Table top warp

I built a mahogany table with a 60" round top back in the summer for our church. It has recently turned off cold with low humidity and the top is curling down on the sides. It has a piece of round plate glass laying on top of it to protect the surface, and the mahogany top has curled down on either side perpendicular to the direction of the boards as much as an inch on either side. I am planning to take the top off and gradually clamp it down to straighten it, hopefully without breaking it, and then add two additional braces to the bottom to try to hold it flat. I put two braces on the bottom when I built it hoping to prevent warping from occurring. I am also planning to put more coats of finish on the bottom to equalize the amount of moisture the bottom and top absorb. I originally put 3 coats of Barkley's gel varnish on the top and one coat on the bottom. Any thoughts on whether the glass on top, which is held off the actual wood by small round spacers, is causing the problem, or recommendations on how to straighten the top and prevent this from happening again? Any and all help would be greatly appreciated.
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What is the configuration of the boards? Was it just cut from a glued-up panel or is it segmented with edges? This seems like a lot of movement so I'm wondering if there are some stresses in the design from opposing grain matchups, etc.
You might first try just taking the glass off and letting it rest for a week or so. It seems like maybe the bot dried out while the top couldn't breath and stayed moist.
It would be interesting to do a moisture test on the top and bottom, only problem being you'd need a pin typoe and have to prick the surface finish on top.
BW
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I had a 60 inch panel glued up from flat sawn 4 qtr kiln dried mahogany. The individual boards were from 3 to 5 inches wide and were alternated to some degree as far as the growth rings are concerned. I had the panel glued up at a wood supply business that specializes in cabinet grade hardwoods because they had the glue up racks and a belt sander to level the panel after the glue up. I have built a lot of furniture over the years and have never had a piece curl like this one has. This is the first piece I have made however that has had glass laid over it. I will remove the glass and see what this does. Thanks for your comments.
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at
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I know how distressing the problem is and I know you built the table out of mahogany with good intents but a good way to stabilize any solid surface is to counterveneer it .That is one veneer across the grain [a cheap veneer such as poplar] then another laid on top with the grain [say mahogany]. Do the same to top and bottom.and you should have a stable surface .
The core needs to be flat and unstressed before commencing this operation . mjh
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wrote:

I don't think clamping is likely to do any good. Most likely yopu have unequal moisture loss due to unequal application of finish and especially the glass top. Any idea what the moisture content was when built?
As a first attempt i would remove glass and give it time to see if it starts to straigten. If the finish is easy to remove i would get it off to help speed up moisture equalization to both sides of top.
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if that works to get it back to flat, here are some ideas to keep it that way...
if the glass is sitting right on the wood with no airspace between add some transparent rubber bumpers when it goes back on.
whatever you do to one side, do to the other. adding rails to the bottom is unlikely to fix the problem.
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I removed the glass top yesterday and will see if this will help. I had plastic spacers between the glass and the top so there should have been some air movement between the two, but possibly not enough. I did not check the moisture content of the wood but it was suppose to be kiln dried to approx. 6 %. However, I have no idea how long the wood had sat stacked in a warehouse after it was kiln dried and absorbed moisture from the atmosphere. The humidity here in Ky. was really high this summer due to all the rain we have received this year. I really appreciate everyone's input and hope that the solutions offered work so I don't have to remake the top. Any other ideas or suggestions will definitely be appreciated.
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"Frank Korb" wrote in message

some
approx.
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Sorry to say, but for the "as much as an inch on either side" cup that you describe, I don't hold out much chance of a lasting and easy fix.
An "inch" is a LOT of cup!
Without seeing it, my bet is that the choice of stock, very likely the way it was cut and from whence it was cut in the log, is playing a bigger role than what as been ascribed to thus far, along with the mentioned changes in RH.
If I was mine, and things don't look up quickly with the other suggested fixes, I would seriously consider ripping off the offending stock and replace same with something that is likely cut to be more stable, IOW, quarter sawn, or with grain as perpendicular to the face as you can find in your wood pile, and perhaps with narrower pieces, foregoing any artistic grain matching attempts.
That said, I would be perfectly happy to be totally wrong and off base here.
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