How to set up and use keyholes bits
When you want to hang a shelf, picture frame, clock, or candle sconce on a wall but don't want the mounting screws to show, use an inexpensive keyhole router bit to cut screw slots instead. Then, when you hang the piece, it will appear to float on the wall. And, with keyhole slots you don't need special hardware. Once you've got the tools set up, you can cut multiple slots quickly and easily. Here's how.
Table-rout for precise slots
First, choose a keyhole bit with an entry-hole cutter slightly larger in diameter and thickness than the head of your screws. The slot cutter should be slightly larger than the screw's shank. (Doing all this allows the screw to drop in and slide along the slot easily.)
These slots demand precision -- especially when using more than one -- because if they're off by a little, your project could hang crooked. So we like to rout them on a router table; a handheld router could veer off course while cutting, compromising the slot.
One or two slots should hold most projects sufficiently. To make keyhole slots, first lay out their locations. For projects with a single horizontal keyhole slot, locate the centerpoint of the project in the middle of the slot to allow you to adjust it for level. For a single vertical slot, make sure it's centered side-to-side and located on the upper half of the project.
Next, decide where to plunge the entry hole for each slot. For a keyhole slot that will be on a vertical element of your project, make sure to locate the entry hole at the bottom of the slot. For horizontal slots, it doesn't matter which direction you plunge and cut as long as you do all slots the same. Mark the center of the entry hole and transfer that "start line" to the front of the workpiece.
Install the bit in your router table, and set its height to leave at least a 3/16"-thick shoulder. (See the keyhole slot in center photo.) Position the fence to center the bit on your workpiece's thickness. Mark a "stop line" across your workpiece's top edge to indicate where you'll stop routing the slot. In most cases, 1-1 1/2" proves sufficient for slot length. Now mark a line on your fence that aligns with the center of the bit, such as the one shown in bottom photo.
Now rout the slots
With the router running at maximum speed, place the workpiece above the bit and against the fence with the start line and bit centerline aligned. Gripping it tightly, slowly pivot the project down onto the bit, as shown top right. Once the project rests on the tabletop, slide it to rout the slot, stopping when the stop line reaches the bit centerline (center right). Without moving the project, use one hand to shut off the router. Wait for the bit to stop spinning before backing the board off the bit. Failure to do so could damage the slot and project. Repeat for the remaining slots.