REAL-TIME CONTROL OF SHEET STABILITY DURING FORMING

During the stamping of complex three-dimensional sheet metal parts, th e in-plane compressive stresses created often lead to failure by buckl ing. These are typically suppressed by binding the material at the per iphery to provide a tensile bias. In practice, these biases are diffic ult 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, mater ial, or geometric changes occur. In this paper two methods are present ed for controlling the blankholder force in-process to ensure optimal forming conditions at all times. This is effectively a signature-follo wing method based on replicating either a previously determined optima l forming-punch force trajectory or a normalized average thickness tra jectory. The method is implemented using closed-loop control of these quantities, and subjected to experiments where various disturbances ar e presented. Previously reported results for axisymmetric shapes indic ated the ability to eliminate the effect of uncertain initial blankhol der force settings, friction variations, and blank placement errors. I n this paper, the work is extended to include material property change s and thickness variations, both of which require a scaling of the opt imal trajectory based on simple process mechanics. The work is then ex tended to include nonsymmetric parts, in particular a square dish-shap ed part with corners of unequal radii. Results from these experiments are essentially identical to the axisymmetric case, with a virtually c omplete elimination of common process disturbances on forming stabilit y.