3 Fundamentals of press design

Sheet metal forming lines
As early as in the part design stage, through close cooperation with
the component design and production departments, part simplification
can lead to a situation where several different forming or blanking
operations can be grouped together in a single forming station. The
more operations can be combined in this way, however, the more complex
the die itself becomes. This may lead to higher costs, reduction of
working speed and higher susceptibility to malfunctions. Therefore, an
ideal compromise has to be found when defining the number of forming
processes or presses in a production line. Press lines consist of four
to eight presses, although, depending on the range of produced parts,
four to six presses are most commonly used.
Lead press: blank holder slide or draw cushion
While the lead press is frequently specified as a double-action press, the
downstream presses are single-action, some of them incorporating a
draw cushion in the press bed. In the case of double-action lead presses,
separate draw and blank holder slides are available and they are driven
either mechanically or hydraulically from above (cf. Fig. 3.1.8 and
3.2.4). The blank holder slide contacts and holds the blank firmly at the
outer contour prior to the initiation of the forming process by the draw
slide (cf. Fig. 4.2.2). In order to avoid wrinkle formation, the four pressure
points can be separately controlled, i. e. different blank holder
forces can be set at the four corners. In certain lines, it is possible to vary
the blank holder forces during the drawing process using the pinch control
(cf. sect. 3.2.8). These measures help to reduce the try-out period
required for a new set of dies, and also improves drawing results. The
double-action design of the presses allows the optimum way to achieve
large draw depths. The drawback is that generally the part has to be
turned over for the subsequent operation. The turnover station
required after the first press limits the overall output rate of the press
line (Fig. 4.4.21).
New CNC-controlled hydraulic draw cushions which simulate the
function of the blank holder slide have been developed. These cushions
allow to form similarly deep and complex pressed parts on single-action
mechanical or hydraulic lead presses (cf. Sect. 3.1.4).
In this case, the formed part does not require turning over after the
drawing operation. Consequently, the output rate of a press line can be
enhanced through the use of a single-action lead press with draw cush-
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