Design for temporal and spatial variation of blank holder pressure in sheet metal forming

A complete, systematic approach to the design of elastic blank holders and multi-point blank holder pressure (BHP) control systems for sheet metal forming operations is proposed. A design sequence, testing procedures, and experimentally obtained performance characteristics of a single-cylinder prototype of a cost effective, hydraulic/nitrogen blank holder force (BHF) control system are presented. A FORTRAN program is developed to calculate the BHP distribution and magnitude and to select the blank holder thickness. The blank holder is modeled as an elastic layer supported by an elastic foundation, where the foundation represents the combined elastic properties of the sheet metal blank, die, and press. The program is based on an axisymmetric model of an infinite, elastic, isotropic layer supported by a Winkler foundation and loaded by an arbitrary axisymmetric pressure distributed over a finite circular area. A numerical procedure based on Gaussian-Laguerre integration formulas is developed to evaluate the integral expressions for the contact pressure (i.e. the normal stress component) between the layer and the foundation. Conclusions are drawn about the applicability range of the axisymmetric model to the blank holder design.; An investigation of the effects of the elastic deflections of blank holders on the quality and formability of tailor-welded and non-welded sheet metal parts from steel and aluminum is conducted. An eight-point BHP control system with a flexible blank holder is designed and built as a part of an experimental viscous pressure forming (VPF) machine. FEM simulations are conducted to fine-tune the control system as well as to predict the forming loads. Experimental results demonstrating the viability of the developed design procedures are presented. The viscous medium "blow through" phenomenon between the blank holder and the sheet during VPF is described. Counter-forces acting upon the blank holder are quantified and recommendations are given on how to compensate these forces by increasing the BHF proportionally to the viscous medium pressure.