TB 3 looses almost 75% of its strength at over 150 degrees. I have had exterior doors fail due to this relatively unknown fact.
We do not use PVA like TB 1-2-3 since we have found that the glue line raises. We level it off - cut or sand, then a day or two later, it raises up. We even have a sample in the shop hanging on the wall with this glue line to show to customers and to remind us why we do not use TB on this kind of work.
Epoxy is easy to starve the joint. It likes it thick, which does not jive with pulling things all together. Plastic resin glue - urea resin type glue - is the best we have found for this work. Check with CP Adhesives to see what they have, characteristics, etc. You have to have 70 degrees (must have!) for wood, shop, glue, spreaders, etc, or it will not bond.
Ordinarily, a joint should be 1.4 times stronger than the wood, so if there is a shrinkage issue, the wood will break first. An open joint indicates an error in surface preparation almost all the time. Hickory retries special care when preping.
To affirm what David has already posted, TB III may not be adequate for a top that has a lot of hot pans put on it. Actually, compared to other PVA adhesives, this adhesive is formulated so that it will not soften with heat or moisture as much as the others do. There is a chemical reaction that occurs that cannot be reversed. So, do you know what the heat and moisture history has been since the piece was made and put in service?
Second, there is a very good chance that you did not make a strong joint initially. The individual pieces might not have been tightly fitting (0.006" maximum gap). Did you machine the pieces and then glue them within 15 minutes? Because hickory is so dense and moves with even a small change in moisture content, we need to prepare the surface and glue within minutes. Further, was the surface preparation ideal without excessive heat (knives or saws were sharp, no heating, etc.)?
Did you have someone examine the inside of the open joints? Oftentimes, we can tell what is wrong from this sort of exam.
So Gene, we used to wipe the mating surfaces of hickory with acetone then misting the wood before applying PUR glue then clamping. We have had no failures with this for years. We know of the benefits of machining close to the gluing time, but this is easier for us to do. We always make sure mating surfaces are as perfect as we can make them before starting the gluing process. Have others had success with this method ?
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