Round Joint Fit Tolerance

Pennington Windsor Chairs writes in, “Hey, Brian. At Rundell, and Rundell and I were discussing in our class recently how tight the 5/8 mortise-and-tenon for Windsor and ladderback chairs should be after super-drying the tenon and using hide glue. Your thoughts?”
Well, the language used in fitting joints is often similar to the question I answered in the Impact of Moisture in Joinery post, how tight it should feel, and truth be told, until you’re putting numbers on it, it’s really hard to have a valuable conversation. What I’ve done is, over the years, is just gotten used to using a dowel caliper, and I have toyed with different tightnesses of mortises. Early on in my career, I was told by several master chairmakers that 10 thousandths’ oversize for a tenon, in other words a tenon being 10 thousandths larger than the hole, is a really good size for a green wood joint, and I’ve kept that in my green wood chairs.
In fact, I’ve got one here.

This guy is as tight in its joints as it was the day I made it, and there is no glue at all. It’s just using differential movement, super-dry hickory for rungs, and fairly damp, about 15 to 18% leg wood, and that would be soft maple. That moisture content is enough to hold the chair together, and that oversizing wasn’t so much that I weakened the leg or cracked the leg, but it’s important to consider which woods you’re using. If I’m going with a hide glue joint, which I do in my three-slat ladderback chairs, if I’m going to make them out of walnut or cherry or even hickory, where I’m not getting the advantage of the characteristic of hickory and the characteristic of maple marrying together inside that joint through a lot of movement, if I need a hide glue, or if I need a glue bond, and I want to use hide glue, I would not oversize the tenon at all. There’s no advantage in that whatsoever.

I would make the tenon as close to a perfectly sized fit. In other words, it would ultimately be about two thousandths of an inch smaller than the hole, so you would be able to push it in there. It will feel snug, but you should be able to bottom it out, and when you pull it out, it should pop like a cork. That’s extreme, but a good chairmaker can do that, and it’s best … I found that I can get a good feel, fit with just turning it using a Bedan tool and a wrench to size it with, but I prefer to turn it about four thousandths oversize and then sand it really quickly to even the surface so that I have more fibers of the tenon in contact with the fibers of the leg, and that contact is actually just … It’s actually the wrong term, because they can’t contact if there’s a glue body there and the tenon is smaller than the mortise. You’ve got about a thousandth of an inch thick layer of hide glue in between, and then the rung’s going to swell pretty quickly and get super tight in there.

As long as your rung is around 4% moisture content and the hole is not drier than that, which is going to be nearly impossible, then that should be fine. The rung’s going to swell up, fill that hole, and be tight, and it should be tight for as long as the piece is taken care of. Those are hard dimensions to hit, particularly in the environment in which Windsor and other post and rung chairs are made. What attracts a lot of people to that kind of chairmaking is how primitive the approach can be and still get a successful chair, and I think the green wood chairmaking is, whether it’s Windsor or post and rung like the one I just showed you, is very forgiving, much more forgiving than when you’re trying to get an actual strong glue joint, because if you’re using soft maple for the legs and hickory for the tenon part, or soft maple for the whatever is housing the mortise and hickory for the tenon, you’re going to get a malleability in that joint that will absorb a little bit of oversizing without destroying the leg or the mortise part.

So there are a lot of variables that impact that, but ultimately, the perfect hide glue joint is a super-dry tenon, not super-dry mortise, and about a two-thousandths undersizing. That’s going to feel a little loose with hide glue on there. It’s just going to slide right in because it’s lubricated, but it will bond very, very well.