The more I think about this the more I'll bet the problem was the MC when you built it. You said the lumber had air dried for 2 years. If that was outdoors the wood wouldn't have been below about 12% if you live in the northern and eastern quarter of the country, even higher if it's the S-E quarter or PNW. If that's true, then the MC when you built it will depend upon the EMC of your shop and how long the wood was in your shop before you built it, but I would be hard pressed to think you could get the wood below about 10% in an outbuilding shop unless you put it somewhere warm and dry.
37" of plain sawn walnut drying from 12% to 6% MC is going to shrink 5/8" according to the shrinkulator. That's a lot, but it should still be OK if the top was not rigidly attached to the aprons and dried uniformly top and bottom.
If there was hot air blowing under the table I would expect it to curve up in the middle. Since it curved down in the middle then it must have dried more quickly on top. Doesn't matter, it's a problem either way.
You finished the bottom of the top before attaching it to the aprons, right? If you used them along those parts, how close did you set the clips to the aprons running the length of the top? The top could shrink 5/16" on each side of center to the edge, so maybe 1/4" to the apron. The clip would have to be set with at least that much gap to avoid a problem.
Here are two pix: the plan, and the top with a carpenter's level showing how it has bowed down 1/2 inch. Thanks for the replies. I will take the table apart and remake as necessary, but I don't plan to post any more on this thread. --Peter
(01-08-2018, 01:44 PM)petertay15 Wrote: Here are two pix: the plan, and the top with a carpenter's level showing how it has bowed down 1/2 inch. Thanks for the replies. I will take the table apart and remake as necessary, but I don't plan to post any more on this thread. --Peter
Don't take it too hard! It happens to all of us. Chalk it up to a learning experience and move on.
(01-08-2018, 08:25 PM)toolmiser Wrote: I wouldn't have guessed it bowed that way, I would have thought across the grain. Must have been storing Anvils on it.
I keep looking at the pix and thinking that it looks more like the ends are cupping up rather than the center (and supporting rails) somehow all bowing down. How could that be?
I made a chess set top and it is bowing downward in the middle after a year. My thought is that I didn't seal the bottom and this super dry wood expanded on the bottom and not the well sealed top because the unsealed wood absorbed the humidity.
01-09-2018, 09:54 AM (This post was last modified: 01-09-2018, 09:56 AM by loosetoe.)
I’m not certain, but I think that piece at the end that is glued on to increase thickness is glued crossgrain. The end of the bottom board appears to be end grain to me.
As the top is trying to shrink in winter, the upper part is decreasing in size, but the bottom is not able to. The glue joint appears to be failing, but the wood is cupping first.
Both the top and bottom boards look like end grain to me, as the OP said, but it there definitely are cracks opening up between them. I don't see the aprons being bent, though, as someone else said. If that were happening then the legs would splay out, too. In any case, it ain't good.
(01-09-2018, 09:54 AM)loosetoe Wrote: I’m not certain, but I think that piece at the end that is glued on to increase thickness is glued crossgrain. The end of the bottom board appears to be end grain to me.
As the top is trying to shrink in winter, the upper part is decreasing in size, but the bottom is not able to. The glue joint appears to be failing, but the wood is cupping first.
A better picture would confirm.
I also notice in the picture there is a sort of heater there. I wonder if that heat source so close to the table exaggerated the problem.
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