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On a crosscut sled, the fence must be absolutely square to the blade. Here’s a super-accurate method of testing for square. It makes errors easier to detect by magnifying them by a factor of four. The same method also works for a miter gauge, using smaller test pieces.
Start with a piece of plywood about 16″ x 16″ that’s more or less square. Cut one edge using your sled and a sharp crosscut blade. Rotate the board so that the freshly-cut side is against the fence, and cut again. Rotate and cut two more times. Rotate the piece one more time, back to the original cut. This time, cut off a sliver about 1/16″ thick.
Use a caliper to precisely gauge the sliver’s thickness. If your fence is perfectly square, this sliver will be the same thickness over its entire length. If your fence is not square, the sliver will be thicker at one end than the other. The difference in thickness indicates four times the amount of error in each cut. Keep adjusting the fence by minute amounts, repeating the procedure until the last sliver is perfect. –Joe Emenaker
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