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Thread: Options to prevent table top cupping

  1. #1

    Options to prevent table top cupping

    I'm building a dining room butterfly leaf extension table of walnut and am thinking about how to stabilize the top against cupping. The top will consist of two pieces that are 36" by 42" and the leaf, which I will ignore here. The grain is oriented perpendicular to the length of the table so the width across the grain for the top pieces is 36". There will be two 1" by 2" cleats screwed to the undersides with suitably elongated holes to accommodate width expansion and contraction across the 36".

    Two options I'm considering:

    (1) use 3/4" thick stock for the tops and count on the cleats to hold everything flat

    (2) buy 5/4" thick stock, resaw it in half, flip one of the halves over and glue the halves back together so that the growth rings oppose each other to resist cupping.

    Option (2) seems safer but will cost more in materials and time. Is it worth it? Is it needed? I buy from a good lumber distributor so I think the wood should be properly seasoned.

    Thanks!
    Tom

  2. #2
    Quartersawn material, if you can get it, would be more stable than flatsawn. Make sure the moisture content is suitable for in-service conditions before you process it, regardless of its source. Resawing and regluing is likely to be problematic and a waste of time. 3/4" seems a bit light for those dimensions. Don't count on cleats to hold flat material that is likely to cup because of inappropriate moisture content.

  3. #3
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    Applying the same amount of finish on both sides of the table top will also help reduce cupping.
    Lee Schierer
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  4. #4
    Thanks for the good suggestions. I'm still undecided and am considering a couple more options:

    (3) shop-sawn veneer on Baltic Birch core. I have a vacuum bag and have done this before. I know it will work but it's more involved

    (4) go with 5/4 or 6/4 S3S lumber from my local supplier. A lot less work but I'm counting on properly seasoned wood. It will also be heavy and costly.

    Right now (3) looks like my preferred route.

    Tom

  5. #5
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    Quote Originally Posted by Tom Linton View Post
    ...(3) shop-sawn veneer on Baltic Birch core...
    This, if you veneer both sides for a balanced assembly.
    "Anything seems possible when you don't know what you're doing."

  6. #6
    A balanced veneered panel will be more stable than solid wood, if the substrate is flat and acclimated to service conditions to begin with. Thicker lumber will be stronger/stiffer but not necessarily more stable. I don't count on any supplier's material to be properly seasoned without checking the moisture content before use.

  7. #7
    Having a hard time understanding your idea with the cleats, and agree that the second idea just seems like a lot of work.

    Would c-channel be an option here?

    I had a large ash table top a while back that came out of the glue up slightly cupped and I had a hell of a time getting it to go back flat. Sought advice here and received a ton of advice, but little optimism outside of re-cutting and re-jointing and essentially starting over. I left it clamps with cauls for a couple weeks letting it acclimate as much as possible, put two large pieces of c-channel at each end, and it has held almost completely flat for close to a year now.

  8. #8
    I wouldn't resaw if you're concerned about stability. Those pieces may cup.

    Personally, I'd just select the pieces carefully. You could even rip out the crown of any flatsawn pieces so that you're left with narrower, but rift and qs pieces.

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    Quote Originally Posted by Prashun Patel View Post
    Personally, I'd just select the pieces carefully. You could even rip out the crown of any flatsawn pieces so that you're left with narrower, but rift and qs pieces.
    Very wise comment from Prashun IMHO. This is a very good idea, coupled with a c-channel you should be good. Ideally you acquire rift/qs material, but what Prashun mentioned is what I do as a matter of practice on any wider panel or table-top.

  10. #10
    Another benefit of that is you generally end up with pieces that are easier to grain match. When I started out I was always trying to keep the widest boards possible.

    Also, think carefully about c channel. It requires a PITA mortise channel.

    The bigger risk in these tops isn't cupping (in my experience), it's seams opening up. The c channel doesn't help with this - certainly not aluminum, and even not steel (DAMHIKT).

    Again, the best thing is to properly select, then acclimate/mill/acclimate/mill your boards.

  11. #11
    What is the climate differential between your shop and house? IME walnut is one of the more stable species. I just don't think cupping is going to be a big issue if the wood is KD, acclimated and you've selected the stock well. Also avoid really wide boards especially flat sawn. That said, I've been known to sticker lumber inside our house especially in the summer b/c my shop is not climate controlled.

    I would start with 5/4 material only b/c I always start thick to allow for flattening.

  12. #12
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    [QUOTE

    (3) shop-sawn veneer on Baltic Birch core. I have a vacuum bag and have done this before. I know it will work but it's more involved

    [/QUOTE]

    This option is safest. Solid wood can be hard to restrain and 3/4" cleats will not be enough. Any movement will really show at the center joint where the halves meet, and the leaf will add to the problem. BB ply may want to warp but it is easy to restrain. And fastening the top is much easier.

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