Cupping of resawn lumber
The two reasons wood cups when resawn. 1998.
by Professor Gene Wengert
Q.
I've recently experienced cupping problems when I've resawn lumber. Is there anything I can do to reduce (or eliminate) the cupping?
A.
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.
Click on Wood Doctor Archives to peruse past answers.
If you would like to obtain a copy of "The Wood Doctor's Rx", visit the Wood Education and Resource Center Web site for more information.
Would you like to add information to this article?
Interested in writing or submitting an article?
Have a question about this article?
Have you reviewed the related Knowledge Base areas below?
KnowledgeBase: Lumber & Plywood: Storage
KnowledgeBase: Primary Processing: Kiln Operation
KnowledgeBase: Solid Wood Machining: General
KnowledgeBase: Wood Engineering: Wood Properties
KnowledgeBase: Knowledge Base
Would you like to add information to this article? ...
Click Here
If you have a question regarding a Knowledge Base article, your best chance at uncovering an answer is to search the entire Knowledge Base for related articles or to post your question at the appropriate WOODWEB Forum. Before posting your message, be sure to review our Forum Guidelines.
Questions entered in the Knowledge Base Article comment form will not generate responses! A list of WOODWEB Forums can be found at WOODWEB's Site Map.
When you post your question at the Forum, be sure to include references to the Knowledge Base article that inspired your question. The more information you provide with your question, the better your chances are of receiving responses.
Return to beginning of article.
|
