Q: We are having trouble with warping. We take 9/4 thick lumber, plane it lightly on both faces, and then rip it into strips about 3 inches wide. At this point, the ripped strips are straight. We then cut thin (about 1/2-inch thick) strips (3 inches wide) from the planed face, so we get three strips from 9/4 lumber. The problem is that the two outer strips warp right away. What is this?
A: Such stress can arise from three basic causes: a moisture gradient; longitudinal drying stresses (also called casehardening); and longitudinal growth stresses. I will briefly discuss each one. Let me know if you require further information.
If a piece of lumber is a little high in moisture content for the environment that it is in, the outer surfaces will begin to dry and cause a small amount of lengthwise shrinkage. If a strip is sawn from the outside, it will curl away from the remaining piece of lumber. As time goes on, this piece will straighten partially or fully as the moisture gradient disappears.
Longitudinal drying stress
During normal drying, the outside fibers dry first and try to shrink, but cannot shrink due to the wet core. As a result, these fibers dry in an enlarged condition. When the core shrinks, these fibers are pulled on (compressed). This stressed condition that exists at the end of drying is called tension set longitudinal drying stress, or longitudinal casehardening. All three words mean the same thing. Longitudinal means lengthwise. When a strip is sawn from a piece with longitudinal casehardening, the strip will curl in toward the remaining lumber. This amount of curl will not be reduced with time. Such stresses can (and usually should) be removed in the kiln drying process by proper conditioning or stress relief. (Note: Most kiln operators worry about transverse stresses, but pay little attention to longitudinal stress. The prong test only measures transverse stress! Many drying manuals don't even talk about longitudinal stresses - a serious oversight!)
Longitudinal growth stress
In the standing tree there are many stresses. Sometimes these stresses are quite large and will exist in the lumber. Lumber from such trees is often bowed when first sawn and may have large end splits. These stresses appear different from drying stress in that the strips, when sawn, can curl up, down and/or sideways. They do not change with time. Longitudinal stress relief techniques do help relieve growth stresses too.
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