3 Fundamentals of press design

44 Fundamentals of press design
bulky, cost-intensive turning device to be installed downstream from the
double-action press. There are advantages in forming a part in the reverse
position, i. e. the bottom of the part facing upwards. In this case part ejection
is simpler through a slide cushion in subsequent forming stations.
The alternative is to work with a draw cushion or nitrogen cylinders.
Furthermore, the centering in subsequent stations is improved, since the
parts can be positioned over their entire inner shape by the bottom die.
In addition, the sensitive surfaces of the material – for example when
working with automotive body panels – are less likely to sustain damage
during transportation in the press and when centering in the die. It is
also possible to install the blanking punches at a lower level for subsequent
operations if the part has previously been turned over.
In the case of single-acting deep drawing presses, in contrast, the
blank holder force is transmitted from underneath by a draw cushion
(Fig. 3.1.9 and cf. Fig. 4.2.3). Both the forming and the blank holder
force are applied by the slide, as the slide forces the draw cushion downwards.
After the drawing, without “turning over”, the part is ideally
positioned for the subsequent operations.
When used together with a hydraulically driven slide movement, the
draw cushion can also be used for active counter-drawing (Fig. 3.1.10 and
cf. Fig. 4.2.4). From the point of view of energy savings, it is beneficial
that the slide holds the blank while the draw cushion applies the forming
force.
In order to achieve as great a degree of flexibility as possible, doubleaction
presses can also be configured with a draw cushion.
3.1.4 Draw cushions
The function of the draw cushion is to hold the blank during deep
drawing operations in mechanical and hydraulic presses (cf. Sect.
2.1.4). The cushion thus prevents the formation of wrinkles and, as a
result of the ejection function, raises the parts to the transport level
during the return motion of the slide. Modern draw cushions can be
either hydraulically or pneumatically actuated. A major difference
between the two is the larger force of the hydraulic draw cushion. The
cushion force is initiated during downward travel of the slide by, for
example, four hydraulic cylinders via the lifting bridges and com-
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998