Sheet metal forming is gradually changing from a craft based to an engineering based process. The review acknowledges the metallurgical improvements that have been made in sheet metals and in tool steels, and concentrates on research aimed at understanding and describing the mechanical properties of sheet metals in a more quantitative manner. The need to forgo reliance on simple parameters, and to obtain more comprehensive data on plastic yielding and work hardening, is clearly recognized. The pronounced effect of the shape of the yield locus on the strain distribution in stretch forming and in deep drawing is discussed. Innovations and improvements in the engineering design of tooling now enable the production of long shells by deep drawing and redrawing, or by deep drawing and ironing, in a single stroke of the ram and without inter stage annealing. Drawing into a pressurized fluid ensures greater control over deformation, and permits the forming of tapered shells in one operation, using thinner gauges than previously. General pressings are discussed with particular reference to galling problems, and to the different tooling requirements of various sheet metals.
Material properties of blank before forming These include chemical, physical, and mechanical properties. Consistency of these properties is important if frequent adjustments in tooling and lubrication are to be avoided. Interaction between tools and work piece. The important variables are the surface finish of the sheet metal and the tooling, the materials from which they are made, and the clearances allowed. Effective lubrication between work piece and tooling is equally important and, in general, is dependent on temperature and strain rate developed in the process.
In some cases lubrication between tools and workpiece is critical; thick film lubrication can lead to wrinkling, and can give a poor surface finish to the product, while poor lubrication can result in 'pick up', with serious damage to the tooling. Tooling In choosing a tool material, a compromise might have to be made between wear resistance and fatigue life, or between wear resistance and effective lubrication. Rigidity and alignment of the tooling are of great importance, especially in the deep drawing of thin materials and in ironing processes. Properties of product These are a function of the initial properties of the blank, the strain rate during the process, the interfacial contact between workpiece and tooling, and the geometry of the process. The most important properties are metallurgical, surface finish, dimensional accuracy, and residual stress distribution. Possible chemical reaction between the product and the lubricant may also have to be considered. 6. Machine tool A choice may have to be made between a hammer, a hydraulic press, or a mechanical press. In many instances a specialized machine may have to be designed to form a restricted range of products.
*Courtesy of IHS engineering 360