Universal shearing machines
(nibbling machines)
In these machines the cutting is done by a pair of
very narrow blades, one of which is fixed while the
other moves up and down from the fixed blade at
fairly high speeds. Generally the blades have a
very steep rake or angle to permit piercing of the
sheet for internal cutting. Since the blades are very
narrow the sheet can be easily turned during cutting.
The term ‘universal’ is apt, since the machine
will cut straight or curved shapes inside or outside.
These machines are powered by compressed air or
an electric motor, and are held in the hands during
the cutting operation. In the machine the top blade
is fixed to the moving member or ram and the bottom
blade is fitted on an extension which is shaped
like a spiral so as to part the material after cutting.
The ram and blades are driven at a speed between
1200 and 2100 rev/min, depending on the metal
thickness and whether air or electrical power is
used. The maximum thickness of metal to be cut is
about 4.5 mm, but the machines find their widest
application on lighter gauges of metal 1.00 mm
and 0.8 mm. The lighter machines have a maximum
cutting radius of about 15 mm and the heavier
machines one of about 50 mm.
The most important points to remember when
using the machine are:
1 See that the blades are set for the correct thickness
of metal being cut.
2 Keep the blades sharp (a blunt blade tends to be
dangerous).
3 Hold the machine correctly with the blades at
right angles to the metal being cut.
Guillotine
A 1.25 m treadle guillotine (Figure 5.7) will meet
the requirements of the average panel shop for cutting
sheet metal. The guillotine is probably the
most widely used of the straight-line cutting
machines. The principle employed in the cutting
action is very similar to that of the hand bench
shears. A concentrated load is applied across the
width of the sheet or plate as cutting proceeds. All
have a bottom cutting blade which is flat and is
fixed horizontally, and a top blade which is
mounted at a rake to the bottom blade and is fixed
to a moving beam. The top beam and blade can be
brought to meet the bottom blade by hand, foot,
mechanical, air or hydraulic power, depending on
the type of machine.
The guillotine consists of a bed, a foot treadle,
two cutting blades, and a front, back and two side
gauges. The face of the bed sometimes has a graduated
scale which permits the cutting of sheets to a
specified size. The side gauge, which has steel bars
bolted on each side of the bed, can also be graduated
for measuring purposes. By placing the edge of the
sheet against one of these side gauges, a cut can be
made at right angles between the side gauge and the
bottom blade. The front gauge consists of a rectangular
bar which slides in the bed and can be set at
any required distance from the blades. For cutting
long sheets, extension arms are fastened to the front
of the machine. Slots in the extension arms permit
the movement of the front gauge away from the bed
to accommodate the larger sheets. The back gauge
has an angle-shaped bar which slides on two rods
fastened to the rear of the machine. This bar is used
to cut a number of pieces of the same length by the
adjustment that is allowed on the rods. The lower
cutting blade is fastened to the bed of the machine,
Figure 5.7Treadle operated guillotine
(Selson Machine Tool Co. Ltd )
Metal forming processes and machines 171
and the upper cutting blade is attached to a beam
which is moved by stepping on the foot treadle. This
beam is connected to the treadle by heavy springs
which return the treadle to its original position after
the metal has been cut. Also attached to this beam
and blade is a hold-down device which is located in
front of the top blade, and as the treadle is pressed
down it clamps and holds the metal in place ready to
be cut. A guard is used in front of the upper blade to
prevent the operator’s fingers from coming into contact
with the blade or hold-down device. The standard
guillotine blades are usually made from plain
carbon steel and are suitable for cutting all types of
mild steel and non-ferrous metals. For cutting
higher-tensile steels such as stainless steel, alloy
steel blades are necessary.
Guillotines should never be used to cut metal
which exceeds the capacity of the machine. The
capacity range for these models is: a 2 m blade,
which will cut up to 1.2 mm; a 2.5 m blade, which
will cut up to 1.00 mm; and a 1.25 m blade,
which will cut up to 1.6 mm. Most foot operated
machines will cut up to 1.6 mm metal. Under no
circumstances should the guillotine be used to cut
wire, rod or bar, as it is intended to cut only flat
sheet metal.
When using this machine, make sure that the
angle finger guard, which is fitted in a position
between the operator and the cutting blades, is set
in such a way as to protect the hands from both the
crushing action of the hold-down device and the
cutting action of the blades. Accurate cutting is
accomplished by leaning over and sighting the
scribed cutting line on the metal at a point perpendicular
to the cutting edge of the bottom blade.
Check before using the machine that the clearance
between the top and bottom blades is set correctly
for the thickness of metal to be cut. Do not cut
metal of heavier gauge than the machine is
designed to cut. Always hold the metal flat and
firmly on the base plate of the guillotine; this will
prevent drag (the effect of pulling the metal in
towards the blades) which often occurs as the cutting
edge loses its sharpness.
The guillotine is used more in the coach building
and building of new vehicles than in the body
repair side of the industry. It is ideal for cutting a
number of sheets to a predetermined size because
it guarantees greater accuracy than is possible with
hand methods.
The following safety precautions should be
observed when working with guillotines:
1 Keep the blades of the machines sharp.
Remember that blunt blades drag the metal and
can cause the loss of a finger.
2 Hold the metal firmly on the bed plate of the
machine to ensure shearing action and also to
prevent drag.
3 All guards should be kept in place. They are
required to be there by law in the interests of
your safety.
4 When using a treadle guillotine make sure that
any assistant or onlooker has not got his feet
under the treadle.
5.7 Bending theory
Many metals and their alloys may be formed by
bending. When considering metals with regard to
their bending properties, the following facts are
most important.
Behaviour of the metal in bending
In bending sheet metal to an angle, the inner
fibres in the metal on the bend are compressed
and given a compressive stress, while the outer
fibres are stretched and given a tensile stress
(Figure 5.8). The boundary line in the metal
thickness between the two areas of stress is called
the neutral axis. The position of the neutral axis
may vary with the properties of the metal and its
thickness.
Figure 5.8Bending action
172Repair of Vehicle Bodies
Calculation of the bending length
The formula for calculating the bending allowance
is shown in Figure 5.9. An approximate workshop
method for calculating the bending allowance is
(r/2) _ T.
clamp to which is attached the blade around which
the metal is bent, and is operated by a hand lever
fastened to the end of the machine. The clamping
action applies necessary pressure to hold the work
to the bed or base of the machine, in order to prevent
movement while the bending is in progress.
The actual bending of the metal is done by swinging
up the front part of the bed which carries the bottom
blade on a hinged centre with the top blade.
The following procedure should be used:
1 Mark the bend lines on the metal.
2 Open the upper blade by pushing the clamping
handles backwards.
3 Place the sheet of metal between the upper blade
and the bed of the machine with the bend line
directly under the edge of the top blade, then
clamp the metal by pulling the clamping handle
forwards again. Some adjustment to obtain the
correct clamping pressure may be necessary.
4 Raise the lower bending blade to a position that
will produce the desired angle of bend. Move
the blade slightly beyond the required angle to
allow for springback of the metal.
5 Drop the bending blade to its normal position
and open the clamping handles, which will
release the work.
Edge folder
As the name implies, this machine is used for turning
the edges on sheets to make hooks for grooving
seams or for wire insertion, or for making small
flanges, but generally only in mild steel of 1.00 mm
or less.