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Old 11-09-2006, 02:09 PM
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Can I use heat to relax metal after stretching?

I've seen this done on custom headers, once they were welded up, they turn the heat on the tubing, to relax it, and prevent flange warping.

Can I do this with sheet metal. I am working on making some curved panels. I am using a hydraulic jack to generally get the shape I want, but was thinking of using to heat on the panel. Either using it before stretching, during or after to "help" the metal hold it's new shape.

I think a flat piece of sheet metal, is going to not accept as much heating as headers. That being said I wonder if a lower amount of heat would even do anything?

Maybe a combination of hammering and dollying and heat? I am not 100% sure about hammering and dollying, as it is in a more specific location.

Suggestions? opinions?

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Old 11-09-2006, 02:20 PM
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Heating and dabbing a wet rag on it should shrink it......................
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Old 11-09-2006, 02:43 PM
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I'm sure a lot of Body guys will answer this one, but I will use a Porta Power and get the Metal where I want it, and then use a Body Hammer and tap the surrounding Metal taking the stress out that way-
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Old 11-09-2006, 02:52 PM
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Quote:
Originally Posted by 35WINDOW
I'm sure a lot of Body guys will answer this one, but I will use a Porta Power and get the Metal where I want it, and then use a Body Hammer and tap the surrounding Metal taking the stress out that way-
Thanks

Would you use a dolly to back the areas where you are tapping? And do you tap out, towards the stretch or does it matter?
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Old 11-09-2006, 03:31 PM
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In my experience, it does not matter-I will tap (half hard smack) the surrounding Metal-don't use a Dolly-
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Old 11-11-2006, 06:48 AM
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What are you making? There is a big difference between pushing the metal with a jack and shaping the metal in 3 dimensions. You can heat the metal to make it easier to bend (rarely necessary with sheet metal), or to shrink it.

John
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Old 11-13-2006, 07:09 AM
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I think I know where Ripped is going with this.

Heat releaving a built up header is a different ball game then the sheet metal.

The header is a built up structure. You have this fairly solid plate that weld some tubes to. Every weld on the tubes is a shrink point causing stress on this plate. So after you do the welding, you then take the tubes up in tempurature, but leave the plate realitivly cool and flat, so that they normalize the stresses and not pull on the plate.

You do not have the same interaction on sheet metal. Heating sheet metal is sometimes done to anneal the metal when it is work hardened. This process will also cause the sheet metal to shrink in the area you heat it and could cause other problems. It is a complex decision that take experience to understand and use.
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Old 11-16-2006, 12:14 AM
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Thanks for the tips and advice. The project this time involves minitubs and reshaping some of the inner wheelhouses etc.

I can certainly see what you say is happening. I hammer and stretch the metal. As I weld the tendency is for it to want to shrink back. I find the best way for me so far is to hammer the metal. Hold it in place with self tapping screws strategically placed. As I weld and the metal shrinks back, I am quickly grabbing the hammer and tapping it back while the weld is hot.

It's going not too bad. I am getting better with my technique. The biggest problem so far; sandblasted rusty sections are thin, so occasionally I blow through, and have to turn down the heat and go back.

I am experiencing better results with the wire speed on the slower side. Even when I have a low heat setting, I can take my time and weld.
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Old 11-16-2006, 12:01 PM
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Let me refine what you have said.

The metal does not tend to 'shrink back as it cools'.

When it gets hot it pushes out and the pressure from the cooler metal is pushing in. This causes the bulge you commonly see when heating metal. When the metal gets to its plastic state the pressure from the surrounding metal causes the plastic area to collapse in on itself. When you take the heat away the area cools below the plastic point long before the pressure goes away. This traps the metal that has collapsed in on itself.


The metal also shrinks as it cools, but the action of taking the metal temp up beyond the plastic point is what causes the metal to take on a permenant shrink. This is different than the normal expansion and contraction from heating cooling when the temp is kept less than the plastic point.

For a point of reference. The plastic point starts where you have enough heat to turn the metal a little blue.

You really want to cut beyond the rusty parts. But when that is not possible you need to think heat control. Copper can be used to back the weld area and control the heat. For filling holes you can also use a regular welding rod with the MIG. For heat control you can start the arc on the rod and feed the rod into the arc. This also works for filling smaller holes mid panel.
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Old 11-16-2006, 01:46 PM
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The metal shrinks after welding. This is almost all you need to know about it to correct the problem.
From my perspective, terms like "plastic limit", "work-hardening", and sometimes even "annealing" are great for scientists, but not very helpful for learning to work with metal unless you like the theory of it. For practical purposes it helps to keep it simple. Using terms like "surface area" as in stretched or shrunken parts of a panel, "form" or "arrangment" of the panel as in how it is rolled, bent, or twisted are more useful to the understanding of what is happening.

You do not need to quickly hammer welds while they are still hot. You can, but it is not necessary. They will stretch cold.

(The metal shrinks at temperatures that are slightly lower than blue heat as well).

Again, just my perspective.

John
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Old 11-17-2006, 12:36 AM
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Thanks again for the expert advice. Maybe I should clarify, there are no rusted metal sections on the car. It was all sandblasted, with large sections being cut away. Some metal which had a large amount of surface rust, is thinner in areas, so can blow through at times.
I am getting a little better at turning down the heat and controling the weld penetration.

Anyway there are some pics in my photo album/ gallery of the one side of the wheelhouses 1/4 panel I have been working on.

I have a bit more to do before switching to the other side.
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