Using a special bit to cut a narrow slot in a board’s edge is a router technique often used in joinery and furniture-making. Slots are often used to hold door panels or to contain a wood spline to stiffen a joint, especially a mitre joint. Slot-cutting bits (or slotting cutters) comprise three pieces: a two- or three-wing bit to cut the wood, a bearing to control depth of cut, and an arbor on which cutter and bearing are mounted. Slot depth is controlled by the bearing diameter, and slot width is controlled by the thickness of the bit.
Choose a slot-cutter bit to cut the specified slot width. Slot-cutter bits come in a range of sizes identified by kerf width. The kerf is the width of the slot that the bit will cut, such as 5/64 inch.
Choose a bearing that will create the desired depth of cut. Measure the slot-cutting bit diameter (standard size is two inches) and subtract the bearing diameter. The slot depth created by the combination is half the difference. Example: a bit diameter of two inches less a bearing diameter of one inch will cut a slot ½ inch deep.
Assemble the bit pieces on the arbor. Put the slot-cutter on first, followed by a shim, the bearing, another shim, and the nut. Tighten the nut securely with an adjustable wrench. Verify the cut depth by measuring the distance from the bearing to the tip of a wing on the cutter.
Unplug the router, place the assembled arbor in the collet and tighten securely. Adjust the distance between the router base plate and the cutter wing to the desired depth. Plug in the router and don safety glasses.
Test the slot location and depth on a piece of scrap wood with the same dimensions as the finished piece. Adjust cut height as necessary, and then cut the slot(s) in the finished pieces.
A router table with a fence allows you to cut slots of various depths without having to measure bearing diameter. Mark the front of all pieces that have a matching slot and cut all slots with the marked side facing up. Even if the slot is slightly off-centre, all the pieces will line up smoothly.
Do not cut slots deeper than 3/8” in one pass. Always wear safety glasses and hearing protection to use power tools.