Cupping of resawn lumber

      The two reasons wood cups when resawn. 1998.

by Professor Gene Wengert

I've recently experienced cupping problems when I've resawn lumber. Is there anything I can do to reduce (or eliminate) the cupping?

There are two reasons for cupping of resawn lumber. First, if the cupping occurs when resawing (or if any warp occurs while machining) and there are no severe moisture gradients from shell to core, the cause is drying stress (also called "casehardening," although the case is not harder than the core). Drying stresses are normal--they result because the outside wood fibers begin to shrink very early in drying, but are restrained by the core, which isn't yet ready to shrink. Thus, the outer fibers are left in a "stretched out" condition, which is called casehardening, when drying is completed. The second cause, characterized by the warp showing up an hour, week, or month after resawing, is moisture content differences between shell and core.

Many try to reverse the damage done to hardwood lumber by drying stress through "conditioning," a simple steaming treatment performed after drying. In the conditioning process, moisture is quickly added to the outer fibers, which try to swell but cannot because the core... This is the reverse of what caused the stress originally. Now the opposite stress has developed, which offsets the casehardening stress. Conditioning is so fast and easy, it is hard to imagine why anyone would not deliver lumber that is free of transverse (across the grain) drying stress.

Hopefully, you didn't pay much (or anything) for this improperly dried lumber.

It's interesting to note that resawning can introduce stresses along the length (longitudinal) of the board that cause the piece to bow or side bend when ripping. These stresses are more difficult to remove.

It is axiomatic in the wood drying business, however, that the best dried wood is wood that you dry yourself. Another rule is to resaw green, if at all possible, and dry the thinner material.

Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.

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