Force-controlled sheet metal stretch-forming process based on loading at discrete points

Stretch-forming based on loading at discrete points (SF-LDP) is a new stretch-forming process, by controlling the loading trajectory at each discrete point; the process can fulfill the sheet metal deformation along the prescribed forming path. In this paper, force-controlled SF-LDP process was proposed and discussed. The sheet metal deformations in SF-LDP were numerically analyzed and it is shown that the strains remain in a fixed ratio during the forming process and the three principal axes of strain are stationary and in the longitudinal, transverse, and thickness directions, respectively. Based on the characteristic analysis on the SF-LDP process, a precision design method for the loading trajectory described by stretch-forming force was presented, and the time course of the stretching force at each loading point is determined by taking into account the effect of material hardening and the friction between the sheet metal and stretch-forming die. A force-controlled SF-LDP process for a spherical surface part was designed and the loading trajectories were employed in the experiment, the measured results on the formed part demonstrate that the force-controlled SF-LDP process achieves qualified product without forming imperfections.