U-channels and box sections

A U-channel, as the name implies, comprises

two right-angled bends, one at each of the opposite

sides of a piece of metal (Figure 5.4).

Much more strength can be gained by making a

U-channel instead of a single-angled bend in

any reinforcement section. The U-channel is also

the most common type of section used in the

construction of car frames. Yet another method

of increasing strength is known as box construction,

which consists of two U-sections welded

together to form a square pillar or box. Box

sections are used in conjunction with U-channel

construction in the manufacture of car chassis

frames, underbodies, subframes, cross members

and any construction where great strength is

necessary.

Figure 5.2Strength in crowned surfaces

Figure 5.3Strength in angles and flanges Figure 5.4Strength in U-channel and box sections

Metal forming processes and machines 169

5.4 Machines for sheet metal fabrication

The mass produced, all-steel bodies of both private

cars and light commercial vehicles call for the production

of involved panel shapes on a massive

scale. This requires heavy power presses employing

mating die sets which in effect press the sheet

metal and deform the structure of the metal to a

degree where it takes on a natural shape and retains

that shape. However, since all pressed panels have

some degree of tension in the material structure, it

is important that this should be taken into account

by the body repairer.

It is often necessary to manufacture sheet steel

parts by metal forming machines. The most common

of these include wheeling machines, guillotines,

fly presses, folders and forming presses. In

body repair shops the fly press is often hand operated,

and is a most useful piece of equipment. Tools

for the fly press fit into a central spindle which is

moved down in order to pierce, blank and to a limited

extent form the metal parts required. Another

widely used machine is the folder or bending

machine; adjustment of the setting of the blades of

this machine enables sharp acute or obtuse angled

bends to be produced in long lengths of sheet metal

for the making of sections. The swaging machine is

used for swaging, wiring, joggling and closing

sheet metal work edges. Finally, the body shop is

not complete unless it possesses a wheeling

machine, which is used to produce double curvature

panels from flat sheets by passing the sheets to and

fro between rollers or wheels in order to stretch the

sheets and to create a curved shape.

5.5 Shearing theory

If a piece of sheet metal is placed on one bottom

cutting member or blade, and the top cutting blade

is brought to bear on the metal with continuing

pressure, after a certain amount of deformation the

elastic limit of the metal is exceeded and the top

cutting blade penetrates and cuts the surface of the

metal (Figure 5.5). Fractures begin to run into the

metal thickness from the points of contact of the

top and bottom cutting blades, and if these are positioned

correctly relative to each other, the fractures

meet and the metal is sheared before the top member

penetrates and cuts the whole thickness of the

metal. The horizontal distance between the two cutting

members is called the clearance (Figure 5.6),

and it is this distance that determines the quality of

the cut and the load on the machine or hand tool

being used. There is only one correct clearance

allowance for any particular sheet, and this

depends mainly upon the thickness of the metal

being cut and its physical properties. This clearance

allowance is usually 5–10 per cent of the metal

thickness.

5.6 Cutting machines

Hand lever shears

There are the simplest sheet metal working

machines. For cutting mild steels they are used

with blade lengths up to 500 mm for cutting 3 mm

material, or with a length of about 200 mm for

material up to 6 mm. The cutting members are flat

blades and there is generally no provision for

adjustment of blade clearance. The bottom blade is

fixed to the machine frame, while the top cutting

blade is fixed to a moving member which is pivoted

in the main frame and operated by a hand

lever via a simple link mechanism. An adjustable

Figure 5.5The cutting section showing a sheared

piece of metal

Figure 5.6Blade clearance

170Repair of Vehicle Bodies

hold-down is usually fitted to the top member to

hold the sheet down during cutting. The body of

the shears may be cranked or offset to permit the

cut sheets to be forced beyond the blades so that

sheets longer than the blades may be cut. The body

of the machine is usually fabricated, and this construction

is to be preferred.