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This
material first appeared in the I-CAR Advantage Online, which is published and
distributed free of charge. I-CAR, the Inter-Industry Conference on Auto
Collision Repair, is a not-for-profit international training organization that
researches and develops quality technical education programs related to
collision repair. To learn more about I-CAR, and to subscribe to the free
publication, visit www.i-car.com.
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Part 1 (see
Straightening Panels Part 1 ) of this two-part series discussed the basic science and theory behind metal
shaping and how it relates to straightening damaged panels. Part two will
discuss the procedures and techniques used to apply this knowledge during the
straightening process.
Restoring Two-Dimensional Shape
Remember that the first step in repairing a
damaged exterior panel should be restoring the basic two-dimensional shape, or
basic length, height, and curvature in each direction individually. To do this,
the structure of the vehicle that the panel is attached to must first be
dimensionally correct. If the vehicle structure that the part is attached to has
moved, the part will be placed under stress that causes changes in its
two-dimensional shape. Restoring the vehicle structure to proper dimensions will
remove the stress from the panel and correct the two-dimensional shape changes
in the panel, as long as all of the induced damage is within the elastic
range.
Once the vehicle structure is verified as
dimensionally correct, any remaining changes to the two-dimensional shape of the
part can be corrected. There are a variety of techniques that can be used to do
this, depending on the location and type of the damage. Remember that when
restoring two-dimensional shape, the repair technique should not stretch the
metal. For damage that has pushed a large portion of a panel straight in, a good
choice is to use hydraulics cushioned with rubber pads or sand bags to push from
the backside. Pulling on a large area of two-dimensional shape change from one
small spot will tend to stretch the metal where the pulling force is attached.
Pushing the damage out, with the force spread over a large area, will help avoid
stretching the metal.
If the exterior skin of a panel has been hit in a
way that its length has been shortened, it may be necessary to pull the proper
length back into the panel. Try to pull from a large cross-section of the panel
and move a lot of the damage back at once. Gently stress-relieve the buckles
that have been induced into the panel while pressure is still applied to the
pull.
When using striking tools to restore the
two-dimensional shape to a panel, be sure that not to stretch the metal. A
common mistake when straightening a panel is to grab a hammer and start pounding
on the damage from the backside. Often times the damaged panel will already have
areas of stretched metal, and doing this will only add to the problem. Use
wooden- or plastic-faced striking tools when restoring two-dimensional shape. A
wooden slapper used with a small hand-held sandbag as a dolly works well for
moving the metal without stretching it. Wood “corking” tools can be shaped on a
band saw and belt sander to work in tight areas or to restore feature lines.
Corking tools are similar to blunt-ended chisels that are struck with a hammer
to force the metal into a particular shape. The end of the corking tool should
be shaped to match the contour of the area, or feature line being repaired.
Picks and pry tools similar to those used for
paintless dent repair can be used to gently lift damage out from the backside.
Use tools with rounded tips, and work slowly from the outside of the dent inward
to avoid creating dimples in the panel where the pressure is applied.
When using steel hammers and dollies to restore
two-dimensional shape, many light hammer blows should be used instead of trying
to move the metal with a few heavy blows. The off-dolly technique should also be
used. Off-dolly hammering is where the dolly is held tightly against a high spot
and the panel is struck on a low spot on the opposite side with the hammer, or
vice versa.
For restoring two-dimensional shape, the hammer
should never hit the panel where the dolly is being held. Striking the panel
where the dolly is being held is called on-dolly hammering and will cause the
metal to stretch where it is being hit.
Patterning a Panel
When restoring shape to a damaged panel, how do we
know when we have it right? There are a number of ways that a panel can be
patterned to ensure that the shape, both two-dimensional, and three-dimensional,
of the repaired panel are correct. Paper can be used to make a simple pattern of
the panel shape. The paper can be cut, tucked, and taped together where the
metal is shrunk, and cut, spread, and taped together where the metal is
stretched.
Layers of masking tape can be built up on a panel
surface and peeled off creating a mould of the panel shape. Both of these types
of patterns can be made off of the panel on the undamaged side of the vehicle
and then inverted for use on the damaged side.
