[ATM] The mouse in the mirror

[ArtfulBodger <artfulbodger@earthlink.net>]

Thanks, folks, those of you who helped me with spherometer/sharpie 
issues.  The sharpie showed me what I didn't want to see, and it took 
considerable grinding to get things spherical; but now the sharpie is 
happy and the grinding's done.

So, polishing.  I'm using a cerium/water slurry, pitch lap, pyrex 
blank.  (Two laps, actually, on two mirrors, one 8" f/4.5, one 8" 
f/5.5.)

Now, I know guys who seem to use a fair bit of the cerium gravy, they 
keep everyting pretty wet.  Use enough so that the mirror slides well 
on the pitch.  Also they mix the cerium and water somewhat on the 
thick side.  I can't tell you the actual proportions they use, the 
mixture just seems a bit rich to me.

I tend to keep the mix thinner and use less of it: I want the mirror 
to be a fairly hard push -- but smooth, not grabby! -- across the 
lap.  It seems to me that that's when the cerium is doing its work: 
not when the mirror is skidding over the lap, but when it's pushing 
across it, hard and slow.  Fairly hard and slow.  Not from weight on 
the mirror, not pressure on top, but friction on the surface.  I 
don't let the lap get dry, but I use as little gravy as I can.

I don't know if this is good or not.  It seems so.

And when the push is slow and smooth, and the lap is not Not NOT 
dried out, and the mirror starts to chirp like a bird or squeak like 
a mouse -- is that an evil thing?  Does it just mean good friction 
and a good fit between mirror and pitch?  Or does it mean I'm 
polishing a chirpy, wavy surface texture into the mirror?

Or what?

I've read some guys (websites) who mention the squeak in passing, as 
if it were normal; but I dunno.  Could just add more gravy, but I'm 
not so sure about that, either.

So.  The squeak.  Evil noise and cry of pyrex pain?  Or sign that the 
lap is working good and hard?

Pete

-- 
Artful Bodger
http://www.artfulbodger.net
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