Like many home-shop woodworkers, I use a router jig to cut mortises and a dado set to form tenons. This creates the classic “square peg into a round hole” problem, leaving me either to square the corners of the mortise or round the corners of the tenon, both of which require time-consuming handwork.

This simple jig works with the same table-saw setup I use to form the tenons, quickly chamfering their corners so they will fit into rounded mortises. While this approach sacrifices a tiny bit of glue surface, it’s far from enough to compromise joint strength. The jig is a 45° channel in a straight, flat board, which forms a cradle for the tenoned workpieces. The channel is made with two simple ripcuts, with the blade set at 45°.

To use the jig, leave your tenoning setup in place so the rip fence provides the same length of cut. This ensures that the dado set will simply brush the tenon shoulders, leaving clean corners there. You’ll need to adjust the height of the dado blades for the chamfer cuts, but that step is fast and easy, even if the tenons are off-center.

—BILL JANZEN, Chapel Hill, N.C.
Drawings: Dan Thornton
From Fine Woodworking #310.