Process monitoring for sheet metal stamping using force and strain signatures.

Sheet metal stamping is the process of forming various panel shapes such as automotive doors, hoods etc. It includes a forming operation that sets the panel shape and dimension. The forming process is complex and requires successful interactions between its several input variables (blank, dies, drawbeads etc.) for yielding good quality panels. However, in current practice, there is insufficient monitoring for continually produced panels. Panel dimensional control is rarely done, resulting in quality problems in the final car assembly. Moreover, there is a lack of scientific process understanding, making the trouble-shooting of panel defects (splits, wrinkles etc.) difficult. A trial-and-error approach of the die makers is relied upon, which is time consuming and expensive. The primary contribution of this research is the development of a process monitoring methodology using the available output process signatures, namely, panel strain (epsilon) and punch force (f). Towards this objective, an adaptive strategy for efficient strain measurement has been developed, and in addition, a deeper understanding of the impact of drawbead faults has been made. The process monitoring methodology is a tool applicable in production environments for identification of the root cause that results in a defective panel. Pertinent information (or features) such as input work, peak strains etc. are extracted from the signatures to characterize the process. The methodology involves the development of a linear mapping between the input variables and the selected output features through systematic training. The trained mapping is later applied to the features from a defective panel for root cause identification. The methodology is demonstrated and validated for a cylindrical cup deep-drawing, hemispherical punch stretching and box drawing processes. The adaptive strategy for strain measurement involves etching uniform small square grids over the panel, and measuring strains adaptively based on the deformations of larger squares comprising many smaller grids. The adaptive scheme uses FEA strains as input and accounts for experimental noise. The superiority of the methodology is demonstrated in that the strain (both low and high) features are measured accurately, and in addition, measurement time is saved significantly. For a better process understanding in the presence of drawbead faults, a generalized 2D FEA modeling procedure is adopted to study the restraining forces in both straight and curved drawbeads. The results indicate that the restraining force for a curved drawbead is close to the sum of the contributions from an identical straight drawbead, and the metal flow-in deformation of the curved flange. In addition, the sensitivity of the restraining force for the curved drawbeads, is similar to that of the straight drawbeads.PhDApplied SciencesIndustrial engineeringMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131995/2/9938530.pd