It’s important to know, conclusively, that when you’re shooting for a 90-degree cut you’re going to get a precise 90-degree cut. While many woodworkers set up their saw by positioning a square between the blade and fence, I prefer to check the set up by making a cut. I don’t want to know if the saw is set at 90-degrees when it’s sitting still. I want to know if it’s precise when it’s cutting wood.
Get two boards ready. You need one perfectly straight edge on each board. The best way to ensure that is by jointing the edge. The wider the boards, the better. Your test will be more accurate if you cut boards that are close to the maximum width your saw will cut.
With the jointed edges against the fence make a cut. It’s OK to cut both boards at the same time. But get in the habit of helping the saw make a good cut. It’s best if you set up the cut so that the blade has material on both sides, rather than just skinning the end grain. If you merely skin the end grain the blade can deflect away from it, adversely affecting the cut. If there’s material on both sides of the blade you won’t get deflection, and the cut will be straighter.
Place the jointed edges on a flat surface and position the cut ends against each other. Check for gaps. These boards show a gap at the top of the cut, so the saw can’t be cutting at 90-degrees. Since both boards were cut the gap amplifies the error. The size of the gap is twice the amount your saw is off. One of the things I like about this approach is that it makes small errors look big, allowing you to really dial in the accuracy. Check your owner’s manual and adjust your saw as needed. Make another test cut.
You’re done when the cut edges touch uniformly across the width of the board.
If you have a compound miter saw you also need to make sure the saw cuts a perfect 90-degrees perpendicular to the table. With jointed edges down on the table cut two boards. Again use material that’s close the max your saw will cut. Check the cut ends the same way you did on the previous step, and adjust the bevel angle until it’s perfect.
OK, second verse, same as the first. With jointed edges against the fence and the saw set to 45-degrees, cut through two boards.
Hold the miter closed and position a square in the inside corner. If the edges of the square are in full contact with the material, you’re good to go. If you see gaps, you need to correct the angle of the cut.
You’re gonna love this. Make it easier to accurately position your material on the saw by adding a line-of-cut fence. This consists of a sacrificial fence you fasten to the miter saw fence. There are typically holes in the saw fence you can run screws through, into your sacrificial fence. Mine is made out of melamine. The ruler between the sacrificial fence and the saw table provides a small gap under the fence that gives sawdust a place to go so it doesn’t build up in the corner.
After the fence is screwed on, cut through it.
You’ve now made it very easy to accurately position material on the saw. Mark your material with a lay out line that’s near the edge, and position the line on the kerf you created in the fence.
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