I have always had less than stellar results using a miter gauge to make tenons, either the stock slipped, or I never quite got lined up correctly to my cut line.

My solution was to make a zero clearance sled that runs in the miter gauge slot. First, i cut a runner 3/4″ wide by 1/2″ thick from some scrap poplar. With my planned bit in the router (in this case, a 3/4″ Freud mortising bit #DR16104), I locked the router table fence about 1/8″wider that the bit side of the miter slot to bit distance and ran a 1/8″ deep groove across a 10″x12″ piece of 1/2″ birch ply. I then glued the runner into the groove.

After the glue dried, I raised the router bit (in stages) and pushed the sled past the bit using the miter gauge slot. This formed a perfect zero clearance table. Next, I took a scrap piece of 3/4″ red oak and carefully glued it exactly 90 degrees to the newly machined surface of the sled to form a fixed backstop. I took another piece of 3/4″ red oak and attached it on edge to the backstop using pocket screws. This piece allows me to use a clamp when milling the “tall” end of the tenon. Lastly, an adjustable clamp was added to hold the work solidly to the sled.

Use is simple. I first mark all 4 sides of the workpiece, adjust my bit height, then work up to my mark against the edge of the zero clearance sled.