I struggled to cut mitered frames with my standard tablesaw miter gauge, getting ugly gaps in the joints no matter how carefully I tried to set the miter gauge to exactly 45º.

Finally it occurred to me that, if all the cuts in a mitered frame are made in the same way on the same miter gauge, any errors are incremental. An error of 0.5° on eight cuts (for a rectangular frame) adds up to a 4° gap. You could see that gap from across the room. The solution, I realized, was to cut the joinery in a way that instead caused the errors to cancel each other out. If I made a small error on one cut, it could be corrected by making an opposite error when cutting the mating face.

To do this, I made a miter tool using a Shinwa combination miter square. These stainless-steel tools are very accurate, and are available at Amazon and at woodworking outlets for around $27. I attached foam pads at the top on both sides, so the tool will lie horizontally on a saw table. I also glued a hardwood strip to the square’s flanged base to distance the metal square from the blade and added an auxiliary fence to my miter gauge.

To use the miter tool, I set the miter gauge to zero, clamp the miter tool to the left side of the fence, and cut one side of the miter joint. Then I flip the miter tool over to the right side of the blade and cut the miter on the mating piece. Done this way, the second angle will be the complement of the first. When joined, the two cuts will be a perfect 90º.

mitering jig

click to enlarge

mitering jig

click to enlarge

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November 15, 2011