Q: This sounds like a funny question, but I think that it makes sense. We make an oak product that is similar to flooring. It is machined to a precise size and at a precise moisture content (MC). Our customer is now complaining about the pieces being too wide, by a small amount. The customer says the MC is correct now (7.5 percent). So, either we set the machine wrong, or the wood was too dry when we machined it and there was swelling. Can you help with the swelling calculation?
A: Did you use a pin-type moisture meter to check the MC? If so, these meters will not accurately read below 6.5 percent MC, so the wood you machined could have been quite a bit drier and you wouldn't have known. There is an easy way to calculate the size change for a given moisture change, or the moisture change for a given size change.
Change in size = [Original size] x [MC change] x [coefficient]
The coefficient depends on whether you are going perpendicular (calledradial) or parallel (tangential) to the annual rings (when looking at the end grain). For red oak, the coefficients are: .00158 radial and .00369 tangential. For white oak, .00180 and .00365. For Southern oak, the tangential value is larger - up to 50 percent. (Readers: I have coefficients for all important U.S. wood - let me know what you need.) As an example, assume that you machine a piece of red oak to 2 inches in width (tangential) at 5 percent MC. When the piece is at 8 percent MC, the size change will be [2.50"] x [3 percent MC] x [0.00369] = 0.028 inches (or nearly 1/32 inch). Is this the magnitude of the size change problem that your customer is having?
Have something to say? Share your thoughts with us in the comments below.