The Big Advantage Of A Tapered Joint

I received this question in episode 1 of the Ask Brian Boggs Show:

When using turned legs, is it necessary to have tapered tenons and reamed holes… Or can a straight wedge tenon hold fine?

This question opens up a whole lot of what I’ve been doing in experimenting with joints, how they behave, how glue works, and how do you assemble a tight joint where the sides of the tenon are parallel to each other. Whether it’s a round tenon, a rectangular tenon, or a bridle joint, the issue of assembling is key. The question of whether or not it’s tapered, while that’s important, what’s more important is the precision of both parts, the mortise and tenon to fit, the quality of the surface on both parts so that it’s not torn up, so that it’s a good clean cut, so as you can get a good bond, and the type of wood you’re using for that joint.

For example, when we’re building our post and rung chairs, for many years, I was using hide glue. The rungs are 4% moisture content, the legs are anywhere from 12% to 18% moisture content. We’re using hide glue so we can put a thin coat of glue on both parts, let that dry, reactivate it with a hot hide glue, which can bond really well and allow the joint to slip together, even though it’s tight, without shearing the glue off of either part. Yellow glue in that situation doesn’t give as good a bond, because too much of the glue gets sheared off and shoved to the bottom of the hole, or peeled off the tenon and left on the surface of the chair.

That’s the biggest advantage of a tapered joint. It doesn’t get tight until it’s almost all the way home, and then it’s pressing the wall of the tenon against the wall of the mortise with a clamping force that bonds that glue really well, even against end grain. That’s the biggest advantage of the tapered joints in Windsor chair making. I don’t know a bit that will cut tapered holes appropriate for a post and rung joinery.

A wedge tenon, I think, is only theoretically good. I think you’re causing more problems than it’s worth, while you just don’t need to wedge a blind tenon. A well-fitted tenon is about as good as it gets. If the fit is good, the moisture content is correct, and the precision of the surface is such that you get a good glue bond, a wedge is just not going to help the situation.

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