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Re: [ATM] Polishing to a sphere

Often times, the temperature of the room will have  tremendous effects on whether the lap will have an impact. Sounds like  you are pressing well enough, but I am going to suggest the following  courses of action:
  
  (1) Get some netting (like the type that onions, garlic, citrus fruit,  or potatoes are often sold in); slit the bag into a single sheet. Dunk  the lap into some uncomfortably warm water for about 10 minutes (say,  130 to 140 dgrees, the hottest that normally comes out of a tap); then  lay the netting on the lap, and press the still-room-temperature mirror  (which has some cerium oxide slurry just applied) onto the lap hard  enough to make the impression of the netting into the lap facets. BTW,  with this method, you can tell easily whether the lap is really making  good contact or not.Then cold press with your pressure for about 5 to  10 minutes, and then polish normally for about 15 minutes. I suspect  you will find the drag (friction) from your lap will be dramatically  increased, and your figure will improve.
  
  (2) If this still doesn't make enough of a difference, then you might  need to soften the lap itelf a little bit by either raising the ambient  air temperature or by chipping off all of the old pitch, putting it  into a large coffee can (or whatever), melting it back down, adding  about 1 teaspoon of turpentine, and casting it again and doing what you  ahve been doing in the past.
  
  (3) Try different methods of applying weight on the mirror; try pads of  cut-up carpet on top of the mirror; you could have one with a hole cut  out in the middle, looking like a ring; along with a piece of plywood  on top of that; and then a barbell weight.
  
  I hope some of this helps. It pays to try different approaches until you find the one that works for you.
  
  Guy
  

Gilbert Gagne <ggagne@clearwire.net> wrote:  Hi all!
I've been manually polishing a 12" plate glass mirror trying to get to a 
sphere. The back has been ground to the same radius of curvature as the 
front which is 134.4" (f/5.6) and it is 0.75" thick, with max. wedge of 
0.0005". It ultimately will be a "flex" mirror, but first its figure 
must be made spherical. It is completely polished (CeO) out but its 
figure seems to be resisting all efforts to be spherical. The polishing 
lap has been properly prepared full size with gugolz 64 pitch and 1" 
squares (not touching). Polishing was done with 1/3 stroke MOT 
center-over-center. The last hour of polishing has been done in 10 
minute spells alternating with half hour cold pressing at about 20 lbs. 
To get uniform pressure on the convex top when cold pressing, I use a 
12" beach ball nearly filled with water. The Ronchi-grams below (80 
lines/inch) were taken with a webcam and red LED after allowing several 
hours cooling time. They are separated, in time, be three polishing 
spells. Its hard to see much change. I must be doing something wrong, 
but what?

http://www.atmlist.net/contrib/ggagne-at-clearwire-dot-net/113-1.jpg
http://www.atmlist.net/contrib/ggagne-at-clearwire-dot-net/113-5.jpg

Constructive comments will be greatly appreciated.
Gil Gagne
Port Orange, FL
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Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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