Tools required for hot shrinking

Before commencing to heat up the stretched section,

it is essential that all tools and material

needed to carry out the shrink should be conveniently

placed so that they can be brought into use

quickly. The tools required are as follows:

Wire brush The wire brush and scraper are

used in preparing the panel prior to shrinking. With

the scraper any anti-drum or underseal compound

must be removed before applying the heat.

Mallet or shrinking hammer The mallet face has

a soft surface much larger than that of a planishing

hammer. If used together with a suitable dolly

block it will bring the metal down to a level

surface whilst hot, and also avoid stretching the

panel as the mallet is made of wood and not metal

like the panel hammer. Another tool that can be

used is the shrinking hammer; this is similar to a

planishing hammer but the faces have cross-milled

serrations, which reduce the tendency for the hammer

to stretch the metal because of the very small

contact area between the points on the serrations

and the panel.

Planishing hammer This should be used to complete

the shrink after malleting, as the hammer

is better for levelling out the surface of the panel

than the mallet.

Dolly block This should not be fuller in shape

than the actual panel being repaired. Also it should

be of a rather light weight so that it forms a

relatively weak backing for the malleting, thus

reducing the stretching during the levelling operation.

A grid dolly has a serrated face to reduce the

possibility of stretching in the panel.

Damp cloth This is helpful in checking the

spread of heat, thus reducing the risk of panel

distortion.

Hot shrinking process

Arrange your tools so that they are within easy

reach, as it is necessary to change quickly from

one tool to another when performing a shrinking

operation (Figure 13.42). Locate the highest point

in the stretched section of the panel with which

you are working. Light the torch and heat the spot

to approximately 10 mm diameter in the centre

of the high spot or stretched area to a cherry red,

using a circular motion when heating the spot.

Be careful not to burn through the metal by overheating.

As soon as the spot is cherry red place

the shrinking dolly (grid dolly), which is usually

fairly flat, under the spot and strike several sharp

blows with a mallet around the heat spot; this

will force the surplus stretched metal into the heat

spot. Then the blows are directed on the centre

of the heat spot, which pushes the metal down

Figure 13.42The technique of hot shrinking

366Repair of Vehicle Bodies

while it is still red hot. At all times the dolly block

must be held loosely against the underside of the

panel, exactly centred under the heat spot. After

four or five mallet blows, the last being on the

centre of the heat spot, the heat spot will turn

black, and should be quenched immediately with a

water filled sponge. Repeat this operation taking

the next highest spot in the stretched section of the

panel, until the bulge or fullness is finally shrunk

down below the level of the surrounding surface.

It can then be brought up to its correct level

by planishing with a hammer and dolly and then

finishing with a panel file.

The following points should be noted:

1 Never quench a red hot heat spot; wait until the

metal has turned black.

2 Never heat an area greater than that which can

be hammered with mallet and dolly.

3 Never use anything but an oxy-acetylene welding

torch for heating a stretched section.

4 Never attempt to shrink a panel until it has

been roughed out.

5 Always hammer the stretched section outwards

before applying heat.

6 If the stretched part of the surface is small,

make a smaller heat spot.

7 It is possible to shrink metal without quenching

each spot. However, the shrinking operation is

much faster when each spot is quenched with

water, as fewer heat spots are required if the

heat expansion is drawn out by quenching than

by additional spots.

8 In some panels it is possible to use a spoon

for the backing tool for the hammering operation,

especially on door panels or over inner

constructions.

9 When performing a shrinking operation, take

care to avoid overshrinking the panel. This will

cause the metal to warp and buckle both in and

out of the stretched area, owing to overheating

of the stretched section. If this does arise, heat

a small spot in the area where the panel is

buckling, apply a dolly block or spoon with

enough pressure to hold the buckling section

up, then allow the metal to cool. Do not use the

mallet or water in this case. In extreme cases

like this it may be necessary to repeat this

operation in several different places in the

buckled section.

Shrinking aluminium

Sheet aluminium can, with the application of heat,

be subjected to shrinking. The process is similar to

that used for sheet steel except that the work

should be carried out faster than when shrinking

steel. This speed is essential because aluminium is

a good heat conductor and the spread of heat must

be prevented from distorting adjoining panels.

Unlike steel, aluminium does not change colour

when heated and, because of the melting temperature

differences, great care must be taken not to

melt holes in the panel while it is being heated.

Shrinking equipment

Shrinking can also be carried out using shrinking

equipment, which can be either a specialized piece

of equipment or an attachment to a MIG welder.

Basically the equipment consists of a power source

to which two cables are attached: one is to the

shrinking torch fitted with a tungsten or carbon

electrode (which is interchangeable), and the other

is to the earth return clamp for completing the

circuit.

To use the equipment, first position the earth

clamp, making sure of a good connection. Switch

on the power source, which in some cases has a

built-in timer and then apply the tip of the electrode

to the highest point on the surface of the

stretched panel to be shrunk. The resulting arc will

produce an extreme concentration of heat to the

small area on the panel surface. Heat until bright

red or to the preset time, then immediately cool

with water. This high concentration of heat in one

area together with rapid cooling is extremely effective

in reducing high spots by heat shrinking in

vehicle repair.

13.10 Cold shrinking

Cold shrinking is another method of repairing

stretched sections on vehicle body panels. In this

case a hammer and special dolly block are used in

conjunction with one another. This dolly block is

a shrinking dolly and is shaped like the toe dolly

but has a groove running along the full length of

its top face. The dolly block is placed under the

high spot formed by the stretched metal of the

damaged panel and the panel is hammered down

into the groove with hammer or mallet to form

Craft techniques and minor accident damage 367

a valley, care being taken not to make the valley

any longer or deeper than is necessary to draw

the stretched metal back to its original contour.

The valley formed is then filled using body solder,

which is filed to give a good finish. From the

reverse side of the panel it will be seen that a rib

has been formed, which will in effect give strength

to the damaged area should this be required. This

method is also most useful in reducing welds: the

weld on the face of the panel is depressed into

the channel of the dolly to form a valley below the

line of the face of the panel, and the valley is then

filled by the body soldering method.

13.11 Body soldering

Body soldering (or loading, filling or wiping, as

it is sometimes termed) has become a general

practice where owing to the structural design of

the all-steel body, the use of normal panel beating

methods of repair using hand tools is not possible.

It is ideally used to hide a lapped joint in the

construction of a body shell, and is very useful to

the panel beater repairing a windscreen or door

pillars, or when a dent or crease is backed by a

bracket or is double skinned, which prevents the

placing of a dolly behind the panel and beating

the section out. Body solder was first used in the

motor trade in the mid 1930s and was introduced

into the repair industry by the car manufacturers

who were using it on the mass production of car

bodies. It was during this period that the all-steel

body was developed, along with rapid advances

in streamlining. To finish a panel in the area of

the production welds, which in most cases were

lapped, spot-welded joints, body solder was used

and found most useful. This method has been continued,

and today solder filling plays a major part

in the finish and repair of the modern car body.