A plunge router and a simple mortising jig make a fast, accurate combination for cutting mortises. This setup is especially useful when a furniture project calls for multiple mortise-and-tenon joints and you need every mortise to be positioned consistently.
I originally used this jig while building the Industrial Bookcase, which required 40 mortises in its long posts. Rather than setting up each mortise individually, the jig allowed me to establish the router's position once and then repeat the same accurately aligned cut at each mortise location.
The jig itself couldn't be much simpler. A hardboard baseplate replaces the router's standard baseplate, while two adjustable guides capture the workpiece. Once the guides are adjusted, the router slides along the workpiece without shifting from side to side.
Add a stop block to control the length of the cut, and you have an easy way to rout clean, straight, repeatable mortises with a plunge router.
How This Router Mortising Jig Works
The key to accurate router-cut mortises is controlling the router in two directions.
The jig uses a pair of adjustable guides attached to a hardboard baseplate. These guides fit against opposite sides of the workpiece, capturing it between them.
Once adjusted, the guides perform two jobs. First, they position the router bit accurately across the width of the workpiece. Second, they prevent the router from wandering sideways as you make the cut.
The router and jig can still slide freely along the length of the workpiece, allowing you to rout the mortise between your layout lines.
For multiple mortises in the same workpiece, leave the guides locked in position. This keeps each mortise on the same centerline without having to repeat the setup.
Setting Up a Plunge Router for Mortising
Start by installing the mortising jig on the router and selecting the router bit.
Choose a Spiral Upcut Router Bit
For cutting mortises, I prefer a spiral upcut bit.
As the bit cuts, the upward spiral helps pull chips out of the mortise. That's particularly useful as the mortise gets deeper and chip removal becomes more difficult.
Choose a bit that matches the required width of the mortise.
Lay Out the Mortise
Mark the location and length of the mortise clearly on the workpiece.
These layout lines give you the references needed to position the router bit and establish the beginning and end of the cut.
If you're making several mortises, lay them all out before you begin routing. That way, once the jig is adjusted, you can move directly from one mortise to the next.
Clamp the Workpiece Securely
Clamp the workpiece firmly to the bench while leaving enough clearance for the mortising jig to slide freely along its length.
Pay particular attention to clamp placement. The router and its guides need an unobstructed path over the entire mortise.
How to Adjust the Mortising Jig
With the bit installed and the mortise laid out, place the plunge router and jig over the workpiece.
Loosen the two adjustable guides and slide them against the sides of the workpiece.
Don't tighten them yet. First, you'll use the router bit itself to fine-tune the position of the jig.
Center the Router Bit on the Mortise
With the router turned off, lower the bit until it's just above or lightly touches the surface of the workpiece. Lock the plunge mechanism in this position.
Now move the router slightly from side to side until the bit is positioned between the mortise layout lines.
Once you're satisfied with the alignment, slide both guides snugly against the sides of the workpiece and tighten them securely.
Check the Fit of the Guides
Before turning on the router, slide the jig along the workpiece.
The guides should keep the router accurately aligned without allowing side-to-side movement. At the same time, they shouldn't fit so tightly that the jig catches or binds.
You're looking for a smooth sliding fit with no noticeable slop or wobble.
Once that's established, leave the guides locked in place. If you're cutting several mortises in workpieces of the same thickness, this setting can be used for each one.
How to Cut a Mortise with a Plunge Router
With the jig adjusted, you're ready to rout the mortise.
Rather than trying to cut the mortise to full depth in a single pass, remove the waste gradually.
Rout the Mortise in Multiple Passes
I like to cut approximately 1⁄4" deeper with each pass.
Position the router over the mortise and make the first plunge just inside one of the end layout lines.
After plunging, rout back carefully to the end line. Then move the router forward through the mortise toward the opposite end.
This is easier than repeatedly plunging the router into exactly the same starting point.
Once the first pass is complete, increase the plunge depth by about 1⁄4" and repeat the process. Continue stepping the cut deeper until you reach the final mortise depth.
Use a Stop Block for Consistent Mortise Length
The guides control the mortise from side to side, but you still need a reliable way to stop the router at the correct point along the length of the cut.
A simple clamp-on stop block solves the problem.
Position the block so one of the jig's guides contacts it when the router bit reaches the end of the mortise.
Now the jig controls the router in both directions. The two guides keep the bit aligned across the workpiece, while the stop block prevents you from routing beyond the end of the mortise.
This means you can concentrate on controlling the plunge and feed rate rather than watching several layout lines at once.
How to Rout Multiple Mortises Accurately
This jig really earns its keep when a project requires a series of mortises in a long workpiece.
That's exactly what happened when I built the Industrial Bookcase. I laid out all the mortise locations on the posts before routing.
After positioning the jig for the first mortise, I left the guides locked in place. Since every mortise needed to share the same alignment across the post, there was no reason to repeat that adjustment.
For each new mortise, I simply repositioned the router along the post and moved the clamp-on stop block to the next layout location.
The result was a repeatable process that allowed me to work my way along the posts while maintaining the same mortise alignment.
By the time I was finished, I'd routed all 40 mortises needed for the bookcase.
Square the Mortise Corners with a Chisel
A router bit naturally leaves rounded ends and corners in a mortise.
Depending on how you're making the mating tenon, you can either round the tenon to fit or square the mortise with a chisel.
For traditional mortise-and-tenon joinery, I prefer to do a little cleanup with a sharp woodworking chisel.
The router has already removed nearly all the waste, so there's very little handwork involved. Carefully pare the rounded corners back to the layout lines, keeping the chisel square to the workpiece.
After that small amount of cleanup, the mortise is ready for its mating tenon.
Why Use a Router Mortising Jig?
There are plenty of ways to cut a mortise. You can drill out the waste and clean up the walls with chisels, use a dedicated hollow-chisel mortiser, or rout the mortise.
The advantage of this simple plunge-router mortising jig is repeatability.
Once the guides are set, the router stays aligned with the workpiece. A stop block establishes the mortise length, and the plunge router controls the depth. That takes much of the guesswork out of making repeated mortises.
And the jig doesn't require an elaborate fixture or complicated setup. A simple hardboard baseplate and two adjustable guides turn a plunge router into an accurate mortising tool.
For a project with one or two mortises, you may choose another method. But when you're staring at dozens of them, this jig can save a lot of time while helping keep every mortise straight, clean, and consistently positioned.
