Obtaining the right-size shims for a job can be quick, easy, and precise if you put in about 10 minutes’ preparation. Begin with the shim stock.
A proper shim has grain running its entire length—no end grain allowed. To that end, I glue 3-1/2-in.-long 2×4 blocks together edge to edge, as shown in the drawing. The number of blocks depends on the length of bar clamps available for gluing. I label the glued pieces “Shim Material” so that this wood isn’t used for something else.
Next, on a piece of masking tape affixed to the miter-saw fence, I draw index marks exactly 3 in. left of the blade. These marks represent both sides of the sawcut. If I wanted to cut a shim 13/32 in. thick, I would mark the shim material at 3 plus 13/32 in. from the right end, as shown. Then I would align the mark on the wood with the right index mark on the miter saw. When I cut the wood, a piece exactly 13/32 in. thick is the result.
It’s also easy to cut wedged shims this way. A useful rule of thumb is that for every mitered degree you cut through a 3-1/2-in.-wide piece, the wedge becomes 1/16 in. thicker on one end than it is on the other.
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