Stretching during axisymmetrical forming of sheet metal

In sheet metal forming, the surface area of the blank normally is increased simultaneously with the reduction in the overall blank thickness. The distribution of and overall change in the thickness and, consequently, the efficacy of the forming process are influenced by forming parameters such as the blank size, blank holding load, lubrication and tool geometries. In most forming processes, drawing-in of the flange is unavoidable, unless this is deliberately prevented using a serrated die set and a very large blank holding load. This paper demonstrates that forming processes may be viewed as mixtures of two basic processes, namely, ideal stretch forming and ideal deep drawing. The ideal stretch forming process is typified by extensive thinning, due to bi-axial stretching, of the blank within the die throat, with drawing-in of the flange being absent. Ideal deep drawing contrarily requires the complete drawing-in of the flange into the die throat, with negligible change in the overall thickness of the blank. In the light of this, a parameter χ, describing the extent of stretching relative to drawing, is proposed. The contributing factors on sheet formability are subsequently discussed with reference to χ; this approach should assist die designers in selecting the appropriate parameters for successful forming operations.