Re: [ATM] Polishing questions - stop polishing, keep polishing, or go back to fine grinding?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Jerry wrote:
> It is probably fully polished.
> 6 hours can be enough for a 6 inch, but 12 isn't really so much on a first
> mirror. With experience you can get more efficient. 
> If you have to use a 60X microscope to see these "pits" and they are really
> pits, they should polish out.

Agreed.

> Are you polishing with weight?  Even if you finished with 25 micron, if you
> polish efficiently you can polish from there.  I had a friend that polished
> after grinding with 30 micron and he would get polished out almost as fast
> as I would after grinding to 9 micron.

Hold on a minute there.  It takes me 6 or more hours to really get a 
good pit-free polish after grinding through 5 micron grit, and that's 
on a machine!  Working by hand takes longer.

I would never encourage someone (especially a beginner) to go to 
polishing after 30 micron.  I expect this would roughly triple 
polishing time (possibly worse) depending on how spherical the figure 
was.  A mirror becomes a better and better sphere as the abrasive 
becomes finer, and polishing from a better sphere is far more 
efficient.  (A side note - if your mirror polishes unevenly, you are 
not polishing efficiently.  If part of the mirror takes much longer to 
polish out, it was probably not spherical after fine grinding.  Once 
you see a mirror polish out evenly in half the time it took you on 
previous tries, you will understand!)  Fine grinding through 5 micron 
is a better use of time IMO, and requires less physical labor.

But I do have a theory - the 30 micron used by your friend was broken 
down in the grinding process, yielding smaller grit (probably still 
larger than 10 micron, though), and yielding pits that polished out in 
a reasonable period of time (though not as quickly as they could 
have).

Did you check you friend's final polish or compare it with yours?

Lately, I have had good results going to 3 micron for the last 20-30 
minutes of fine grinding.  I believe that it saved me quite a bit of 
hand polishing time on a piece that couldn't conveniently be polished 
by machine (a large elliptical flat).  Normally I finish fine grinding 
with 5u, but 3u seemed to have benefits.  I have tried 1u, but it 
seemed that the grit/water layer was too thick to have it work 
efficiently, and other precautions need to be taken to prevent 
scratching.  I felt it was too risky.

	Mike Lockwood

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