cupped panel

[Mike Steinhilper]

Ok these questions today are all related to the same project obviously. I went home at lunch only to discover that my table top has bowed. It's 1" thick, 42" x 21" panel that was made with 4 cherry boards. My shop is in the garage and that's where it was glued up. Then I brought it in the house for about 36 hours. It was flat. Yesterday I took it back out to the garage and started cleaning it up a little with the plane. It was in the garage overnight last night, and today at lunch... bowed. I'm assuming that since I planed only one side and it was very humid last night that is the cause of the bowing. If that's the case, how should I remedy? Bring it inside? Plane the other side? Weigh it down? My instinct is to keep flipping it back and forth and planing until there's nothing left. But whatever caused it to bow is gonna keep bowing it regardless of how often I flatten it. what now?

[Michael McCoy]

It could be as simple as the way you stored it after the glue up. I've had tops bow when I got side tracked and laid them flat on a surface without stickers. I've also had a couple straighten back out when I gave them time to acclimate.

[Pedro Reyes]

How bad is it?

Also is it bowed or cupped?

/p

[Mike Steinhilper]

sorry, ... it's not too bad and it's actually cupped across the width. probably 1/4" cupped across the 21" width.

[Pedro Reyes]

Ever consider using breadboard ends?

/p

[Mike Steinhilper]

Breadboard ends are definitely worth considering, but i would really like to get this thing back in shape. I hope if I plane the underside tonight and bring it inside in the dryer air, I might get some flattening. Is there any reason to think that it won't?

[Mike Steinhilper]

follow up to this... maybe this will help me understand better. Let's say I could run this panel over a jointer wide enough to handle it. I end up with one flat face. Would it cup again, and again, and so on? Then I would be dealing with different thicknesses along the width. Am I over thinking it? Is this something that you just have to learn to deal with?

[Marcus Ward]

Just turn it over from how it was laying last night and see if it goes back. I had some huge cedar planks I laid on top of eachother that turned into canoes in a week so I flipped them over and they turned back flat (separated them before they canoed the other way). Don't remove material if it's how you stored it, you might be able to fix it by flipping it over.

[Mike Steinhilper]

here's a pic. It's not attached, just sitting on the frame.

[Thom Sturgill]

The picture seems to show that you have the boards all oriented the same way with the arc of the rings opening down. They should have been alternating. As has been said, if it set with one side flat against a surface it could have dried unevenly and this can be corrected by reversing the condition. Long term, the grain arrangement can cause just this kind of warping. Alternating the grain between pieces minimizes this.


I would defer to others here, but it may be worth sawing the joints apart and flipping a couple of the boards.

[Bob Smalser]

Take a break. Move the top inside the house for a week and see what happens. The good news is you have lots of overhang.

If that doesn't work rip the top either in half or thirds depending on how badly it has cupped....then joint, flip the end grain cups and reglue.

Next time store your wood in the house for at least two weeks before dimensioning. The same applies to sensitive stock like stiles, tops and panels....don't leave them in the garage for a week, move them inside. Even with kiln dried wood. If you are using airdried wood, season it there even longer. Then if you still have some moisture differentiation in the boards with drier outside wood and wetter inside wood, when planing insure you surface both sides equally. But the best path to zero surprises is to leave it indoors until it is completely dry. The same applies to finish....if the top gets 6 coats, so the bottom must.

[Steve Jenkins]

I've found that it is best to never leave panels lying flat and uncovered, even on sawhorses. If you get a significant humidity change over night the side that is up will change moisture content and result in the panel bowing. I either leave the panel on edge or cover with a piece of ply or even plastic. anything to keep the top surface from gaining or releasing moisture. The key is either no air circulation or equal circulation on both faces.
To correct your panel I would try spritzing the concave face with some water to straighten it out or even bow it the other direction then stand it on edge to dry for a few days.

[Richard Niemiec]

Bring it inside and let it stabilize, from what you said, it seems to set itself right. Someone else mentioned ring orientation, so next time.....

