Q: A fellow employee suggested that I contact you to see if I could obtain statistics on board crook (also called side bend). I am looking for information that would tell me what percent of lumber has more than 0.5 inch crook, by species (hard maple, cherry, walnut, white oak, birch) and by grade.
A: Actually the amount of crook varies more by how the sawyer saws the log (more if he saws it incorrectly) than by species or grade. In other words, you can get crook from every species if the lumber is not correctly sawn from the log. Having said that, the species you listed, except for birch, which I do not have much experience with, do not have excessive crook. Cottonwood could indeed have more than these species. We also know that crook is more severe in lower grade lumber. Let me explain the reasons in more detail.
Crook results because the lumber is shrinking lengthwise (not a normal event) and this lengthwise (or longitudinal) shrinkage is not even, but one edge is shrinking more than the other. Longitudinal shrinkage is somewhat normal or often found in wood from near the center of the log, so if the sawyer produces lumber that has one edge closer to the center of the log than the other edge (such as with quartersawn or riftsawn), then the edge that is closer will shrink while the other edge does not. The closer this edge is to the center of the log, the greater the risk. As the center of the log is also likely to produce lower grade, we see more crook in lower grade lumber. (This crook-prone lumber is produced because the sawyer is turning the log incorrectly from face to face. He needs to use 180 degree rotation from face 1 to face 2. This will keep the rings centered edge to edge.)
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