Why did this board cup?

[Glen Blanchard]

I know that this is something from Woodworking 101, but I’d like to see if I have a handle on it. Indulge me if you will.

I bought a (approx.) 54” board of 4/4 Jatoba on e-bay not long ago. It had been sitting in the shop for 3 or 4 weeks, so I assumed it had acclimated to the shop. A few days prior to Christmas, I milled this piece to ½” thickness in anticipation of using it for my next box. I then left the newly milled and perfectly flat board untouched (Christmas, vacation, etc) for about 10 days in an unheated shop. Upon my return home, I went into the shop to check on this board. To my disappointment, it is now cupped beyond my ability to use it for its intended purpose. Let’s see if I have a good handle on what happened.

I assume that this board was not as dry as it should have been (I don’t own a moisture meter) and that upon re-sawing, the board continued to dry - albeit unevenly - and thus the cup.

Could it have had anything to do with my unheated shop? Was there anything I could have done differently to avoid this? I assume that had I finished my project right after it was milled (and prior to the cupping) that the wood would still have attempted to cup, and would have ruined any assembled project for which this board was used. Does all this sound about right?

[Kelly C. Hanna]

Jatoba is not stable to my experience. I had the same thing happen to three 1"x8"x40"'s that had already been milled [but not planed yet]. One twisted up like a potato chip [well it wasn't that bad I guess], but I still made a sign out of it!

[Jim Becker]

Did you take off equal amounts of material from both sides when you thicknessed it? If not, the moisture on one side may have been very different from the other and then the moisture got released after you did your thing...I

[Dan Forman]

Did you take even amounts off of each side (1/4"), or just saw it in half? The former is the preferred technique, the latter can encourage wood to do strange things due to the fact that the recently exposed inner portion probably has more moisture, and now it is leaving the wood at a faster rate than the other side. Also if you didn't sticker it or leave it on edge, so that both sides had exposure to the air, it could move because the side exposed to the air would lose more moisture than the other.

The ideal solution would be to face joint and plane evenly, but leave enough extra to do the same if moves after initial milling, then come back when you are ready to begin work and if it's still flat, plane, if not, joint and plane.

Don't quote me on this, but I think if done this way and your joinery is sound, unless the wood is extremely unstable or your project very delicate, you should be able to use it even if it still moves a bit, as the project will hold itself together. Hopefully if I'm wrong, someone else will jump in and point that out.

Edit...Oops, looks like Jim beat me to it.

Dan

[Steve Schoene]

When you left it was it laid on the bench, or was it stickered with air circulation on all sides.

At this point you should sticker the board, with a bit of weight on the top. If the problem that caused the cupping was differential moisture between the sides, it will rectify itself and the board will flatten out.

Unfortunately that is not the only, nor even the most likely, cause of the warping, given the circumstances as you describe them. Boards that have not be properly dried develop internal stresses, even when the moisture levels have equilibrated throughout the boards thickness. Consequently, when the board is milled the stresses become imbalanced and the board warps. This warping is more or less permanent, and you are stuck using the lumber to make only small objects.

[Glen Blanchard]

Equal amounts from both sides......Hmmmmm.......Let me think......That would be a NO!! Just cut off one side (resawed in an attempt to conserve the timber...HA!!) and planed the rest to dimension. That could explain some or all of it.

[Mike Armstrong]

Everybody's pretty well covered it, so you probably won't make that mistake again :o, but, don't worry, there's plenty more left :D.

I'd add that had you built your box (and especially applied a finish), it wouldn't necessarily be ruined, as the wood movement would probably not be as dramatic. So, how about a box out of 3/8" material?

Mike

[David Kauffman]

Glen the bottom line is, when working with thin pieces 1/2 inch or thinner, unless your shop is climate controlled and the humidity does not change from day to day, the wood is possibly going to move a little, no getting around it. I have found that the thinner the stock, the more likely it is to cup or twist slightly over the course of a few days just setting there all by itself. However, if you did plane that down and use it right away in a box or something glued/screwed/dovetailed etc, it might have been fine, and would not necessarily have pulled your whole project out of whack that much.

