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The keyhole slot is made with a keyhole router bit. The bit has a cutter at the end that makes the hole for the screw head to enter. The smaller cutter above it cuts the slot.
Use a plunge router (Image 1) to cut the slot vertically in the center of the plaque. Use 3/4" material and make sure to plunge deep enough to hold the screw in tight. This project requires the use of a plunge router. A fixed base router cannot be used for this. An edge guide will need to be used to keep the keyhole slot straight.
If you cut a horizontal slot, slide the screw into one side and adjust the plaque by sliding it over the screw until you find the center spot (Image 2).
When making a horizontal cut, plunge the bit into the wood, then push the router along to make the slot. Turn the router off, then back the bit out of the slot and lift it out through the hole.