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

["Jerry" <wa4guu@verizon.net>]

I wasn't encouraging. I was not discouraging. After 12 hours of polishing,
even if he finished with 25 micron he could be fully polished by now. If it
is not fully polished, and the earlier grades were done properly it would
not be necessary to go back to grinding.

My friend polished by hand in the particular instance I am thinking of. I
don't know that that was his regular procedure, but he did it that time and
it didn't take much longer to polish out. Try it sometime from 30 and see
how long it takes. If you can't bring yourself to try that, maybe try from
20 or 25 micron. You will be surprised. It is important that the prior
grades and the last be done properly, as is always the case.

I don't finish grinding at 20 or 30. I wouldn't recommend it, but I know it
can be done and still fully polish in a reasonable time. I used to go to 5
micron but saw no benefit in time or quality over only going to 9 micron. I
usually only go to 9 now. Last 6" fully polished from 9 micron in 6 hours by
hand.

I polished through a bubble by hand on a 12.5", actually 3 or 4 bubbles,
much bigger than "pits". It took a while but not near as long as you would
think. I am not recommending this, unless you want to learn something about
turned down edges.

Jerry



-----Original Message-----
From: Mike Lockwood
 
Hi,


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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