Indirect hammering (off the
Dolly block)
Indirect hammering is another technique which
uses hammer and dolly to level a panel surface.
A low area can be raised by hammering round
the outer edges in such a manner that the rebound
action of the block tends to push the low area
upwards to its original contour. This in fact is
achieved by the sequence of hammering just off, or
at the side of, the dolly block; hence the name of
indirect hammering (Figure 13.36). This technique
is used in conjunction with direct hammering or
planishing to achieve a final finish on the panel
surface. Metal that has not been excessively hammered,
displaced or stretched will have a tendency
Figure 13.34Using a high-crowned dolly
Figure 13.35Using a low-crowned dolly
Figure 13.36The technique of indirect hammering
Direct hammering requires skill in directing the
hammer blows and close observation of what you
are doing so as not to hit the metal too hard,
thereby displacing it. Perfect coordination between
your two hands is necessary to enable you to move
the dolly around under the damaged area and still
continue to hit squarely over it with the hammer.
Start hammering by using light blows; these will
not do the job, but will show you whether or not
you are hitting squarely over the dolly. Do not
forget to let the dolly just lie in your hand and to
grip the hammer loosely. A true ring will be heard
if you are directly over the dolly otherwise the
sound will be dull. Increase the force of the blow
gradually until you have found just the right force
Craft techniques and minor accident damage 361
to return to its original contour of its own accord.
This is due to the internal strain imparted to the
metal by the forming dies in manufacture. If the
metal is prevented from springing back by other
strains imparted to it by additional bends or creases
that have been formed by accident, the metal can
then be restored to its normal contour by relieving
whatever new strain is holding it out of position.
In direct hammering a dolly block having the
correct contour to match the original shape of the
panel is held under the low spot, and a series of
light blows are aimed around the outer edge of this
low spot, and slightly off the dolly block. The light
blows will not displace the surrounding undamaged
area, but the force of the downward blow will
be transferred to the dolly block. As a result of
receiving the hammer blows indirectly, the dolly
will rebound and the hand holding the block will
automatically bring it back in place so that it
imparts a light push upwards on the area. The
centre of the damaged area will slowly rise until
the original contour is restored.
Spring hammering
This is another technique of using hand tools to
smooth and level a panel surface. In this case only
a hammer is used, and it is not supported with
a dolly block. The technique is used to reduce
high spots which sometimes form as a panel is
planished. In some cases these high spots can be
reduced by careful, controlled hammering which
spreads the force of the blow over the area of the
metal, thus reducing the high spot. When a crown
or curved surface is formed in a metal panel, it
becomes strong in that it resists any change to
its shape. The strength of this crowned surface
can be used to support the surface being hammered
without the use of a dolly. This type of
hammering is called spring hammering, and can
be used to correct high spots on metal panel
surfaces (Figure 13.37). To take advantage of a
great amount of the natural support provided by
the crown of the metal, the force of the hammer
blow is spread over a larger area. Once the metal is
back to its original contour, additional hammering
will cause the surface to sink below its original
contour line, and it may not be possible to raise
it readily. Always start with light blows, and as
the repair nears completion, inspect the work after
Figure 13.37The technique of spring hammering
(Sykes-Pickavant Ltd )
362Repair of Vehicle Bodies
each blow. This will reduce the possibility of sinking
the surface too low. Keep the surface of the
hammer face clean and highly polished. Any marks
on the surface of the hammer will be transferred to
the surface of the metal and create additional work.
Removing low spots
Low spots can be removed in several ways, the two
most common being the use of a pick hammer or
a dolly block. When using the dolly block, start
by holding it so that it can strike the underneath of
the low spot on the panel with one of its rounded
corners. It must be noted that if the operator does
not hit exactly in the centre of the low spot, he will
raise metal in some unwanted place. Accuracy is
therefore essential, and can be achieved by holding
a finger in the low spot and lightly tapping the
underside of the panel with the rounded corner of
the dolly until you feel that it is exactly beneath
your finger, then strike a sharp blow and raise the
metal at this point. After each low spot has been
raised in this manner, these points can be filed
to check that they are level with the surrounding
panel surface.
The second common method of raising low
spots is by pick hammering. Bringing up low spots
with a pick hammer is more difficult than by the
use of a rounded corner of a dolly block. With a
pick hammer more accurate placing of the blow is
required. Likewise greater control over the force of
the blow is necessary. Start using the pick hammer
in a manner similar to the dolly block. Hold the
end of your finger in the low spot. Tap the under
surface of the panel until the pick is directly below
your finger. Then strike a light blow from beneath
the panel, of sufficient strength to form a pimple in
the low spot (Figure 13.38). Care must be taken to
avoid overstretching the metal by using too hard a
blow. These pimples represent stretched metal, but
in being formed also raise the surrounding metal.
When all the low spots have been raised with a
pick hammer in this manner, the pimples can then
be lightly hammered level by direct hammering,
and finished by filing.
13.7 Filing
Filing is one of the most important aspects of
finishing a body panel. It is carried out using an
adjustable file holder, fitted with flexible blades
which can be adjusted concave or convex to
suit most contours on the average vehicle body.
Initially the>
prior to sanding and locating high and low spots.
With the introduction of body solder and later
metal and plastic fillers, filing took on an even
greater importance in the finishing of repairs on
body panels. Filing indicates any irregularities in
the repaired surface of a panel, and is carried out
as the panel is planished. First of all fasten the
correct file blade to the file holder with the cutting
edges of the teeth facing away from the handle
or operator. Adjust the contour of the file holder so
that it is almost, but not quite, matching the contour
of the surface on which you intend to work.
