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Thread: Stitch welding

  1. #11
    Registered Member MP&C's Avatar
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    Quote Originally Posted by 5557mad View Post
    thank you i understand that now. what do you do if you cant get a dolly behind the weld. just learning to run my welder

    As an example, I have seen some outer rocker repairs posted online that install less than a full rocker as the rust damage seen appeared to be confined to one end or an isolated area. The entirety of the rocker is held in with spot welds around the perimeter that in effect minimize the weld distortion if the replacement involves the entire rocker. Perform a vertical weld with the installation of a partial patch, absent any planishing because you can't reach the back side, and your result will be a guaranteed low area as the vertical weld pulls into a valley. Any effort you thought was saved by doing a piece meal job just gave you more work in final finish. I have also seen where many people suggest you install as little of the patch as needed to repair the rust. My preference is to repair what is needed to put the weld seam in the location that makes it most accessible for planishing, locates the seam where panel features best resist distortion, and do yourself a favor and leave the panel size as secondary to these considerations. You'll have a better repair in the long run.
    Last edited by MP&C; 04-26-2017 at 07:44 PM.
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  2. #12
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    Since the heat from welding causes shrinkage, would it help to remove the heat from the HAZ quickly by cooling each tack with compressed air?

  3. #13
    Registered Member chevynut's Avatar
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    Personally I don't think cooling does much of anything. Its good to keep the panel cool so you can see where it's going to end up, but the HAZ is created during the welding, not during cool-down. Once the arc is stopped, the metal immediately starts to cool so I don't see how cooling it faster helps. I just jump around the weld seam and let it cool naturally. If you cool it too fast you could even harden the steel. At least that's my opinion.
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    Quote Originally Posted by chevynut View Post
    Personally I don't think cooling does much of anything. Its good to keep the panel cool so you can see where it's going to end up, but the HAZ is created during the welding, not during cool-down. Once the arc is stopped, the metal immediately starts to cool so I don't see how cooling it faster helps. I just jump around the weld seam and let it cool naturally. If you cool it too fast you could even harden the steel. At least that's my opinion.
    Thanks. I saw a guy on youtube doing it while welding in a floor pan, and it made me think about it.

  5. #15
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    The fact that the heat was applied is what causes shrinkage. How fast it cools doesn't matter with one exception. Cooling it faster lets you see the end result quicker.

    This applies to both welding and applying heat to shrink a high spot. Also you can straighten (or intentionally bend) a round tube by applying heat to one side.

  6. #16
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    Sorry Rick, its not often that I disagree with you
    We just recently had a conversation about mig welds being harder (more brittle) than a tig weld, and that is because of the mig does not heat the metal very much so the cold surrounding metal quenches the weld much quicker the tig weld. That is to say that when a weld is cooled too quickly, it becomes harder and more brittle.

    In that same conversation there was mention of speed welding with a tig. Its done without filler rod and it produces almost no shrinkage. Additionally, if you think about the shrinking disc, you can increase the degree of shrinkage if you use a water soaked rag to cool the metal after using the disc.

    If you could heat a complete panel to several hundred degrees evenly--as in an oven--and then let it cool slowly to room temperature, there would be no shrinkage. Paint and rust strippers do that all the time to burn the paint and undercoating off.

    So I would recommend allowing the weld to cool naturally, even without the help of compressed air.

  7. #17
    Registered Member chevynut's Avatar
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    The shrinkage is caused by the "upsetting" off the metal meaning that it compresses where it's heated. When you heat the center of a sheet of metal, the surrounding metal keeps it from expanding and that's when it "upsets". If the entire sheet is heated and cooled, as you mentioned, it won't upset anywhere, the whole piece will just expand and contract with temperature.

    You can shrink metal cold if you can upset it as you do with a shrinker/stretcher. The "upsetting" happens a lot easier at high temperatures when the metal is softer such as when it's heated during welding, shrinking disc, or a torch. Once it starts to cool, it doesn't matter how fast it cools because the "upsetting" has already happened. So no, using water with a shrinking disc shouldn't make the metal shrink any more than without it in my opinion.
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    this is all very interesting since i just got a new welder and am going to teach myself how to use it. thank you to all

  9. #19
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    Quote Originally Posted by 5557mad View Post
    this is all very interesting since i just got a new welder and am going to teach myself how to use it. thank you to all
    Buy scraps of steel in various thicknesses, and practice, practice, practice. Lots of videos on youtube to get tips from.

  10. #20
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    I've been practicing a lot the last couple months. Downsized my MIG from a Miller 200 that weighed at least that much, and capable of welding all day long to a new Miller 211 inverter, dual voltage, and only weighs 38 pounds with plenty of power for most stuff. Welds differently than my old one, and have a new learning curve. So after getting bored welding scrap over and over, I thought practice would be much more fun if I started making stuff instead.
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