3 Fundamentals of press design

244 Sheet metal forming and blanking
medium-sized parts, in which two parts are formed from one blank or
two adjacent blanks (Fig. 4.4.34 and cf. Fig. 4.9.2). Double parts produced
from a single blank are separated as required in a subsequent
operation. The production of double parts could refer to any suitable
parts, for example passenger car doors. This kind of production doubles
the output per press stroke and increases considerably the economy of
the system, compared to gripper rail transfer systems. Moreover, dual
production helps to better utilize the press load capacity when forming
a single large and unstable part does not fully require the available press
load.
Large-panel crossbar transfer presses can be structured in different
ways. Ideally, each die should be assigned to a separate slide to ensure
optimum peripheral conditions for the die and for the forming process.
The design is based on a modular structure comprising individual
machines in which all the drive systems are connected to the main
press drive system by means of central longitudinal drive shaft and
intermediate couplings (cf. Fig. 3.2.9). This ensures synchronous running
of all the stations: The transport system can be operated by a continuous
transfer with only a minimal safety clearance to the top die.
To transport large unstable parts in transfer presses, a two-axis transfer
system equipped with crossbars and suction cups is used (Fig. 4.4.34).
Unlike the off-center positioning of parts on feeder or robot arms as used
on press lines (Fig. 4.4.35),here the parts are held directly by suction cup
carriers located above the part. Due to the symmetrical arrangement of
the suction cups relative to the center of gravity of the parts, higher
accelerating forces acting on the part are permissible. In comparison
with feeder mechanisms, stroke rates can be increased from 13 to 15
parts per minute with transport steps between 2,000 and 2,600 mm.
The crossbars are fastened on carriages which execute the longitudinal
feed step from one die to the next. The carriages run on the two lift
beams of the transfer which move in the vertical direction to raise and
lower the part. No supplementary aids, such as lifters or ejectors in the
die, are required for part transport.
The time-motion diagram differs markedly from that of the tri-axis
transfer (Fig. 4.4.36). In contrast to a tri-axis transfer, the crossbar transfer
cannot return during the forming process, as the crossbars are located
between the dies. When the dies are closed the crossbars are located
in a parked position outside the die area. The die spacing must in any
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998