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.