One of the most common joints a woodworker might cut when building a project is a miter joint, such as when two pieces of trim meet at a corner or when making a picture or mirror frame. Cutting the two pieces so they meet perfectly is a simple matter of using a compound miter saw. However, using a compound saw requires you to set up the saw twice, once for each piece. If you are not exactly dead on with the angle setting, the two pieces will not meet at 90º. On a single corner this may not be as noticeable, but when you are building a picture frame, by the time you get to the fourth corner, there will be little chance of the pieces closing properly.

This jig employs the principle of complementary angles taught in junior high math class, and while not as exciting an addition to your shop as a nice new compound miter saw, we think it will become a favourite. By cutting a perfect square to act as the fence on this jig, the principle of complementary angles assures that when one of the pieces is cut on one side of the jig, and it’s mating piece is cut on the other side, the two pieces will form an exact 90º angle. If the angle on one piece is not exactly 45º dead on, don’t worry, the jig will ensure that the second cut is the exact difference between the cut you just made and 90º.

Making the Jig

• Cut the base (A) of the jig to size for your saw. This version is sized to fit a Delta Unisaw; you may need to adjust the base size and placement of the runners to suit your table saw.

• For this jig to be accurate the fence (B) that the material registers against must be cut at an accurate 90º. Use a cross cut sled to cut this piece (see “Cross Cut Sled”, Feb/Mar ‘07, Issue #46).

• Set your table saw miter gauge to 45º and cut the fence diagonally to create a triangular piece.

• Place the triangular fence piece on the base and fasten it in place. If you choose to use brads or screws, ensure that they are not located in the path of the saw blade. Using glue alone ensures there is no chance of damaging the teeth of your saw blade by hitting a nail or screw.

• Cut a blade guard (C) and fasten it over the center section of the fence. This will ensure that the blade is completely buried in the wood after you make a cut.

• Mill two runners (D) and ensure that they run smoothly in the table saw miter slots.

• Attach the runners to the bottom of the base.

• Fasten a set of toggle clamps (leevalley.com), item #88F05.01, to the fence section close to the cut line to hold the material in place during the cut.

Using the Jig

To cut a perfect miter, lay your pieces out and mark one side of the joint with an ‘L’ and the other piece with an ‘R’. When standing at the saw, place the ‘L’ piece on the left-hand side of the jig and make the cut. Place the ‘R’ piece, on the right-hand side and make the cut. The result will be a perfect 90º miter joint. Most miter joints will be cut on stock that will adequately register against a shallow fence as shown. If you need to cut a number of joints at the full depth of the blade you might consider building a jig with a taller fence.