Sheet hemming by using rolling tools

Sheet hemming is a joining operation which generally allows the assembling of two overlapped sheets by plastic deformation of the outer part. For example, sheet hemming is often used in the automotive industry to join the engine hood or the door panel with frameworks or to assembly the inner and the outer panel of an white good. The hemming operation is the results of a 90° sheet flanging, a pre-hemming (up to approximately 135°) and final hemming (the outer sheet is bended up to 180°). Usually the hemming process can be carried out by using rigid dies or shaped rollers moved by robots. This last solution, which is starting to be diffused, becomes an interesting alternative not only in pre-series or prototype activities, but also in all flexible productions in order to minimize set-up time and cost. Moreover, it is not necessary to realize expensive dies requiring only a support and a blocking system for the sheets (this can be realize with low cost materials) together with the rollers mounted on a robot. By changing the tool path it is possible to hem two sheets following different 3D hemming trajectories so obtaining a flexible production system. The present research, carried out by the Technologies and Manufacturing System (TMS) Group of Brescia and Bergamo Universities (Italy), deals with this technique. Many experiments have been run to study the influence of the hemming process parameters such as flange geometry (height, fillet radius), distance of the inner panel from the flange, tool path sequence on Fe 220 BH 0.7 mm thickness sheets. These tests were carried on two straight sheets by using a roller mounted on a horizontal CN milling machine installed in the Brescia University Lab. This process has been also simulated by using the explicit commercial code PAM-STAMP 2G 2003®. Simulations predict very well the behaviour of the outer sheet deformed by the roller and allow the optimization of the actual process and to gain the knowledge to realize complex curvilinear trajectories (managing 5 axis movement). In fact limits and defects of this technique, together with the possible solutions, have been highlighted. For this reason the implemented FE model will be applied to sheet hemming operations along curvilinear trajectories.