Drying and Cupping in Table-Top Boards

      Here's a discussion of moisture-related wood movement and table construction. November 27, 2007

Question
The company I work for makes custom tables. The tops are around 48" wide, usually made of pine, maple and reclaimed pine (barn boards). They insist on using wide boards. The tops start to cup a day or so after glue up. Any ideas on how to keep them from cupping, or at least how to reduce it?

Forum Responses
(Furniture Making Forum)
From contributor J:
If the whole top is cupping, it's likely because it isn't sanded and finished equally on both sides. The more coarsely sanded and/or less finished side will absorb more moisture and swell enough to cup the top. The side absorbing more moisture will always be convex. If individual boards are cupping, there could be many reasons. Maybe suggest they purchase quartersawn boards, or something not as prone to warping. The old solid hardwood furniture and millwork was often made from quartersawn boards to minimize expansion/contraction/warping.



From contributor K:
Aside from what has been mentioned, and without knowing more about the design, I would suggest routing several T slots across the bottom of the boards right after they have been milled, and have a cross member that fits into the slot to keep them flat. To keep the glue from sticking to them, I usually use an oil finish on them, and wax the heck out of them before gluing the top planks together with the T-Stiffener in place.


From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Wood in use does not change its size or shape unless the MC changes. So, if they are cupping, it means that the MC is changing. Specifically in this case, the top surface is losing moisture faster than the bottom surface.

You have two choices... Get the wood at the correct MC before gluing, or else finish the top right after gluing so that the finish will buffer the changes, making the top and bottom similar in MC. This second option is not too realistic for most operations.

So, check the MC of each piece before gluing and make sure they are all between 6.5% and 7.5% MC. Then keep the shop at 35% RH until you have a good finish on the top. (Remember that the finish only slows the MC change and does not stop it.)



From contributor V:
I recently glued up a table top. It sat in the clamps on end for about a week. It was flat at the time I removed the clamps. I set it on the workbench to sand, and the next day it was cupped. So I flipped it over and the next day it was flat again. That tells me the top side is drying out more than the down side. My hope is that when I get it assembled to the lower portion of the desk and finished, it will be okay. I think the T slot is a good idea; maybe a sliding dovetail would fit the bill too. Do not glue the entire length of the slot, just a dab on one end to hold it in place.


From contributor J:
Gene, am I wrong?


From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
I do not believe your post was incorrect, as you mention the difference in MC from side to side. I think that it is most likely that the tops are too wet overall when first made. Contributor V has the same experience, and flipping them over helps to even out the MC difference that was developing from one face to the other. But the real cure is to get the correct MC from the start.


From contributor J:
I'm just thinking that the overall MC isn't as important, vis-a-vis cupping, as the relative MC, surface to surface. Of course, the MC overall is important, especially with the use of wide boards, but given even a high MC, equal sanding and finishing of both sides should minimize cupping.

Also, the selection of grain orientation, especially with wide boards, will have an effect. Flatsawn boards will tend to cup toward the center of the tree from which they were sawn, despite sanding and finishing. Alternating the grain orientation of the boards towards and away from the center of the tree from which they were sawn will help, but if the boards are wide, you may wind up with "S" shaped surfaces.

The best solution, in my opinion, is to use narrower boards, quartered, and sanded/finished equally on both sides.



From contributor W:
Not to be argumentative, but I thought boards cup toward the bark, not the pith.


From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
As wood dries, it cups toward the bark.

Also, if there is no MC change, there is no cupping. So, although what contributor J says is basically true, there would be no MC gradient, face to face, if the MC is correct initially.

One small exception to what contributor J says with regard to higher MC pieces... In addition to equal planing, sanding, and finishing (which can be hard to do), there will still be uneven MC face to face if the exposure conditions are not the same. So, the top piece in a pile has the top face exposed to dry air while the other face is touching other panels and is not drying. That is why flipping the top panel seems to work. Note that this is happening before sanding and finishing.

Further, a finished table may have a plastic table cloth at times on the top, which will stop the top from changing MC, while the bottom is exposed to dry air.

Incidentally, the difference in swelling or shrinking from bark face to heart face of a piece of lumber is very small. The real issue is the difference in MC from face to face, which is many times bigger in its importance.



From contributor B:
Just a couple of simple solutions. In wide boards, rip them to allow them to dry in the center and re-join with biscuits, etc. Use a thin kerf blade to minimize alterations in grain. Always keep the bark side of the tree up. It is not always the prettiest side, but it is the most durable. Moisture content has a lot to do with it, but so does relative humidity. Dry days, boards will reach up; humid days, they will reach down. Try to control this if possible. What controls this in the end is how it is attached to skirt or pedestal. Make sure to board it underneath, gluing in the center up to four inches and screwing it every two inches the rest of the way if using pedestal. If on a skirt, be sure to cut groove in skirt all the way around and attach with "Z" brackets, allowing it to conform but float.

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