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Q: I am a metal fabricator and welder by trade, and I have a 1948 Thames sedan delivery I have been working on for several years. I am in the process of replacing the rear fenders, which were stamped in one section at the factory. The section goes from the rear door jamb all the way back to the rear door opening and from the top opening for the roof insert down to the bottom of the fender. The driver's side fender went well, but the passenger side warped all over the place when I started patching some rusted areas. I've been hammer shrinking the metal to get it straight again.
My question is, does excessive hammer shrinking work harden the metal to the point where it's no good to work with? I have some areas 3-inches wide by 12-inches long with overlapping hammer shrinks. I have about 20-30 shrink spots in this one area.
The way I hammer shrink metal is to heat it with an oxy-acetylene torch in an area no larger than the head of the body hammer. I heat it until it's cherry red, then place my dolly directly behind the hot spot, and hit it with the hammer in a circular motion, always hitting the spot with the hammer toward the center of the hot spot. I always overlap the hot spots as I go. I have had some areas of the body metal that were bulging out from its normal position about 3/8 of an inch. When I hammer shrink it, its amazing how the metal returns to a close proximity of its original position, but my main concern is whether or not it work hardens the metal when I shrink it like this. One other note, I do not quench the metal after I shrink it with water or air, I let it cool naturally.
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A: Shrinking sheetmetal is one of the most difficult metalshaping techniques to master. If you simply heat a small section of a piece of metal, that alone will cause some shrinking. When the metal is heated, it will bulge out from the expansion of the heating - if the bulge is hammered flat while the metal is still hot, you will increase the shrinking dramatically, as long as you hammer OFF-DOLLY. Hammering on-dolly on hot metal will stretch it very rapidly! I prefer to call this process "heat shrinking", but despite our difference in terms, your process is fundamentally the same as mine.
Your technique sounds quite good! For areas that don't need the maximum amount of shrinking, you might experiment with heating smaller areas, and not getting them cherry red-any amount of heat that causes a color change is enough to accomplish shrinking. I try NOT to overlap shrinks, but that's a matter of personal preference. Many people cool the metal (either by quenching with water, or blasting with compressed air) after adding heat, or after hammering a hot spot. Although this does cause the metal to shrink a tiny bit more, I don't recommend speeding the cooling in any way, since this accelerated cooling will harden the metal. Just heating it, or heating and hammering off-dolly won't appreciably work-harden the metal, but hammering the metal at room temperature WILL harden it, significantly.
I've found that spots I've heat shrunk are slightly harder than the untouched metal adjacent to them, but I think this is caused more by the fairly rapid cooling of the spot that was heated, not by the heating or the hammering. Steel can be completely annealed (softened) by heating it to red temperature, and letting it cool slowly (over a period of several minutes). The faster it cools, the harder it gets, and that's why I don't recommend accelerating the cooling with water or compressed air.
Q: What's the difference between an auto body planishing hammer and a shrinking hammer? Wouldn't the planishing hammer do the same job?
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A: So-called "shrinking" hammers come in different styles. There is a type that has grooves going across the face in two directions-right-to-left, and up-and-down, leaving sharp pyramid peaks in the areas between the grooves. There is another type that has a pinwheel pattern of grooves on the face, and a cam that is designed to rotate the hammer face as it strikes the metal. I've tried both types, and in the tests I've done, they don't seem to really shrink metal any more than a smooth-faced hammer will, and they leave marks on the metal that I find objectionable, although other people might not mind the marks if they will be covered with plastic filler anyhow.
Any hammer, with either a smooth or textured face, will shrink metal in some instances. The higher the crown of the panel, the greater strength the panel has. On a fairly high-crown fender (like an early Chevy pickup) if there is an "up" dent caused by a rock thrown against the inside of the fender by the tire, it will most likely be possible to flatten the dent successfully by hammering off-dolly, even with a smooth faced hammer.
On a low-crown panel such as a door skin, the panel doesn't offer nearly as much support, and only the tiniest dents can successfully be hammered out. If the dent is too large, it will pop right back after it's tapped down with the hammer.
To shrink a medium or large dent on a low-crown panel, you'll probably have to use heat, as described above.