Date: 2011-01-25
by Gene Wengert, gwengert@wattnet.net
Q. One of our customers has been purchasing hard maple KD S2S 1-5/8 inches thick and 1-1/2 to 2-1/2 inches wide. Here are their problems: 1. Movement of the rips within a minute or less after ripping and prior to gluing. 2. The glue stave moving prior to gluing. 3. The glued-up piece moving prior to finishing. 4. A few glue lines “opening up” after staining, where they can’t see it until the first coat of sealer is applied. A quick moisture test indicated 8.8 percent MC average.
A: Based on your brief introduction, it sounds like we are dealing with two problem.
Any movement that occurs after machining is always moisture related.
The first, based on description 1, is lengthwise (longitudinal) stress that will cause immediate warp just after ripping. A change in stress relief procedures in the kiln (achieving higher RH more quickly) will cure this. The test that shoud be conducted at the kiln or before lumber is ripped is that a piece about 8 inches wide and 2 feet long is ripped in half. The two 4-inch pieces are fit back together (minus the sawdust). If there is no stress, then the fit should be essentially perfect.
The second, based on descriptions 2 to 4, is the difference between the MC of the lumber and the EMC of the air. Note that any movement that occurs after machining is always moisture related. With this drying, some warping can occur at times. Next, moisture drying just from the ends will cause the ends of the individual staves to dry out and shrink a small amount after ripping. Then when glued, the slightly larger gap between pieces, due to the shrinkage of the ends, will lead to a weak joint. Unfortunately, a few wet pieces can really affect this defect, so we are worried more about the few wet pieces then the average MC, although both are important. It is also pieces that are a bit wetter than the average that will have longitudinal stress.
So, it is my guess, based on the limited info, that the kiln operation does not have the best moisture sampling procedures...specifically, the wettest lumber is not being sampled or the samples are not being properly placed within the load.
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.
Updated: Jan 17, 2012
This article appeared in CabinetMakerFDM, March 2011. ©Copyright 2012, All Rights Reserved.