3 Fundamentals of press design

Sheet metal forming lines
(cf. Fig. 3.4.3) for automatic die change. For each die station there are
two moving bolsters. One die set is used for production inside the press,
the other set is located outside the press being prepared for the next
production run (Fig. 4.4.38 and cf. Fig. 3.4.4). This allows the die and
part-specific tooling to be prepared while the press is still producing.
While the lift beams stay in the press, the crossbars and the tooling
are separated by automatic couplings from the lift beams for die change.
They are then positioned on the moving bolster of the die and moved
out of the press at 90° to the direction of part flow. Every die set is
equipped with its own crossbar and suction cup tooling. When settingup
the moving bolsters outside the press, the crossbars assigned to the
moving bolster are equipped with the tooling required for the next part.
The steps performed when changing dies and resetting all relevant
parameters of the press are executed automatically. These steps include:
deposit and unclamping of top dies, release of crossbar couplings, raising
of lift beams to clear traversing of moving bolsters, exit of moving
bolsters; entry, lowering and centering of new bolsters, clamping of
upper dies, replacing the next crossbars with tooling and contoured
nests of the universal stations.
All the die change parameters are automatically set by a programmable
logic control. Accordingly, all the adjustable parameters of the
peripheral press systems such as the destacker or part stacking are
recalled and automatically set (cf. Sect. 4.4.11).
A complete die changeover requires about 10 min, as various resetting
processes run simultaneously. The individual phases of the die
changing process are locally and centrally displayed on screens for
monitoring purposes.
When manufacturing large parts such as one-piece body sides, in particular,
individual and direct control of the blankholding forces is very
important to achieve good part quality. Just as in tri-axis transfer presses,
power is transmitted via pressure cylinders to the blank holder
frame. Generally, systems with four displacement cylinders are used.
The fundamental advantage of the hydraulic displacement system,
compared to pneumatic draw cushions, is that the draw cushion force
can be optionally controlled at each individual hydraulic cylinder during
the drawing process by means of microprocessor-controlled servo
valves. The pressure can be adjusted between 25 and 100% of the rated
pressure (cf. Fig. 3.1.12).
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
249