| Summary: | Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999. === Includes bibliographical references (p. 239-246). === Today stretch forming of sheet metal parts and associated tool design methods used in the aerospace industry are still more an art than a science. Few analytical tool design techniques exist and even fewer are used in practice. When they are, they only address the result but not the cause of the forming anomalies. Rigid tooling, fixed-configuration facilities and dependence on tool designers' expertise are the major drawbacks of modern tool design. tool fabrication result in long lead times, high manufacturing costs and insufficient product quality. In response to this, a methodology combining a self-tuning shape control algorithm and reconfigurable forming tool is developed and implemented for stretch forming. This methodology is based on empirical estimation of the process characteristics from the calibration forming trials, and is capable of generating a tool shape that produces, upon the impact of unloading and in-process variations, the final part shape of desired precision. The methodology is based on a new system identification strategy, where the effect of decreasing signal magnitudes on the transfer function estimate is greatly alleviated. The assumption that the change in tool shape is linearly related to the corresponding change in part shape proved to accurately capture the stretch forming characteristics. The relative stability analysis shows that the final part shape converges to the designed shape despite significant alterations of the calibration die shape and proportional controller gain. Moreover, a sapient selection of the calibration dies improves convergence of the tool design process without sacrificing the part accuracy. Finally, the same methodology is employed to compensate for combined shape distortions caused by the stretch forming, chemical milling and trimming operations. The experiments involving various compound curvature stretch formed skins and cylindrical parts ensuing from the successive stretch forming, chemical milling and trimming operations are used to validate the effectiveness of this methodology to reduce shape errors. The final maximal shape errors were reduced below a tolerance (0.010 in.) in just one or two closed-loop forming trials regardless of skin shape. The major obstacles in achieving even higher final part shape fidelity are in the mechanical properties of the interpolation, and in one case, in the stretch forming press design. === by Marko Valjavec. === Ph.D.
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