Have you ever wondered why there are specific rules for the sizes of mortise-and-tenon joints? Did you know there are rules? If you consult the 19th and early 20th century texts, they state that tenons should be one-third the thickness of your stock. And that the tenons should be five times as long as they are wide.
So if you were cutting a tenon on 3/4″ stock, you would make it 1/4″ thick and 1-1/4″ long. Is this arbitrary? And why do more modern texts call for tenons that are one-half the thickness of the stock? Recently I’ve been cutting a lot of mortises by hand to test some mortise chisels. And as I get more comfortable with the chisels, the old rules for this old joint seem to make sense.
For example, when I work in 3/4″ stock (especially anything slightly fragile, such as cherry or walnut) a 3/8″ mortise chisel will absolutely destroy my work. It’s just too much steel plunging into the wood, and the 3/16″ walls of the mortise are simply too fragile. When I step down to a 5/16″ chisel, things become very manageable in oak and ash, for example, and workable in cherry, walnut and mahogany). And when I step down once more to a Ã?Â¼” chisel, I pick up serious speed in the harder woods and more control in the softer ones.
So what about that modern rule that has us all cutting 3/8″ mortises in 3/4″ stock? I’m starting to think that’s a machine perspective. Boring a 3/8″ mortise with a hollow-chisel mortiser is a simple thing in 3/4″ stock. In fact, I find that chip clearance with a 3/8″ hollow mortise bit is more efficient than with the 1/4″ bit.
And then there’s the issue of the tenon’s length. Why do we make tenons five times as long as they are wide? Was this to provide more gluing surface with less-reliable hide glues? Perhaps. Was it to ensure more of an interference fit between the tenon and mortise to beef up a sloppy hand-cut joint? Sounds good to me.
I think it also has something to do with drawboring (surprise, surprise). Here’s my highly questionable theory: You have to locate the drawbore hole a certain distance in from the edge of the mortise. I like 3/8″. Any closer and you risk cracking your stile when you drive in your wooden peg. And if your tenon is any shorter than 1-1/4″, then you risk blowing out the grain in the tenon, ruining your mechanical fit. There are probably other valid reasons for the rules that govern this joint. Have a theory? Let me know.