Reply by brtech

The first thing you need to do is to be able to cut a 90 degree angle and know it’s really square. There is a technique called a “5 cut” that allows you to confirm it, but you use a dial indicator on a bar in your miter slot to align your table saw. No need to be fancy. Get a Harbor Freight $10 Dial Indicator, cut a hunk of wood that you mount the DTI (Dial Test Indicator) on, and clamp it to your miter gauge so the tip is in contact with the blade near the edge of the blade. Mark the spot on the blade with a sharpie, rotate the blade around so the spot is on the other side, and move the DTI in the miter slot to touch the same spot. You are measuring the distance between the miter and the blade in two places. You want the difference to be zero. The absolute measurement doesn’t matter. The DTI doesn’t have to be square to the blade. All that matters is that the miter gauge doesn’t wiggle in the slot, and the DTI/bar doesn’t wiggle on the miter gauge.

Then turn the bar with the DTI around and measure the distance between the fence and the slot. Measure in a couple of places up and down the slot. There should be zero difference (actually, some jocks like the back edge to be a thou or two wider than the front edge).

For extra credit, while you are at HF, pick up the mag base for the DTI. Take off your blade, mount the mag base so the DTI can reach down onto the arbor. Find a smooth spot and watch the DTI as you rotate the arbor by pulling on the belt. You should see zero movement as you rotate. That’s “runout” and you are looking for less than .001.

Take a 24” or so square of ply (rectangle is okay), or MDF, or sheet good you have. trim one edge, making sure you keep it flat against your fence. Mark that edge. Rotate 90 degrees clockwise. Trim again. Rotate 90, trim again. Repeat 2 more times. At that point, you have trimmed all four sides against your fence. Rotate again and you are back to where you started, and your square is a bit smaller than it was. Now cut off an inch or so of the square.

Now, you want to measure the width of the cut off piece at the top and the bottom. The best tool for this is a caliper. The $20 HF is okay. Measure carefully.

There should be zero difference. If there is any, your fence is not parallel to your miter slot and/or your blade is not parallel to your miter slot. The error is 1/4 of what you are reading. Go back to step one. William Ng has a great video that shows how to do this.

Then you need to confirm your miter gauge is exactly 90 degrees to your blade. While you can use the triangle to see it fairly well, do the 5 cut test with the miter gauge instead of the fence and confirm.

When you really can get a square cut, exactly 90 degrees, then you want to set up to cut your 45s.

The triangle is an excellent, cheap tool. A digital angle gauge is an excellent, not so cheap tool, but they go on sale for $20 every once in a while.

You should have a good combination square, and the head on it is an accurate 45. You can set your blade with that too. Be careful that you are measuring on the flat part of the blade, and the teeth are not throwing off your measurement.

We are assuming you are cutting your 45 with the blade at an angle, rather than cutting with a miter gauge set to 45. If you are doing the latter, the same tools work to confirm you are at 45 degrees exactly. A sled with a miter bar is a better tool for this than a miter gauge.

You have to hold the work piece really stable as you pass it through the blade. If you are using the blade canted over, a cross cut sled is a much better tool than the miter gauge to hold the work. The sled needs its fence exactly at 90 degrees, and you use the same 5 cut method to set it.

At least for me, being anal about the basics is the key to good miters. The faces have to be square, the miters have to be exactly 45. Miters are one of those things where “close” isn’t good enough.

Then all you need to do is get the lengths right :).