Stretch Roll Forming

This paper documents the development of a novel CNC metal forming process called "Stretch roll forming". It is an extension of the three roll forming (or slip-roll) process, wherein many pairs of roller-like dies mounted on positioning slides are used to approximate the instantaneous geometry of the part being formed, and to guide the formation of extrusions and sheets into desired shapes. The revolutionary enhancement is that, a stretch force sufficient to induce tensile yielding of the entire section is superimposed on the workpiece while it is being formed, by applying appropriate traction forces on the workpiece at each of the contact points. Tensile stretching avoids bucking of the section, and increases the strength of the part while reducing spring back and residual stresses. A further revolutionary improvement arises from replacing rollers with short die segments. The shape of the die can be changed to accommodate different cross-sections of extrusions. Dynamic changes in the location of the die can be used to produce parts with varying curvature along the length of the part. In addition to forming workpieces to desired geometries, this process can also be used to straighten them. 2D and 3D finite element analysis has been carried out using LS-Dyna to simulate the stretch roll forming of flat sheets and T-section extrusions. Results indicate that this process can also be further extended to impart a controlled amount of compressive residual stress on the outer layer of parts, to improve fatigue life. A proof-of-concept machine has been developed and used to stretch form flat, T-section and L-section extrusions with uniform stretch along the length of the part. The uniform stretch results in parts with uniform strength and cross-section, which permits weight reduction of the parts. Parts with constant radius as well as variable radius along the length have been formed. This CNC metal forming process eliminates heat treatment and produces higher quality parts while eliminating the need to use part specific dies for a wide class of parts. Significant savings in tooling and setup costs can be realized, thereby reducing lead time and cost of producing the parts and allowing "just in time" production.