Not only is the material removed part but you also have to consider the weight on the mirror is important. In addition is the amount of overhang also part of the equation.
Usually I find that digging a hole with a small tool relative to the mirror (a 4" pipe end cap would be something in the right size and is fairly cheap) and then making a dental stone and tile tool to finish the grinding.
Another thing that I will note is that if you are using a full size tool, you want to stop grinding with the Sag. at about 80% of the needed value although with that .2", you may want to go a little further. The rest of the grinding will improve that Sag. to the desired .2" You will also be wanting to finish most of that needed digging with the 120 and 220 grit, finishing up the 220 or 320 grit with the short spherizing stroke. If you go all the way to the desired Sag., you will end up going too far as the finer grits will still keep on digging a hole as long as the mirror is on top.

• Thanks Bob - a follow up question - billw123 - November 22, 2002 - 01:38 UTC
  • Followup - bobmay - November 22, 2002 - 19:45 UTC