Altan T. Metal Forming Handbook
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In the unit illustrated in Fig.4.4.17,the stack of blanks is deposited on
a roller conveyor without using a pallet. The stack is transported into
the turnover device using this feeding roller conveyor and another
roller conveyor in the turnover drum itself. The blanks are held in place
by adjustable stops at two sides of the stack. A third roller conveyor
comes down onto the stack from above, and the unit rotates by 180°.
After opening the turnover device and releasing the stops, the stack is
moved out. Here, too, fast set-up is guaranteed as the stops automati-
cally move to lean against the stack.
Fork turning device for multiple stacks
Multiple stacks of blanks which are stacked in a defined position on a
pallet and must be positioned again exactly after turning require a
highly precise turnover system (Fig.4.4.18). The stacks are initially
loaded together with the pallet on a carriage. To raise the first stack, the
carriage travels towards the turning device, allowing the lower forks to
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Fig. 4.4.17 Blank turning device for complex blank geometries
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
engage between the pallet and the stack. The forks close and the stops
automatically come to rest against the stack. A rotation around the hor-
izontal axis of the turning device moves the stack of blanks to the
desired position for uplifting. After the forks are opened, a second car-
riage with the pallet takes over the stack. Fully automatic changeover of
the pallet surface and the transport directions ensures fast set-up and
also allows to position several stacks of blanks on one pallet. Here, the
stacks are turned over in sequence and deposited back onto the pallet in
a defined position.
4.4.5Press lines
As a rule, series production of medium-sized and large sheet metal parts
is performed on press lines or large-panel transfer presses. The former
consist of a series of mechanical(Fig. 4.4.19)or hydraulic (Fig. 4.4.20)
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Fig. 4.4.18 Blank turning device for multiple stacks
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
individual presses arranged in sequence and interlinked by means of
handling devices. In many cases, press lines are being replaced by large-
panel transfer presses. However, their high degree of flexibility assures
that automated press lines still play a major role in the processing
of complex parts which do not conform to any particular part family
and which call for a high standard of quality. These include, for
instance, parts requiring large draw depths, complex dies with cam
action (cf. Fig. 4.1.17)or parts that require major repositioning between
two successive operations (cf. Fig. 4.1.11).
Modern press lines are expected to satisfy the stringent demands
concerning output, part accuracy, die service life, equipment uptime
and flexibility. These demands can be fully satisfied using mechanical
driven presses with link drive systems. The use of link drive helps to
reduce the impact speed during die closure to around one third of a
comparable eccentric drive system (cf. Fig. 3.2.3). This has major advan-
tages to offer for the production process: With the same nominal press
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Sheet metal forming lines
Fig. 4.4.19 Mechanical press line with noise protection
(double-action lead press, nominal press force 20,000kN,
follow-on single-action presses, nominal press force 15,000 kN)
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
force and force-displacement, a link-driven press can be loaded within
the work range considerably higher and further away from the BDC
than comparable eccentric presses, because the link-driven press has a
steeper force-displacement curve (torque characteristic).
Consequently, a link-driven press can achieve higher stroking rates
than an eccentric press depending on the drawing depth – the greater
the drawing depth, the greater the increase. If the stroking rate is not
increased, the lower impact speed results in improved drawing condi-
tions. Even low grade sheet metals can be successfully processed. In
addition, there is less wear and tear on the die and draw cushion, as
well as on the clutch and brake.
Under practical conditions, the motion curve of a link-driven press
with automatic part transport is configured so as to reduce the impact
speed. Therefore, and also due to the use of double-helical gears in the
drive system, noise is additionally reduced by up to 8 dB(A).
Number of presses
The number of presses which make up a line depends on the number of
operations required to process the part in question (cf.Sect.4.1.1). In
general terms, an attempt is made to reduce the number of presses and
thus also the number of necessary dies.
