You can do the hole digging stroke (a lot of overhang on the tool) with the tool on top and it will come down. The speed will be about 1/2 of what you were doing with the coarser grit. I would also suggest that you also get a cross check with the flashlight method at the center of curvature.
One of the guys in the grinding group is doing a 4.25" F15 and it`s now a F14.5 due to polishing alone!
Don`t forget to get back to spherical (the hole digging stroke makes a very rough surface) with the shorter strokes that you should use with the finer grits.
BTW, I get .03125" for a sag. on the mirror and .025" would be a good stopping point for the 80 grit although .020" probably would have been better as the 120 grit still does a lot of moving on the ROC and you will probably want to do another .003" for the finer grits. It is really amazing how far you need to stop coarse grinding in order to get to where you want to be.
Since you have already gone way down in the grits, I suggest that you get to the correct radius with the 220 grit and if you want to, stop when you are about .002"-.003" short on the sag and take most of the rest out with the next grit.

• RE: RE: Overshot radius of curvature - Walt Hamler - May 19, 2000 - 20:03 UTC