Q: Over the years we have learned that when we machine a profile into a piece of wood, that we also have to machine the reverse side. If we just do the one side (the face), then the pieces will often warp; when we do both sides, no warp. Two questions: First, is machining both sides really doing something? Second, if this machining is really doing something, can you explain what is happening?
A: The answer to your first question is, "Yes, machining both sides is an effective way to prevent warping when machining."
I will answer your second question, but I hope you will forgive me if I get too technical. The explanation is indeed quite complex; but if you understand what is happening, this understanding will be invaluable in developing a quality product.
When lumber first begins to dry, the outer fibers (we call this the shell) will dry first and begin to shrink. However, the wet core will resist this shrinkage. As a result, the shell cannot shrink as much as it wants to; the net effect is that the shell dries in an enlarged shape. As drying proceeds, the core will eventually begin to dry and shrink. As the core shrinks, it pulls on the shell, compressing it. This is the final stress pattern at the end of drying. So, now, when you machine the lumber, if you just take off the compressed shell on one side, this unbalances the stress pattern. The compression in the shell on the un-machined side can now cause the piece to immediately warp toward the machined side; that is, the shell on the un-machined side will expand back to the enlarged size. But now, if we machine the shell on the un-machined side (that is, machine both sides of the lumber), then the stresses in the remaining piece of lumber will again be balanced and the piece will be flat.
The stresses in the shell before the lumber is machined are sometimes called tension set by the academic community. However, the more common name in the industry is casehardening. Casehardening can be removed in the dry kiln as part of the standard drying procedures. The removal of casehardening stress is called stress relief or conditioning. Most kiln operators are familiar with the process and it has been discussed in this column frequently. However, most operators are only familiar with transverse casehardening, which results in cup when machining. Lengthwise warp (bow or side bend) when machining is the result of longitudinal casehardening, which requires more care and attention when conditioning, and requires a different type of stress test to make sure that such stresses have been adequately removed. Send me a SASE for a copy of the specific instructions for kiln operators on conditioning.
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