Machine
In these machines the end frames in which the
clamping beam is fixed have small gaps so that
folds can be made in sheets of unlimited length,
provided that the depth of the bend is below the
width or height of the gap, which is usually about
50 mm. The maximum length of the blades in this
type of machine is 2 m, and the maximum gauge
which can be bent is about 1.6 mm. The lift of the
clamping beam and top blade is about 3850 mm
in height. The clamping beam and top blade
are operated by an eccentric shaft running across
the front of the machine; this shaft has handles at
Figure 5.9 Bending allowance
Figure 5.10 Bending machine (Selson Machine
Tool Co. Ltd)
5.8 Bending machines
Angle bender (clamp folder)
The essential factors in producing a clean bend on
sheet metal are that the edge or blade over which the
metal is bent should be straight, smooth and fairly
sharp and that the pressure applied to bend the metal
over this edge should be equal throughout the length
of the bend. The most generally used machine for
bending sheet metal up to 1.6 mm is the angle bender
or clamp folder (Figure 5.10). This consists of a
Metal forming processes and machines 173
each end which lift the blade up and down. The
bending blade is lifted by a handle at the front of
the machine. In larger machines there is provision
for counter-balance weights to be fitted to the
bending beam. The bending machine is only suitable
for single bending operations.
Box and pan bending machine
(universal bending machine)
It is possible to form boxes with more than one bend
using the clamp folder if there is sufficient clearance
between the clamping beam and the top blade to
allow the workpiece to be removed. However, most
box forming is done using the box bender, which
differs from the clamp folder in the design of the
upper blade (Figure 5.11); whereas the clamp folder
has a single solid bending blade, the box bender has
a series of individual blades known as fingers. The
advantage of this type of machine is that any number
of finger blades can be removed, permitting a
great variety of bends to be made. The actual operation
of the machine is the same as the clamp folder.
Both machines are used to advantage in body work
as they are capable of bending to shape many articles
suitable for both new and reapir work.
2 Check on the path of the swinging counterbalance
weight, warning anyone working close
by of your intentions to use the machine.
5.9 Rolling machines
Bending rolls
Bending rolls (Figure 5.12) of 1 m capacity are well
suited for panel work. They are used mainly to
form curves in panels and to roll complete cylinders,
but they can also be used for breaking the
grain of coated sheet metal, such as tin plate or galvanized
sheet, before it is worked. Machines are
made in all sizes: bench rollers for light tin plate
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work, hand powered rollers for general sheet metal,
and motor driven rollers for heavy gauge sheet
steel. All rolling machines comprise two front
rollers which lightly grip and draw the metal
through, and a free roller at the rear to set or bend
the metal to the desired radius. They can, however,
differ in their mode of operation, and bend rolls fall
into two main types: pyramid rolls and pinch rolls.
Figure 5.11 Box and pan bending machine
(Selson Machine Tool Co. Ltd)
Figure 5.12 Bending rolls (Selson Machine
Tool Co. Ltd)
Safety measures for working
Bending machines
1 Before locking the clamping blade down, make
sure that your fingers and those of your assistant
are clear.
Pinch rolls
The rollers (Figure 5.13) are of the same diameter
and are usually geared together to give the same
feeding speed and rotation. The two front rollers
are adjustable for different thicknesses of metal;
the third roller at the rear is also adjustable to
provide for various degrees of bend required.
A swinging-type end support can be opened, permitting
the top roller to be pivoted upwards so that
completed cylinders can be removed. Nearly true
174 Repair of Vehicle Bodies
cylinders can be formed using pinch rolls; the
slight flat which occurs on the leading edge of the
plate can be reduced by initially reversing the
rollers and feeding the flat plate back into the
rollers a little way, which will preform the edge
ready for rolling. Pinch rolls grip or pinch the
metal between the first two rollers and give good
contact during bending, but if these rolls are too
tightly adjusted the edges of the metal will be
stretched and the result will be an uneven edge.
Pinch rolls are used for rolling all types of thin
gauge sheet metal up to 2.00 mm.
Pyramid rolls
This machine (Figure 5.14) has two rollers of
equal diameter mounted side by side which rotate
in the same direction. The third roller, which is
situated above the other two, is of larger diameter
and is adjustable in the vertical plane in order to
control the radius of bend. It pivots at one end to
permit easy removal of the finished work.
Pyramid rolls tend to leave more flat surfaces
than pinch rolls. Again this may be reduced by
rocking the plate between the rollers, but it may
be necessary to preform the edges of the metal by
hand before rolling. This type of machine is best
suited for the heavier gauges of metal 3 mm and
upwards.
As already stated, common metals used for
panel work have a certain degree of elasticity. This
means that before a metal sheet can be formed
into a curve or cylinder, sufficient force must be
applied to deform the structure of the material
beyond its elastic limit so that the sheet assumes a
permanent curved shape. Some heavier material
may require presetting by hammering to provide a
lead over the rear roller and also to prevent flats at
the ends of the rolled surfaces. However, small
flats may occasionally prove to be of assistance in
the welding of thin sheet metal. The slight ridge
formed by the flats tends to act in a similar manner
to a swage and has the effect of stiffening the
area to be welded. Excessive distortion is checked
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by the flats and, after welding, the smooth curve
of the cylinder can be restored with a mallet.
Bending rolls are sometimes used to repair flat
aluminium panels by rolling them, then repairing
them in this state, then rolling them flat again.
Cone rolling machine
Cone rolling machines are mostly used for rolling
small conical-shaped fabricated articles in light
gauge metals. They consist of a pair of conical
rolls, geared and mating together and acting as
pinch rolls. There are no special sizes. They are
usually made to the particular requirements of the
company (Figure 5.15).
Figure 5.13 Pinch rolls
Figure 5.14 Pyramid rolls
Metal forming processes and machines 175
