Altan T. Metal Forming Handbook
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This point corresponds to the point calculated using the perimeters.
Blanking clearance between punch and female die
The blanking forces ascertained on the basis of the equations provided
here assume that the correct blanking clearance has been selected
between the punch and the female die. The precise size of the blanking
clearance u [mm] depends on the thickness of the sheet metal and its
tensile and shearing strength, as well as on the blanking speed, the type
of blanking plate perforation (with or without clearance angle a[°]) and
the required quality of the cut surface (Fig.4.5.12).A larger blanking
281
Blanking processes
666
55
66
7 7 8 8 9 9 10 10 11 11 12 12
5
6
789101112
=−
=
⋅+⋅+⋅+⋅+⋅+⋅+⋅+⋅
+++++++
.x amevalueaswith sthesame xis
y
lblblblblblblblb
llllllll
y
S
S
(s a),a ya )
SS
S
R
y
enterofgravityS thecoordinatesystem
=
⋅+ ⋅+ ⋅+⋅+⋅+⋅+⋅+⋅
+++++++
=
⇒
62832062832094255096070904580909045800
6283628394256090809080
4325
6664325
....
...
.
(.;.)Cinc.
Fig. 4.5.11 Example to determine the surface center of gravity S
R
:
apart to be produced
bdetermination using lengths of the perimeters
cdetermination using line centers of gravity
yy
x
x
S
3
U
3
S
1
U
1
a
3
a
b
3
a
7
S
7
S
5
S
2
=U
2
a
2
a
6
b
5
l=l
56
b
9
l
9
b
10
l
10
l
11
l
12
b
8
b
l
8
b
7
l
7
a
4
b
4
a
10
S
6
S
4
S
U
4
S
9
S
S
10
S
11
S
12
S
8
c
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
clearance generally reduces the necessary force and work requirement,
and thus also tool wear; with a smaller clearance, in contrast, a qualita-
tively improved cut surface and greater part accuracy are frequently
achieved. In conventional blanking with blanking speeds in the region
of 0.1 to 0.2 m/s, the optimum blanking clearance is between 2 and 10 %
of the sheet metal thickness (cf. Fig. 4.7.6),whereby the lower value
applies to thinner or softer sheet metals.
When piercing sheet metal, the dimension of the pierced hole is
determined by the blanking die; the opening in the blanking die must
accordingly be selected to be twice the blanking clearance greater than
the blanking punch. When blanking external contours, by contrast, the
female die determines the size of the blanks. The punch must accord-
ingly be configured smaller by twice the blanking clearance.
Example:
A 3mm thick medium-hard steel sheet is to be processed in a progressive tool
(cf. Sect. 4.1.1). Initially a hole with a diameter of 10mm is to be pierced, after
which the circular blank with a diameter of 28mm (external contour) is to be
blanked out of the metal strip (Fig. 4.5.13):
1st stroke – piercing:
The piercing hole punch is given the nominal dimension 10mm. The female
blanking die has the diameter
2nd stroke – blanking:
The female blanking die has the workpiece dimension 28mm diameter. The
blanking punch has the smaller diameter:
282
Sheet metal forming and blanking
a
a
1
u
Fig. 4.5.12
Dimensions for piercing and blanking on the punch and the
female die
dmm
mm
mm mm mm=+⋅
⋅
=+=10 2
36
100
10 036 1036
%
%
..
dmmmm mm==28 036 2764–. .
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
283
Blanking processes
Fig. 4.5.13 Workpiece and die dimensions used in the manufacture of a pierced disk
28
10
10.36
10
0.18
0.18
27.64
28
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
4Sheet metal forming and blanking
4.6 Shearing lines
4.6.1 Slitting lines
The parting of coiled sheet metal stock to produce several narrow strip
coils is performed using waste-free cutting with rotating circular shearing
bladesin so-called slitting lines. Materials processed in this way include
hot and cold-rolled and stainless steels, NF metals, in particular alu-
minium, and coated sheet metals.
Alongside the decoiler and recoiler, the major components of a slit-
ting line include the circular shear and the brake frame. The starting
coil is pulled tightly off the decoiler, cut to narrow strips with the cir-
cular roller shears and coiled again to form firm rings by the brake
frame and the recoiler (Fig.4.6.1). The material processed in this way has
generally a thickness of 0.2 and 10mm. The starting coils can weigh
anywhere between 5 and 30 t. The minimum width of the individual
coil depends on the width of the blade and the thickness of the sheet
metal being processed. Circular shear blades are generally between 10
and 20 mm wide and have a diameter of 250 to 500 mm. The maximum
number of cuts depends on the diameter of the shear shaft and the
shaft length. Shaft deflection must not exceed a certain value.
