Wood Movement

[Patrick Harper]

I have a quick question regarding wood movement. I recently purchased a DW735 power planer. I decided to test it out on some flat sawn white-oak I had laying around. The wood has been in my dry shop for several months. It was straight, but fairly cupped to the bark side of the tree. I jointed the concave side and then ran it through the planer with the jointed face down. The board came out extremely flat and I was very pleased with the surface quality. I left the piece in the garage for several days. About a week later I checked the piece again and it had cupped a bit on me. The wood is pretty dry and I only took about 1/32nd off of each side. Is this fairly normal?

[Mel Fulks]

If it's flat on a table flip it over for a while and it will probably straighten. After it does ,stand it up so that air is on both
sides ,or leave it flat and cover it with more wood.

[Andrew Pitonyak]

It seems to me that almost anything you can do to a piece of wood may change the internal stresses on the wood and cause it to change shape. It is difficult to say if the wood changed shape because of internal stresses, or, if it had to do with how storage conditions might affect moisture entering or leaving the wood (like if it is laying flat on a surface so the exposed side has more exchange or the bottom side absorbs something or has something leeched from eat).

I often see advice such as. Mill your wood close to final thickness and then let it sit for a day or two (some people claim one week) to accommodate wood movement and then mill it to final thickness.

Another rule of thumb seems to be that you want to take similar amounts off both sides of the board. So, if you need to remove 1/2", don't take 1/16" from one side and 7/16" from the other. I often violate this principle when there is some sort of damage on one side, but this can prove to be a problem later.

I will admit that I do not have an ideal setup, but, for much of what I do, absolute precision is not overly important. For example, I recently built a box using wood that was a wee bit problematic

http://www.sawmillcreek.org/showthre...-a-locking-box

There was some wood movement after final milling; not a bunch, but some. When I assembled and glued, I used some clamps to pull things tight, which removed the slight cup (or what ever movement there was). I don't know if it will mater for a dove-tailed box, it might fail or crack, but for this I simply don't care. I have not had any pieces fail when I do this, but I don't do it often, it is not a large correction, and my pieces have generally not had more than a few years (less than 10) to fail; I started wood working later in life.

[Andrew Pitonyak]

If it's flat on a table flip it over for a while and it will probably straighten. After it does ,stand it up so that air is on both
sides ,or leave it flat and cover it with more wood.

Oh yes, I forgot to mention that some people attempt to remove the cup by introducing moisture (but probably not overly effectively)... For example Flexner recommends quickly wetting the convex side. The water causes the wood to expand (or try to expand), but it cannot so the board develops a compression set (some cells that fail to expand squish). Do not wet for more than a few hours. Remove the water and then let this convex side dry (not too fast, such as using a hair drier) and shrink. Hot water works better. The problem with this technique is that it is hit and miss as far as how much water to add.

[Prashun Patel]

Yes it's entirely normal.

Assuming it was fully dry before being in your dry shop for several months, I think the cupping had more to do with putting the piece in your garage than in planing down 1/32". Take it into your dry shop and see if it improves. I would also cut the pieces close to their final lengths/widths now before jointing them to perfection. You'll have to remove the least material this way. You may even determine there 'good enough' to use without further correction.

[Patrick Harper]

Prashun, my garage is my shop. It's pretty dry in their most of the time. The board was left flat on a bench top. As others mentioned, it's possible that it gained/lost moisture unevenly because of this.

[Jim Becker]

It's also important, whenever possible, to remove similar amounts of material from both sides of a board to help have a stable moisture content left in that board through the thickness. When flattening a cupped board, for example, you do not need to take it all the way on the jointer face planing. You take enough passes on the jointer so that it will run flat and true on the planer and then take light cuts on both sides to bring to your final thickness, making a judgement call around the so-called "equal amounts" from each side. It may take a little more time, but the end result pays.

[John TenEyck]

Yes, if you left the wood flat on your bench then it likely gained/lost more moisture on the open top surface than on the bottom which caused it to cup. I make it a habit to leave my planed boards on edge or leaning against my bench and that eliminates most tendency to cup. If your wood is at EMC with your shop you can plan off as much as you like from one side and the board won't warp, at least not due to non uniform moisture. The problem is many folks don't have climate controlled shops so their wood is never really at EMC. So if you plane off a lot more wood on one side than the other the MC is likely to be different on the two faces and the board will cup. Since you work in a garage shop where the RH will shift with the weather it would be good to plane off relatively equal amounts from both faces.

As for straightening cupped boards by wetting one side, clamping, whatever - it's a crap shoot. It might work if the stars align just right, but generally the board will go back to the way it was after the moisture content equalizes again.

John

[Danny Hamsley]

I am with John. If left flat on the bench, and if the board was not in equilibrium with the garage environment, one side lost/gained more moisture and the result was wood movement. Wood only moves if it is not in equilibrium with the environment. Laying it flat on a bench, floor, etc. where one side is exposed to air and the other is not can create stress that leads to cup and warp. Storing the board where the air can circulate on all sides is a very good practice.