Installing a half-mortise lock used to give me fits. I’d end up with the keyhole off center, the lock plate set too deep, or the selvedge set below the surface of the drawer front or door edge. Over time, I’ve developed a series of routing steps that result in a great fit each time. The best tip I can give you here is to use a sharp, small-radius router bit (1/4″). A bit this size allows easy freehand guiding of the router or trimmer.
Begin installing a half-mortise lock by finding the center of the piece (unless you plan to have the lock off center). You may be interested to know that the lock’s key pin is not centered in the lock. I found that out the hard way! So make sure that you place the pin at the center and draw the outline of the lock onto the panel or drawer front. Hold the selvedge flush with the edge of the material.
Next, set the depth-of-cut of the router to the depth of the selvedge. But before you rout, use your saw to define the edges of the lock area. By cutting this first you will eliminate the flake-out of the material as the router cuts. Create the recess for the selvedge and clean the ends (the area remaining from the radius cut of the router bit) with your chisels as shown at right.
Now rout the area for the lock plate. Set the depth-of-cut to the thickness of the lock plate. Before you begin to rout, use a utility knife and square to cut the outline of the lock. I like to use a marking gauge to get to the back line , it allows me to square the lock to the edge by running the gauge against that edge. Carefully rout out the lock plate area by hand, getting close to the lines without touching, and finish the area with your hand tools. The lines should cut away cleanly because of the use of the knife and gauge.
To lay out for the lock body, position the lock into the recessed area and trace around the outside of the lock body. Make sure that you leave enough material to catch the screws that secure the lock into the panel. Using the trim router, I ran into a small problem at this stage because its base was not large enough.
If your lock is on the large size you’ll have an area for the lock body that allows your base plate to fall into the plate-routed area, off of the panel. To keep this from happening, you can either use a larger router base plate or cut the lock body recess prior to cutting the recess for the lock plate. Generally, it’s not an issue.
Now set the depth-of-cut for the router bit to the thickness of the lock body and create the recess. Because this area will never be seen once the lock is installed, you don’t need to achieve routing perfection.
Now you’re ready to locate the pinhole for the lock. The key pin protrudes from the lock body and if you rout the body cavity as shown, you’ll have no trouble finding the exact location of the pinhole.
– Glen D. Huey