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Shrinking sheet metal
Not finished yet:
Most natural surfaces of a body or fender have at least a slight curve ; therefore, if the metal is shrunk too much it will cause a flat place at that point and sometimes cause buckles to appear elsewhere in the panel. The flat place would then have to be stretched to the proper level with hammer and dolly, adding labour cost to the job.
The first heat should be applied in the approximate centre of the bulge heating a spot to a cherry red, about 1/4 to 5/16 inch in diameter, depending upon the size and height of the bulge. This spot is then hammered down while red hot, directing the blows against the red spot, using the flat face of the dolly underneath the ding hammer (picture 3). After this initial heating and hammering operation, the bulge assumes a crater-like shape, as shown.
The next step is to shrink the rim of this crater in successive stages as numbered in top picture. Selecting the highest point of this rim, the heating operation is then repeated, and hammer and dolly used as on the first spot.
On the next high spot which is No. 3 (top), the preceding operations are again repeated. It will be observed by this time that the bulged surface is being brought down perfectly and begins to assume its normal shape. Continue this procedure, taking successive numbered spots until the entire bulged area is brought down to the level of the first spots.
Due to the expansion of the metal while hot, the surface will be slightly bulged after the last spot is hammered down, and will continue to shrink until the metal is thoroughly cool. Therefore, it is very important to allow the metal to cool before the final dinging is done after which it can be determined whether the surface is at its proper level. Should there be one or more high spots remaining these should be shrunk again, using very small heat spots. After allowing it to cool again the dinging operation is repeated.
The panel surface is then ready for filing or sanding with an electric polishing disk of No. 60 or 80 grit, producing a finished surface. It is suggested here that the inside of the shrunk panel, be cleaned and painted to prevent rust or corrosion.
If a bulge is unusually high, it will be necessary to shrink the entire bulge down about halfway first, following the foregoing procedure ; then, after it cools, reduce the balance by heating the same heat spots used the first time.
Remember that the heat is always applied on the high point and the dolly always held directly under the heat spot which is hammered down while cherry red.
In case of a long narrow stretch caused by a rather sharp object rubbing across the surface, the shrinking is started at either end instead of the centre.
The examples illustrated herein were on a bulged area about 6 inches across. Larger areas may require a greater number of heat spots but at further distances apart, depending on height of the bulge.
Aluminium shrinking follows the same general procedure as steel except that care must be used to avoid burning through the metal at point of contact with the flame because this metal absorbs heat much more rapidly than steel and does not change colour under heat. It also cools off more rapidly.
Just as soon as the metal starts to blister where the flame contacts the metal, flame must be removed and hammering done quickly with a wood or fibre mallet.