Reply by brtech

The 5 cut method doesn’t depend on starting with a square. It only depends on starting with a reasonably straight edge.

The sled cuts with the fence 90 degrees to the blade, and you are trying to determine if it’s exactly 90. You start by cutting with the straight edge against the fence. This gets you another straight edge 90 degrees (we hope) from the first edge. It doesn’t matter if the original board had a 90 degree angle, we cut a new one. You rotate so the new cut edge against the fence, and do it again. And again it doesn’t matter what the original angle is. You do that 4 times. In theory, you now have a board that has four 90 degree angles.

Then you cut a strip off that board. In theory, it’s rectangular, because in theory it too has four ninety degree angles.

If it doesn’t, then what happens is that one edge is a different width than the other. If they were exactly four 90s, then it would always be the same width top and bottom.

The thing is that the angle of the fence doesn’t change on each cut. So, if it was originally, say 91 degrees, you would have four 91 degree corners, and, of course, that means the top and bottom wouldn’t be the same width.

That 5th cut isn’t really necessary, but it lets you have a board bigger than your measuring tool, which is good. You cut off a strip narrow enough to measure with a caliper or micrometer. And, of course the error is 5X the actual error, because you compound the error 5x.