by Professor Gene Wengert
I own a small furniture manufacturing plant that specializes in pine bedroom furniture. For the tops of the case goods we use a 2" x 6" #2 com ponderosa pine that nets at 1-1/2" thick. After the panels are glued we plane them to 1-1/4".
Our problem is warping and cracking along the glue lines. We clean up both edges on a table saw with a power feed attached, then rip it down the center to make two 2-1/2" rippings. The speed on the power feed is on the slowest setting and we're using a 10" 40 tooth blade. We are careful to alternate the growth rings and use adequate pressure using our 'Doucette' clamping system. We apply the glue with a small Black Bros. glue spreader.
We are getting frustrated with our hit and miss attempts to solve this problem. We have measured the moisture content on the warped panels and they are no different than the panels that turn out fine.
When you check the MC of a split panel, is the check made AFTER the panel has split? (Recall, you stated "We have measured the moisture content on the warped panels and they are no different than the panels that turn out fine.")
The reason I ask is that a check made after the split occurs will always show the same MC as an unsplit panel--if the panel was initially too wet and then dried out, causing shrinkage, a check made after splitting and warping is a check made after the panel has dried.
What is needed is a check before anything happens. We also must remember that wood doesn't shrink, swell, or warp except for one reason--change in MC, which is related to changes in relative humidity (not temperature).
Also, keep in mind that a glue joint is always stronger than the wood itself--a broken joint means that the two pieces didn't glue well and the #1 cause of a bad glue joint is that the two pieces didn't mate perfectly, which in turn could be that with wetter pieces, especially the freshly ripped edges, there is some drying out, shrinkage, and un-true edges within 30 minutes after sawing.
Several other questions:
How do you know the MC of the incoming wood--If you check it, how do you do it? How is the wood stored--what is the RH? How long after ripping before the wood is glued?
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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