Re: ATM 21" f/6 report: status and questions

["Jeff Arnold" <scoper10@hotmail.com>]

Bill:

>I'd sure like to get some pointers from somebody who's >perfected fine 
>micron technique with clumsy, big mirrors >and tools.

Well, perfected is a stretch but the latest mirror just finished was ground 
past 3 micron with a 16 inch tile tool on top of a 22 inch F/5 pyrex blank. 
The "Trick" is to bevel the sharp perimeter edges of all the tiles so when 
it's time to drag the tool off that last little "lift" will not cause a 
sharp corner of a tile to pivot on the glass. Also, I mark the top of the 
tool so I remove it from a long row of tiles rather than the corner of one 
tile using 1 inch square porcelain tiles.

>As far as I can remember the 3 and 5 micron mirrors >didn't polish 
>dramatically faster than the  9 micron >mirrors.


I've found that going to 3 and even 1 micron DOES save some time in 
polishing but it really is about being sure those 9-5 micron pits are gone 
and being careful not to lay down a scratch. I decant the 3 micron some what 
so I'm sure it's all small stuff. The 1 micron is really crazy - "unit may 
seize at any time" - should be the warning. I apply the 1 micron at the end 
of the wet using 3 micron and slow things down considerably. That last 1 
micron wet is for 10 minutes. The mirror gets a simi polish and shows an 
orange Sun from a perpendicular reflection off the glass. The drag on the 
tool is almost as strong as a pitch lap so you've got to watch out that the 
mirror doesn't lift up off the table from the force of the tool pulling 
across the mirror. This is all done on a machine. By hand, I've done 1 
micron too. It's just a little scary. A tile tool helps relieve the fear of 
a stuck tool. Mostly my advice to others is to stop at 5 or even 9 micron. 
That's all pretty fine a finish and doesn't save a whole lot of polish time.

It is really just my style - most should stop at a minimum of 5 micron. 9 
micron is safer !

Polish was complete with the 22 inch in only 5 hours using a 14.5 inch lap 
and Gugloz 64.

Jeff


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