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Old 05-06-2013, 11:39 PM   #1
e-tek
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Default Heat Shrinking Metal

A lot of times - when you are welding-in patches, or repairing damaged panels, you'll often have to deal with a stretched area in the metal that needs to brought back in line with the rest of the panel. Heat shrinking is a method of putting heat into a high spot, then using the hammer and dolly to work the area, thus "shrinking" that area back down. It's one of those "tricks of the trade" that is rarely talked about, but is critical to being able to get the panel straight enough to finish properly.

The attached video may help you understand the process better. It was taken by my 8 year old son, so I'll apologize in advance for the quality, but we had fun doing it. The key to shrinking is to use a neutral flame, heat a dime-sized spot to a dull red, then fairly quickly, with a dolly in behind, tapping the area just around and on the heat-point. This hammering, along with the cooling action, will pull the metal molecules tighter together, thus shrinking the metal.

When I was first taught this method - in the 70's - we used a wet rag to "quench" the area. This method has now fallen out of favor, due to the quenching causing the metal to become brittle and work-hardened. The heating, then hammering, does everything you need it to, so don't bother quenching it with water.

The other key point I'd make is to assess after your second or third shrink. Like anything, too much of a good thing can go bad. Too much shrinking will either over-flatten the area, or cause it to bow inward, when what you want is to just remove a bulge, leaving a smooth transition to the surrounding areas.

So take a peak at the vid and if you have any questions, fire away and I - or someone else - will help clarify the process.

Heat shrinking proud metal

Last edited by e-tek; 05-07-2013 at 10:23 PM.
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Old 05-06-2013, 11:54 PM   #2
rld14
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Default Re: Heat Shrinking Metal

Great video, thanks for posting that! Heck, even I can understand it
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Old 05-07-2013, 12:40 AM   #3
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Default Re: Heat Shrinking Metal

Thanks for posting that.
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Old 05-07-2013, 05:49 AM   #4
chopolds
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Default Re: Heat Shrinking Metal

Good idea, but the theory is wrong.
When you weld metal, you shrink it....there is no difference in apply heat to weld a panel, as it is to apply heat to shrink a spot.
When you weld a seam, you need to STRETCH it to get it back in alignment. You can work the weld hot, to flatten the seam, but as the panel cools, you begin to stretch it, if working hammer on dolly.
You can also do this cold, if you like, but you won't flatten the seam as easily, but it can be done. I prefer doing it cold, as you usually have more control, as it happens more slowly.
Good idea, and good work, but if you understand the theory better, you would have more options, as in cold stretching. This doesn't work as well with MIG welding, but it is fine with gas or TIG.
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Old 05-07-2013, 07:55 AM   #5
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Default Re: Heat Shrinking Metal

Good video,

I get my better half to hold the torch.


Ago
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Old 05-07-2013, 08:02 AM   #6
Texas Webb
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Default Re: Heat Shrinking Metal

Your son did a good job.
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Old 05-07-2013, 08:58 AM   #7
larry k
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Default Re: Heat Shrinking Metal

we all know that when you put heat into metal it gets bigger,and when it cools it gets smaller,well it gets 5 to 7 times smaller ,and it
also shrinks the same amount if it is cooled fast or let cool slow, the hammer and dolly are used just to align the metal..
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Old 05-07-2013, 06:26 PM   #8
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Default Re: Heat Shrinking Metal

Sheet metal can get stretched by collision damage and/ or some moron pounding out a dent with a ball peen hammer then finishing on-dolly.....
You end up with a big football in the panel. Shrinking is required, either by shrinking disc or the torch method shown. I've had to do it many times.

Nice video, and good to see you are getting the kid involved!
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Old 05-07-2013, 08:34 PM   #9
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Default Re: Heat Shrinking Metal

I've had some good success using the little spot heat fixture on the stud gun to do this too. nice tech post.
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Old 05-07-2013, 10:21 PM   #10
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Default Re: Heat Shrinking Metal

Quote:
Originally Posted by falconsprint63 View Post
I've had some good success using the little spot heat fixture on the stud gun to do this too. nice tech post.
I've seen that method used too - looks like a good way to keep the heat where you want it. Thanks.
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Old 05-07-2013, 10:38 PM   #11
George G
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Default Re: Heat Shrinking Metal

You don't quench it with water?
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Old 05-07-2013, 10:42 PM   #12
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Default Re: Heat Shrinking Metal

That works for me,, finally something I can understand,, Thanks
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Old 05-07-2013, 10:47 PM   #13
e-tek
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Default Re: Heat Shrinking Metal

Quote:
Originally Posted by chopolds View Post
Good idea, but the theory is wrong.
When you weld metal, you shrink it....there is no difference in apply heat to weld a panel, as it is to apply heat to shrink a spot.
I think my first sentence was just a little confusing (I fixed it). What I meant was that when you are repairing and/or cutting/welding in patches you can end up with stretched areas that need to be shrunk. The weld itself will cause a shrink area, but sometimes the patch itself may need a shrink or the areas around it. Regardless, the tech was how to shrink metal.

Quote:
Originally Posted by George G View Post
You don't quench it with water?
We USED to quench it, but now we know that quenching causes metal to get brittle, making it hard to work. You get the same "shrinkage" without water anyway, so no need to.

My son will be thrilled to see the great feedback - cheers gents.

Last edited by e-tek; 05-07-2013 at 10:53 PM.
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Old 05-08-2013, 09:31 AM   #14
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Default

I had my driveshaft redone by a guy a long time ago and he insisted working it till the spin balancer read dead zero. Initially he would weld on washers, followed by extra little tacks. When he was really close he would hit the shaft with a torch and quench it with a wet rag, so probably a little less dramatic than pouring water on. Worked beautifully.

I really like the idea of doing little "tech articles"

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Old 05-08-2013, 10:17 AM   #15
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Default Re: Heat Shrinking Metal

Good article. Your not left handed are you? Good job anyway. I can use that this afternoon. Thanks
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Old 05-08-2013, 10:18 AM   #16
oldrelics
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Default Re: Heat Shrinking Metal

Stud gun with shrinking tip, no water, very controllable. Trust me, I fix ALOT of my mistakes this way
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Old 05-08-2013, 10:20 AM   #17
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Default Re: Heat Shrinking Metal

great you teachin your son, put them to work keeps em out of trouble
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Old 05-08-2013, 10:25 AM   #18
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Default Re: Heat Shrinking Metal

compressed air cools it nice .....enough to feel the results right after
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Old 05-08-2013, 10:27 AM   #19
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Default Re: Heat Shrinking Metal

nice vid btw.

the guy that showed me used the edge of the hammer and left "donkey tracks" .....when he hit with the air nozzle you could see the metal pucker.
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Old 05-08-2013, 02:01 PM   #20
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Default Re: Heat Shrinking Metal

Thanks for the video, your son did a great job.
JJ
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