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Fig. 4.4.20 Hydraulic press line automated by robots
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
As early as in the part design stage, through close cooperation with
the component design and production departments, part simplifica-
tion 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 com-
plex 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 form-
ing 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 press-
es, separate draw and blank holder slides are available and they are dri-
ven either mechanically or hydraulically from above (cf. Fig. 3.1.8and
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 pres-
sure 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 con-
trol (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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ion. However, in this case the energy requirement is greater compared
to a double-action press because the slide is additionally required to
press against the draw cushion (cf. Fig. 4.2.3).
In press lines, the design of the clutch and brake system is important
because automated press lines are frequently operated in the controlled
single-stroke or intermittent mode. As a result of demands for greater
performance, the cycle times of the large-panel presses are continuous-
ly being increased and today can reach as many as 15 strokes per
minute. Consequently, hydraulically actuated clutches and brakes are
frequently used. In combination with a heat exchanger, this design
alternative can sustain a higher energy rate than pneumatically actuat-
ed units, resulting in a higher stroking rate when operated intermit-
tently, in the controlled single stroke mode. The engagement of
hydraulic clutches can also be damped in order to reduce wear and tear
on the drive system and to reduce noise (cf. Sect. 3.2.4).
Automation
The work sequence of a press line begins with the destacking of blanks
by the destacker, cleaning, lubrication and feed of the blanks to the lead
press (cf. Sect. 4.4.4). On completion of the forming operation, the parts
are fed for further processing such as restriking, trimming, flanging or
piercing. Stacking of the finished parts takes place at the end of the
press line.
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Fig. 4.4.21 Interlinkage of presses with swing arm feeders
swing arm
feeder
blank
destacker
blank
feeder
intermediate station with turnover station intermediate station with longitudinal transport
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
In modern press lines, workpiece transport between the individual
presses as well as blank feed are automatically executed. Automatic
stacking of finished parts is used wherever approximately identical
stacking conditions, part shapes and sizes exist (Fig.4.4.41).A number
of different automation systems are used: swing arm feeders, CNC feed-
ers or robots. The type of automation most suitable for any particular
line depends on the range of parts being processed, the stroking rate
and the available space.
Swing arm feedersare driven by electric motors via cams and links
(Fig. 4.4.21).The feeder at the rear of the press removes the drawn part,
which has been raised in the die by ejectors, by means of suction cups
and a suction cup carrier and places it on an intermediate station. From
here, the workpiece is turned over if required and loaded into the next
die by the feeder of the downstream press. Swing arm feeders have fixed
motion curves, making them suitable for small press distances, medi-
um-sized parts and similar part types. They can transport between 10
and 12 medium-sized components a minute.
CNC feedersare electrically controlled and as such can be freely pro-
grammed on two axes and used for a wide variety of part types. Drive is
performed by means of two spindle mechanisms or timing belt-driven
carriages (Fig.4.4.22).They are able to remove the part from the die
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Sheet metal forming lines
Fig. 4.4.22 Interlinkage of presses with CNC feeders and timing belt-driven carriages
washingmachine turnoverstation intermediatestation
CNCfeederCNCfeeder
blankfeeder
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
without the need for an ejector. The output of a press line making use
of a CNC feeder is between 8 and 10 large-sized parts per minute.
When using both, swing arm and CNC feeders, intermediate stations
are required. These intermediate stations are equipped with three to
five programmable axes for part transport in the longitudinal direction,
for transverse and height adjustment and also additional tilting move-
ments.
Automation using robots offers the advantage that it eliminates the
need to deposit parts between the presses (Fig. 4.4.20). The robot’s grip-
per arm positions the part directly from the die of one press into the die
of the next. This means that only one robot is required to service each
press without the need for intermediate stations. The main drawback of
robot-linked press lines is that heavy parts can only be transported
slowly or not at all due to the centrifugal forces and the long transport
paths. For this reason, output is largely dependent on part size and cur-
rently lies between 6 and 8 parts per minute. The freely programmable
axes of the robot arm, in contrast, ensure that widely differing part
shapes can be transported into many different part positions.
