Bodies of automobiles, ships and spaceships are composed of doubly-curved sheet metal. As methods for manufacturing doubly-curved sheet metal, there are various kinds of processes, such as, stamping process, incremental forming process, roll-forming process, and so forth. Though the stamping process is very popular, high setup cost and inflexibility make it inapplicable to production of doubly-curved sheet metal. The incremental forming process provides flexibility so that sheet metal products with various radii of curvature can be made efficiently. The roll-forming process requires large-scale equipment and its application is limited to thin-walled channel shape or pipe section products. In addition, in the roll-forming process the deformation of the sheet metal is not much attainable.
In this study, in order to make doubly-curved sheet metal effectively, a sheet metal forming process has been developed by adopting the advantages of the incremental forming process and the roll-forming process for manufacturing doubly-curved sheet metal : i.e., inherent flexibility of the incremental forming process and slight deformation of the roll-forming process. The developed process is an unconstrained forming process using the principle of bending deformation. In this study, the experimental equipment is set up with the punch-set instead of the roll-set. The punch-set consists of two pairs of lower support-punches and one upper center-punch. In the preliminary experiments using lead sheets, it is found that the curvature of the formed sheet metal is determined by controlling the distance between supporting punches in pairs and the forming depth of the center-punch. In the main experiments using aluminum sheets, the edge-forming method is proposed for forming the sheet metal into the balanced shape and the equation using process variables such as the distance between supporting punches in pairs and the forming depth of the center-punch is proposed for the prediction of the radii of curvatures of the formed shape. The equation is corrected by the experimental results and the FEM simulation results about whether springback takes place. It is found that, according to the simulation, there is a certain set of the distance between a pair of supporting punches and the forming depth of the center-punch, which causes a little springback. It is thus shown that the radii of curvatures of the formed sheet metal can be predicted by the corrected equation unless significant springback occurs.
