Q: We have some cypress 4/4 lumber that the customer returned as being unsatisfactory after he has already put it into use. The surface is corrugated and very bumpy, but I know that it left here smooth as a pancake. Can you tell me what has happened? Also, in a few places, the grain where the grain is very flat has separated and you can lift up long slivers with your fingernail.
A: You are describing the defects that we call grain raising (which makes the surfaces rippled like a washboard) and ring separation (sometimes called shelling, separated grain or grain separation).
Grain raising is actually a machining defect. When machining (planing or moulding) flatsawn lumber, the knife has a choice of either cutting away the fibers or pushing them out of the way. With a wood like cypress that has the hard dense wood in part of a growth ring and also very soft light-colored wood in the ring, the knife, when it hits the hard wood, will often push this wood into the softer wood underneath; the softer wood cells are actually crushed. The wood will initially look very smooth, but when the wood is subjected to wetting, then the crushed cells will recover and push the hard wood upward, creating the ripple effect.
Shelling looks like shake, in that it runs with the annual rings and not across them, but shelling is much more localized and is almost always clearly related to grain raising and is restricted to the face of flat sawn lumber. In extreme cases of raised grain, when the hard wood is pushed down excessively, the cells will actually separate from each other, creating the ring separation or shelling.
Although both of these defects are related to the wood structure, they are accentuated by low MCs when machining, high feed speeds, dull knives, small rake angles, carbide knives and high stock removal rates. A little self-examination should nail down which one or two items are causing the problem in your case.
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