Warping plate?

  1. Dragnet Member

    Messages:
    57
    Location:
    Cornwall, UK
    I've been making some brick trays to cover existing manholes. I've got 3mm x 50mm x 50mm angle for the frame, mitred and stick welded together. I was quite pleased with that. I had some 2mm plate to fill the interior (450mm x 900mm) which I've MIG'd all round, both for the practice and so that the brick-bedding sand doesn't run out. The plate now has a warp set into it which must be because I didn't go slowly enough to let the heat dissipate. If one steps on it, there's a great bang as it bends out the other way. This would be fine if the weight of the bricks kept it down, but they don't.

    Short of cutting around the whole thing and welding it again, can I release the tension with a torch, or perhaps put a bar across for strength? I had hoped that welding the plate to the frame would give it some of the strength of the flanges on the angle, but obviously that was pie in the sky.
     
  2. Screwdriver

    Screwdriver Member

    Messages:
    6,254
    UK London
    Yes. Flame straightening would work. Sounds like you just need to shrink the middle of the plate. With oxy kit should be straightforwards.

    FWIW, I didn't understand the mechanism but it's quite simple when you dig into it. You heat the material until it gets hot enough to expand (as all metals do) but when it expands it can't move because it's restrained so it deforms in thickness (upsets). When it cools back down to room temperature, the thicker material effectively uses up the linear dimension and the plate/rod/form appears to shrink. The "trick" to it is that as it cools the material stiffens and cannot stretch so easily as it is compressed.
     
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  3. nickk Forum Supporter

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    2,287
    Location:
    Hay-on-wye
    If you haven’t got any heat,weld some stiffness on the underside,like 30/40 flat bar on edge ,2 bars corner to corner.
     
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  4. Dcal

    Dcal Forum Supporter

    Messages:
    2,004
    Location:
    Antrim Northern Ireland
    What you describe is called "oil canning" and is a result of the thin plate expanding as the weld is laid down and the extra area of the hot expanded metal being locked in because the weld solidifies before the plate cools and contracts.

    You need to shrink the metal or just hit it with a hammer to put a hard dent in it to loose the stress.
    It is not easy to shrink thin metal well (ask anyone that works on car bodywork) but it is easy to get rid of the extra if you don’t care what it looks like.

    Batter it down fill it with sand and it will never be seen.
     
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  5. tom2207 Member

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    1,604
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    uk northern ireland
    That hits the nail on the head ,, easiest fix is to put a ring of pipe in the centre ,, and beat a dent into it.
     
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  6. Morrisman

    Morrisman Forum Supporter

    Messages:
    2,071
    Location:
    Staffordshire, England
    The frame and edges have shrunk from welding, making the centre too big, as it were. Put a 2” circle of weld in the middle, it will pull the sheet together and stop the oil canning.

    But if it doesn’t work you can simply grind it flush and deny everything. :D
     
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  7. TechnicAl

    TechnicAl Member

    Messages:
    7,477
    Location:
    Rotherham
    Did you weld it all the way round without stopping?
     
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  8. 500e

    500e Always buy fire insurance a flood is hard to start

    Messages:
    4,584
    Location:
    SWest UK
    @tom2207
    Its easy enough to shrink thin but doing it in the correct place is the skill :D I have chased the stretch all over big flat panel vans like the Morris EA series
    @Dragnet
    Oil canning
    The method is to heat up a circle red hot with a dolly behind bang\tap around the edge of the heated bit then bang\tap centre flat.all while red hot
    The size of the red hot dot differs according to amount of oil caning.
    Have seen a guy shrink 1\16 sheet with a dished plate on a angle grinder it appears to be an American thing
     
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  9. Morrisman

    Morrisman Forum Supporter

    Messages:
    2,071
    Location:
    Staffordshire, England
    A shrinking wheel. No idea how they work but they seem quite common over there.
     
  10. tom2207 Member

    Messages:
    1,604
    Location:
    uk northern ireland
    Yep , I do lots of it , as I do a wee bit of tin bashing , for a long time it was a well kept secret , but there are videos out there now showing how its done , Ive tried the disc shrinking , but I fall back to hammer shrinking most of the time , again oxy acet ,, but the op wanted something fast and handy , and as it wont be seen ,,, hit it a thump .
     
  11. tom2207 Member

    Messages:
    1,604
    Location:
    uk northern ireland
    same idea really ,, the high bits get rubbed and friction generates heat , then they rag the hot spots to shrink them down , no hammer no dolly , just slow old going .
    you can shrink with gas that way , but I prefer to wait for the bump to rise then drive it down with a flat planishing hammer ,, There are loads of fancy spiral and knobbly even dotted hammers , but a flat one does just fine.
     
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  12. Dragnet Member

    Messages:
    57
    Location:
    Cornwall, UK
    Thanks all. Some very good tips. I tried using a blowtorch to heat areas which I then cooled with a wet rag, and that made far more of a difference than I expected. Fascinating! I've been left with heat colours in the steel that makes it look like a magnified micrograph of a bacterial colony. The stiffness still isn't there, so I think the idea of bar or angle underneath in a cross is called for so it doesn't flex when one walks over it.

    I had gone very slowly, welding an inch or so at a time all around the plate, but I must have rushed at the last and done too much at once. For the next one I'll try welding a circle underneath the centre. Does it matter which side the weld is done on, or the torch is applied? I know that a run of weld will cause shrinkage towards it, but if it's held all round the edge perhaps the heat does the trick whichever side it's applied.
     
  13. 500e

    500e Always buy fire insurance a flood is hard to start

    Messages:
    4,584
    Location:
    SWest UK
    If playing with heat red is the colour either side is it still popping if so try and work out where the stretch is heat that bit, it is difficult to explain where you need to shrink it comes with practice & a lot of frustration:(
     
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