|Home » Forums » Solid Wood Machining Forum » Message||Login|
You are not logged in. Consider these WOODWEB Member advantages:
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?
I do not believe glue would cause this on 4/4 Walnut. It could be that the wood used had a few fine cracks or weak areas you did not see in the boards prior to glue up. Letting them sit for 3-4 months would have given them enough temperature and moisture changes to cause the cracks. You may not have seen them prior to machining or they were inside the glue up.
One of the problems with letting unfinished wood sit so long is that the center of the boards that were cut to make your posts are not as dried as the ends of the boards. Those new ends that we're cut dried out and cracked. This is even worse when there are defects that are now exposed to air.
If the cracks were all along the glue line between laminations, i'd say adhesive issue. Along the wood itself, wood fiber breakage. Both, makes me wonder if the laminations followed a counter-pressure method of back to back rings--if your adhesive is strong enough, but the stacking not back to back, it could tear the wood/laminations like that, once subjected to time/warpage.
Just a thought. Could be a number of things, really. Got pics? Could help.
Another thing could be not cutting off enough end from the raw stock--what maybe a hairline fracture at glue-up can get split apart under pressure after sitting for a while.