A mortise-and-tenon joint is one of the standard methods for building furniture. This joint creates a rock-solid joining of two pieces of wood and, in the process, aligns the work piece as it is being put together ensuring perfect 90-degree angles. In furniture factories and large woodworking shops, mortise-and-tenon joints are done on machines. But the average home woodworker can also make mortise-and-tenon joints by hand using only a router.
Set the 1/2 cutter bit into the router and tighten with wrenches. Set the depth of the bit at 1/4 inch. Place the guide fence on the router and set it at 1 inch to the inside edge of the bit.
Lay the 2 1/2 by 2 1/2 inch legs down on the worktable, clamping them securely to the table about 6 inches apart. Center and make a mark 3 inches down from the top of each leg with a pencil. Take the router in both hands, position it over the top of the first leg and turn it on. Gently ease it down, tipping it slightly making sure that the fence makes contact with the side of the leg first, then tip the router up to square and let the bit cut into the wood. Once the router base has settled onto the leg, gently pull the router along the leg, using the fence as the guide. When you get to the 3-inch mark, stop. Repeat on the other leg.
Re-set the router depth to 1/2 inch and repeat the procedure on both legs. Re-set again at 3/4 inch and repeat again until there is a clean slot 3/4 inch deep running centred down both legs.
Take the 1/2 inch bit out of the router and replace it with the 3/4 inch bit. Set the depth of the router at 1/8 inch. Slide the guide fence of the router until the fence just barely makes contact with the bit.
Lay the apron down on the table and clamp it securely. Set the router down on the end of the apron, letting the fence sit parallel with the end of the apron. Turn on the router and gently ease it along the end of the apron, letting the cutter trim the end. Turn the board over, clamp it down, and do the opposite side, creating a 1/2 inch tenon. Do both ends of the board.
Round the bottom corner of the apron tenon, coordinating it with the 1/2 inch diameter mortise in the leg. Slide the tenon into the mortise.
Do a test run on the tenon with a scrap board the same dimension as the apron. Try the fit, and then adjust the router accordingly.
Be careful when running the mortise. Ease into it, and make absolutely sure that you keep both hands on the router, and that you are holding it square.