Altan T. Metal Forming Handbook
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4.6.3High-speed blanking lines
Blanked and stamped parts made from thicker sheet metal such as chain
link plates, cup springs, aluminium and coin blanks (cf. Fig. 6.8.7)as well
as sheet metals used in electric motors (Fig. 4.6.20)are manufactured in
large quantities, calling for high-performance lines with high stroking
rates – so-called high-speed blanking lines (Fig. 4.6.6).
In this type of press, the blanking process causes sudden changes in
press forces due to the rapid nature of material breakthrough when the
shear strength of the material is exceeded. These forces can result in
major dynamic displacement in the dies (Fig. 4.6.7).Because of possible
negative effects on part quality and on service life of the dies, high-
speed presses are subject to particularly high precision requirements
(Fig. 4.6.8).
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Shearing lines
Fig. 4.6.6 Blanking line with 1,250 kN high-speed press and die changing cart for automatic
die change (long version)
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
The productivity of the blanking line is determined by the mecha-
nism used for sheet metal feed, the efficiency of the press itself, work-
piece removal and the equipment used for resetting. The coil material
is fed by a coil line whose coil speed is automatically adjusted in line
with the blanking rate of the press (cf. Sect. 4.3).
To produce coin blanks, an additional coil thickness unitis required
(cf. Fig. 6.8.23).The sheet thickness is continuously measured by a sen-
sor and, where appropriate, a command is transmitted to the control
system of the high-speed blanking line, allowing faulty parts to be dis-
carded automatically.
A major criterion when it comes to achieving high output is the effi-
ciency of the feed system by means of roller feed. Mean feed rates of up
to 100m/min are achieved where continuous adjustment of the feed
length is possible for a ratio of 1 : 10. The adjustment process is motor
powered and can also be carried out during press operation. The feed
length is either manually preselected at the keyboard and digitally dis-
played, or where an automatic tool change system is used, it is accessed
by the press control system depending on the die set in use.
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Sheet metal forming and blanking
Fig. 4.6.7 Force-time curve for blanking of sheet metal
stripper force
time t
force
F
material flow
vibration decay
breakthrough
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
Another important aspect determining the output of a high-speed
blanking line is its stroking rate. High stroking rates can only be
achieved where there is acounterbalance of massescreated by the hori-
zontal and vertical forces (Fig. 4.6.8).Counterbalance systems in which
the forces of the slide and upper die are completely eliminated by
directly opposing masses have proved particularly beneficial. The mass
compensation of presses with stroke adjustment, which are also suit-
able for smaller forming operations, is automatically adjusted together
with the stroke. Mass counterbalance is an essential prerequisite for
subcritical arrangement of the press on rocker elements, which ensures
that the vibrations initiated by the stamping process are transmitted
only minimally to the foundation.
The systems of part removal,transport and interlinking with the
downstream production process increase in significance as the press
293
Shearing lines
Fig. 4.6.8 Construction of a high-speed press (short version)
roller bearing
mass counterbalance
eccentric shaft
hydraulic brake
feed drive
feed
press body
spring body
variable DC drive
flywheel
hydraulic coupling
double fork
connecting rod
penetration depth
control device
pressure point
slide gib
slide
bed plate
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
and die output increases. Conveyor belts for transportation of blanked
parts are frequently arranged under the press. The parts drop out of the
female die onto an ejection channel, which is frequently oriented, onto
the conveyor belts. There is a separator integrated in the ejection chan-
nel which segregates defective parts from the start of the coil, the end
of the coil or parts of the coil which lie outside the specified tolerances.
Link-up between this device and the downstream processing stations
for washing, annealing, polishing, inspection, coating or packaging of
parts is increasingly implemented using conveyor belts or handling
devices (cf. Sect. 4.6.4).
Stamping scrap is often collected in boxes and transported away. More
satisfactory than this method is continuous disposal by means of con-
veyor belts positioned underneath the press. The scrap can either be fed
away directly through the female die or outside the die by means of
scrap chutes. If a scrap web is created, this is generally chopped by a
shear at the outfeed side of the press.
In order to reduce storage of material and capital tie-up to a mini-
mum, small batch sizes also have to be produced economically on high-
speed blanking lines. Short die change and resetting times are an essen-
tial requirement here. Resettingfor the manufacture of a different part
includes set-up of the feed and removal devices, the preparation of dies,
die change, conversion of the press and a final check of die and press
setting data (cf. Sect. 3.4).
As the blanking process is the major factor in determining the preci-
sion of punched parts and also theservice lifeof dies, steps must be taken
to minimize the vibration occurring between the punch and the female
die in the vertical and horizontal direction at the moment of sheet met-
al breakthrough by executing the necessary measures at the machine
(Fig.4.6.7).
Vertical vibrationsare created as a result of the play existing in the
force flow of the press and of the elastic properties of the entire press
system. They cause an increase in the penetration depth of the blank-
ing punch in the female die, the extent of which depends on the
stroking rate. The result is increased punch wear (Fig. 4.6.9).
Compared to a press with friction bearings, vertical play is consider-
ably reduced when using an eccentric shaft running in roller bearings
(Fig.4.6.8).In conjunction with the hydraulic slide adjustment clamp,
the total vertical play is reduced to a minimum. A high degree of verti-
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Sheet metal forming and blanking
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
cal rigidity is provided by the solid press body, by the slide drive system
using a double fork connecting rod in short presses and two generously
dimensioned connecting rods in long presses, and by the bending resis-
tance of the slide. However, as the stroking rate-dependent increase in
penetration depth has a physical cause and does not depend on the sta-
tic and dynamic behavior of the press, a control device is frequently
used on high-speed presses. This device detects the increase in penetra-
tion depth and automatically corrects it to the set value(Fig.4.6.10).
