Sunday Toolfoolery - My Ever-Evolving Router Mortise Jig

Most of the time I enjoy fully immersing my head and hands in the work of furniture building.  But with when faced with a set of rote tasks, the "craftsmanship of certainty" becomes mighty appealing.  So when I choose to create joinery by machine, all style points are thrown out the window - I just want accuracy, ease of set-up, and repeatability.

Over the years I've created any number of second-rate router jigs that have haunted me through various projects.  Remembering where the tolerances were off, which way I had to compensate, and what moves had to be avoided, has become exhausting.  So I decided to build (what I hope to be) my last router mortise jig.

In some ways this is a response to the last one-off jig I made to mortise the through tenons for the Arts and Crafts Coffee Table.  This jig used a collar in a slot and encased the workpiece to ensure correct registration on every cut.  Besides the fact that it was purpose-built for one cut, it's top plate reduced the router's effective depth of cut and you couldn't flip the workpiece end-to-end to come at the mortise from each side.  This meant that you had to rotate the piece 180 degrees and register off the other side to cut a complete through mortise.  No matter how accurate the jig, you still needed to sand the inside of each wall to get a truly square mortise.  Way too much work.

The new design (cadged from a Fine Woodworking article, some other blogs, and my own experience) looks to address these shortcomings.  It really comes down to two things:  How do you hold all sizes of workpieces stock still in the jig, and how do you move the router across the workpiece in a smooth fashion with no slack.  The body of the jig is about eight inches tall by about two feet by about 3 inches wide.  The exact dimensions are not important, but make sure you can attach it to your bench via the dogs in a vice and a hold-down.

Holding the Workpiece:
The system begins by routing vertical grooves to hold two pieces of t-slot in the jig's body.   This houses 1/4"-20 T-slot bolts that attach to a shelf to hold the piece to be routed.  The shelf is three pieces of  face glued 3/4" plywood.  The T-slots allow you to place the workpiece on the ledge and move it to a position level with the top of the jig.  I secure the bolts with star knobs.

The ledge takes most of the downward force from the router, but you still need to hold the workpiece against the vertical face of the jig.  I rout a slot across the ledge and install another piece of T-slot track.  Incra makes some nice hold-downs that you can install in the slot, but I made mine from pieces 1/8" by 3/4" steel available at a big box store.  I cut a 6" length, drill a 1/4" hole and bend a 30 degree crook on the end.  I have found that the pressure from a star knob hold everything in place.

Installing the Router
This was the trickier bit.  I knew that I wanted to use the adjustable fence on my Dewalt 621 to guide the router across the jig.  At one level this works well on its own, but I wanted to add a degree of accuracy that didn't depend on constantly applying pressure against the jig.  Experience has shown me that, particularly when plunging the beginning and end of a mortise, the router can shimmy and widen the slot by some fraction.  This is fine for loose tenon joinery, but less so when creating visible through tenons.

I'll spare you the prototypes that failed, but I finally settled on a piece of sliding T-slot track, routed in place so that the fence of the router guide (with the plastic doodads removed) lined up with the sliding portion of the track.  I roughed up both bits and epoxied them together in situ.  Once cured,  I drilled holes and inserted bolts for more stability.

The last step was to install stops to define the travel of the router.  This is achieved by installing (more!) T-slot track and using wooden blocks that can be moved and set via T-slot bolts and star knobs.

I've been quite happy with the jig in the short time that I've used it, and the fact that my Festool dust extractor is reverse engineered to match the port on the Dewalt, is a real plus.  Please feel free to send me a message if some of these ramblings are confusing!