METAL BODY REPAIR - OD1653 - LESSON 1/TASK 1
displaces the molecules of metal. The high pressure of the press squeezing
the metal locks the molecules in a new position.
They will resist any
subsequent force which tries to change their arrangement.
Metal has some
elasticity; that is, metal can be bent or twisted without permanent
disarrangement of the molecules.
When the force thus applied is removed,
the elasticity of the metal will cause it to resume the shape that it had
when the molecules of the metal were locked together in the dies. The body
repairman must avoid destroying this natural tendency of the metal to return
to its original shape. This is not always possible in collisions where the
normally smooth surface is badly creased.
Creases represent a portion of
the metal that has been distorted beyond its elastic limit and in which the
molecules have been displaced.
They are no longer locked together in the
same arrangement that was given to them in the forming die.
(2) Hot Shrinking.
(a) Hot shrinking is a simple process, but it requires careful timing
and proper tools. Tools required are those which are used for many other
operations in body work; i.e., a welding torch that is equipped with the
same size tip as used for welding the thickness of sheet metal being worked
on; a body hammer or mallet; shrinking dolly; a container of water, and a
sponge or rag.
(b) Arrange the tools so they are within easy reach. It is necessary
to change quickly from one tool to another when performing shrinking work.
(c) Locate the highest point in the stretched section of the panel
with which you are working. Light the torch and heat a spot the size of a
nickel in the center of the high spot (A) (figure 30 on the following page)
to a bright red. Use a circular motion when heating the spot, being careful
not to burn through the metal. As soon as the spot is bright red, place the
shrinking dolly under the spot and strike it several sharp blows with a
hammer or mallet (B).
(d) After driving
the center down, hold the dolly block loosely
against the underside of
the panel and exactly centered under the hotspot.
The hammer blow must hit
the spot accurately and with sufficient force to
push the metal down while