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Q. My son bought me a shrinker-stretcher for Father’s Day. I inserted a piece of 22-gauge steel in the shrinker jaws and gave it a few cranks. The steel is 2 inches wide and 10 inches long. The length and width both tipped down toward the floor. All edges went the same direction: DOWN. I have most of your DVDs, and have never seen that happen to you. How about a quick primer?
Via the Internet
A. I’m not sure what you’d expected your metal strip to do after shrinking an edge, but it sounds to me like the shrinker is doing exactly what it’s designed to do! Let me give a brief description of how shrinking and stretching affect the shape of metal. Let’s start with a piece of metal with a domed shape – imagine the top of a 1949 GMC front fender, for example. Any portion of the metal that is stretched will go up. One of the most common ways to stretch metal is by hammering on dolly.
Here’s a mechanical shrinker being used to curve a piece of sheetmetal angle.
If we put a rounded dolly under the fender, and strike with a hammer in such a way that the fender metal is compressed against the dolly, the hammering makes the metal slightly thinner, and since the volume of the metal can’t really change much, the hammered metal “squeezes out to the sides” (somewhat like a penny that is run over by a train). The metal reacts to this on-dolly hammering by doming up more, creating a “bulge” in the area that was hammered. In simple terms, we can say that stretching makes the metal go up (and makes it thinner).
Shrinking is the opposite. There are two common ways that the top of the GMC fender can be shrunk. One is by using the heat-shrinking process, which has the ability to shrink the metal a lot, and the other is by hammering off dolly, which normally can only shrink the metal a little. If we shrink a spot near the top of the fender, we will lessen the crown of the fender, and if we shrink it more, we can actually make a flat spot. For another example, if you take a piece of metal with a gradual dome (imagine a Moon disk here), and shrink the edge with a mechanical shrinker, it makes the edge go down.
So, we might say that shrinking makes the metal go down (and this process makes the metal thicker)! Is this making sense so far? Well, now it’s time for the caveats.
I’ve been talking about “up” and “down,” with the reference of looking at the outside of a domed shape. If we take the Moon disk, and turn it so the concave side is up, then the basic rules I listed are reversed. So, a more universal description of the effects of shrinking and stretching is probably in order.
It’s more accurate to say that if we stretch any part of a domed shape, the stretching increases the doming in that area. In a similar manner, the effect of shrinking is the opposite – it will reduce the amount of doming.
Now that we have a description that seems to work well for domed panels, we can move on to the special case of flat metal. If you stretch or shrink a flat piece of metal, it often doesn’t know which way to go! Saying this another way, it is just as likely to go up as down, unless there is something that favors it moving in a particular direction.
I believe in the case of the flat strip of metal you experimented with, 2 inches wide and 10 inches long, gravity was pulling on the metal enough to draw the edges down, and once the shrinker started to work, the metal just kept going in that direction. If you start with an identical piece of metal, and put the tiniest amount of upward pressure on the ends, you’ll get the opposite effect! Give this a whirl, and contact me again if the metal is still not behaving the way you want it to!