3 Fundamentals of press design

Part transfer
For cold forming processes, the gripper units are mounted on clamping
rails which are fixed in turn on a base rail (Fig. 6.6.1). When resetting
for producing a different part, all that has to be exchanged are the
clamping rails including the pre-set grippers. The front grippers are
equipped with proximity switches to monitor part transfer.
Parts with a higher L/D ratio (approx. 3 to 8) must be transferred
between stations by a 3D transfer. This involves the use of an additional
horizontal pusher which moves the part together with its retainer
into a position from where it can be raised vertically. Here, too, the level
of the base plate on which the parts are positioned is located below
the transport plane. Shaft-shaped parts with a billet L/D ratio of > 6 can
also be moved into the uplift position using special devices such as
rotating sleeve retainers.
In warm forming, the loading station has the additional function of
segregating parts which either have not been fed in correct synchronization
with the press, or which have an incorrect temperature. To
allow these parts to be ejected, an opening is released in the base plate
of the loading station through which the parts drop down into a chute
for removal.
6.6.2 Transfer study
Before conducting a transfer study, data on the kinematics of the slide
movement must be available in the form of a time-displacement diagram.
The stroke is defined based on the range of parts, the required
forming process and the die layout. The slide curve is given by the stroke
height and press kinematics (eccentric drive, knuckle-joint drive, modified
knuckle-joint drive) (cf. Fig. 3.2.3). The slide curve, whose bottom
dead center is at 180°, can only be displaced vertically for transfer study.
The ejector stroke is determined by the range of parts being processed
and the respective position of the forming stations in the die. The
largest necessary ejector stroke is equal to the sum of the inlay depth of
the part in the die and the part length. The ejector stroke can be
achieved by means of a mechanical and an additional pneumatic displacement.
The stroke covered mechanically must eject the part, which
is stuck in the tool as a result of elastic deflection of the container. In
addition, the pneumatically generated displacement is able to raise the
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
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