When you purchase a lock miter bit be sure you get a cutter that matches the thickness of material you’ll be working with. Bits typically cover a range of thicknesses.
When prepping your project material be certain you make practice pieces that are exactly the same thickness as your project pieces.
In setting up the bit, don’t try to make the bit height and fence location perfect. We’ll first concentrate on bit height, then tweak the fence location.
When first setting the height of the bit, try to get the center of the bit close to the center of the stock. Center of the bit, for many cutters, is the bottom corner of the groove cutter. If you are using 3/4'” stock you can use a 3/8” setup bar to get close.
Test cuts will accurately determine if the bit is a little too high or too low.
Position the fence such that the top of the material, the face of the fence, and the 45-degree angle of the bit are all aligned. Start by moving your fence out so about a quarter of the bit is exposed. Position your material against the fence and, with the cutter pointed out from the fence. Lay a straight edge on the top of the stock and then slide the straight edge inside the corner and over the bit. Move the fence back until The bottom of the straight edge barely drag on the carbide bit. Lock the fence.
Make test cuts by machining one long grain edge of two practice pieces flat on the table. We are not making a vertical cut at this time. . Flip one piece over and slide the cut edges together. The goal is to have the two faces flush. If the long point of the miter is higher than the face, the bit is too low in the table. If the long point is lower than the face, the bit is too high. You need to adjust the cutter height by ½ the amount of the offset. Adjust the bit height as needed. You may want to tweak the fence position every time you change bit height, though this is not critical at this point.
When the two pieces fit together perfectly flat the bit height is correct.
After the bit height is perfect follow the setting the fence steps above. When setup is correct, there will be a crisp edge on the 45 degree edge.
Note, begin by leaving a small blunt on the edge and then move the fence back until the edge is crisp. It takes very little adjustment of the fence.
The parts are machined by cutting both ends of one piece with the piece horizontal on the router table. The mating piece is machined by being held vertically against the fence. You can handle chipping on the exit side of the cut a couple ways. One approach is to leave the pieces too wide and, after the joint has been machined, rip to final width to eliminate the chips. You can also follow the project pieces with a sacrificial piece that will support the exit side and eliminate blow out.
Some type of sled is strongly recommended when machining the vertical pieces.see example
Note, the tolerance is very tight between the two halves of the joint. Because of that you have to be very careful to spread the glue in a thin film before assembly. Puddles of glue will prevent the joint from closing. Clamps are only required in one direction, the same direction in which you assemble the two pieces.