In general, the tonnage necessary to draw a shell of round or rectangular shape is a function of the area of metal in the wall (i.e. = mean perimeter and metal thickness). This is true because at any given time, the force necessary to deform the metal equals the area of metal being deformed multiplied by the yield strength of the material. (In calculating these forces tensile strength is used, since this will either draw the part, push out the bottom or tear the walls). This is generally true for parts having odd shapes, although there are other factors involved which complicate the operation.