Joseph Moxon in his 1678 Mechanick Exercises struggled with proportioning the basic mortise and tenon joint. He observed that "if one be weaker than the other, the weakest will give way to the strongest when an equal Violence is offer'd to both. Therefore you may see a necessity of equallizing the strength of one to the other, as near as you can. But because no rule is extant to do it by, nor can (for many Considerations, I think,) be made, therefore this equallizing of strength must be referred to the Judgement of the Operator."
Proportion is more critical than size in mortise and tenon joints, for enlarging one element means reducing the other. Tenons can be quite small if they are just holding a timber in place, but in equally sized timbers, I follow the old rule of thumb, making the width of the mortise and tenon more than a third, but less than half of the timber's thickness. For 2-inch-thick timbers, this would be 7/8 inch, but it you have only a 3/4-inch or 1-inch chisel, that's probably the width you'll use.
The mortise and tenon joint is founded on the width of the chisel, because all the work of mortising is repeated cross-grain chopping, taking out chips until the mortise reaches its final size. For smaller joints, there are specific mortising chisels, but in carpentry, it's another job for the framing chisel.
The chisel itself can serve as the layout guide. If the sides are parallel, you can align it on the timber and trace down the sides to indicate the width of the mortise. Since the mortise and tenon must be the same size, the chisel serves as the template on both pieces.
The chisel can give you the width, but not the placement, of the mortise and tenon. The framing square serves well, and if the timbers are smooth enough, so does a gauge with two teeth set to the width of the chisel. Gauges with
adjustable teeth and fences are more common in joiner's work, while carpenters more often make a fixed gauge that won't lose its adjustment when stepped on, dropped, or thrown.
As always, gauge from the face side on both the mortise and the tenon pieces. Because a mortise can't easily be made smaller, or a tenon made bigger, I generally cut the tenon first, sawing the shoulders and splitting and shaving the cheeks. This done, I lay the tenon on the mortise lines of the other timber and mark its crossing points.
You can make a neater mortise if you start shallow and work deep. Starting perhaps 1/4 inch in from the ends of the mortise, set the framing chisel across the grain with the flat face to the end. Give the chisel a whack with the mallet to drive it in no more than 1/4 inch. Reset the chisel a little farther along and whack it again. The chip should split free and move into the space made by the previous cut. Continue this shallow cutting down the length of the mortise, stopping 1/4 inch from the other end.
Now you can start mortising in earnest. The process is called "chopping," an odd-sounding name until you think about felling trees with an axe — the cross-grain chopping action is the same. Some folks like to start in the middle and work outward. Others march back and forth. In either case, the bevel of the chisel faces the uncut wood as you advance. Each strike of the chisel goes a little deeper, because the preceding strike has made room for it.
The mortise needs to be a bit deeper than the tenon is long to allow for shrinkage. As you go deeper, the waste builds up and has to be dug out with the chisel levering against the ends of the mortise. Don't think you can't break a framing chisel when using it as a lever. Levering out the deep chips dents the shoulders of the quarter inch of wood left at the ends of the mortise—but that's why you left it there. You take out that last bit at the ends only when all the rest of the wood has been chopped and cleaned to full depth.
Now and then you may have to turn the blade with the grain to macerate the chips. This is a good time to contemplate the magic of mortising. If you were to drive the framing chisel hard enough, you could split the timber—but only if the edge were aligned with the grain, acting as a wedge pushing between the fibers. When mortising, chopping across the grain, the edge is dominant, severing a chip and pushing it aside. You can strike across the grain as hard as you wish, as often as you wish, and never split the timber.
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