One hand is used to hold the file handle, while
the other grasps the knob at the opposite end. The
file should be applied with long, straight strokes,
pushing it away from you along the length of
the panel (Figure 13.39). Short, jabbing strokes
should never be used, as these will only scratch
the panel and will not indicate low spots. If the
file digs in, too much pressure is being applied
and hence a need for reduction is essential. At the
end of the first stroke, raise the file and, without
dragging it over the metal, bring it back to the
starting position and make a second stroke. Repeat
this procedure until the area has been covered,
making the file marks parallel to one another. This
Figure 13.38Pick hammering used to remove
low spots
Craft techniques and minor accident damage 363
is termed line filing, and indicates the levelness of
the panel in the direction in which it has been
filed. At this point both the high and low areas
will show up. The high spots can be corrected
by spring hammering and the low spots by direct
hammering, pick hammering, or in some cases by
using the corner of a dolly block. Line filing indicates
curvature in one direction only, and as most
panels are double curved the panel surface must
be cross filed to give an accurate contour check.
Cross filing means a change in the direction of
the file strokes so that the file is moved at an
angle between 45° and 90° over the previous file
strokes, thus checking the accuracy of the curvature
in that direction.
After filing, and prior to refinishing the panel, the
damaged area is sanded using a fine-grit sanding
disc which leaves a smooth, even surface ideally
suited for painting.
13.8 Sanding or disc grinding
Use of grinder
Several general rules govern the use of the disc
grinder. If these are observed they will enable the
operator to become proficient very quickly in the
use of the grinder. The rules are considered good
shop practice and are directed towards the safety
of the operator. In the first instance, if the device
is electrically operated see that it is properly
connected and earthed. Shop floors are usually
of cement; they are generally moist and, therefore,
relatively good conductors of electricity. If the
grinder is not properly earthed it is possible to
receive a fatal electric shock when the machine is
in use. Always wear goggles to protect the eyes
from flying particles of metal and from small
abrasive particles that come loose from the grinding
disc (Figure 13.40). Always replace worn
discs as soon as a tear is noticed; torn discs may
catch in the work and twist the grinder out of
your hands.
Figure 13.39The technique of filing
(Sykes-Pickavant Ltd )
Figure 13.40Ear and eye protection when sanding
(Motor Insurance Repair Research Centre)
Always maintain a balanced position when using
the grinder. This position not only permits perfect
control over the machine at all times, but it
will also produce less fatigue over longer periods.
When operating the grinder, hold it as flat as
possible without permitting the centre connecting
bolt to come in contact with the surface being
ground. Hold the grinder so that only 40–65 mm
of the outer edge of the disc is in contact with the
surface being ground. The grinder must never be
tilted so that only the edge of the disc contacts the
364Repair of Vehicle Bodies
surface. Failure to observe this will cause gouges
or deep scratches in the metal which will be hard
to remove. Move the grinder from left to right,
overlapping the previous stroke with each new
stroke. Make the cutting lines as clean and straight
as possible. Move the grinder in the same manner
whether using it for the removal of paint, rough or
finish grinding. For most grinding operations, finish
grind in the longest direction possible on the
repaired surface.
Sanding discs
The coated abrasive disc is the part of the sander
that does the actual sanding, and selection of the
right grit and coating for each job is important.
There are five different minerals which are commonly
used for manufacturing abrasives. These are
garnet, flint, emery, aluminium oxide and silicon
carbide. Aluminium oxide is the most important
of these. Because of its toughness and durability
it is used in the motor body repair trade, where it
is chiefly used on metal. The abrasive is fixed on
a backing which is either of paper, cloth or a combination
of the two. For dry grinding or sanding,
high-quality hide glues are used for anchoring the
abrasive grains to the backing. For wet sanding,
resins are used as the bonding agent.
Coated abrasives fall into two additional classifications
based on how widely the minerals are
spaced. If the minerals are close together the
abrasive is close coated. When they are widely
spaced it is open coated. In close-coated abrasive
discs, the abrasive is applied in such quantity as to
entirely cover the backing. In open-coated abrasive
discs, the backing is from 50 to 70 per cent covered.
This leaves wider spaces between the abrasive
grains. The open coating provides increased
pliability and good cutting speeds under light
pressures. Open-coated discs are used where the
surface being ground is of such a nature that
closely spaced abrasive materials would rapidly fill
up and become useless. When grinding a body
panel, always use an open-coated disc up to the
time that the area being sanded is completely free
of paint. Then use a close-coated abrasive to grind
the metal to the point where the surface needs no
further correction. The final finish is accomplished
with a fine grit to get the surface smooth enough
for re-finishing by the painting department.
13.9 Hot shrinking
One of the most important skills in the repair of
damaged panels is that of hot shrinking. It is
important because in most cases of collision
causing the damage of body panels, stretching of
the metal takes place. The actual process is
carried out by gathering the stretched metal into
a common centre or area and then by heating
this section. The panel steel is then at its best
condition to be hammered down, thus reducing
the surface area and so making shrinking possible
(Figure 13.41).
Figure 13.41Hot shrinking panel (Sykes-Pickavant )
The oxy-acetylene flame is used as a means of
heating the panel, and care must be taken to reduce
the spread of heat to the surrounding area of the
panel. This can be done by cooling the panel with
water after every shrink. The advisable welding
nozzle size when using for a 1.00 mm panel is a
number 2 nozzle.
Craft techniques and minor accident damage 365