Repairing Aluminum Part 2
Straightening Tempered Aluminum Sheets

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Straightening Tempered Aluminum Sheets

As with a steel part, a good start is made by pushing or bumping out with the appropriately contoured tool. However, hammering should be done judiciously and with caution. The objective is to work smart and not so hard. With each blow of the hammer, spoon or slapper rings on the dolly, stretching occurs (Fig. 1). For this reason, care should be taken to work off the dolly as much as possible (Fig. 2). Bumping and lifting to a roughly correct contour will prevent excessive stretching. Use the properly contoured tool for the push and bump (Figs. 3~). Work until the springy resistance prevents metal movement. Now, find the high spot and get ready to shrink.

Since cold shrinking is almost impossible on tempered aluminum, we will discuss hot shrinking. On difficult areas, heating the area to temporarily reduce properties can mean the difference between success and failure (see heating guidelines that follow).

Hot shrinking is the "legendary" shrinking method of simply heating up and dashing water for a quick chill. This will create a cold aluminum area or a hard steel one with very little resulting shrinkage. Do not reach for the waffle-face club either, as it only limits the stretch at the high cost of "tenderizing" the panel-with some nice marks for the painter to deal with as well.

The following shrinking method is easier and simpler on aluminum than on steel. Remember, stretching is easy. Just hammer too hard, push too hard, or hammer "on dolly" too much and you'll get bags of stretched metal. Shrinking is more difficult. It's a "black art" that becomes easier with practice. To shrink, first follow these two guidelines:

Choose the right contour, size and weight hammer, spoon or slapper and dolly. Most hammers have large (11/2 in. diameter) or medium (11/4 in. diameter) faces, flat or medium crowns, and are heavy ((1/16 oz.) or medium (8-10 oz.) in weight. Two additional striking tools are the traditional slapper and spoon (Figs. 7 and 8). The spoon, driven by a hammer, spreads the force over a large area, leaves few if any marks and works well on relatively flat surfaces (Fig. 8-A). By itself, the spoon hits lightly and evenly, shrinking large gentle high spots very effectively (Fig. 8-B).

Use mechanical advantage. The trick to quick, efficient shrinking and straightening is mechanical advantage. In Figure 9, the mechanical advantage of off-dolly work is seen as the simple leverage of working one low spot against a high one.

However, in Figure 10, a greater advantage is gained with the use of a slapper. By covering two high spots at once, the slapper brings up the low more precisely. (see Slappers available from TM Technologies)
Using these two guidelines and information previously given, the next step in shrinking is to cross-hatch the high spot with the low temperature indicator crayon required, and set up the oxy-acetylene torch in the following manner:

Use a #2 tip with the pressure regulators set to 5 pounds and 5 pounds, respectively. Adjust the flame to slightly soft or reducing, and heat rapidly to the desired temperature. Working quickly, bump the high area down with the hammer while supporting the neighboring low area with the dolly (Fig. 2). Work the metal until the area temperature falls to normal, because the metal continues to shrink until it is cool to the touch. Having the high spot is very necessary as it offers the mechanical advantage particularly needed for the shrink.

Note: I have routinely reduced bulges higher than one full inch in 5052 H34, 6061 T6, and 3003 H14 by this method.

This warm-and-bump system may be used again and again until the proper contour is achieved. A wooden dolly block may be employed in more critical situations where a greater downward driving force is needed for a very specific change in shape. The wood absorbs hammer shock rather than reflecting it and so better avoids the stretch. The woods of choice are soft, not hard, and the side grain is used, rather than the end grain.

Over the past 50 years, many roughfaced tools have been sold as shrinking hammers, but none do any more than limit the stretch to some degree at the cost of beating up the work. The two magic tools for shrinking are practice and careful observation.

The more difficult shrinks to pull will be those of subtle definition (low mechanical advantage) and thicker, less workable alloys. There is hope, however, even if welding is necessary. Persistence with careful heating and quick hammer-mg will succeed where there is sufficient mechanical advantage. When the area's high spots have been shrunk down and there is still considerable roughness remaining, the next step is planishing.

Planishing is the traditional term used for the process of smoothing a rough panel to a finished condition simply by hammering (or rolling). For centuries, metalsmiths the world over have worked sheets of gold, silver, pewter, brass, bronze, iron, and steel to polished condition (with very little filing or sanding) by planishing.

Note: Metal finishing, also traditional, refers to the process of filing and sanding the work. In the context of autobody work, metal finishing would bring the planished piece to a degree of finish appropriate for painting, plating or polishing.

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