Q. We are having, and so are our neighbors, quite a bit of grain raising, which seems to show up after finishing. We keep our plant in the low 40 percent RH.
A. It is difficult in a few words, to respond completely to your questions. Appreciate that grain raising is basically due to fibers that were crushed or pushed into the wood surface during machining. These fibers absorb moisture and then swell and spring back to their original position. Water-based finishes make this effect worse. What can you do?
First, most homes and offices during the heating season are around 30-35 percent RH. By running at 42 percent RH (8 percent EMC), a manufacturer is potentially swelling the dry surface fibers in pieces that dried to 6 percent MC and also pushing any problems to the customer’s home or office where it is drier. So, I suggest 35 percent RH is a better value for a manufacturer.
Grain raising will be worse if kiln drying uses a 40 degree F depression or larger, or under 5.0 percent EMC at any time in the schedule. This dry condition makes wood fibers quite strong and more prone to being pushed over rather than being cut. Sharp knives are essential for best cutting.
A big factor in grain raising is almost always the result of dull tools, especially sandpaper. Many people do not appreciate that sandpaper dulls, so it needs to be changed maybe 30 percent sooner than what is presently done. I would encourage you to run a test. Right after changing sandpaper, note if grain raising stops or is greatly reduced.
We also see grain raising from excessive "land" on a knife. Land results when knives in a planer are "jointed" to get them in the same cutting circle. Carbide knives often have too much land as they cannot be sharpened as keenly as HSS.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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