Flattening a welding/fabrication table

  1. JonE Forum Supporter

    Messages:
    92
    Location:
    Central Scotland
    Am just putting a new table together, with a box section frame & a waterjet cut 1400x800x15mm mild steel top. All good except the plate turned up a bit more warped than I'd like. Is probably only +/- 2mm, but enough to be an annoyance when clamping larger pieces.

    Considering my options for getting the top flatter, and wondering what has the best chance of success? I have no experience of trying to move anything this thick.
    1. Add some additional 50mm box section cross members, bolt down the high points & pack up the low points. (What will move first, the table top or the frame?).
    2. Try to press it flat with some C channel and a bottle jack.
    3. Invest in some oxy propane kit, and have a go at heat straightening. (Have no experience, but always keen to learn new skills).
    Open to any suggestions suitable for a home workshop, thanks!

    Screenshot from 2019-12-04 13-14-31.png
     
  2. Brad93

    Brad93 M J B Engineering

    Messages:
    6,072
    Location:
    Essex
    You can get steel sheets “ploughed” from some suppliers. Then bed your plate on the frame with a bedding compound and jack screws.

    If you buy plate from SSAB they’re guaranteed to a certain flatness.
     
    JonE likes this.
  3. hotponyshoes Member

    Messages:
    1,391
    Location:
    Somerset. Uk
    I would not think 50mm box will stand a chance of bending 15mm plate but if you have some you could try laying it onto the side that is risen in the middle and seeing if the edges will pull down to it with some clamps?

    You might be able to weld up an H or I beam run from the 50mm but I would think you would stand more chance with rsj for that thickness.

    You might be able to jack it back flat, maybe a couple of big ratchet straps around the ends, bricks or blocks to make enough space to get a jack in.

    I would not like to try heat on something like that but only because I would not want to mess it up. I am sure it would work nicely if you knew where to heat it. I suppose you could do a practice run on a scrap piece first?

    Personally I think I would make up the frame first, I would move the top rails in your drawing right up to the underside of the sheet and maybe add another rail below with a center support or maybe some braces running from the centre of the top rail diagonally onto the legs, drop the sheet on top and see if I could clamp it down to the frame to flatten it. If it goes easy then bolt or weld, if I thought it was just going to pull the frame up i, would put it on the floor with the center as the high spot and either drop a ibc container on it with the forklift or drive the truck over it a couple of times.
    If it was springing back I would pack under the ends with some 1mm or so sheet and try again.
     
    JonE likes this.
  4. knighty Forum Supporter

    Messages:
    1,769
    Location:
    Sunderland
    if you build it like the picture I think it'll sag in the middle over time anyway

    build the frame nice and square/flat

    bolt the top down to it, countersunk bolts down from the top or weld a fed bits under it to bolt to

    then add some shims under any high bits

    if you only need to pull it 2mm over a decent length then I think it should work pretty easily/well


    if it's low in the middle jack it up from the floor and leave it to sit a while bent the wrong way.... if it's high in the middle flip the top over before you fix it down :-)
     
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  5. TechnicAl

    TechnicAl Member

    Messages:
    6,727
    Location:
    Rotherham
    At 2mm its not bad........the standard for flatness allows 7mm over 1000mm......the tightened "special" standard is 3mm
     
  6. JonE Forum Supporter

    Messages:
    92
    Location:
    Central Scotland
    Thanks all, have been out experimenting tonight.

    I finished the frame last weekend (so can't easily move the existing rails), but haven't fixed the top down yet. The plan is for countersunk bolts from the top into the legs.

    The plate was sat on top with the bow largely upwards. Using another length of box under the rails, and some all-thread through the fixture holes, was pleasantly surprised how easy it was to pull it down by a few mm in the middle (am sure the box was flexing as well).

    I've got spare 50x30 box, which I could get some extra rails out of, and add some threaded bosses to make a grid of jacking screws. Then could flip the plate over so the bow is downwards and try pushing it flat off of these..
     
  7. Screwdriver

    Screwdriver Member

    Messages:
    5,108
    UK London
    Flame straighten it. Google it and hit it with oxyacetylene and some wet towels!

    :D
     
  8. TechnicAl

    TechnicAl Member

    Messages:
    6,727
    Location:
    Rotherham
    Its very difficult and Im not convinced that even a seasoned practictioner could do it to within 2mm. Ive seen it done in shipyards but they were saying 6mm is fantastic and 10mm is difficult.
     
    mtt.tr likes this.
  9. dannyp Member

    good luck with that......

    i built a tabel a few years ago thats 2.5 mtr x 2.5 mtr heavy box frame 90x90 with lots of crossmembers the same size from memory. with angel tabs off the sides

    i drilled and tapped the sheets and bolted up through the tabs to the sheets and sanded any of the bolt that poked through off flush and pulled the sheets down flat with the bolts it wasent perfect but it got it a lot flatter but id do it diffrently if doing it again, id go throgh the box more faff but i think it'd work better, id also use bigger box or rectanguar section to get more depth as its supriseing how much itll pull the box upto the plate. and bring the frame out to within 50mm of the edge of the plate

    big mistake with mine was tabs round the outside of the frame as over a few years the outside 6" has crept down about 5mm
     
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