Nearly all vernacular chairs use straight (not tapered) cylindrical tenons throughout.
Before I owned a tapered tenon cutter and reamer, I used a 1”-diameter auger to make the mortise in the seat, and I shaved the tenons on the legs to size with planes. This requires great care.
You can greatly speed the process by using a plug cutter in a drill or a brace to cut the tenons. These inexpensive tools ($16 to $38) are widely available. They are supposed to be used “only in a drill press,” but you can use them safely in a handheld drill or brace as long as you first taper the tip of your tenon.
I make this taper with a jack plane. First I drill a shallow 1”-diameter hole in the center of the end of the leg to act as a target to work toward. Then I use a coarse-set jack plane to waste away the top of the leg until the tenon cutter just barely fits over the tip of the leg.
Next I level the leg in my vise, and I level the tenon cutter (I use a bubble level I epoxied to my drill’s body). Then I drill. It makes a nearly perfect 1”-diameter tenon (0.995”). The tenon has a shoulder, which I like for mechanical reasons. I like to lock the shoulder against the underside of the seat and lock the tenon in the top of the seat with a wedge. But you can plane away the shoulder if you like. Here’s a short movie that shows the tenon-cutting process.
More tips to come.
— Christopher Schwarz