STAMPING

Operations called stamping, pressing and sometimes drawing involve clamping a sheet at it edges and forcing it into a die cavity with a punch as shown in Figure 17.1. The sheet is stretched rather than squeezed between the tools. Pressure on the draw beads controls how much additional material is drawn into the die cavity. In some cases there is a die, which reverses the movement of material after it is stretched over the punch.

DRAW BEADS

Draw beads (Figure 17.2) are used to create tension in the sheet being formed by preventing excessive drawing. As a sheet moves through a die bead it is bent three times and unbent three times. Each bend and each unbend there requires plastic work. Over each radius there is friction. Bending and unbending create resistance to movement of the sheet. If the resistance is sufficiently high, the sheet will be locked by the draw bead. The restraining force of the draw bead can be controlled by the height of the insert.

The restraining force has two components. One is caused by the work necessary to bend and unbend the sheet as it flows over the die bead and the other is the work to overcome friction. A crude estimate can be made of the restraining force per length resulting from the bending and unbending with the following simplifying assumptions:

work hardening, elastic core, movement of the neutral plane, and the difference between engineering strain and true strain (ε = e) are neglected.