member and less than the diameter of the tube. The lower die has an entry opening which opens to the top surface of the die and is defined by angularly disposed funnel walls which converge to intersect with the cavity. The round tubular blank is placed onto the lower die so that the blank rests on the angularly disposed funnel walls above the cavity. The upper die is lowered onto the lower die and has a lid wall which forces the tubular blank downwardly into the cavity while the angularly disposed funnel walls guide the blank to collapse in a manner preventing pinching of the blank between the upper and lower dies. The application of internal hydraulic pressure then expands the collapsed tube outwardly to the shape defined by the cavity of the lower die and the lid wall of the upper die. The lower die preferably includes rounded corners at the intersection of side walls and a bottom wall to provide rounded corners on the box shape tubular frame member. However, the edge surfaces of the lid wall of the upper die are rounded in the reverse direction away from the side walls of the lower cavity for ease of die manufacture, so that the rounded corners of the box shape tubular frame member are provided by controlling the magnitude of internal hydraulic pressure to a level sufficient to properly expand the tube against the side walls, the bottom wall, the lid wall, and the lower corners, while not so great as to force the upper corners fully into the reverse rounded upper corners of the die cavity.