Q: How can warp be prevented when resawing 5/4 basswood into two thinner pieces?
A: The key to preventing warp when resawing is to have lumber that is without stress. If the lumber has transverse (across the grain) stress, then the lumber will cup when resawed. If the lumber has longitudinal (lengthwise) stress, then the lumber will bow when resawed. Both forms of stress can be nearly 100 percent eliminated by proper drying procedures, called conditioning. It is common that many kiln operations will only check for transverse stress. (The test uses a small piece of wood cut from the lumber. This piece is then further sawn into a piece shaped like the letter U. If the legs pinch together, then there is certainly excessive stress.)
If you have transverse stress and the resultant cupping, it is clear that the lumber was improperly dried. Such stress would be unacceptable throughout the secondary hardwood manufacturing industry.
A separate test for longitudinal stress needs to be made, especially for lumber that will be resawn. It would be common, without a longitudinal stress test having been made, that longitudinal stress could be present without the kiln people knowing it.
The key for longitudinal bowing is communication with the kiln people to make sure that they properly condition the lumber to relieve all stresses. Conditioning involves aggressive steaming of the lumber at 180 degrees F at the end of drying. Excellent stress relief requires a little more than just average drying quality. If you buy from a broker, longitudinal stress would be common, as the kiln people didn't know that the lumber was to be resawn, meaning that the longitudinal stress also had to be relieved.
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