Q: In our manufacturing of mouldings, we use red oak and are in the process of trying to monitor casehardening. What is the easiest test to do for this? Prong? Cup? Other? Also, what is the frequency recommended for doing this test? And should the supplier be able to provide us with this information normally based on their testing or drying process?
A: You are probably more concerned about "across the grain" stress (and resultant across the grain warp when machining) than lengthwise stress (and resultant lengthwise warp). Therefore, the prong test, as documented in most drying manuals, is appropriate. You should probably detail the procedure for conducting the test for your suppliers, however, to make sure that they and you use the same procedure.
Make sure the prongs are about 6 inches long and are each 1/4 of the thickness of the lumber, with the middle half being removed. Note that the test is accurate only if there is no moisture gradient, so a kiln operation needs to use a microwave oven to eliminate the gradients in the prong before they read the results. By the time you get the lumber, all gradients should be gone.
I would check about 10 pieces in a load, chosen randomly throughout the load. If the prongs are straight (within 1/8 inch typically), then you are fairly certain that stress is not an issue. Certainly, more tests will increase your confidence. If any of the first 10 do not stay straight, then I would cut 20 more to ascertain if indeed there is a problem. Being fairly strict on stress is worthwhile when dealing with small pieces of wood, such as mouldings. For larger pieces, a little more, but not much, stress can be tolerated before creating a problem.
A good dry kiln operation should be able to eliminate all stress and should also keep records to document that they did so. Oftentimes, they get erratic stress removal when the final MC is not uniform, so they should also be able to document, and you should be able to confirm by making your own readings, that the MC is uniform. If you find widely varying MCs, expect poor stress relief.
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