3 Fundamentals of press design

172 Sheet metal forming and blanking
Draw beads
With very shallow, arched parts, for which the drawing force will be relatively
low, a very high blank holder pressure is required so that friction
between the blank holder and the die acts to restrain the flow of the sheet
metal. Thus, the tension stresses are increased and the material is formed
around the punch without forming any wrinkles while it is stretched to
achieve the required strength level. In such cases, in order to lower blank
holder forces, draw beads are provided in the tooling. Beads are incorporated
into the die as per Fig. 4.2.6 or into the blank holder as per
Fig. 4.2.7, while a corresponding recess is allowed for in the matching side
of the die. The sheet metal is pressed around the bead by the action of the
blankholder and thus must flow over the bead as part of the drawing
process. The force required for this deformation is added to the drawing
force. Draw beads assure that the material flows in a more uniform way,
especially in deep-drawing non-circular work parts. They have the task of
distributing the longitudinal stress in the drawn part over the whole
perimeter. For this reason, they are positioned in locations which are lightly
stressed during the drawing process such as, for example, on the longer
sides of rectangular parts. If a single bead is inadequate to achieve the
required braking effect, several beads can be located one next to the other.
In single-action drawing with a draw cushion it is possible to control the
blank holder force during the forming stroke or in different areas of the
blank holder in order to carry out the forming operation (cf. Sect. 3.1.4).
With double-action mechanical presses, the blankholding force can be varied
by altering the pressure in the pressure points of the blank holder slide
(cf. Sect. 3.2.8).
blankholder punch
die draw bead
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
Fig. 4.2.6
Draw bead in a die