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When cutting joints, make sure the router table is clear of sawdust. The debris under the wood can cause the cut to be misaligned. The tongue-and-groove bits are a matched set. The smaller bit on the right cuts the groove. The larger bit cuts the tongue. Washers can be used to adjust the size of both bits if needed. Put the groove bit in your router table and set up the cut. An indexing line on the bit shows you where the two plates match together so you can locate the center of the cut.
To make the matching cut, use the bit for cutting the tongue. This pass removes a large amount of material, so take it slowly. The two pieces should fit together to form a nice joint.
Specialty tongue-and-groove bits make easy work of creating the joints, but they can be a bit expensive. If you don’t have the budget for those bits, a simple straight flute bit can be used to cut a tongue-and-groove joint. The same bit can be used to cut both pieces of the joint. Cut the groove first. Set the fence on the router table so that the bit will cut approximately through the center of the board. Turn the board around and make another pass. The board now has a cut through the center. For the tongue, move the fence forward so that you’re cutting away the edges of the board. Your goal is to leave the same amount of material on the tongue board as you cut away on the groove board. This will take at least two passes. Once the cuts are made, put the boards together to get a tongue-and-groove joint.