Study on the free bending of metal sheets based on the model of modified indices

In the free bending process for sheet metals, determining the holes of lower dies can be difficult due to the different thicknesses of metal sheets, which has a considerable effect on the forming quality of the bent metal sheets. An improved mathematical model of modified indices was proposed in this study to reveal the relationship among metal sheet thickness, lower die hole, and bending angle at the time of free bending and to identify the optimal hole of the lower die for sheets with different thickness during free bending. First, relevant data were obtained through field experiments and processed using a three-point moving average. Second, the data were divided into the growth period and the saturation period according to the different changing rules of the experimental data. The dimensional model and the model of the modified indices were employed to establish the piecewise fitting modelling. The influence law of the ratio of the lower die hole to sheet thickness on the quality of free bending was also obtained. Finally, ABAQUS was employed to verify the proposed model, and the ratio range of lower die hole to sheet thickness for the optimal quality of free bending was given. Results show that the fitting degree between the influence function of the free bending forming quality established by the improved mathematical model of modified indices and the actual data is 99.8% while the ratio of lower die hole to sheet thickness for the optimal bending quality of free bending is 7-9. This study provides a certain reference for the selection of the lower die hole for the free bending of metal sheets with different thicknesses. © 2020 School of Science, IHU. All rights reserved.