Hi everyone,
I was reading an article in the summer of 2009 issue of WOODWORKING magazine on wood movement as you cut. The article goes on to say that the stresses that cause movement while you rip a board are caused by improper kiln drying. They refer to it as casehardening an go on to give a good description of what is happening during the drying process to cause casehardening.
I have experienced this phenom many times. I’ve had boards open up as mush as 4″ and close up tight enough to stop the table saw blade. I usually set these boards aside and use them in applications where stability is not critical.
My questions are: “Is there any way to know if a board is casehardened before you cut it?”, “Is there anything that can be done about casehardening?”, and “If you use the board, will it continue to warp over time?”.
Scott
Replies
Scott,
Not all tension that leads to warping when resawing is caused by case hardening, stresses in the wood can also be caused by the growing conditions of the tree the plank came from and/or the current distribution of moisture in the plank, even if the wood was properly dried.
In general, any board that shows dramatic movement when sawn, especially if it has had a chance to acclimate several weeks in the shop before being sawn, is too unstable for use in furniture.
John White
Shop Manager for FWW Magazine, 1998-2007
I think the article you mention includes a diagram of the "tuning fork" cut, right? You cut off about 1-2" of the end of the plank, then cut out the middle so that the piece is shaped sort of like a tuning fork. Improper drying of the wood will show up as curvature of the "tines" of the fork.
In practice, there's not much you can do about it. In principl, additional heat and moisture treatment can relax the tension, but it's pretty unpredictable.
-Steve
I don't think that I buy the argument that every board that opens up (or closes up) as it's ripped was incorrectly dried.
Drying stabilizes the wood, but stable doesn't mean that internal stresses were eliminated. Internal forces can still exist that are resisted by a 5" wide board, but will cause movement in the ripped pieces when the cut passes thru the stressed region.
I don't get overly concerned about small amounts of movement (~1/4") and have seen ripped pieces actually straighten (or curve even more) if left alone for a while.
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