You can’t have a tenon without a mortise. They go together like peas and carrots. (Or insert your favorite “go together” cliche here.) Uh, anyway, I described how to cut a tenon by hand in a previous post, and in this article, I’ll describe the process for making a mortise in a timber by hand. A mortise is basically a slot for a tenon, which is later pegged to secure the joinery. It’s simple and timeless.
Mortises are arguably more difficult to make by hand than tenons. I’ll describe how to do it with a boring machine and chisel.
How To Cut a Mortise in a Timber
I won’t try to describe the layout process for a mortise. The process will vary if you are making a mortise for a post or a brace, but the process of cutting the mortise is basically the same across the board. Read ahead for some tips.
Once your mortise is laid out on your timber, you may want to mark a scribe line down the direct center of the mortise with a marking gauge. A little cut of the gauge will help guide the tip of the boring machine bit into the center of the mortise.
Before drilling, however, you will also want to chisel along your pencil lines, making a shallow cut to prevent the boring machine bit from tearing out the grain when you begin to bore.
Once you have your center scribe line and your chisel lines cut, it’s time to begin drilling. Set your boring machine depth stop to the appropriate depth (not all boring machines have them, but the really sweet ones will), and set up the machine towards one end of the mortise. Drill down, and keep the chips out of the way. Once your first hole is complete, slide the machine to the opposite end of the mortise, and bore again.
Tip: It’s always better to make sure you get the ends bored before the middle of the joint.
Now you can go ahead and remove the middle of the mortise, and be sure to overlap your holes as much as possible. Be careful, though: too much overlap and your bit will slip. It’s best to get as much material out of the way with the boring machine to eliminate extra chisel work.
With all of your holes bored, grab your favorite chisel and mallet, and remove excess material right up to your line. A corner chisel will make quick(er) work of corners, and I definitely recommend picking one up if you plan to do any significant amount of joinery, or mortise making by hand. Mine is on the smaller side (3/4″, I think?), and I prefer it because it doesn’t feel quite like bashing a massive hunk of metal through grain in two directions at the same time. In hard woods like oak, it’s more difficult to remove larger quantities of waste at once.
A slick makes sweet work of cleaning up the mortise, and getting right to your layout line.
A swan neck chisel or a lock mortise chisel allows you to clean up the bottom of the mortise, and flatten things out a bit. This is not always essential, but if you cut things a bit close, you may need to take out some depth. You can sometimes flip your chisel upside down to achieve a similar effect, although it is more awkward.
Once you think you’re close, grab a framing square and use the 2″ wide leg (assuming your tenon is 2″, use the smaller end for a 1 1/2″ mortise), and attempt to slide the square through the length of the tenon. If it slides without any tweaking or force, you’re in the clear! If not, keep taking shavings off the walls of the mortise.
There ya have it. You could do a mortise by hand with as few as two tools (not including a mallet), but a corner chisel and slick really help things out.