Novel Die-less Method for Variable-Section Shaping and Bending to Manufacture Curved Rectangular Tubes with Longitudinally Varying Cross Sections

Abstract

There are two main types of body-frame structures for automobile bodies: monocoque structures, which are made by welding several panel parts together, and space-frame structures, which are made by joining closed-section tubes together. Space-frame structures have high strength and stiffness. Therefore, that can reduce vehicle weight. Furthermore, if the dimensions of the rectangular cross section of the curved tube can be varied in the longitudinal direction, further weight reduction is expected. The bent tube with variable cross section can be formed by the existing method, hydroforming. However, hydroforming requires large dies that fit the shape of the product, and the forming machine is large and expensive, because of the large clamping force required. In this paper, we adopt the roll forming method, which is smaller than the conventional method, has smaller forming load, and can form square tubes continuously. Then, we propose the novel die-less tube bending method in which the straight circular tube is continuously bent and formed into various cross section by means of the rectangular opening formed by the four forming rolls installed in two directions. Using a prototype device, we verified that the bent tube with variable cross section in the longitudinal direction can be formed.