Modern continuous rolling mills produce large quantities of thin sheet metal at low cost. A substantial fraction of all metals are produced as thin hot-rolled strip or cold-rolled sheet;this is then formed in secondary pricesses into automobiles,domestic aplliances, building products,aircraft food drink cans and a host of other familiar products. Sheet metals parts have the advantage that the material has a high elastic modulus and high yield strength so that the parts produced can be stiff and have a good strength-to-weight ratio. A large number of techniques are uesed to make sheet metal parts.

Common forming processes of stamping die

Blanking and piercing. As sheet is usually delivered in large coils, the first operation is to cut the blanks that will be fed into the presses; subsequently there may be further blanking to trim off excess material and pierce holes. The basic cutting pricess in shown in Fig. 3.1. When examined in detail, it is seen that blanking is a complicated process of plastic shearing and fracture and that the material at the edge is likely to become hardened locally. These effets may cause difficulty in subsequent operations.

Fig3.1.

Bending. The simplest forming process is making a straight line bend. Plastic deformation occurs only in the bend region and the material away from the bend is not deformed. If the material lackes ductility, cracking may appear on the outside bend surface, but the greatest difficulty is usually to obtain an accurate and repeatable bend angle. Elastic springback is appreciable.

Stamping or draw die forming. The part shown in Fig.3.2 is formed by stretching over a punch of more complicated shape in a draw die. This consists of a punch, and drawing ring and blank-holder assembly, or binder. This process is widely used to form auto-body panels and variety of appliance parts. Much of the outer flange is trimmed off after forming so that it is not a highly efficient process, but with well-designed stamping tooling, vast quantities of stamping sheet metal parts can be made quickly and with good dimensional control. Stamping die design requries the combination of skill and extensive computer-aided engineering systerms, but for the purpose of comceptual design and problem solving, the complacated deformation system can be broken down into basic elements that are readily analyzed.

Fig.3.2.

Deep drawing. In stamping, most of the final part is formed by stretching over the punch although some material around the sides may have been drawn inwards from the flange. As there is a deeper parts, much more material must be drawn inwards to form the sides and such a process is termed deep drawing. Forming is simple cylindrical cup. To prevent the flange from buckling, a blankholder is uses and the clamping force will be of the same order and the punch force. Lubrication is important as the sheet metal must slide between the stamping die and the blankholder. Stretching over the punch is small and most of the deforamtion is in the flange; as this occurs under compressive stresses, large strains are possible and it is possible to draw a cup whose height is equal to or possibly a little larger than the cup diameter.

Above are the press process of stamping die.