To test whether your miter saw is cutting true, you will need various pieces of parallel and straightcut material, blue tape, and a razor knife to help indicate a mark. If the tool doesn’t have its own adjustment wrench, you need a way to adjust the tool itself.
Take a white piece of S4S or pre-primed baseboard material to look at how the saw is cutting square relative to the fence. Set your saw to 0° on the miter scale and place the piece of straight and parallel edged wood against the fence and cut it. Once the pieces are cut, there’s a cut seam. If you flip one over, you can tell if it’s not cutting square. An open gap on the flip piece indicates that the saw is not truly cutting 90°. That leads us to how to true-up a saw properly with a 45° cut.
Take a similar material and cut four triangles. Set the saw to 45° as indicated on the miter scale and if you cut four repeated triangles, it should make a crosshair with no gaps in it. Set the four cut pieces on a flat surface and put them in a square pattern. If they don’t come around on a very tight crosshair, that indicates an error in the saw.
Your brand of miter saw ought to have a way to adjust the gauge. To make the miter adjustment, place tape over both the miter scale and the saw body in a place somewhere away from both the screws and the miter lever. Use a razor knife to separate the tape very carefully. In my video, I want to open an angle away from the fence. I know that the blue tape must move in that direction, and it must move enough that the gap will close. I don’t want to make a large change, but I do want to make a change. Use the fact that tape on the miter scale will move but the tape on the saw body will not, to indicate how much of an adjustment you made, and in what direction. Tighten the bolts again to avoid moving
the miter scale while cutting. Set the saw to 45° and repeat those same four cuts again. That small adjustment hopefully will close the gap down to a perfect crosshair. If it’s not perfect the second time, do it a third time or however many times it takes. You may overshoot the adjustment, but not to worry. You know to go in the other direction based on your tape offset.
Do this test when your project includes any simple mitered cuts, as well as any decorative inlay work, borders, or parquet patterns to make sure you’re cutting true on the angles that are indicated on the gauge. I do this on any saw I have not operated personally, even in my own company, because you don’t know what happens to your saw when it’s out of your care. It could get dropped, bumped, or bounced around, and vibrations can cause things to get loose. It’s always good to verify your saws are operating well and cutting true before you start any project.
Watch it!
See Lenny Hall complete this process.