174 Section 2 Nonstructural Repairs
Copyright Goodheart-Willcox Co., Inc.
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 arrow-
heads. 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 undamaged but held out of position by
the arrowhead buckles. To correct the damage, pinch
the arrowheads as shown in Figure 9-1. 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
approximate 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 immo-
bile flat panel. Overall panel length is reduced. In an
immobile 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 combi-
nation 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.
Reading the Estimate
The first step in any repair is to read the estimate
or repair order. Study each line of the estimate. Find
out which parts will be repaired and which parts must
be replaced.
Goodheart-Willcox Publisher
Figure 9-1. Pinching the arrowheads on this soda can
will release the buckles.