flatness makes a difference
Because, the blade is set at 90 degrees, in order to make a cut that is square the table must be flat.(laws of geometry) look at an equal lateral triangle. The miter slot is cut so that it is perdedicular to the table surface so it will not effect the angle of the cut. :) small differences proably are not for major concern,but the flatter the better in my opinion.
How to make corrections for flatness?
If I may ask, what sort of adjustments can you make? Now that the cold weather is coming, it's going to be fairly cold in my garage - not quite as cold as outside, but still around the freezing mark and colder on the really cold days. After seeing this thread, I went out and laid a long aluminum ruler across the top of my saw, and now I see a VERY noticable gap. I hadn't made this sort of check before, so I don't know if it happened because of temeratures or what.
For what it's worth, I used 3 different aluminum levels as a base for a dial indicator that I use for the telescope mirrors I grind, and averaged the differences to adjust my table. I estimate that it is within .002" flat and true PTV (peak to valley, the highest point to the lowest point), and less than .001" RMS (root mean square) across the entire surface, which is close enough for the work I do. Still, with the cold front that's moving through the area today, that might all change. I'll recheck flatness in the spring, again when it starts getting really hot, again in the fall, etc. It takes me about an hour to check for flatness and make adjustments.
The gap that I'm seeing is really disconcerting. I checked the center table, and the center table itself is reasonably flat. Each of the wings seems to be flat too, however, along the join line between the wings and the main table, the wings seem to be angled upwards! The angle is high enough that if I run a 4' level from the left edge of the left wing to the right edge of the right wing, there is 1/16" to 3/32" between the bottom of the level and the surface of the main table. It seems to be the same elevation from front to back, so I wonder if it's not square along the edges of the table...
No idea what caused it, and I'm not sure when it happened or if it was there from the start. Any ideas on what I should do to fix it (or perhaps diagnose a cause)? Is it possible that it's the temperature doing this? It has been unseasonably warm around here lately, so I can't imagine what it'd do when it drops 30-40 degrees over the coming months....