Slitting is the operation of cutting a wide strip
into a number of narrow strips by passing the
strip between circular cutters or blades, as
shown in Figure 6.
Figure 6: Cutter arrangement for slitting
5.1 Slitting Line Configurations
Slitting lines consist of an uncoiler for holding
the wide coil, one or more slitters, and a coiler
for simultaneous recoiling of all the slit strips.
Slitting lines can be of two main types, pullthrough
In pull-through slitting all the power for slitting
and coiling comes from the coiler drive system.
In driven slitters the power for slitting comes
from the slitting machines, while uncoiler and
coiler are separately driven.
The choice between the two types depends on
steel sheet strength and thickness and on the
quality of slit strip required. Driven slitters are
often used where the strip to be slit:
a) has a low strength and is, therefore, likely
to be locally stretched, or
b) is thinner than 0.40 mm and likely to tear
in the pull-through.
While the lower cost pull-through system is the
more commonly used method for the general
range of steel products, the versatility of the
driven system is a definite advantage in the
slitting of products where optimum edge
condition, minimum surface damage and a
freedom from camber, distortion and residual
stresses are required. Residual stresses can be
present in strip slit by a pull-through unit and
may cause problems where a high degree of
flatness from edge to edge is necessary.
Combination pull-through and driven slitters
are sometimes used where the versatility to slit
a wide range of material types and thicknesses
In many slitting lines the material forms a free
hanging loop either before (preloop), after (postloop) or before and after (double loop) the
slitting head. Double loop slitting, using a
driven slitter, is preferable for slitting electrical
steels, since the stresses induced in pullthrough
slitters can be harmful to the magnetic
properties of the steel. Residual or locked-in
stresses are generally the cause of bowing in
narrow sections formed in a press brake from
pull-through slit strip, the thickness/width
range of 1.6-2.0 mm/150-250 mm in low
carbon steel being more prone to this effect.
Cutters are arranged on parallel arbors. The
distance between cutters is set by spacers.
They are usually made from similar material to
shear blades (see 2.5).
The clearance between cutters needs to be
closely controlled to achieve optimum width
accuracy and edge condition. Recommended
clearances vary from 10% of material thickness
to 7 - 8% of thickness where minimum burr is
required. Suggested clearance conditions based
on experience and published information are
shown in Table 3.
Vertical overlap or penetration must be
sufficient to give complete separation of the slit
strips. However, excess penetration can
increase cutter wear and increase burr height
In general, optimum horizontal clearance will
result in a Type 3 edge with minimum burr.
Insufficient clearance will result in a Type 5
edge and excessive burr or edge crimping.
Excessively tight clearance can result in
damaged cutters or machine overload.
Excessive clearance will result in a distorted,
rolled edge with high burr.
The properties of the steel workpiece influence
the horizontal clearance. Fully annealed low
carbon steels usually require less clearance
than high strength steels.
As with other sheared edges, a slit edge always
exhibits a burr which increases in height as the
cutters wear. Burr height can usually be
minimised by close attention to rigidity of
slitter head design, setting of vertical and
horizontal clearances, and cutter resharpening
Table 3: Vertical and horizontal cutter
clearances for slitting1
STEEL SHEET THICKNESS (mm)
(a) HORIZONTAL CLEARANCE
|(b) VERTICAL OVERLAP
The shearing action between the cutters tends
to separate the slit strips. To maintain the strip
in a horizontal plane requires the use of
strippers. Two commonly used stripping
a) timber or metal fingers inserted between
b) rubber or Neoprene rings fitted over
the spacers that occupy the space
between the cutters.
Clearance between cutters and strippers should
be minimal. Excessive clearance between the
stripper and cutter results in a rolled edge on
The use of rubber or Neoprene strippers for
thin, soft steel sheet can cause surface
indentations. For these applications timber
fingers, with protective covering where
necessary, are preferred. Tallowood fingers can
be used for uncoated steels or for coated
electrical steel where the insulating coating
(Coreplate) has lubricant properties and is
resistant to scratching. However, for most
coated products timber fingers can cause
surface scratching or scuffing, and therefore a
protective covering is beneficial. Light felt and
vinyl have been used to avoid damage to zinccoated,
ZINCALUME® zinc/aluminium alloycoated
and COLORBOND® prepainted steels.
After passing through the cutters the multiple
strands from the slitter are wound onto a
mandrel. There are a number of alternative
mandrel designs all of which must meet the
a) The mandrel surface must be sufficiently
smooth and free from irregularities to
prevent marking of the wound slit coils
b) It must hold the end of the slit strip
securely, and in such a manner that there is
no marking of subsequent wraps
c) It must provide a reliable expanding and
collapsing mechanism to allow trouble-free
removal of the slit coils.
Two aspects of coiling which require special
consideration are strip tension control and
separation of the slit coils.
Both hot-rolling and cold-rolling produce strip
which is characteristically slightly thinner near
the edges than in the centre. As a result, on
coiling the slit strips, the thicker slits are
wound more tightly than the thinner slits.
Uniform tension can be achieved by inserting a
drag pad or bridle arrangement between the
slitting head and coiler. However, the diameter
of the coils from thin slits will then be less than
those of thicker slits, with the result that excess
length will accumulate between slitting head
and tensioning device of the thinner slits. The
excess length is usually accommodated by
provision of a postloop pit.
With both drag pad and bridle rolls, care needs
to be taken to avoid marking of products with
critical surface finish, particularly prepainted
products. Drag pads are covered with industrial
felt or rubber-backed carpet to minimise
damage to zinc-coated, ZINCALUME® steel or
Where provision for uniform coiling tension is
not made, operators have developed the
practice of inserting cardboard fillers between
the wraps of loose coils. However, extreme
caution is needed to avoid damage to soft, thin
products when using this method.
Slit coils are separated on the coiler mandrel (as shown in Figure 7) either by large separator
discs which fit between each slit coil on the
mandrel, or more commonly by overarm
separators, using tapered steel separator discs.
In either case damaged separators must not be
used as they can cause damage to the edges of
the slit strip.
The distance between slitting head and coiler is
important as it determines the angular
displacement of outside slits relative to the
centre line. Excessive angular displacement can
cause camber in the slit coils.
Figure 7: Coiling slit strip with separators
The use of lubricant, even sparingly, reduces
cutter wear, improves stripping and enhances
the effectiveness of drag pads in controlling
uniform coiling tension. Lubrication can also
reduce any tendency for drag pads to damage
prepainted coatings. Do not use lubricants for
COLORBOND® steel surfaces unless absolutely
necessary. If it is absolutely necessary, use
SHELLSOL T or an equivalent.