Need to make mortises for an upcoming project? One of the easiest ways to make them is by using a drill press equipped with a Forstner bit. The resulting mortise requires a little chisel clean up.
Mortise and tenon proportions
When calculating the dimensions for your mortise and tenon joint, start with these guidelines. The tenon is typically made 1/3 as thick as the rail, and twice as long as it is thick.
For example, if you’re working with 3/4" thick rail stock you would make a tenon 1/4" thick and 1/2" long. Once you’ve calculated the joint sizes, cut the mortise first, and then make the tenon to fit the mortise.
Setting up your drill press
You’ll need to equip your drill press with a fence (see Sources) so there’s a solid backstop for the material. The fence guarantees that every hole you drill will be exactly the same distance from the face of the board, and the mortise will be parallel to that face.
The best style of drill bit for this operation is a Forstner bit (see Sources). Forstner bits, unlike brad point or twist bits, are very good at drilling overlapping holes; the precise characteristic we need for making a mortise.
If you only want to get a Forstner bit or two, 1/4" and 3/8” are very common mortise and tenon sizes.
Position the fence to correctly locate the mortise on your material. The easiest way to do this is to determine the center of the mortise, then measure from the center point of the Forstner bit to the face of the fence. Set the depth of the mortise by setting the stop on your drill press.
I typically make mortises slightly (1/16”) deeper than the length of the tenon. This provides a glue pocket in the bottom of the mortise, which can make joint assembly easier.
Cut the mortise
Start by drilling two holes; one at each end of the mortise. This defines the overall length of the mortise. A side note: when working with walnut (or other dark woods) I do my layout with a charcoal pencil instead of a conventional pencil (see Sources).
Charcoal pencil lines are much easier to see on dark wood.
Drill a series of overlapping holes. As you drill these holes go for a “snowman” shape, keeping the center point of the Forstner bit in portions of the wood you haven’t drilled yet.
The next step is to clean out the high spots. Hold the part tightly against the fence and align the Forstner bit with the peaks that remain in the mortise. Plunge down to remove them.
We’ve eliminated a lot of ridges, or at least made them quite a bit smaller, but we can clean the mortise up a little more.
Do additional clean up on the mortise by repeating the previous clean up process; hold your stock tightly against the fence, move the material to position remaining high spots under the bit, and plunge.
If you’re careful, you’ll be able to get the walls of the mortise nearly flat. This isn’t a horrible mortise, but the strength of a mortise and tenon joint comes from the glue bond between the face of the tenon and the inside face of the mortise.
Any remaining ridges will adversely affect that glue bond.
Chisel work is next
Secure your work in a vise and use a sharp, wide chisel to further eliminate high spots. Use light pressure and be careful to not dig into the sides of the mortise.
A wide chisel, rather than a narrow one, makes it much easier to ride the high spots and avoid digging in.
Skewing the chisel, cutting at an angle across the high spots, also makes it easier to avoid digging in.
This process results in a mortise with rounded ends. When it comes to matching up your tenon to your mortise it’s a lot easer to round the corners of the tenon than it is to square the ends of the mortise.
Related Article: Check out this article on Cutting Tenons and Half Lap Joints