making perfect miters can be a challenge, but with this fixture you only have to measure a miter angle once.
Start with a piece of ½" or ¾" plywood measuring 12" x 22". Cut a hardwood guide bar sized to your miter slot that will mount to the underside of the plywood so the plywood extends ¼" past the line-of-cut of the table saw. Double-check the guide bar for straightness (so it doesn’t bind in the miter slot), and attach it to the plywood with glue and flat-head screws.
Place the fixture with the guide bar in the miter slot and cut off the excess along the cutline of the table saw – the edge is now perfectly parallel to the guide bar.
Make two hardwood fences measuring 1" x 1½" x 14". Glue and screw one fence to the top of the fixture at 45 degrees to the cutline edge (measure perfectly, then measure again to ensure the angle is exact). Using an accurate framing square, attach the second fence at exactly 90 degrees to the first fence.
Glue 80-grit sandpaper to the outside fence faces to keep workpieces from slipping when you make your 45-degree cuts.
