The mortise-and-tenon joint is one of the strongest and most reliable joints in woodworking. And when making one, I almost always start with the mortise. Once the mortise is cut accurately, it's much easier to cut or adjust the tenon for a precise fit.
There are several ways to cut a mortise, from chopping it entirely by hand to using a router or dedicated mortising machine. But one of my favorite methods combines the speed of a drill press with the accuracy of hand chisels.
The process is straightforward: Lay out the mortise, drill away most of the waste, then use sharp chisels to square the sides and ends. This combination of machine woodworking and hand-tool work produces a clean, square mortise without requiring a dedicated mortising machine.
How to Cut a Mortise in Three Steps
My approach to cutting a mortise follows the same three-step routine:
1. Lay out the mortise. Accurately mark the length and width with a combination square and marking knife.
2. Drill out the waste. Use a drill press and a Forstner or brad-point bit to remove most of the material.
3. Square the mortise with chisels. Finish the sides, ends, and corners by hand for a clean, accurately sized mortise.
The steps themselves aren't complicated. The key to getting good results is how you approach each one.
Step 1: Lay Out the Mortise
Accurate mortise-and-tenon joinery starts with an accurate layout. There are two goals here. First, the mortise needs to be positioned and sized correctly. Second, you need crisp layout lines that are easy to follow when drilling and chiseling.
For this, I use a combination square and a sharp marking knife. The square establishes the dimensions while the knife creates a precise line that also gives the chisel a positive registration point during cleanup.
Mark the Ends of the Mortise
Start by locating the two ends of the mortise. Use the blade of the combination square to measure the mortise length and make a small knife mark at each endpoint.
Then position the square across the workpiece at each mark and use the knife to score a crisp line across the mortise.
Lay Out the Mortise Cheeks
Next, mark the long sides, or cheeks, of the mortise.
For a centered mortise, use the graduations on the combination square blade to position the blade the correct distance from the edge of the workpiece. Make a short test mark from each face and measure the distance between them.
If necessary, adjust the square until the two lines establish the correct mortise width and are centered on the workpiece.
Once the setting is dialed in, slide the combination square along the edge of the workpiece while holding the tip of the marking knife against the blade. Repeat the process from the opposite face to mark the other cheek.
You should now have a crisp knife line defining all four sides of the mortise.
Step 2: Drill Out the Mortise
With the layout complete, head to the drill press. The goal here isn't to create the finished mortise. Instead, you're simply removing as much waste as possible so there's less work to do with the chisels.
A good drill press setup makes this step faster and more accurate.
Use Fences to Keep the Holes Aligned
Set up the drill press with the workpiece captured between two fences. The workpiece should slide freely from side to side while the fences prevent it from moving toward or away from you.
This keeps every hole on the same centerline and makes it much easier to drill a straight mortise.
Choose a Bit Slightly Smaller Than the Mortise
I prefer a Forstner bit or brad-point bit that's slightly smaller than the finished width of the mortise.
For example, for a 1⁄4"-wide mortise, a 3⁄16"-diameter bit works well.
Using an undersized bit gives you a little room for error. You don't have to center the bit perfectly, and if it wanders slightly while drilling, it won't cut beyond the finished mortise walls. The remaining material will be removed with chisels.
Set the Mortise Depth
Set the drill press depth stop so the holes are about 1⁄16" deeper than the length of the tenon.
The extra depth provides a little space for glue and any remaining debris during assembly. More importantly, it prevents the tenon from bottoming out before the joint closes completely.
The Best Pattern for Drilling a Mortise
There's a simple trick to drilling out the waste efficiently: Don't drill a continuous series of overlapping holes from one end to the other.
Instead, give the drill bit solid wood to register against for as long as possible.
Start by drilling a hole near each end of the mortise, staying safely inside the knife lines. Then drill a series of staggered holes between the first two, leaving a small bridge of wood between each hole.
Finally, go back and drill away those remaining bridges.
Take these final cuts slowly. Because part of the drill bit is entering an existing hole, the bit has less support and is more likely to wander.
You don't need perfectly smooth mortise walls at this stage. Leave the final cleanup to your chisels.
Step 3: Square the Mortise with Chisels
After drilling away most of the waste, return to the workbench to finish the mortise by hand.
For this step, I like to use two sharp chisels and a mallet. A wider chisel works well for paring the long cheeks of the mortise, while a narrower chisel makes it easier to clean up the ends and corners.
Clamp the workpiece securely to the bench before you begin.
Establish the Mortise Walls
Start with shallow cuts around the perimeter rather than trying to chop immediately to full depth.
Place the wide chisel directly in the knife line defining one cheek of the mortise. A crisp knife line gives the chisel a positive place to register.
Tap the chisel lightly with a mallet and work from one end of the mortise to the other. Repeat the process along the opposite cheek.
Switch to the narrower chisel and establish the two ends of the mortise in the same way.
At this point, you're creating a short, clean shoulder around the perimeter of the mortise. This gives the chisel a solid reference for the deeper cuts that follow.
Work Down Gradually
Once the perimeter is established, begin working deeper.
Make slightly more aggressive cuts at the ends of the mortise, but don't try to remove a large amount of material at once. Then switch to the wider chisel and pare deeper along the cheeks.
As you work, the waste left behind by the drill press will begin to break away.
Continue alternating between the sides and ends of the mortise. This back-and-forth approach creates clearance for the chisels and makes it easier to keep the walls straight and perpendicular to the face of the workpiece.
Let the original knife lines guide your chisels. They—not the drilled holes—determine the finished size of the mortise.
Clean Up the Bottom & Corners
As you reach the full depth of the mortise, use the narrow chisel to clean out any waste remaining in the corners.
Turning the chisel bevel-side down allows you to reach into the bottom of the mortise and lift out small chips. You'll need to approach the ends from both directions to clean them thoroughly.
There's no need to make the bottom of a blind mortise perfectly smooth. Concentrate instead on removing loose waste and making sure nothing prevents the tenon from seating fully.
Finish by checking the mortise depth with a rule or combination square.
Tips for Cutting a Clean, Accurate Mortise
A few details make this drill-press-and-chisel method much more reliable.
Stay inside your layout lines when drilling. Let the chisels establish the finished walls rather than risking an oversized mortise with the drill bit.
Keep your chisels sharp. A sharp chisel is easier to control and leaves a cleaner, straighter mortise wall.
Work from shallow cuts to deeper cuts. Establishing the perimeter first helps prevent the chisel from wandering beyond your layout lines.
Drill slightly deeper than the tenon. About 1⁄16" of extra depth provides clearance and makes final assembly easier.
Cut the mortise before the tenon. Once you have a clean, accurately sized mortise, you can cut the mating tenon to fit it.
A Simple Way to Cut Mortises Without a Mortising Machine
A dedicated hollow-chisel mortiser or router can certainly speed up mortise-and-tenon joinery, but neither is necessary for getting accurate results.
Using the drill press to handle the bulk of the waste removal and chisels to establish the finished walls gives you the advantages of both machine and hand-tool woodworking. The drill press makes the process fast, while the chisels give you precise control over the final dimensions.
Once you've established a repeatable layout, drilling, and cleanup routine, this technique makes it surprisingly easy to produce clean, square mortises and accurately fitted mortise-and-tenon joints.
