Re: ATM [long] Getting the smoothest mirror (no dog biscuit)

[papilio1@juno.com (Michael Pankratz)]

A few more notes on the subject...

It's very true, using a thick polishing mixture will fool you into
thinking that the lap is in good contact when it may not be.  In fact,
even when thinned, the Ceriums which I've used -- the best so far being
that from WB (as I said, I haven't tried the premium ones from Salem yet)
-- are still somewhat forgiving of less than perfect contact.  When I
think I've got good contact with Cerium, and then switch to rouge, the
lap usually grabs horribly.  It takes quite a bit of patience and care to
get the drag smooth with rouge, but once you do -- magic!  All the
roughness vanishes in minutes, using fairly long, slow strokes with
plenty of pressure, and avoiding *short* figuring spells.  Also, I
generally alternate 2 minutes of polishing with 1 minute tapered cold
pressing -- but since the lap stays on the mirror, I don't think this
constitutes short spells.  Even TDE seems impossible to maintain/produce
with rouge and a reasonably hard lap -- I've *never* had a mirror
finished with rouge show anything less than a perfect "diffraction edge".
 (By *hard* lap I mean #73 Gulgoz, even though it's listed in the
catalogs as *medium* -- and my shop temps are usually 70-80 degrees.)

Whatever the arguments for over/under-sized laps, I don't think this has
much to do with smoothness.  (FWIW, I think oversized laps are great!)

Also remember -- a thick, hard lap will behave in many ways like a softer
thin one, as far as smoothing and figure control.  On the other hand,
soft pitch may induce TDE and hard pitch may scratch, no matter what the
lap thickness.

All this said, just remember that, here in ATM-land, what works for one
person will often have the reverse effect in someone else's hands!  Laps
in particular seem posessed of strange and temperamental demons.

Cheers,

Michael Pankratz
Tesuque, NM
papilio1@ juno.com