K.I.S.S. Mortising Jig

Comments (0)

This article is from Issue 67 of Woodcraft Magazine.

Edge clamps put pressure where it’s needed most

I’ve built hundreds of jigs over the years, and my “keep it simple…” design ethic often means that a jig gets tossed when the job’s done. A “keeper” jig must not only be accurate; it also has to be easy to use, even after spending months on a shelf. This mortising jig meets my “keeper” requirements nicely.

Made from plywood, T-track hardware, and a few Bessey edge clamps, my jig fixes a workpiece against a sturdy fence for plunge-routing edge or end mortises. The broad top of the fence provides a stable platform for the router base. Adjustable stops on the T-track set the length for your mortises. By extending the jig’s plywood base beyond both fences, I created “feet” for clamping the jig to a workbench. Although the photos show me using an oversize auxiliary base on my router, the jig will work with any mid-size router equipped with a standard edge guide. 

Simple and strong. Removing the locking screw from a Bessey edge clamp permits the metal tab to hook around a cleat. If clamps are needed for other uses, they can be freed from the jig by unscrewing the clamping block from the base. Faceting the clamp handles (I used my belt sander) makes them easier to slide out of the jig.

Make the jig, then make it work for you

The fence and clamp bar are laminated from strips of 3⁄4" and 1⁄2" birch plywood. To ensure straight, square inner faces, I cut the strips 1⁄8" oversize, and then trimmed the working faces flat after assembly. To make matching dadoes for the clamp-holding cleats, cut a single dado in a larger piece of plywood, then cut out individual dadoed strips. Gluing and screwing the strips together is faster than using clamps. Attach the upper fence to the base so that its inner face is flush with one edge of the base cutout. When screwing the clamp bar to the base, use a spacer to keep this part parallel with the fence.

Edge Mortising in 3 Steps

1. Clamp the workpiece in the jig. Make sure that the mortise layout is roughly centered and the workpiece top edge is flush with the top of the fence. To save time for identical mortises, cut a spacer to rest under the workpiece, and mark the end of the workpiece on the inside face of the fence.

2. Adjust edge guide & stop blocks. Place the router on the fence, and adjust the router’s edge guide to register against the fence and center the router bit inside your mortise layout. Adjust the stop blocks to set the location and length of your mortise.

3. Plunge-rout in stages. Adjust the plunge depth to rout the full depth of the mortise in a series of shallow (about 1⁄4"-deep) cuts. Rout from right to left so that the bit’s rotation pulls the guide toward the fence. Raise the bit at the end of each pass, then rotate the depth stop turret to set up the next cut.

Add a fence and clamp for end mortising

Like the jig’s upper elements, the lower fence and lower clamping block are glued and screwed together. When making and attaching these parts, pay attention to two critical details: 1) The broad working face of the lower fence should be flush with the inner face of the upper fence; and 2) The vertical rail should be perpendicular to the top face of the upper fence. The lower fence and lower clamping block are secured to the jig’s base with 5⁄16" hex-head bolts that are driven into threaded inserts (see drawing, previous page). I made the attachment holes about 1⁄8" oversize to ensure proper positioning of the two lower parts.

Bolt these to the base. Align the lower and upper fences when tightening the bolts.
Set, slide, cinch. Set your stock against the vertical rail, slide the top end flush with the upper fence, cinch both clamps, and you’re ready to rout.

Easy repeatability. With stopblocks locked in place on the T-track and the workpiece held fast against the vertical fence, the jig is set for quickly routing end mortises in multiple rails.

End Mortising

After bolting the fence and clamp mount to the base, cantilever the jig off your bench as shown in the photo at right. From here, end mortising isn’t much different than edge mortising: Lay out a mortise on the end of your first piece, clamp it against the rail, set your stops, adjust your router, and take the plunge.

(Note: To direct more clamping pressure to narrow pieces, set a wider piece of scrap between the clamp and workpiece.)


Write Comment

Write Comment

You must be logged in to write a comment. Log In

Top of Page