I had a similar problem with a wide maple top of a cedar lined chest. Longer term, if the design does not allow for breadboard ends, and there is clearance available above any drawers you may have in the design, a couple of battens might be in order, attach with the end grain facing the bottom of the table top, and this will help to minimize further bowing. Putting on a finish would help the moisture absorption.

PS: Edit: I don't mean end grain, I mean just not the flatsawn side.... sorry for confusion.

[Jeff Hallam]

I'm surprised no one mentioned this yet (or maybe I missed it) but I also work in a garage / workshop and I consider my dehumidifier a necessary tool just like dust collection. I leave it set, it keeps the RH at 45 -50%, and I have a drain hose attached so I don't need to remember to empty the bucket. It allows me to leave my lumber in the shop to acclimatize (There's no way SWMBO would have a pile of lumber in the guest room :D)

They aren't too expensive and you can usually find them used. Just a thought.

[Mike Steinhilper]

good thought. My garage/shop isn't insulated or climate controlled in any way, so I don't know how effective it would be, but that is definitely worth checking into. The other thing that I didn't mention is that the place I buy my lumber is basically outdoors as well. It's kept in a greenhouse type shed that is open on one end, and I don't think it's climate controlled either. So moving my lumber from there to my shop shouldn't be too different. Regardless, I will win this battle.:mad:

[Grant Lasson]

Michael,

Sorry about your problem. I have had some recent experience with the same. Two things really helped me: 1) plane both sides at the same time and 2) store the panels in a way in which air flows freely on both sides (vertical or horizontal).

For you right now, I'd wait and see if won't flatten out somewhat on its own. If not, you're going to lose some thickness but you'll get it flat again.

Also, there's a respected group that doesn't believe in the "ring alternation" theory. I'm not respected but I don't do it either. I go for grain matching first. Once I get a panel flat and dry on both sides, I haven't had trouble keeping it flat.

[Bob Smalser]

Also, there's a respected group that doesn't believe in the "ring alternation" theory.

Yes. But only with qsawn. Otherwise they too fall flat on their faces unless they carefully select and acclimatize their wood to where it will live and leave it there . And regardless of care, once the finished piece moves from the wet Pacific NW to Arizona or vice versa, they always regret denying Mama Nature her due.

Here's how your boards will move seasonally with even minor changes in humidity based on how flat or vertical the grain is. I guarantee it. ;) This USDA graphic tells you both when and how you should be flipping cups.

http://pic20.picturetrail.com/VOL12/1104763/3075040/189555324.jpg

[Mike Steinhilper]

That's a great diagram Bob, thanks for posting it. I think I'll blow it up to poster size and hang it in my shop! That diagram illustrates what the real rule is... there are no 'rules'. Every board is different.

[Marcus Ward]

Hey Mike, take a look at that diagram again. I think what it shows that logs sawn from various portions of the tree will shrink in certain directions, and depending on the shape of that cut, the grain orientation is important. If you look at the quartersawn log (lower left) you see it has very little shrinkage in any direction, but the same thing in a round piece (lower right) will cause it to become ovoid. The plank at the very top, flatsawn, will cup because the rings are oriented across rather than up and down. Etc etc. At first glance it appears random, but it's not.

[Thom Sturgill]

And the reason I mentioned ring orientation was that it looked like his boards were all flat-sawn like the center top example. Also remember that these are generally either short-term (if the wood is wet) or long-term as the wood air-dries over the life of the table. Just because a piece survives 4 or 5 years in the same house does not mean that it will not cup over 40 or 50 years, or if moved across country.

[Mike Steinhilper]

I know it's not random, I just meant that it's extremely variable. Depending on where the board comes from, e.g. the angle of the grain can vary even within quartersawn or flatsawn, etc. And Thom you are correct, however I think I've got 3 flatsawn like the one at 12 o'clock, which seem to be stable -- they are still lying flat. The problem board, which is creating the cup, is the board at the 10 o'clock position. And it is responding as indicated in the diagram. I expect over time that the 3 12 o'clock-type boards may cup, but if the overall table remains flat, I can deal with it because it will create some small peaks, rather than cupping the whole table top. The one at 10 o'clock fortunately is on the end so it might be easier to deal with. Make sense?