166 Section 2 Nonstructural Repairs
Introduction
This chapter explains how to straighten damaged
sheet metal. It begins by discussing the skills needed
to perform basic straightening operations. It then
describes the basic nonstructural repair operations:
analyzing damage, planning the repair, accessing the
damage, roughing out, and finishing. These operations
are then combined in a section on repairing specific
types of nonstructural damage.
Fundamentals of Damage
Repair
In this section, an empty soda can will be used to
demonstrate the damage repair process. The side of
the can is a high-crown surface. Lateral force on a high-
crown surface will cause arrowheads. Press on the side
of the can with your thumb to make arrowheads. Look
closely at the damage. The arrowheads point away from
each other. The metal between the arrowheads is flat,
and the flattening extends out to both ends of the can.
This flattened area is displaced metal, which is undam-
aged but held out of position by the arrowhead buckles.
To correct the damage, pinch the arrowheads as shown
in Figure 8-1. The buckles at the arrowheads will be
released, and the can will pop back to its normal shape.
Next, damage the can severely by pressing one
side in so far that it touches the opposite side. Notice
how the ends of the can move toward each other as the
crown collapses. To straighten this damage, first pull the
ends away from each other. This will begin to restore the
length, and the flattened metal will begin to rise. Pinch
the buckles out as you pull on the ends of the can. The
metal may tear from metal fatigue, but the approxi-
mate shape will be restored by pulling on the ends and
pinching the buckles.
Of course, vehicle panels are not made of thin
aluminum and generally do not have a high crown like
the soda can, but you can learn about vehicle repair
from this type of demonstration. Buckles hold damage
in place. If the buckles are released, the displaced
metal will return to its proper position. Also, if a
change in length is present, length must be restored
as the buckles are released. You would not be able
to straighten the severely damaged can if you did not
restore the length as you released the buckles. Keep
these simple can demonstrations in mind as you read
the rest of this chapter.
Chapter 4, Fundamentals of Collision Damage,
explained that there are only two ways a panel can be
damaged, longitudinally and laterally. Longitudinal force
causes folds or simple hinge buckles in an immobile
flat panel. Overall panel length is reduced. In an immo-
bile panel with body lines, a longitudinal force causes
collapsed hinge buckles, again reducing overall length.
Lateral force on a flat panel first moves the ends of
the panel closer together. It then stretches the metal,
increasing the surface area.
On a high-crown panel, lateral force causes arrow-
heads. In an arrowhead, the crown is flattened and the
surface area is reduced.
All buckles are the result of grain rearrangement.
The grains in the metal are too small to be seen, but
their arrangement determines the shape of the metal.
A force greater than the yield point will rearrange the
grain set, causing buckles and changing the shape of
the panel. A stretch, or an increase in surface area,
means that the grains have been thinned, flattened, and
elongated. An upset, or a reduction in surface area,
means that the grains have been shortened, thickened,
and bunched together. A stretched area is repaired by
shrinking the surface area. An upset area is repaired by
stretching the surface area.
Nonstructural Panel Repair
Steps
The following section outlines the individual steps in
the nonstructural panel repair process. A combination of
these operations is used to complete most nonstructural
panel repairs. The entire repair sequence involves reading
the estimate, analyzing the damage, planning the repair,
accessing the damage, roughing out, and finishing.
Figure 8-1. Pinching the arrowheads on this soda can
will release the buckles.
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