Q: We are using four pieces of 4/4 hard maple to form a thicker piece of wood. When we look at these pieces after they are glued up, we notice that they are warped lengthwise. We call it bow. Oftentimes they have bowed so much we cannot use them. What are we doing wrong?
A: I would have to do some testing to be 100 percent, but my initial answer is that the lumber has drying stresses in it. These stresses are lengthwise stresses in your case and would not show up in the common “clothes pin” stress test. (Drying stress is also called casehardening, but nothing is harder.) The water in the adhesive you are using releases these stresses on the glued edge, but not on the other edge. As a result, you will see lengthwise warp. The cure is to get these stresses removed in drying by properly using a process called conditioning. Conditioning is a brief steaming treatment at the end of the drying cycle.
The best way to test for lengthwise (also called longitudinal) casehardening is to take a piece of lumber about 8 inches wide and 24 inches long. Rip this into two pieces each about 4 inches wide. Put the two pieces back together. If there is too much stress, there will be a gap between the pieces when you put them back together.
Note that some species of wood, such as yellow poplar, and many non-native species, such as eucalyptus, have stress in the tree which will cause splits in the lumber and will show warp even before drying begins. These growth stresses behave similarly to longitudinal drying stresses.
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