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walnut table top movement3/13
so i am a building a solid walnut table top 75" x 36" by 1.125". Each individual piece is approx 6-8" wide and kiln dried. I glued up the wood slab alternating the ring direction and all was near dead flat when it came out of the clamps. I used biscuits to help aid with the glue up alignment. After about 2 weeks I put the slab on my cnc router and cut deep underside bevels on all four edges. The bevel is ~ 8 " long and ends with the edge being around 1/2" thick. After leaving the slab in my workshop for a week i have noticed that all four corners are pulling down ~.125" towards the bevel. At the middle of the table it is still flat. I would of thought that if anything that the curve would be away from the bevel. Any advise as to how i might be able to fix this. I would have thought if anything they would have curled up. Right now i have the topside clamped in reverse curve blocks trying to correct.
thanks for any advice.
Was the top fastened to a base ?
How did you store it for that week in the shop? You need to have air circulate all around the glue up. If you let it sit on a bench, the top will respond to the humidity and heat faster than the bottom. Cupping will result. Just because it was in a kiln, it doesn't mean it is dry or correctly dried. It could have had some internal stresses, or maybe the core was not the same moisture as the surface. I suspect it was the way you stored it. Keep air all around it while it's in the clamps.
Did you check the shell and core MC? If the core is a bit wetter than the shell, when you machine away the shell and expose the wetter core, the wetter core will then shrink causing what you see now. Having low uniform MC is critical and it is hard to fix this now.
Incidentally, reversing grain direction neither helps or hurts. The key is the MC at the time you make the table. Also, you want the table not to change MC when first put into use.
As much as possible, do the same thing to the top as to the bottom.
Strongly agree with Rich C, stored flat on top of eachother I have had panels warp overnight , now on glue ups I stand them up so air flows all around .
Regardless of shell mc or core mc a similar result will occur when you lay a piece like this down flat to store unless it is held flat .
A key point is that wood does not move in the short term unless its moisture changes. The exception is if the wood moves immediately when machining, in which case we have stress. As the top curled overnight, then we know it is moisture.
It is not just the moisture that causes this cupping .
Yes, we are saying the same thing because if the moisture escapes unevenly, then we have different moisture contents. So, it is indeed moisture that makes this happen...uneven moisture and the resultant change.
Dr. Wengert has this dead on. Higher moisture content core is exposed to the air, dries out, shrinks across the face of the taper causing cupping in the direction of the newly exposed tapered surface.
He is after all the doctor.
The ability of the moisture to evenly escape is what makes these panels warp when stacked without stickers.