Even when working at extremely high stroking rates, this system allows
to achieve a die service life that is similar to that achieved when blank-
ing in the lower stroking ranges.
In addition to the behavior of the press in the vertical direction, the
vibration characteristics of the slide in the horizontal directionalso influence
die life. As the blanking clearance between the punch and female die
amounts to only a few hundredths of a millimeter, highly stringent
demands are made on accuracy of slide guidance and on the horizontal
rigidity of the press body. The degree of horizontal vibration is also
directly influenced by play and by the elastic behavior of the press. The
horizontal play can be eliminated through the use of slide gibs with
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Fig. 4.6.9 Influence of the press stroking rate on the penetration depth of the blanking punch in
the female die
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
rollers, while the horizontal press rigidity is appreciably increased
through special configuration of the press body.
The slide is guided without play by means of rollers at four columns
(cf.Fig.3.1.6).The optimum arrangement of gibs above and on the
blanking plane, in particular, serves to reduce slide vibrations compared
to previous slide gib systems so that a substantial increase in die life is
achieved.
In contrast to conventional presses, the configuration of the press
body is now so compact that upright deflection under load is negligi-
ble, and horizontal forces generated during blanking can be absorbed
evenly over the entire gib area.
4.6.4Lines for the production of electric motor laminations
Rotors and stators in electric motors and iron cores in transformers are
made from individual layered pieces of sheet metal ranging from 0.5 to
1mm in thickness, in order to reduce eddy current loss (Fig. 4.6.11).
The starting material used for this type of application is silicon-
alloyed iron sheet or semi-finish sheet, which is available in coil form
296
Sheet metal forming and blanking
Fig. 4.6.10 Effect of penetration depth control on the increase in penetration depth
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
in widths of up to 1,300 mm. Rotor and stator sheet laminations can
accordingly be produced from a single piece only up to a maximum
outside diameter of 1,300 mm. All larger diameters are composed of
segments which can be used to produce stacks for electrical machines of
any optional size.
Depending on the geometrical shape and production lot size, the
economical production of laminations for electric motors calls for a
variety of die and machine technologies. Figure 4.6.12indicates the
fields of application for single notch, complete blanking and progres-
sive dies. To manufacture rotor and stator laminations in larger lot
sizes, up to a circular blank diameter of 600mm, compound dies are
used in high-speed blanking lines. For diameters up to 1,300 mm, com-
plete blanking dies are used in straight-sided presses (Fig.4.6.13).
Where smaller lot sizes are involved, the laminations are produced
from circular blanks on notching machines using the single notch
blanking method.
Notching machines
Notching machines are generally open-front presses with mechanical
drive systems. The press is mounted in adjustable roller bearings on a
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Fig. 4.6.11 Rotor and stator blank of an electric motor
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
machine bed. An important element of every notching machine is the
indexing device which is also located on the machine bed. This gener-
ates the required notching pitch, i.e. the number of notches per blank.
The punching radius is selected by adjusting the distance between the
press and the indexing device.
In semi-automatic notching machines,the blanking process is performed
automatically, while feed and discharge are manual processes. The index-
ing unit is either mechanically or numerically controlled (Fig. 4.6.14).
The mechanical control device operates with a cam tripping gear and
gears that are exchangeable for obtaining different notching pitches.
When using circular blank diameters between 800 and 1,000 mm, these
low-cost devices achieve a higher output than numerically controlled
systems. In contrast, however, numerically controlled solutions offer
greater flexibility: The notching pitch can be varied as required, making
this type of system ideally suited for more complex notch geometries
such as magnet wheels, pole laminations, rotor rim punchings or seg-
ments (Fig. 4.6.15 below).With the aid of controllable dies, it is possible
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Sheet metal forming and blanking
Fig. 4.6.12 Die engineering methods for electric laminations
no.ofpiecesperyearandtype[parts/a]
outside diameter of the electrical machine [mm]
200
400
600
800
1000
1200
1400
1600
1800
10
3
10
4
10
5
10
6
10
7
10
8
singlenotchingdie
completeblankingdie
progressivedie
segmentblanks
rotor/statorblanks
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
to generate contours, several rows of notches in a single clamping oper-
ation, uncommon notching pitches (relative to 360°) and particularly
wide notches using double punches.
In automatic notching machines and flexible notching systems, the
unnotched blanks are removed fully automatically from a stack of
blanks and fed into the notching machine. Following the notching
operation, the blanks are removed, also fully automatically, and
deposited on a stack. The loading and discharge operations are per-
formed either by a slewing ring, a linear transfer system or a robot. The
six arms of the slewing ring of an automatic notching machine, which
operate using a circular indexing technique, are equipped with central-
ly adjustable transport magnets. Although this process permits the
shortest possible indexing times, it only allows the processing of round
or polygonal blanks. In the standard version, the blanks are transported
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Fig. 4.6.13 Complete blanking die set
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
from one stacking mandrel to the next. With special attachments, pal-
lets can also be used.
On automatic notching systems with one notching machine, it is
possible to carry out either
–simultaneous notching and parting operations or
–only notching operations or
–first stator notching and parting and then rotor notching in two
passes.
However, rotor and stator laminations can be produced in a single pass
on a line with two notching machines(Fig.4.6.16). In the first case,
additional shaft hole punches are inserted in an empty station, in the
second case, the blanks are positioned in the sixth station and passed
outwards to the shaft hole notching device. Slewing arms are the most
economical and most efficient way of automating notching machines
for the production of standard laminations.
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Sheet metal forming and blanking
Fig. 4.6.14
Notching machine for circular
blanks with numerical indexing
drive system (nominal press
force 250 kN)
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998