This simple jig lets you cut the same mortise on workpieces of different sizes. All you need is a plunge router fitted with a guide bushing, a spiral upcutting bit and some wood scraps.
I have versions for common mortise sizes, and I just grab the one I need. The jig works on the sides and ends of a workpiece, so it's ideal for loose-tenon joinery.
Want plans for more jigs like this?
Download A Guide to Guide Bushings.
Make the base as wide as your router's base and twice as long. Attach it to a hardwood fence milled flat and square.
Attach the fence. Leave the fence slightly proud of the base so you can reference off it when cutting the slot for the bushing. Referencing off the jig's fence ensures that the routed slot is parallel to the fence and so when you cut the mortises they’ll be parallel to the edge of the workpiece.
Lay out the mortise slot on the bottom of the template, adding the proper offset to the width and length. Cut the slot at the router table, and try to make it fit the bushing exactly. If you have a bit that's the same size as the outside diameter of the guide bushing, use it. Otherwise, use a smaller bit and cut the slot in multiple passes.
Slot the base. Hold the jig’s fence against the router-table fence and carefully lower the template onto the bit to start the slot.
Some people drill a starter hole for this kind of cut, but I find it unnecessary. I rest the right side of the template on the router table, then carefully lower the left side onto the spinning bit and move the template right to left to make the cut.
How to use the jig
Attach the jig using a vise (as shown below) or clamps.
Make full-depth plunge cuts at the ends of the slot, with a series of shallower passes to clean out the middle. Blow out the chips before making a final pass. Don’t forget to record the bushing size and bit size on the jig for future reference.
Anatomy of a cut
For workpieces of different thicknesses, add shims next to the fence.
Drawings Jim Richey, photos by David Heim