I received this question in episode 7 of the Ask Brian Boggs Show:
“I cut my mortise and tenon joints so that they can be assembled by hand pressure and they don’t fall apart from the force of gravity. I presume that means you’re holding them up so the thing, the tenon would fall out. “Many times I find that if I wait a few days prior to assembly, the joint loosens up so that it no longer holds together. My guess is that even though the wood is kiln-dried, the new exposed wood lost a bit of moisture, which caused it to shrink some. After applying PVA adhesive, the parts swell back up, and the parts fit together firmly again.”
The question is, “Will a joint displaying this behavior likely to be problematic in the long run?”
The short answer is, absolutely it will. There are a lot of things that have to be working right in order for a joint to work. What you’re doing is trying to integrate this, ultimately, integrate the strength of one part into the strength of another and have the least amount of strength lost in that transition. You can’t do that mechanically with a mortise and tenon joint alone without a good glue bond, because, as you said, you can put that in with hand pressure. That means you can pull it out with hand pressure. That’s just not enough to hold a piece of furniture together. You’ve got to have a glue bond. What’s important in order to get a glue bond is that you’ve put enough wood, or ideally as much wood as will work, in that mortise in the form of a tenon. The more wood you can get in that mortise for its size, the stronger the joint you’re going to have, provided that you’re getting a good glue bond.
So there are two problems I’m seeing with what’s going on. One is that it appears that the wood isn’t dry. Kiln-dried does not mean the wood doesn’t have moisture in it. It just means that somebody sent the wood through a process that allows them to charge you more money for that wood than green wood. I have been buying wood from suppliers around the country and across the state, and I have not found a single supplier yet who has failed to send me wood that does not need further drying. Now, they will sometimes send dry wood, but they will sometimes send wet wood. It’s up to me, as a consumer, to pay attention to what I’m buying and put a meter on it before I’m buying it, if I have the opportunity, or at least order it soon enough so that I can meter it when it comes in, and I have enough time to finish drying it. That’s kind of a problem in our industry, and we can’t fix the industry, but we certainly can fix our process, so a moisture meter, a good electronic moisture meter, I use a Wagner. It’s an old, pinless Wagner.
We’ll talk about moisture meters on another show, but getting that moisture down to at least as low as 8%, if not lower, for assembling interior furniture is really important. It doesn’t matter how perfectly you fit that joint. It doesn’t matter how strong the wood is, and it doesn’t matter what kind of glue you use. Well, actually, it does matter what kind of glue you use, but all of those things have to work for you in order to get a joint that’s going to work over time. What’s probably happening is, the wood just wasn’t dry, but what can also happen is, you can put a meter on the surface of an 8/4 board and get a very low reading. Once you cut that open, meter the inside, and check again to make sure that the reading on the outside and the inside are both dry enough to do whatever you’re going to do with it, whatever … We make furniture, sometimes, that doesn’t require bone-dry wood like that.