Circular shearswith a continuous shaft are equipped with circular
blades, rolling rings and distance sleeves (Fig.4.6.2). Alternatively,
hydraulically fastened flanged blades can be used for greater slit coil
widths. To reduce standstill periods, the shears are configured as
exchangeable units that can be pre-assembled outside the running slit-
ting line prior to production. Where electrically adjustable disk shears
are used, the blade holders are positioned by a control system to the
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
programmed width. Width-adjustable shears permit easy trimming of
sheet metal coils of different widths.
The brake frame applies the necessary restraining force to the slit
sheet metal, thus allowing it to be recoiled under sufficient tension.
The specific tensile stress levels in the coiling lines lie between 10 and
20 N/mm
2
. The brake frame used depends on the type of coil stock
processed, the treatment or coating of the metal surface, the necessary
coiling quality and the thickness and strength of the sheet metal. An
underlying difference is drawn between plate-type brakes and rotating
brake systems. The plate-type brake applies pressure through braking
felt-carrying plates positioned above and below onto the surface of the
sheet metal. Because of the 100% slip between the braking felt and the
surface of the coil material, this braking method can only be used on
sufficiently insensitive surfaces. The braking energy generates heat in
the slit coil. Where rotating brake systems are used, slip can be reduced to
5%, avoiding damage of the material surface.
A coil loop must be provided between the circular shear and the brake
frame. This is used to accommodate the length differences in coiling
285
Shearing lines
Fig. 4.6.1 Slitting line
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
which occur between the inner and outer slit coils with each revolution
of the decoiler. The reason for these differences are the thickness varia-
tions in the starting coil material over the coil width.
The production speed achieved by slitting lines ranges from 100 to
500 m/min. The resetting of blades, as well as the removal and hooping
of the slit coils with packaging tape can lead to system bottlenecks.
Improved capacity utilization can be achieved by using automatic
exchange of blades and by recoiling outside the line itself.
4.6.2Blanking lines
Sheet metal parts with medium and large surface areas are not produced
directly off the coil, but from stacked blanks fed automatically into the
forming press. These blanks are produced in blanking lines with cut-to-
length shears or blanking presses.
Blanking lines consist of the following components:
–coil line (cf. Sect. 4.3)
–shear or blanking press
–stacking line
286
Sheet metal forming and blanking
Fig. 4.6.2 Circular shears
rolling ringcircular bladedistance sleeveframe sheet metal strip
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
The blanking process used (Fig.4.6.3)depends on the production
volume and the degree of material utilization. Rectangular blanks can
be produced using simple cut-to-length lines. Trapezoidal and parallel-
ogram shaped blanks require a swivel mounted shear or a swivelling die
in the blanking press, which move the shear or the die to the other
swivel position respectively during each feed movement. The feed unit
is driven either hydraulically or using program-controlled servomotors.
Closed contours can only be produced on presses with blanking dies (cf.
Fig. 2.1.29).
Lines for cut-to-length shearing
In cut-to-length lines,the coil stock is cut by means of stationary, flying
or rotating shears.
In lines with stationary shears, the roller feed indexes the coil stock
forward one step at a time. Cutting takes place when the material is sta-
tionary (Fig. 4.6.4).This represents the lowest cost solution and uses a
fixed or swivel-mounted shear.
Flying shearsare accelerated briefly to match the speed of the coil for
shearing. After completing the cut, the shear returns to its starting posi-
tion. The benefits of this design are continuous feed with a constant
straightening machine speed and elimination of the pit to accommo-
date the coil loop, so ensuring that the material is not bent again after
287
Shearing lines
Fig. 4.6.3 Blank geometries, arranged according to the type of blanking line
straight
shear
swivel-mounted
fixed shear
swivel-mounted shear,
moving during feed stop
contoured
blanking die
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
straightening (cf. Sect. 4.8.3). As the sheared blank also moves at the
same speed as the coil, there is no need to accelerate the blank as it is
the case when using stationary shearing methods. Compared to the
stop and go method used with roller feed and stationary shears, the
return movement involved in the flying shear method results in a
slightly lower output when working with short cut lengths.
Rotating shearsachieve the highest number of cutting operations.
