Analysis, design, and experiments for punch force control in sheet metal forming.

Abstract

Sheet metal stamping is one of the primary manufacturing processes because of its high speed and low cost for high volume production. Challenges are to prevent stamped parts from wrinkling and tearing, to improve their dimensional accuracy, and to maintain consistency in the stamping process. New challenges emerge from the use of less formable materials (e.g., aluminum and thin gage high strength steel) to improve fuel economy in the automobile industry. In sheet metal processes, the blank holder force controls the material flow into the die cavity, which is critical to forming a good part. Process control can adjust the blank holder force in-process based on tracking a reference punch force trajectory to improve part quality and consistency. However, key issues, such as process controller design and reference punch force trajectory design, have not been fully addressed. This dissertation presents an investigation of the relationship between process control and part quality and consistency and a systematic approach to developing process control. The relationship between process control and part quality and consistency can be addressed through comparing experimental data from process control and machine control. Consistency in part dimensions is related to consistency in punch force trajectories and then the tracking performance of a process controller while part dimensional accuracy is related to the reference punch force trajectory. This justifies separating the development of process control into process controller design and reference punch force trajectory design. The systematic development of process control includes a procedure to develop a first-order nonlinear process model, a procedure to design a proportional plus integral process controller with feed-forward action (PIF), and a procedure to obtain an optimal reference punch trajectory. Application of the proposed procedures to U-channel forming successfully leads to a first-order nonlinear process model useful for process controller design, a suitable PIF process controller, and an optimal reference punch force trajectory. Experimental results from cup forming successfully reveal the generality of the developed modeling and process controller design procedures The proposed procedures should be useful in designing and implementing process control in sheet metal forming processes.