| Bending, flanging, and hemming are often used as the last stages of stamping operations. An automotive hemming operation consists of three phases: flanging to approximately 90{dollar}spcirc{dollar}, pre-hemming (bending an additional 45{dollar}spcirc{dollar}) and final hemming flattens the hem. In a car door or hood, hemming is used to assemble the outer panel and the inner panel.; It is critical that the assembly produces a smooth edge and that the clearance between the part and the frame is constant. Moreover, some forming defects such as recoil, warp and wrinkling should be avoided. The tool and process design needs to incorporate methods to eliminate these defects. However, the deformation mechanism, and hence the process, is more complex than simple bending and flanging. Therefore, very little information is available in research literature, and most of the bending, flanging and hemming dies are designed based on experience and trial and error.; The main objectives of this dissertation are to investigate the basics of 2-D and 3-D sheet metal bending operations using flanging and hemming as examples, and to develop a process design procedure to predict and prevent product defects in flanging and hemming. To achieve these goals, systematic laboratory experiments; approximate closed-form and numerical solutions and finite element analysis are used and compared.; Among the many flanging and hemming geometric configurations, straight edge-flat surface (plane strain) and convex edge-flat surface (axisymmetric and shrink) cases were included in this dissertation. A new approximate numerical method to calculate springback and tool loads in straight flanging was developed and validated using limited experimental data. The major process parameters in straight and convex edge hemming were determined and relations between them and some forming defects were established. Possible process and tool design modifications, which may lead to quality improvements in hemming, were investigated, and two case studies on flanging with coining and making of modified flat hem were conducted. A new process and die design method that will reduce the costly die tryout including trimming, flanging and hemming operations has been developed. Consequently, a set of part, process, and die design rules and guidelines has been developed for the industry. |
