Preventing Cupping on a Wide Maple Table Top

      Another discussion on moisture and wood movement (causes and preventive measures). November 3, 2010

Question
I am building end tables from curly maple. The tables have pedestal bases made of 1" stock that contact the underside of the top along a T of 12" by 12". The top is 16" X 20". The solid maple top, 13/16", air dried for three years, is cupping after planing and sanding. I'm considering relief cuts on the underside (the convex side) to relieve the stress, and am interested in your experience with similar problems.

Forum Responses
(Furniture Making Forum)
From contributor S:
The wood needs to be kiln dried to 6-8% MC. Air dry will be maybe 12-16% MC. To dry the piece you have now might not work if you are to finish thickness; it might cup more during the drying process.



From contributor J:
Where has it been stored during those 3 years? If it's been in a hot attic somewhere, it might actually be dry, but if it's been stacked in an unheated garage, shed or basement, it's not likely to be dry enough for furniture use. If you've brought the wood from a humid storage environment into a drier shop or climate-controlled environment, movement should be expected.

Are these tops a single plank wide, or glued up from strips? When they cup, are they sitting on a bench or otherwise exposed to air and/or direct sun on only one side? In any case, relief cuts should not be necessary.



From the original questioner:
MC is between 9 and 10 and the planks are stored on stickers. The tops are single planks, and must be, if I use them, because of the dramatic figure. The shop is not climate controlled, but it is probably drier than the barn where they were stored. I haven't been especially surprised by the cupping, since the tops are as wide as they are. This is why I am considering relief cuts (I would make them just over 1/8" deep and 1/2" wide.)

One other thing to mention is that I may not have taken the same amount off both surfaces when I planed and sanded the planks. They were flat after sanding in a Timesaver, though, and I'm also wondering if planks like these would stay flat if I sealed them at that time.



From contributor W:
There have been many threads here about cupping, sealing and MC. It might be worthwhile reading as much as you can in the Knowledge Base. I have used boards up to 28" wide for table tops with no cupping. I have also used breadboard ends to keep table tops flat.


From contributor D:
Sounds like the wood was already cupping when you started. Taking uneven amounts off of each surface could cause cupping and warping. Even on fresh glued up panels, after surfacing evenly, if you lay them flat on top of each other, they will most likely cup. Stand them up pretty flat, allowing the air to circulate all around each board as the wood acclimates to its new thickness and environment. As far as the tops, once you fasten them to the table apron, they should stay flat.


From contributor J:
It could be that the 9-10% MC you apparently measured was just on the outside. If your shop isn't climate controlled, the humidity in the shop is going to be on the high side unless you live in the desert. If your lumber storage area is even more humid, then the core of the lumber is probably somewhat wetter than 9-10%, and will stay wetter for quite a while even after it's been moved to a drier environment. If you then machine more off of one side than the other, the surface at the side you've taken the most off of will be wetter, and will become concave as it dries.

If you're asking how to prevent this problem, you need to fix the humidity problems in your shop and lumber storage area. If you're asking how to fix the tops that are already warped, then, well, the relief cuts might be worth trying.



From the original questioner:
Right now I have a couple of these tops that are already cupped (1/16" over 16"), and will probably try a couple things (unfortunately, there is no apron since I have pedestal bases).

1) Relief cuts on the convex side,
2) put the tops in a hot veneer press at 70%, not much pressure, for five minutes,
3) seal one side, then the other side a day later.

If none of this works, I'll probably cut my losses and, on the group of similar planks, I'll bring them into the shop and store them on end for a couple weeks, then be more careful to take the same amount off both sides, and seal both sides if the planks are flat after the Timesaver. Unfortunately, I can't afford to address the climate control issue in my shop right now.



From contributor W:
Try putting the plank in the sun cupped side up with the other side on something flat like a piece of plywood. Generally after about 3 to 4 hours, it will start to cup the opposite way.


From contributor N:
I would advocate the relief cuts. In the end it is you who has to weigh the negative aesthetics of relief cuts on the underside of a table versus the possible negative aesthetics of a cupped table. Even if the benefits are very slight, the belt and suspenders approach eases my mind a bit. In my experience with custom doors, one bad warped door is remembered far longer than the thousands of perfect ones.


From contributor P:
Always remember when surfacing wood - take even amounts from both sides. Even MDF will cup if material is only removed from one surface.


From the original questioner:
Good point. In fact, I don't know at this late stage why this should make a difference, but I sanded the tops down almost 1/8" because I didn't like the proportions of the table (the table tops are now a little less than 3/4"), taking off very little on each pass, and being careful to turn them over each time I sent them through the Timesaver. I then sealed the tops, top and bottom and all sides, with one coat of Waterlox Original Sealer and Finish. They are stored on stickers in my shop, and have stayed flat for a week or so. If this hadn't worked I would have made the relief cuts, but it looks like I won't have to.


From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Unless the wood has drying stress (also called casehardening), cupping that occurs over a few hours or longer is due to moisture change. For example, if the wood picked up MC during storage and you dried the surfaces a bit when you moved the pieces into your shop, then when you removed the drier wood from one face, you would expose wetter wood on that face which would dry and shrink a bit causing cup. If the problem is drying stress, then the cup will show immediately when the piece is machined, sanded, etc. and not a few hours or longer after machining.

Relief cuts are okay if the problem is stress, but removing equal amounts is also okay if you make sure both sides are fully exposed to their new environment so they both dry uniformly and shrink or swell uniformly.

Ultimately, the best procedure is to get the pieces down to 7% MC throughout so that the MC will not change in manufacturing or use. Note that if your shop is more humid than the in-use environment, the pieces will try to shrink after manufacturing, especially if the finish is not identical on both faces. We also have to make sure that the frame does not restrict movement of the bottom face while the top is free to move in the annual cycle of humidity and moisture.



From contributor A:
I wonder if treating wide boards with Pentacryl prior to milling would do any good? I have read that it's great for checks and splits but no mention of cupping. I am considering a single board walnut top that's 24"W X 48" long. Anybody had experience using Pentacryl as a stabilizing agent for very wide single board tops?


From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Cupping is the result of one face of a piece shrinking more than the other face. The face closest to the bark, especially in hardwoods and especially when the piece is fairly close to the center of the log, can shrink substantially more than the other face. (In the old days, the wood from the center of the log was not sawn into lumber, so cupping was much less of a problem back then.) This difference in shrinking is not affected by products such as those listed.

Of course, if there is little or no moisture change (the wood is at the correct moisture for its environment), there will be no cupping in use. Incidentally, cupping of finished goods often results because the one face is not finished, so it can respond quickly to humidity changes in the air by changing moisture content and then swelling or shrinking, compared to the other face that has a vapor resistant coating or finish.



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