Cold air holds less moisture than warm air. Thus if the RELATIVE humidity of outside cold air is say 50%, that means there is half as much moisture in the air as it can possibly hold at that cold temperature. Then you bring that cold air inside your house and heat it up. Now that same air CAN hold a lot more moisture because it is warmer, and thus the RELATIVE humidity of that same air is even less, and that air is even dryer. My point to all of this is, when you change the temperature of that shop by much, you change the relative humidity of that shop, and thus wood will tend to move more in that changing environment.

In general, when I need thin pieces, I mill them and use them soon therefter as I find it hard to keep them from moving on me no matter how hard I try and keep the environment from changing. Houses breath... and as outside humidity levels go up or down, so does the air inside your house or shop.

[Glen Blanchard]

In general, when I need thin pieces, I mill them and use them soon therefter

Yep. I usually do the same. In fact, this is probably the first time I have milled a whole board and let it sit for that long. Under normal circumstances it would have been crosscut into shorter lengths and glued up long before those 10 days came to pass. That may explain why I have never had this happen to this extent before.

Thanks guys. Another day, another lesson.

[Aaron Beaver]

Just wondering....Does it help if you only partially joint and plane the board let's say 1/8" of each side, then let it acclimate again before taking it down to the final thickness? Or if taking 1/4" off each side and stickering would of been the best way to go?

[Harold Beck]

Whenever I resaw thin stock from thicker stock and am not using it right away I stack and weight it or clamp it flat. I have had a lot of 1/4" stock to what you called the potato chip flex.

Once it gets fabricated into whatever I am making I try to finish it quickly also.

Once I started putting a couple of bricks or clamps on top of the stack I cured almost all of my problems.

HB

[Mark Singer]

Occasionally even a dry piece of stock will deform after resawing. It is rare, but there are internal stresses in each board and as they are resawed, there may be an unbalance. This will lead to a warping...it often happens immediatley when cutting. You may also see it when ripping at the tablesaw...as you rip the free piece curves off away from the blade, or wants to pinch the blade.. Prior to cutting the forces were in balance and as it is cut there is an internal tension that deforms the cut piece. What I reccomennd is to make a curve object:rolleyes:

[Don Henthorn Smithville, TX]

As has previously been mentioned I think your problem comes from laying the board flat on the bench after machining to size. I would advise that you don't develop that habit. While all the other answers have merit, my first suspicion would be in this direction. A newly machined board will be losing moisture or gaining moisture to be equalized with the surrounding air. If the board is left on a flat surface only one surface will be losing or gaining moisture. The board will almost certainly cup in one direction or the other. My advice would be to sticker the board with a weight on top or set it on edge so that the air can circulate around it and wait for a couple of days or so and see if it straightens out.

[Bert Johansen]

Here's a possible solution to FIX the problem. On a dry, sunny day, wet the concave side and lay it in the sun on a dry, flat surface, wet side down. After an hour or two it will have flattened out. Be careful not to leave it too long--it could cup the opposite direction!

[John Appleseed]

I remember reading somewhere that there is some product that you can spray or wipe on a cupped board and after applying some pressure to it for 24-48 hours it will straighten out and remain that way.....wish I knew what that product is....

[Pete Brown]

I don't get a lot of time to do woodworking, so I often end up milling a lot of pieces and then waiting a couple weeks before I am able to cut them to final length and assemble.

During that time, I'll wrap the pieces all up tight in plastic wrap. There's a photo of that here (http://community.irritatedvowel.com/blogs/pete_browns_blog/archive/2006/10/20/Base-Corner-Cabinet.aspx). I keep them in the house (which was where they acclimated to anyway). Like others mentioned, I try and take equal amounts off of both sides.

The wrap slows down the moisture transfer while also keeping them all tightly clamped together and therefore flat.

All the pieces I've done this with have worked well. The pieces that I left out typically ended up with some bow or twist.

Pete

[Cliff Rohrabacher]

Always cover your wood when you shut down for the night.

When the temprature drops moisture settles and the face of the board laying open and exposed will get the water and swell causing cupping and twisting.
Then when it dries it can retain the cup or twist it picked up.

It was either that or you relieved stresses in the wood and it simply responded.