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Heat Shrinking Metal

Discussion in 'The Hokey Ass Message Board' started by e-tek, May 6, 2013.

  1. e-tek
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    e-tek Member

    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: May 7, 2013
  2. rld14
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    rld14 Member

    Great video, thanks for posting that! Heck, even I can understand it ;)
  3. prewarcars4me
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    prewarcars4me Member

    Thanks for posting that.
  4. chopolds
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    chopolds Member
    1. Kustom Painters

    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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  5. ago
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    ago Member

    Good video,

    I get my better half to hold the torch.


    Ago
  6. Texas Webb
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    Texas Webb
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    Your son did a good job.
  7. larry k
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    larry k
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    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:eek: ,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..:confused:
  8. RMR&C
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    RMR&C Member

    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!
  9. falconsprint63
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    falconsprint63 Member

    I've had some good success using the little spot heat fixture on the stud gun to do this too. nice tech post.
  10. e-tek
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    e-tek Member

    I've seen that method used too - looks like a good way to keep the heat where you want it. Thanks.
  11. George G
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    George G Member

    You don't quench it with water?
  12. Babyearl
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    Babyearl
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    That works for me,, finally something I can understand,, Thanks
  13. e-tek
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    e-tek Member

    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.

    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: May 7, 2013
  14. castirondude
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    castirondude
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    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"

    Sent from my Verizon 4G LTE smartphone
  15. JYPSEA
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    JYPSEA Member

    Good article. Your not left handed are you? Good job anyway. I can use that this afternoon. Thanks
  16. oldrelics
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    oldrelics Member

    Stud gun with shrinking tip, no water, very controllable. Trust me, I fix ALOT of my mistakes:eek: this way
  17. lht
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    lht Member

    great you teachin your son, put them to work keeps em out of trouble
  18. bct
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    bct Member

    compressed air cools it nice .....enough to feel the results right after
  19. bct
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    bct Member

    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.
  20. OLDTGUY
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    OLDTGUY Member

    Thanks for the video, your son did a great job.
    JJ
  21. falconsprint63
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    falconsprint63 Member

    so it's called a shrinking tip huh--imagine that....

  22. oj
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    oj Member

    Yup, they sure do work good. You can actually see it happen.
  23. e-tek
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    e-tek Member

    No I'm not left handed, but it was the left quarter, so you gotta make do!
  24. choprods
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    choprods Alliance Vendor

    Why is it here that when someone is good enough to share bits of Technical info we all need, he gets the reaction of "experts" who contradict it?
    We need to be just a little more appreciative and let the poster get his point across to the masses who are still learning,without critiqing his methods first.
    This is exactly why many long time ,skilled Hamb guys refuse to post tech here.
  25. paul hebert
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    paul hebert Member

    Great! Thanks
  26. blowby
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    blowby
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    Shrinking tip? What's that?
  27. oldrelics
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    oldrelics Member


    Its a blunt copper tip thats inserted into the end of a stud gun(instead of the studs) that when you press against the sheetmetal and you pull trigger you get a 1/4" heated spot in the metal...
  28. blowby
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    blowby
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    Thanks, did you buy one or make it?
  29. oldrelics
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    oldrelics Member


    You buy them for H&S Autoshot unispotter. I'm not sure they fit all brands.
  30. Dave Zapatka
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    Dave Zapatka Member

    Right On ! I'm on that page ! Makes a fellow want to go and beat baby seal !

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