3 Fundamentals of press design

Deep drawing and stretch drawing
blank holder
die
Fig. 4.2.7 Beads restraining material flow in the blank holder
punch
brake bead
Draw energy
For drawing operations on double-action presses, the required draw
energy W d [Nm] is the product of the pressing force F U [N], the drawing
stroke h [m] and a correction factor x [–]:
Wd = x⋅FU ⋅ h [ Nm]
The correction factor, which is dependent on the material and the draw
ratio � = D/d, takes into account the actual drawing force characteristics;
this factor fluctuates between 0.5 and 0.8. The higher level is for soft
materials, which can be drawn with the maximum draw ratio � max and
for flanged shells, which are only partially but not completely drawn
through. The lower level is for a small draw ratio, that is to say for short
drawing depths or for harder grades of sheet metal. When drawing normal
materials, calculations can be based on x � 0.65 up to 0.75.
A sufficiently accurate rough calculation formula for estimating the
draw energy of a double-action press is:
2
Wd FU h Nm
3
= ⋅ ⋅ [ ]
With a single-action press with a draw cushion, the slide force is
increased by the blank holder force F B [N]:
W = ( x⋅ F + F )⋅ h [ Nm]
e U B
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
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