Q: We have some small tables that we make using 1-1/2-inch-thick, edge-glued strips of oak. To reduce the weight and help fasten the legs, we rout the inside bottom of the table, giving us a thick lip around the edge. We are located near Chicago, and shipped these tops to a shop in the northern Rocky Mountains, where it is really dry outside, and as you might imagine, they warped. Can you confirm what we think is happening? Any solution?
A: Here is what is happening. Letme know if this brief explanation needs more detail.
1. Wood does not change itssize or shape unless the MC changes. So, we know there has been a significantMC change from the time you made these tops until the customer called tocomplain. In fact, it sounds like they were acceptable when they first arrivedin the Rockies; so we can say that there has been a moisture change after thetops arrived.
2. You are correct that it isdry in the some locations in the Rockies, with some years being drier thanothers. For example, on the average the outside relative humidity in Boise,Idaho, in August drops to 19 percent RH or drier on three out of four days. Interms of wood, we would say that the air is 4 percent EMC (equilibrium moisturecontent), meaning that the wood will be trying to achieve 4 percent MC. Theoutermost surface of uncoated or unfinished wood will be close to this valuewithin hours, but it may take days or weeks for this drying to affect the coreof the wood.
3. In these sort of problems,there is always the debate of whether the tops were too wet or the atmospherein the customer’s location was too dry. We can expect that almost allcustomers in North America will use these tops at an average relative humiditybetween 30 percent RH (wintertime) to 50 percent RH (summertime); those are theaverage inside humidities for almost all of the U.S. This is equivalent to 6.0percent MC to 9.0 percent MC in the wood. Therefore, it would be"normal" for you to manufacture these tops at a moisture within thisrange.
4. Because you are shipping ataround 8 percent MC, exposure to 4 percent EMC will result in rapid drying andwith this drying will be accompanied by shrinkage.
5. Wood shrinks across thegrain and not along the grain. Because the outer edge of the disk is thickerthan the rest of the piece, any shrinkage will be greater in the outside. Ifthe glue joints were inferior, this shrinkage would result in joint failure. Ifthe joints are strong, then as the rim shrinks, the piece will warp. Technically,when it shrinks, the outer rim has a slightly less radius by warping. If youhad measured the disk's diameter upon receipt and then after warping, you wouldalso see a size change across the grain. With oak, this would be about 1percent shrinkage with a 3 percent MC loss.
6. If the disks had awater-vapor-proof coating applied before they warped, the amount of warp wouldbe somewhat less. That is, the entire piece would be shrinking more uniformly with this moisture barrier on the outside. However,appreciate that it is impossible to seal the wood (except possibly if the pieceis totally coated in thick wax or with acrylic). So, some movement is going tohappen if the MC is not close to the EMC.
7. The only way to eliminate the warp would be to dry thewood used for the tops originally to a very low MC (around 5 percentMC) but then they would glue poorly and machine poorly compared to the normal 6to 8 percentMC. Thenstore this dry wood and the manufactured tops in a plastic bag or wrapped inplastic at all times so that they cannot change MC in the more humid Illinois. If there is no MCchange, there is no warp. (Ofcourse, a design change to eliminate the thick rim would also help, but that isapparently out of the question.)
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