Real-Time Control of Sheet Stability During Forming

Abstract

During the stamping of complex three-dimensional sheet metal parts, the in-plane compressive stresses created often lead to failure by buckling. These are typically suppressed by binding the material at the periphery to provide a tensile bias. In practice, these biases are difficult to determine, and must be addressed with a combination of a priori analysis and die-making skill. Even then, in-process variations will cause parts to begin failing by tearing or buckling as friction, material, or geometric changes occur. In this paper two methods are presented for controlling the blankholder force in-process to ensure optimal forming conditions at all times. This is effectively a signature-following method based on replicating either a previously determined optimal forming-punch force trajectory or a normalized average thickness trajectory. The method is implemented using closed-loop control of these quantities, and subjected to experiments where various disturbances are presented. Previously reported results for axisymmetric shapes indicated the ability to eliminate the effect of uncertain initial blankholder force settings, friction variations, and blank placement errors. In this paper, the work is extended to include material property changes and thickness variations, both of which require a scaling of the optimal trajectory based on simple process mechanics. The work is then extended to include nonsymmetric parts, in particular a square dish-shaped part with corners of unequal radii. Results from these experiments are essentially identical to the axisymmetric case, with a virtually complete elimination of common process disturbances on forming stability.