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An experimental investigation into punch forces during a sheet metal stamping operation was performed for a sheet metal stamping die with a flexible blankholder and adjustable drawbeads. Tests were performed for various combinations of blankholder force and drawbead positions. Punch forces were measured using an instrumented punch and wrinkling in the sheet flange was measured during the forming processes using non-contact proximity sensors located near the blankholder edge. The results were correlated with those of the punch forces. Additionally, using a wrinkle criterion based upon part quality, an operating envelope for the die system was obtained and the relation between drawdepth, drawbead penetration, and blankholder forces was developed for the die system.

Drawbeads have been playing an important role in deep drawing processes. The restraining force exerted by the drawbeads is important in enhancing the part formability. It is found that improper drawbead design will result in defects such as wrinkles and splitting. Thus, the drawbead geometry, especially its end radius, is a critical aspect in the die design. The present investigation focuses on the influence of the drawbead end radius and blankholder force on the sheet deformation patterns. The experimental results reveal that a drawbead with end radius equal to its cross sectional radius will cause severe deformation and wrinkling downstream of its end. With drawbead end radius twice the size of its cross sectional radius, the deformation in the downstream side of the drawbead end becomes smoother, hence improving the severe deformation and wrinkling. A higher blankholding force also suppresses the wrinkling near the drawbead end.

In sheet metal forming, metal flow into the die is determined by the restraint imposed by both the blankholder force and the drawbead penetration. This paper describes an experimental investigation in which both advanced binder force and drawbead technologies are used to study their effect on draw depth in the drawing of an AA6111-T4 generic non-symmetric panel. Multipoint binder loading using individual pin force adjustment applied to especially designed binder structures as well as the use of variable blankholder force were investigated in one laboratory in Germany. In another laboratory in the USA, active drawbeads were applied to the drawing of the generic panel. The results of both approaches, which are shown to be successful, are presented and discussed.

A new press/die system for restraining force control has been developed in order to facilitate an increased level of process control in sheet metal forming. The press features a built-in system for controlling drawbead penetration in real time. The die has local force transducers built into the draw radius of the lower tooling. These sensors are designed to give process information useful for the drawbead control. This paper focuses on developing models of the drawbead actuators and the die shoulder sensors. The actuator model is useful for developing optimal control methods. The sensor characterization is necessary in order to develop a relationship between the raw sensor outputs and a definitive process characteristic such as drawbead restraining force (DBRF). Closed loop control of local specific punch force is demonstrated using the die shoulder sensor and a PID controller developed off-line with the actuator model.