As the name implies, a keyhole slot is a recessed hole that looks like an old skeleton key hole. They are usually found on the back of a picture frame or wall plaque. The keyhole slot features a larger opening at the bottom of the slot to enable the head of a screw or nail to slide into the wood frame. The interior area of the slot is the same size as the hole which accepts the screw head. However, the sides of the slot are more narrow along the upper portion of the slot, so that the screw will not pull out of the slot when the plaque is hung on a wall. This specially designed slot is created with a router and a keyhole router bit.
Mark the location for the keyhole slot on the back of the piece of a decorative wooden plaque. Measure the location of the keyhole slot so that the slot is centred on the wooden plaque. If cutting two keyhole slots into the plaque, mark their locations so that each slot is at the same distance from the top edge of the plaque.
Clamp a wood or metal straightedge onto the top of the plaque. It works this way: when the router is pressing on the edge of the straightedge, the router bit is centred on the keyhole slot position. In other words, if the distance from the edge of the router base to the centre of the key hole bit is two inches, clamp the straight edge exactly two inches away from the centre line of the keyhole slot. This straightedge must run parallel to the keyhole slot's centre line.
Insert the key hole router bit into the router and tighten the collet. Set the depth of the router bit so that the wider portion of the bit and the cutting flutes on the bit's shaft are both visible. Turn on the router.
Press the edge of the router against the straightedge guide clamped to the work piece. Plunge the router bit into the wood at the bottom most end of the keyhole slot location, as if you were using a drill bit to make a hole. Once the bit is cutting into the wood, and the router base is firmly against the work surface, slide the router along the straightedge, upward toward the top of the wooden plaque. Move the router a distance of 1 to 1 1/2 inches. Slide the router back towards the bottom of the hole, and once the router bit reaches the point at which it entered the wood, lift the router upward, removing the bit from the wood's surface. Turn off the router and carefully set it aside.
Using a can of compressed air, blow any wood shavings out of the keyhole slot which were left behind.