Small Angle Bending Using Shear Deformation with Negative Punch-die Clearance

Abstract

In sheet metal bending, bending die is designed in prospect of spring back after unloading. Although accuracy of spring back analysis has been improved, slight variation of thickness or material properties depending on production lot of sheet metal causes scatter of bend angle in mass production, which is a serious problem especially in bending high strength steels or low priced sheets. As a countermeasure, a new method for compensation of bend angle is proposed in this paper. When sheet metal is partially sheared with negative punch-die clearance, it is slightly bent by the gradient of plastic strain normal to bend axis developed in the sheared zone. By controlling this deformation, accurate and adjustable bending was studied. Change of bend angle depending on punch penetration, clearance, tool edge condition and lubrication was investigated experimentally for two kinds of pure aluminum sheets 0.8 mm in thickness. Among test parameters, punch penetration was found to be most influential one and to have linear relationship with bend angle, which suggests good angle-controllability of this method. Strength of bent portion was tested and sufficient strength was assured up to 20% punch penetration against sheet thickness under the optimum condition. Moreover, spring back was little observed since plastic strain is much higher than that by moment bending. These results were studied with strain distribution estimated by elastoplastic finite element analysis.