ATM 16" polishing out

[Anthony Stillman <atmer@flash.net>]

All,

I've now got 8 hours of time in polishing out(1) my Walter's 16" blank. At
this point I've taken several close looks at the glass surface using a desk
lamp and Edmund's hand held 50X microscope to examine the scratches.  Yes,
scratches.  Part of me wishes they would go away, the rest knows that if
they do, I've polished too long.  Still, they have been interesting to
follow.  When I first noticed them around 2 hours into polishing, they
didn't look so bad, at least on the surface.  Alas, sub-surface damage was
clearly evident.  At three and third hours, when I looked again, the center
of the mirror appeared molted, the bottom of the remaining pits forming a
sort of mogul pattern.  The edge was still clearly pitted and the top
material around the scratches had loosed from its setting.  In addition,
there were a few new short shallow scratches here and there.  I scrubbed
the lap gently and pressed.  At this point the worst scratch was quite
broad.  An examination at five hours showed the shallow scratches were gone
and the glass was polished out in the center, with slight pitting at the
edge.  I was then concerned that the ten hours planned might not be enough.
At  six and two thirds hours the pits on the edge were gone too.  This got
me thinking that maybe ten hours was too much but I persevered and have now
reached eight.  The scratches haven't changed much since 6 2/3 hours.
Though they are overall more shallow and even gone in places, the worst of
it is still no less than 0.003" wide. Yea, I know.

In addition to scrutinizing the glass I've kept an eye on its figure. I
first tested it under a ronchi around 3 1/3 hours.  Still pitted, the edge
appeared turned down but more striking was the strong transition around the
60% zone.  At about 6 hours this zone had broadened a bit and worked its
way out to 70%, but it was still striking.  At 8 hours the zone is starting
to wash out leaving a mirror which is somewhat parabolic at the center
becoming hyperbolic at the edge.  That is, under corrected with a very
turned up edge.

The two long scratches through the center are clearly visable in the
foucault.  Though one is starting to look broken up.

My bruised ego already has me continuing to 10 hours as I had planned.  The
state of the figure only helps me confirm(2) this decision.   Still I'm
thinking that with careful stroke selection, it should be possible to
polish out , ready for figuring, a 16" soda-lime glass mirror with a 12.5"
tool, using 15 lbs of weight in under 8 hours of actual polishing time.
This is much less time polishing than is written or implied elsewhere, but
it is consistent with my experience.

For the next two hours I plan to test every two secessions.  This is
typically as much as I will do in a day.  It will allow me to change my
stroke at least twice to get a feel for the effects and, track the thermal
changes, if I can, before I buckle down and start taking measurements.

Anthony



1) I've started working in 20 minute secessions which last anywhere from 24
to 26 minutes in real time. I turn the glass 60 to 110 degrees every 2 to 4
minutes and add compound every other turn, or so.
2) Justify