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  1. #1
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    Default Correcting for warpage

    I wonder if anybody more experienced could give me tips on correcting for warpage. Like, recently I tried to build, basically a three-sided channel. No matter how carefully I aligned the side walls before welding, afterwards, they were out of true. I got a suggestion to fix it by clamping them and then beating on the welds with a big mallet, which worked, but surely that's not how pros do it. What do the pros do? Do they torch the joints after welding and bend the pieces into place? Or do they do something beforehand that prevents the warpage? I see all this stuff out there in the world, and it sure looks square, and I have no idea how it's accomplished. I know that warping is just a part of welding, but if that's true, then how does anybody make anything that's actually a 90-degree angle?

    Now that I write out the question, I wonder if warpage isn't really the right word for what I'm asking about. I know about backstepping and stitch welding and other techniques for correcting warpage. I guess what I'm really asking about is how to compensate for the shrinking/expansion of the weld joint itself, that pushes right angles out of true.

  2. #2
    Platinum Member Reyer Farms's Avatar
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    Default

    Sounds as if some well placed clamps while welding and left until cool will help. Metal moves to the heat. Good luck.

  3. #3
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    Default Re: Correcting for warpage

    Quote Originally Posted by Reyer Farms View Post
    Sounds as if some well placed clamps while welding and left until cool will help. Metal moves to the heat. Good luck.
    Will it? I was under the impression that the contraction of the weld metal that occurs during cooling would put stress on the joint that would pull the pieces even after cooling.

  4. #4
    Silver Member firemanmike69's Avatar
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    You can clamp it and wait till it cools to take the clamps off or you you can start slightly out of square and use the contracting weld to move your piece into square

  5. #5
    Super Member mjncad's Avatar
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    Default Re: Correcting for warpage

    I'd clamp the crap out of the pieces, weld them up, let them cool, and then unclamp. I've never tried FiremanMike's technique; but I suspect it takes a lot of experience to know how much out of square to make your setup in order for the weld to pull itself square. I know I'm not good enough to be successful with that technique.
    Paraphrasing Douglas Adams - So long and thanks for all the bacon.

  6. #6
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    Default Re: Correcting for warpage

    At the shop we would often stitch weld or tack weld small pieces that were clamped.

    The idea was not to get anyone area too much hotter than adjacent areas... sometimes preheating would also be used.

    For critical welds... than entire assembly would go into an oven to stress relieve...

  7. #7
    Elite Member Shield Arc's Avatar
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    Default Re: Correcting for warpage

    Can you post some pictures of what you're building? For some reason I can't visualize it.
    Here is a good video about heat travel on square tube. Well the first 5-minutes are good, then the guy starts playing around. Personally I would never place that many tacks on that small of a project.
    When ever I build something I monitor the measurements, squareness, plumb, level, etc, etc, and place the welds according to where I want to pull it. Weld sequence is really hard to get a handle on. Then the more heat that is applied things change. Even when tacking a project up, I place the tacks so they help pull things in the direction I want. Placement is one thing, the size of the weld, or tack comes into play also.



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  8. #8
    Elite Member Shield Arc's Avatar
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    Default Re: Correcting for warpage

    Some times pre bending a project is a big time saver.
    I snagged these pictures off another site.
    Attached Thumbnails Attached Thumbnails -prebend1-jpg   -prebend2-jpg   -prebend3-jpg   -prebend4-jpg  


    Miller Dynasty 300.
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  9. #9
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    Default Re: Correcting for warpage

    Shield Arc: Here are some pics.

    -2013-02-08-20-30-a

    The last pic shows the "spreader" that I improvised to try to get the thing back into alignment. I set the joint up as a totally non-overlapping corner and then welded the fillet on the outside. I also put a single bead down the inside fillet after welding up the outside, more on principle than anything else--it's not relevant to this discussion as the warpage happened before I did that. One thing that confused me was, I expected that the side-walls would pull outwards as the weld metal cooled and contracted, but they didn't at all. They pushed inwards. I can't for the life of me figure out how that physics worked out. What I did was I put pins through the bushings and then used a clamp to pull the side-walls in so that they bound against the bushings. I hoped that as the weld cooled it would pull the walls back out, but it did just the opposite.

    Oh, and if you are thinking, "Gosh it's dumb that he put the bushings in first and then tried to weld up the walls. There's no way they'll stay in alignment." Yeah. I figured that out. The hard way. Now I am thinking that the right thing to do would be to first weld up the channel and get it all square and true and then cut the holes and insert the bushings, and I'm thinking more generally about how one would get everything square and true in a case like this.

    In your example of pre-bending, I'm surprised that would work, because you have stretched the metal, but you haven't made the bend "take". I would think that once you remove the clamps, the tension that's already in the metal would want to pull it back to flat PLUS the tension from the weld would pull it even further. Is that not how it works?

    Thanks for the help.

    PS: I think I recognize that dime. Is that a 1966?

  10. #10
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    Default Re: Correcting for warpage

    If you can anticipate this weld shrinkage/distortion, then you can design your welds so that you stitch weld on both sides. This will do 2 things:
    1) Allow you to put down short, hot welds with good penetration without heating up the whole piece.
    2) Use the weld to pull the material back to where it should be.

    Or, you could tack some temporary gussets/stiffeners which get cut off after everything has cooled.

    When you plan out your welds, try to weld in an order that eliminates long, slender shapes, favoring a "boxy" structure first.

    When I was doing production fabrication I learned to weld with the stick in my right hand and a 2 lb ball peen in the left, which allowed me to correct the warpage as I welded - basically hot working the shape as I went. If you try this don't forget your hearing protection. What?
    That's the problem with trouble.
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