Q. What causes door rails, especially long ones, and stiles to warp during and after machining?
A. Wood, in manufacturing and normal use, moves (warps, shrinks or swells) for one reason only and that is because its moisture content (MC) changes. There is one exception: wood moves immediately while we are machining it because of stresses; drying or growth stresses. Any movement that occurs over time is related to MC change. Basically, with no or very small MC changes, there will be no warp.
Here is what you need to do for straight pieces. When you receive the lumber that you will immediately machine, check the moisture content. (You might also check the lumber’s MC when you first receive the lumber, but it is very important to check the MC just before machining. The MC can change in storage.) The MC, both shell and core, should be checked with a pin-type moisture meter.
Here are the rules for hardwoods (leaf trees). The MC for every piece should be within 1.0 percent MC of the expected wintertime conditions in the home or office where the wood will be used. In much of the U.S. and Canada, this is expected value is 6.5 percent MC or about 33 percent relative humidity (RH). Because hardwood lumber machines poorly (chipped grain and worse) when under 6.0 percent MC, we want all the wood between 6.0 and 7.5 percent MC.
It is indeed possible that the lumber will regain some moisture in the summertime as the humidity is often close to 50 percent RH, but with dry wood, there is about a 1 percent MC gain before we begin to see any changes in size or shape. Hence, we can be a little more on the dry side safely that is why we target the wintertime RH. This procedure is the best we can do; any wide changes in humidity cannot usually be tolerated without some risk.
The other issue that you may have is that the wood might have stresses. Growth stresses, which develop within the tree, cause lumber to warp during drying. So, be aware that warped lumber is a strong indicator that your small pieces like rails and stiles have a tendency to immediately warp when you machine them.
Likewise, during drying, we can sometimes end up with stress in the lumber caused by normal wood shrinkage; such stress, also called casehardening, is normally removed at the end of drying using a process called conditioning. There are two tests for stress, but for long rails, I suggest taking a piece of lumber about 24 inches long and about 8 inches wide and then ripping that piece in half.
Then place the two 4-inch-wide ripped pieces back together (minus the saw kerf). They should fit tightly; any gap indicates drying stresses that should have been removed. Note that this stress test is best done on the lumber right when you receive it. The stress will not change over time.
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