The contour of the panel can be traced onto
cardboard and then cut out. Again, this is done on the undamaged side of the
vehicle. This works especially well at the panel ends, such as at the door jamb,
tail lamp pocket, or wheel opening area of a quarter panel. The cutout can be
compared to the shape of the damaged panel during repairs. Depending on the
location of the damage, cross-sections can be made in both the horizontal and
vertical planes. The problem with cardboard cross-sections is that they are
difficult to trace out in the middle of a panel. Commercial contour gauges are
one solution to this problem. Contour gauges have either wire or plastic pins
that float on a holding bar. The pins will conform to the shape of whatever they
are pressed against. The biggest drawback to contour gauges is that they are
typically only available in lengths of 305 mm (12 inches) or less. Whatever
method used, having a pattern to guide you can be an invaluable tool when
straightening damaged panels.
Restoring Three-Dimensional Shape
After the basic two-dimensional shape of the panel
has been restored, it is time to refine the repair technique, and repair any
areas that have a loss of three-dimensional shape. Collision damage typically
stretches the metal in the area of the direct impact, and will typically never
cause the metal to be shrunk on the molecular level. What this means is that
restoring three-dimensional shape loss is almost always going to involve
shrinking and not stretching. Shrinking metal is much more difficult to do than
stretching metal. This is why damaged panels with a large amount of
three-dimensional shape change will typically not be good candidates for
straightening.
Using your eye, hands, and patterns, identify
areas where the panel is “fat” and needs to be shrunk. While restoring the
three-dimensional shape, continually monitor the two-dimensional shape as it may
change slightly as the metal is shrunk or stretched and require refining before
continuing.
Shrinking Metal
There are a number of techniques that can be
effectively used to shrink metal. Metal can either be shrunk cold, or by using
heat. To cold-shrink the metal there are a couple of options. One is to use a
wood hammer on a steel dolly or a steel hammer on a wood dolly. The soft wooden
face draws the metal together onto the hard steel. This technique takes practice
to become effective, as the direction and force of the hammer blows are
important to the success of the procedure. On the edges of a panel, small tucks
can be made in the metal and the tucks hammered flat using a wood tool against a
metal backing.
Shrinking with heat is another option. Before
heating a part, check the vehicle maker’s heating recommendations for the type
of metal. The theory behind heat shrinking is that when metal is heated it
expands. The surrounding cooler metal keeps the heat and metal contained,
causing it to bulge up. As the metal cools it contracts and eliminates the
stretched metal. Quick-cooling the metal causes it to contract more than cooling
naturally and will eliminate more stretched metal.
To heat-shrink steel, heat an area about the size
of a small coin to a dull red. An oxyacetylene torch, stud welder with a
shrinking tip, or an induction heater all work well for this. The heated area
will expand and rise up. Using a dolly on the backside, lightly hammer the
raised area almost flat, either starting in the centre and circling out or
starting at the edge and circling in. The area can then be allowed to cool
naturally, or quick-cooled with compressed air or water. This process may need
to be done in a number of closely adjacent areas until all of the stretched
metal has been shrunk.
Another way to heat shrink is with a tool called a
shrinking disc. A shrinking disc is basically a metal disc that is placed on a
high-speed grinder and run over the panel in the area that is stretched. The
friction of the disc running over the metal will create heat. Since the disc
will only ride on the high spots, which is typically the stretched metal, that
is where the heat will be generated. The area is then quick-cooled, which causes
the metal to shrink. Monitor the panel and continue this process until all of
the stretched metal has been shrunk. It will typically require multiple passes
with the shrinking disc to shrink all but the smallest of stretched spots.
Final Finishing
If an area has been over-shrunk it can be
stretched slightly using the hammer-on dolly technique and light blows with the
hammer. In fact it is sometimes actually somewhat beneficial to slightly
over-shrink an area and then go back in with a hammer and dolly and plannish the
entire area. Plannishing is using many light overlapping blows with the hammer,
hammering on-dolly, to cause a very slight amount of even stretch to the panel.
This can be done to every part of the area repaired and into the surrounding
areas of the panel around the repair. Doing this helps to smooth out any minor
irregularities and remove any stresses that have built up in the panel during
the repair process, leaving a very stable and stress-free panel.
With the panel successfully metal straightened, a
skim coat of finishing filler can be used to smooth out any hammer marks and
small irregularities if necessary. Also be sure to repair any damage to backside
coatings on the panel, and ensure that proper corrosion protection has been
restored.
Conclusion
Often a panel that could be straightened is
instead replaced. Many times, replacing is just the best option when all things
are considered. However, with the proper understanding of the theory behind
metal shaping, the right tools, refined metalworking skills, and patience,
damage can often be successfully repaired. For some repairs this means less
intrusion into the vehicle and less factory welds and corrosion protection
disturbed.
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