Sunday, October 26, 2014

Tapered Mortise-and-Tenons: Joints That Get Stronger When Used

Test mortise (left) made with the
reamer (left center). On the right
are two halves of my tenon
template, with a test tenon I turned.
Classic Windsor chairs traditionally use mortise-and-tenon joinery. But, unlike most joinery of this kind, there's a twist. These mortises and tenons are are tapered. The tapering means that as the tenon is forced into the mortise, it gets stronger! This is a great joint to have for a chair, as chairs get beat up probably more than any other kind of furniture. So, that's what I'll be using for my rocking chair.

How does one go about making these mortises and tenons? For the mortises (the holes), I use a Veritas Pro Taper Reamer to create the taper. I start the mortise with a 1/2" drill bit, drilling down to the depth I need. Then, I take the reamer and use it to taper the walls of the mortise. These reamers don't come with handles, so I turned one out of walnut to help get a grip on the thing. You can buy handles along with the reamer, but if you've got a lathe, it's short work to make your own.

These joints are strong! No glue
involved here!
For the tenons, I turn them on the lathe. You can get tapered tenon cutters that will do this work for you (and in fact, Lee Valley sells companion tenon cutters for the reamer I got), and this is certainly the best way to go if you're doing a lot of these tenons and need to do so in a short time. I've got some time on my hands though, and as far as I'm concerned, it's just another skill for me to learn.

A test leg, and a real mortise in the
back of my chair.
With some inspiration from this video, I created a guide using the reamer in a piece of scrap wood which I then cut in half. The guide can be used on the turning lathe to indicate my progress as I go. I pair the very end of the spindle down to the smallest size I'll need (1/2", roughly), and mark with a pencil to the top of the tenon. Then, I use my caliper to rough-measure the largest part of the taper on the template, and keep pairing down at the mark until I hit that size. From there, I use a combination of my pairing chisel and bowl gouge (it's smaller than my other gouges) to smooth the taper down, checking my work with the guide as I go. When I've got it, off the lathe it comes and a test fit.

The test leg fit into the mortise.
For my rocking chair, my designs call for 3/4" tenons. I was a bit worried this might not be large enough, but having done some tests with tenons of that size, there'll be no problems at all.

I used the mirror trick outlined by Fine Woodworking Magazine's Peter Galbert. Using a 1/2" drill bit, I carefully drilled into the chair along my sight lines (which I created in Sketchup), using the two mirrors to keep my alignment the whole way. I drilled about an inch into the chair, not far enough to go all the way through. This gives the tenon room to move further into the chair as it's used and become tighter. It also gives the joint somewhere for the glue to go (necessary to avoid hydraulic back-pressure which will prevent the tenon from going all the way in). Then, with the tapered reamer, I widened out the hole to fit a nice tenon.

Before turning the real tenon on my leg, I decided to turn a test tenon to make sure all would be well. Using the lathe and my guides, I got a tenon that fits perfectly into the mortise! Even the angle is perfect! Now, I just have to do it on the real leg. ■

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