Their output lies at over 150 parts per minute, making them twice as
productive as a blanking press. In the case of rotating shears, the coil
material runs at a constant speed between two cutting tools moving in
opposite directions. The cutting tools are arranged over the complete
coil material width at right angles to the feed direction, so that the sheet
metal is separated transversely to the feed direction when it makes con-
tact with the cutting tools. When using this process, the surface edge
speed of the cutting tools and the speed of the coil stock must match at
the moment of shearing. While the cutting tools are not engaged, their
rotation speed is accelerated or reduced by the drive system in order to
produce the required blank lengths.
As is the case with flying shears, the feed rate and consequently also
the output of rotating shears depend heavily on the blank lengths. For
both shear types, it is necessary to have the supply loop for coil mater-
ial and the pit required to accommodate it, as shown in Fig. 4.6.4.Lines
with continuous feed are consequently shorter in comparison to coil
lines equipped with stationary shears.
288
Sheet metal forming and blanking
Fig. 4.6.4 Cut-to-length lines: coil line, cut-to-length shear and stacking line
decoiler with
coil
cropping shear
straightener
roller feed and
cut-to-length shear
threading unit
scrap container
coil loop
infeed area with lateral
coil guidance
blank stacking cart
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
Blanking presses for contoured blanks
Contoured blankshave a complex closed contour (cf. Fig. 2.1.29)and can
contain holes. These blanks are manufactured on blanking presses,
also known as contour blanking lines (Fig. 4.6.5). Blanking presses are
high-speed eccentric or link drive presses similar to single-action car
body presses with a maximum of 80 strokes per minute (cf. Fig. 4.1.19
right and Fig. 4.6.8). They feature a high degree of rigidity as well as
small bearing and gib clearances. The slide – generally with a maxi-
mum stroke of 300 to 450mm – is mounted using eight guide gibs (cf.
Fig. 3.1.5).
Blanking presses require particular attention to noise reduction.Blank-
ing noise depends on a number of factors, including the impact speed
of the punch on the sheet metal, and accordingly also the slide veloci-
ty in the work area. Investigations have proven that noise emissions
can be reduced by around 6 dB(A) by halving the slide velocity. In the
case of link drive systems,the impact speed reached is only 30% that of
289
Shearing lines
Fig. 4.6.5 Contour blanking line with nominal press force of 8,000kN
(six-link drive)
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
eccentric presses, representing a reduction in the noise level of some
12 dB(A) (cf.Fig.3.2.3). In addition, secondary measures are applied to
reduce the noise emission from blanking shock in presses, for example
by providing a sound enclosure. This can serve to reduce noise by as
much as 25 dB(A).
As a rule, modern presses are mounted on rocker elementsin order to
reduce floor vibrations. With a natural line frequency of some 3 Hz, this
type of solution achieves an isolating efficiency of around 80%.
To allow blanking presses to also produce straight and trapezoidal
blanks, swivel dies are used. These are generally configured in the form
of impact dies. Vertical guidance of the upper die is integrated directly
into the die set. The swivel movement is generated by hydraulic cylin-
ders or electric motors, nowadays generally using servomotors and gear
drive. The PLC control specifies the swivel angle.
Stacking lines
Both in the case of contour blanking lines and cut-to-length lines, the
produced blanks are stacked onto pallets by automatic stacking units
(Fig.4.6.4).Conveyor beltsarranged crosswise or lengthwise, relative to
the longitudinal axis of the press, transport the blanks either lying or
hanging out of the blanking line up to the adjustable stops, where they
are located and ejected onto pallets. The pallets are put on stacking
carts equipped with lifting devices which are lowered continuously in
order to permit precise stacking. Using two stacking cartsin each stack-
ing device, continuous operation is possible.
Die change
In blanking presses, die change is performed fully automatically using
two moving bolsters (cf. Fig. 3.4.3).The moving bolstersmove out of the
press towards the operating side and are arranged in the form of T-tracks
(cf.Fig.3.4.4).Positioning takes place automatically in the machine, on
the set-up station and in the exchange position. This configuration
allows to achieve die change times of under 5 min.
Moderncoil linesare also automatically reset (cf.Sect.4.3).The con-
trol system automatically executes the sequences “fetching the coil”,
“depositing the coil”, “coil insertion”, “finish processing of the coil”,
“welding seam disposal” and “scrap metal disposal”.
290
Sheet metal forming and blanking
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998