Open-end folder or bending

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 38–50 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 5.9Bending allowance

Figure 5.10Bending 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

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 5.11Box and pan bending machine

(Selson Machine Tool Co. Ltd )

Figure 5.12Bending 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

174Repair 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

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 5.13Pinch rolls

Figure 5.14Pyramid rolls

Metal forming processes and machines 175