I've had a few instances where my glued up panels, breadboards etc. have had gaps open up at the edges. This happens a few days or weeks after gluing. The gaps have all stopped at a certain point and the join remains strong, but it looks terrible. I'm hoping to start selling soon and I can't afford for things to fail like that. It seems that the wood is moving after it gets to its new environment and is able to overcome the strength of the glue.
This has happened with multiple woods and multiple glue bottles so I know they aren't problems. I tried using splines to reinforce the joint but it still failed.
To mate the edges to be joined I start with a straight edge off the jointer, thicknesser, or table saw and then hand plane it smooth and flat. I am pretty certain the quality of the mating surfaces at this stage is not a problem.
I then usually put a very slight concavity in the surface, both across its width and its length. I was taught to do this as it supposedly helps the ends and sides of the joint that are exposed to air and moisture mate properly. I then glue with PVA and clamp.
Possible causes that have occurred to me are:
1 - The moisture content of the wood just isn't close enough to what it will reach in the environment it goes into. This seems like a big issue but one that is hard to avoid. The area I live in is fairly humid and my workshop is not well insulated or heated. The pieces mostly have gone into a very modern house which feels very dry which can't help. It seems unlikely to me that commercial furniture is made in a climate controlled environment though. Do I need to relocate?
2 - The concavity I put into the mating surfaces might need to be more extreme. Since the joint is pulling apart where it should be tightest it seems unlikely to me that the concavity is causing the problem. Rather, it seems that I might be making the concavity too subtle. Thoughts?
3 - I might need to use a different kind of glue. It is often said that any of the major woodworking glues are stronger than the wood itself but that isn't my experience; these failures are right along the glue line.
I've done some testing where I glue two bits of wood together, leave it overnight, then break it apart with a hammer. The break would generally be fairly clean with only a bit of splintered wood. This would suggest a glue issue to me.
Perhaps there is something I am doing wrong but I can't imagine what there is to mess up with PVA. I always leave it to cure overnight. Maybe I need to switch to something stronger, like epoxy?
4 - Clamp strength. I've read about glue starvation from excessive clamping pressure but I've also read that it is a myth. I would describe my clamping as "pretty damn tight". Maybe I need to either increase or decrease my clamping pressure.
5 - Reinforcement. I know a butt joint is supposed to be strong enough on its own but maybe there is something I can do to really pin the joints together at the ends? Splines didn't work but then I suspect they may only help with alignment and may not give the joint any extra strength. Is there some way to mechanically lock the ends of a but joint together that is invisible or that is easy to hide? If there is a method I suspect it would only work on thicker boards.