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Cracking on 6" walnut post glue-ups3/12
Anyone with experience doing really thick solid hardwood glue-ups? (6")
We recently made 8 large walnut beds with posts that start at 6" thick before machining on CNC 4th axis. We had 3 or 4 of the 32 posts develop long cracks afterward. We glued up 7 layers of 4/4 stock. Some of these posts had been glued up for 3-4 months (project delay) before machining. 3 of the 4 posts that cracked developed the cracks shortly after machining. We used normal Franklin EZ2 glue for the glue ups. The cracks were long and with the grain, occasionally partially on the glue line but usually not.
My guess is that with 7 layers the glue introduced a lot of water into the post - the cracks occurred due to the water in the glue causing the wood to expand.
I think polyurethane glue would be a better choice next time - any other thoughts? Other glues?
My experience comes from big newel posts. 4" is fine to be solid, but once we get to 5" or larger, we learned to make them hollow to avoid cracking. We turn these on a lathe, so imbalance is a problem, but a CNC would not have that problem. Obviously, you have to do careful layouts so you don't turn into 'air' at the center void. I would say that if my theory is valid, then the larger the void the less likely the cracks.
Had big time trouble with white oak table pedestal at the beginning of my career. That's when I bought a moisture meter. Did you check the moisture level of the walnut? Did you do the machining to flatten before glue up, and were these full length or finger jointed? Rift sawn and quarter sawn will give you less trouble on the glue up. Cracking mostly at the ends of the boards, or equal through out?
One of the points I'm heading towards, is that laminating that many boards will lead you to laminating different cuts of wood. Flat sawn laminated to rift and quarter sawn. You will be different expansion rates on the different boards and then you have trouble.