More Than a Mortising Jig

This multipurpose routing jig is no one-trick pony.

At the JD Lohr School of Woodworking, we place a lot of value on practical jigs. I say “practical” because jig-making can get out of hand if you’re not careful, resulting in contraptions that are overly complicated and that try to do too many things. Ideally, a jig should be versatile without overreaching in its capabilities, and should do its job(s) as simply and directly as possible. This mortising jig is a great example of such a design. 

Used in conjunction with a router outfitted with an edge guide, this jig primarily serves for mortising furniture legs. However, we also employ it for other operations, such as slotting aprons to accept tabletop clips, and as a holding fixture to secure parts for edge-routing profiles. Part of its beauty as a mortising jig stems from its adjustable router-travel stops, which allow for quick, repeatable operation when cutting identical mortises in multiple legs. It works very well as a slotting or holding fixture because its fence-and-wedge system eliminates the need for numerous clamps, which would impede router travel. 

The unit—made from a couple of long plywood strips, some wood scrap, and some inexpensive hardware—is a proven investment. Ours has been in play for years in regular production, and our students get a lot of use from it as well.

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See a video of the jig in action:

Mortising jig drawing

This jig consists of 2 basic components: the base and the stop carriage. The base parts include a repositionable fence and fixed end blocks that are tapered to complement a locking wedge. The stop carriage incorporates adjustable primary stops as well as secondary flip-stops, allowing for efficient routing of 2 mortises with one setup. Except for the wedge and stop carriage bar, the parts are all made from hardwood plywood.

Slotting and edging

The base can be used to secure work for operations like slotting and edge-profiling. The beauty of the setup is that you only need to use 2 clamps, both of which sit out of the way of router travel. Whether you’re working with long boards such as table aprons, or multiple, identically sized small pieces, the jig configuration is very similar. 

Simple setup for long boards. To hold long workpieces for jobs such as slotting these table aprons, begin by repositioning the fence if necessary to create sufficient bearing across the width of the workpieces. Next, sandwich the pieces between a loosely placed wedge at one end, and a clamped stopblock at the other, and then push or tap the wedge inward to secure the work. 

Small multiples. After rounding over the long edges of these pull blanks on a router table, they get mounted in the jig to gang-rout the ends. Spacers of similar thickness are included to provide router support at the beginning and end of the cut. 

Mortising setup

To rout leg mortises, you secure all 4 legs in the jig, with 2 set up for routing, and the other 2 serving as additional router support. After laying out 2 mortises, you set the mortise stops and secure the legs in the jig to rout the first pair of mortises. Then, with the same jig set-up, switch the leg positions and rout the remaining mortises. Note that the carriage bar thickness on this jig is suited to 1-3/4"-square legs. For thicker legs, shim under the carriage bar. For thinner legs shim under the legs.

Mark 2 reference mortises. First, lay out one full mortise (top left) and just the length of another (top right). The width lines are reference for setting your router edge guide. Length lines are for setting the jig stops. Chalk squiggles will do for the rest.

Attach the stop carriage. After locating the carriage to suit stops for legs of a particular length, extend the carriage bar centerline across the edge of the fence for future relocation of identically sized legs. Then screw the carriage to the fence. 

Load the legs. Place 2 legs against the jig fence to serve as router support, offsetting their ends from the carriage centerline. Then load the 2 legs to be mortised, aligning their laid-out ends with the carriage centerline.

Set the stop. Locate the stopblock against the end of a target leg, and clamp the block in place, at the same time clamping the jig to the bench. 

Secure with a wedge. Push or tap the wedge into place at the end of the butted target legs to secure both for routing. 

Set one primary stop. After adjusting the router edge guide for the cut width, locate the bit at the outermost end of a mortise layout line (right), and slide the primary stop against the router base. Lock down the stop with the wing nuts (above).

Then set the other. Position the router bit at the outermost end of the opposite mortise, and locate and lock that primary stop in the same manner.

Flip-stops and extensions. When appropriate, outfit your flip-stops with the matched pair of extensions that suits the length of your mortises. When the stop is flipped down, the bit should be at the innermost end of the mortise on the leg that opposes the stop. We use 1⁄4"-, 1⁄2"-, and 3⁄4"-long extensions, as shown here. 

Making the cuts

Once the jig is set up, you can cut all of the mortises in a set of identical legs without readjusting the jig’s stops or the router’s edge guide. Just cut the first pair of mortises as shown, then reorient those same legs in the jig to cut the mortises on the adjacent faces. Following that, invert the location of the mortised legs with the unmortised legs, and repeat. 

Flip, rout, flip, rout. With one of the flip-stops down, rout in the usual fashion, beginning with the router against the first stop, and ending with it touching the second. 

Next couple, please. After routing all mortises on the first pair of legs, switch them out with the unrouted pair, and complete the work in the same manner with the same setup.