Proposal of a bending anomaly estimation method in sequential press forming processes of sheets and its verification

Sequential press forming of thin sheets is a method in which different locations within a single workpiece are pressed in sequence, and is used to manufacture large structures, such as storage tanks. In the sequential press forming, it is important to estimate bending-defect areas efficiently because a product shape can be corrected not by repairing dies but by only re-pressing the bending-defect areas in the subsequent process. At present, high skill is required to determine bending-defect areas because they are judged by measuring a gap from a template. In this study, a versatile method to estimate bending-defect areas in products obtained with sequential press forming is proposed. Specifically, a cosine similarity is calculated between the product shape and templates of ideal shapes with known defects prepared beforehand. A cosine similarity is calculated based on curvatures. Bending-defect areas are then evaluated from the magnitude of the deviation from the average of the multiple calculated cosine similarities. This paper also presents a procedure to calculate the threshold for estimating bending-defect areas by the proposed method. The effectiveness of the proposed method was confirmed through case studies that considered virtual products and actual workpieces. It was confirmed that the proposed method can estimate bending-defect areas and excess or deficiency of bending even when local deformation and measurement noise exist.