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BODY REPAIR AND METAL
FINISHING
One characteristic of sheet metal is its elasticity; that
is, the ability of the metal to spring back to its
original shape after having been bent. Each type of
sheet metal has a definite "limit of elasticity" de-
pending upon its chemical composition, method of
manufacture, and heat treatment. Metal bent or
formed beyond this limit of elasticity assumes a new
permanent shape and loses most of its elasticity. It
becomes non-elastic. To restore such metal to its
original shape, it is necessary to re-work it by bump-
ing or otherwise. Similarly, if it is desired that sheet
metal retain a bend or curve within its limit of
elasticity, it is necessary to reduce the elasticity and
work the curve into the metal by hammering or
heating, or both.
The ability of sheet metal to resist bending depends
to a certain extent on its physical shape. Metal
shaped into sharp V's or ridges is more difficult to
re-work than is metal shaped in curves or U's. Such
sharp V's or ridges are said to be "locked" into the
metal. In manufacturing the elastic, flat steel sheets
into body panels, the forming dies draw and bend the
steel at strategic points to stiffen the flat steel sheet
and give it rigidity.
FIGURE 44
In Figure 44, shown above, a flat sheet is bent in a
short arc beyond the normal elastic limits of the
metal. The metal at the bend becomes stiff and holds
its new position, while the metal on either side of the
bend remains elastic.
FIGURE 45
In the above Figure Number 45, the elastic metal in the large
panels is held in position by stiff metal flanges, off-set bends,
mouldings, and short curves.
FIGURE 46
Accidents often create buckles like those shown in Figure 46.
The metal is stiffened in the short bends at the valley and the
surrounding ridges. the metal in the slopes still has most of the
elasticity that it had before the accident.
FIGURE 47
Metal made stiff and non-elastic by severe impact, Figure 47,
must be reshaped over a dolly block; and, therefore, bumping and
metal finishing is going to require more time and skill than does
the reshaping of elastic metal.
Elastic metal will nearly reshape itself once the stiff areas of the
damage are forced into their original position by the
BODY MANUAL
47
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