How does a metal shrinker/stretcher work?

Factual Questions
1

Summary: How do these things shorten a length of metal without creating ripples/wrinkles?

I’ve seen these things used to bend angle iron in a radius. The info I’ve been able to find explains it as a pair of clamps that move towards or away from each other (depending on the tools setting). I can picture this working fine for stretching, provided it’s done slow enough (or the material is strong enough) so the metal doesn’t just tear. The metal gets thinner but longer. It’s the shrinking I can’t grasp. I’d think the metal would just fold between the two gripping points.

2

It seems like you would have to have almost as much force holding it flat as you did compressing the length, the part holding it flat would have to have enough give to allow the metal to compress and thicken.

3

I hadn’t heard of these machines, but I looked up a video to see if I could figure it out. First, it obviously doesn’t shrink much on each cycle–maybe only 1/64" or so. Second, the main grip points must be somewhat far apart, because the bulging is fairly smooth–there’s no significant ripple along the width. Third, it appears that the thickness of the piece stays constant, and all the extra volume goes into making it wider.

So I suspect that there are four L-shaped grippers arranged symmetrically, a bit like this:



+----------------+ +----------------+
|                | |                |
|        ________+ +_______         |
+--------                  ---------+        
+--------________   _______---------+
|                + +                |
|                | |                |
+----------------+ +----------------+

The distance between the lower and the upper part of each gripper should be very small; perhaps 1/100" or less. It needs to grip tightly on the outside ends (to hold the metal), while allowing the inner part to slide (as the metal stretches) without buckling. Only in the vertical gap would there be any ability to buckle in that direction, but it is so small (due to the short travel) that it’s not a problem in practice.

4

Part of the strength of steel is in the nature of the crystals … the insides and the skin of them…

You are saying that each crystal can take only so much reshaping… this is partly due to the skin of the crystal imposing a brittleness.

There may be crystal reset processes … erase the crystal boundaries and form them back into large blobs… that can be squashed again…
Besides, angle is somewhat pure steel with little carbon to form carbide… its the carbide of the cast iron that makes it brittle… but with little carbon its quite weak… its not for working a structural (building I beam) into shape… no structural connectors will be made with this shrinking process. Why ? the structural part must be made with the crystals all squashed flat by rolling. You can see this layers when it rusts away…

5

Thanks, that does help a bit. And the side effects seen in that video makes me think the jaws aren’t moving completely parallel to the piece, which seems to make a bit more sense.