Re: ATM 21" f/6 report: status and questions

["Bill T." <twentiethwave@hotmail.com>]

Ken,

Congratulations!  The moral strength you developed hogging that 20" with
serve you well during polishing ;^)

I worry about the Gugolz 64. Even when my summertime shop was pushing 82
degrees, I felt it was noticeably too hard for a large lap -- crumbly,
difficult to press, etc.  Even though Gugolz 55 is very runny at those
temps, I prefer it even in spite of more frequent re-channeling.  But you'll
probably be ok with lots of pressing -- actually a reasonable trade of for
lots of channeling.

2 kilos is about right for a 20" lap. Depending on channeling, you'll get
around 3/8" coverage.

I'd go from 220 grit right to either:
--25 micron then 9T (9 micron, with talc)
or
--30 micron, 20 micron, 9T

You can also do 20, then 9, but you'll need to spend a lot of time at 20. Of
course sequences with more grit steps will work, too, with more cleanups.
Before advancing grits be sure to scrutinize all parts of the mirror with a
strong magnifier, a strong cross light, and an eagle's eye. The outer 1/2"
can be more of a problem on large mirrors than small, so spend a lot of
inspection time there. If you swap positions, you should be able to get
through all the grits after 220 in no more than 5 or 6 hours with a large
safety margin.

Hand pushing a 20" lap against a sphere is not impossible. One guy can do
it, but really only up to the point where contact starts to get dry and most
efficient -- I've measured 60+ pounds of sustained back pressure from a 20"
lap (using Gort's load sensing) when the surface gets "clear" and contact is
super good. Normal but effective wetter pressures are more like 15 to 40
lbs, and still result in polishing. One way to reduce pressure is to reduce
total surface contact with a "sparse" lap pattern. Here is a lap pattern I
like, it has 3/8" channels and 1.25" facets:

http://www.kupercontrols.com/misc/lapmold20.jpg

I've used laps with as little as a 50/50 ratio of facets to channels . With
any sparse lap, the channels gradually close up as polishing progresses, so
rippling is not a problem -- try to time things so you end up with small (or
no) channels. I quesstimate 12+ hours of hand polishing to pass the laser
test. Polishing pressure goes way up as you depart even slightly from a
sphere, so it really pays to keep the sphere by swapping positions often.

I like to froth the cerox/water mix just before putting the mirror and lap
together. This keeps the liquid from flowing quickly away, and gives good
solid 30 minute wets without having to add more mix. I put the lap on its
back, get it completely wet with water, add some cerox/water mix at the
center, rapidly spread it around with my flat hand, then froth it up with
fast circular swipes from a clean, non-rusty 3" paintbrush. To make this
work I need to add about 1 drop/quart of cerox/water mix. This is well worth
the effort. Notice how little material is lost overboard in the picture,
taken after a lot of polishing.

http://www.kupercontrols.com/misc/froth.jpg

Here's a figuring lap that works very well for me:

http://www.kupercontrols.com/misc/figurelap1.jpg
http://www.kupercontrols.com/misc/figurelap2.jpg

This a 10" lap with the outer facets rounded and minimized so it acts like a
smaller star lap. The pitch is on a 3/4" particleboard base which is in turn
pitch-glued to a 25 lb weight. Used by hand, one would just push it from the
wood sides, without applying additional pressure. This lap gives a very
smooth surface. It takes little more than an hour or two of slow, wet
polishing to reach the approximate correction for a long 20". I do all
figuring TOT. I use mainly 11-tangent W strokes, lap overhanging 1.5" to
2.5" on all sides, with about 75 percent of the strokes favoring the central
50% of the mirror diameter. It's extremely important to use lots of tangents
with this technique, at least 11 to cross the mirror -- otherwise you'll dig
a central hole for sure. The W pattern can be widened or narrowed to bring
the overall correction up evenly. If the center seems to come in faster than
the edge, I widen the strokes a bit. If the edge is turning, I apply a few
minutes of very short W strokes with a full sized lap. If the edge is still
lagging after an hour or so, I give it maybe 2 turns of very long
center-over-center strokes with a full sized lap. But I give the figure at
least 45 minutes to start developing before taking any compensating
measures. I try to do all the corrections by simply varying the spread of
the W strokes -- it's almost always possible.

Bill T.