The sequences of operation is that the part is formed from a rough blank in such a manner that the metal folds around the punch to form a recess and consequently the periphery is pulled towards the centre. The circumferential length of the cup periphery is thus less than that of the blank and is coincident with circumferential compressive stresses. The accompanying radial stains limit the depth of draw.
During the draw edge thickness increases and tools must be designed to allow for this unless it is definitely intended to reduce resulting thickness by ironing. The direct action of the punch, which is forced against the centre portion of the blank and cup, causes radial tensions. These are at a maximum near the punch corners and fade out to zero at the periphery. The inner portions of the blank are subject to little or no circumferential shrinking and the metal near the bottom of the recess becomes generally thinner during the deep-drawing process. Sharp radii on punch and die are a cause of tearing since combined radial and bending tensions exist. Also sharp corners on the punch cause tension in the wall of the cup to become greater than in the botton owing to increased friction; tearing can result. Tearing at the botton or at the flange may also result from the formation of deep wrinkles which require excessive force if they are to be ironed out in the gap between punch and die.
Excessive tension results if the ratio of blank diameter to punch diameter is large. The punch force required to form a recess increases with increasing ratio which, if very large will create radial tensile stresses which reach the ultimate tensile strength of the material. The metal will then tear or crack circumferentially (blanking). This limiting ratio is known as the drawability ratio and cannont usually exceed a value between 1-6 and 2-2.