Die change
In addition to the interlinkage of presses in a line, automatic die chang-
ing systems also play a major role. Among die changing systems for
press lines, moving bolsters with left/right-hand rails or T-tracks have
proved to be the most popular (cf.Fig.3.4.4).The former require more
space, but offer the shortest possible die changing times. The upper and
lower die of the die set which is no longer required are moved out of the
press on moving bolsters. Subsequently, the opposing bolsters move
the new die sets into the press. Preparation of the dies for the next pro-
duction run is performed outside the press during production. As a
result of die changing automation it is possible to achieve changeover
times of between 10 and 30min depending on the workpiece transport
system.
Nowadays, press lines are often enclosed in order to enhance safety
and noise protection. Noise emissions can be reduced by around
15dB(A) if the space between the individual presses and the peripheral
equipment is also enclosed (Fig. 4.4.19).
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4.4.6Transfer presses for small and medium sized parts
Mechanical and hydraulic transfer presses are used for the manufacture
of ready-to-assemble components for the automotive industry, its sup-
pliers, the electrical and household appliance sectors, and other
branches of industry (Fig.4.4.23).These presses guarantee high output
rates because all forming operations required to manufacture a work-
piece are included in one and the same line. Furthermore, all these
operations are performed nearly simultaneously. A part is completed
with each stroke of the slide, irrespective of the number of die stations
in use.
Transfer presses are either supplied from stacks of blanks from the
blanking line (Fig. 4.4.15)or they process stock directly off the coil (cf.
Fig. 4.3.1). In the former case, the press is equipped with a destacker; in
the second it is equipped with its own coil line including decoiler,
straightener and feed system. By locating the shearing station for blank-
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Fig. 4.4.23 Mechanical transfer press for the production of wheel hubs
nominal press force: 42,000kN; number of stations: 11; feed pitch: 700mm;
slides: 3; strokes per minute: 10-28
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
ing outside of the press upright, it is possible to choose between lateral
or longitudinal material feed to the press. Lateral coil feed offers the ben-
efit that the scrap web can be rolled into a coil again. The drawback here
is the greater space requirement of a lateral coil feed system. Generally
speaking, however, the space requirement of a transfer press is consider-
ably less than that of a press line; the lower space requirement also
serves to reduce ancillary costs.
Forming and blanking operations can be economically performed
both on mechanical and hydraulic transfer presses: blanking, initial
draw and subsequent drawing processes, contour pressing, calibration,
flat pressing, flanging, trimming, hole punching, etc. On the one hand,
even parts with extreme draw depths and complex or asymmetrical
shapes can be manufactured on transfer presses without any problems.
Horizontal wedge-driven cutting and forming tools are also used (cf.
Fig. 4.1.17).On the other hand, dies can often be simplified by making
most effective use of the existing number of stations. The number of die
stations within the die sets used does not necessarily need to be identi-
cal. Idle stations can easily be bypassed. Transfer presses with a large
number of die stations permit the production of two different parts
simultaneously provided the die sets are able to form the part on fewer
stations. In these cases, the blanks are fed synchronously to two differ-
ent points in the press.
The total nominal force and energy costs of transfer presses are gener-
ally lower than it is the case for individual interlinked presses. Operating
and maintenance costs are reduced to a single unit, and favorable con-
ditions are created for reducing production costs due to the elimination
of transport distances and intermediate stations.
Mechanical transfer presses
Mechanical transfer presses are built with two, three or four uprights.
Depending on the range of parts to be manufactured and the number
of stations, the presses are equipped with one, two or three indepen-
dently driven slides(Fig. 4.4.23). Each slide is designed individually to
satisfy the requirements for size, nominal force and stroke. In particular
for larger presses with different press force requirements at various
forming stations, this type of multiple upright construction offers
considerable advantages. It is customary in transfer presses to integrate
a draw cushion in the bed (cf. Fig. 3.1.11).A hydraulic overload system
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