ATM 21" f/6 - Status report and a question

["Ken Bertapelle" <kenb@cqg.com>]

All,

For those interested, below is a recap of what has happened on my mirror
project.  Here is a question for the more experienced members out
there:

Question:
=========
I'm planning on using gugloz #64 pitch on a full size pitch lap, MOT to do
the polishing.  I think I'll use the method described by Texereau to make
the lap - pouring the pitch into molds and cutting tile squares.  What size
pitch square do you all think is good for a 21" polishing lap?  The book
shows 3/4" squares, but will larger ones work?

Recap:
======
1)  Bought a blank, 1.55" x 21" diameter, both sides blanchard ground.
2)  Beveled edges on both sides.
3)  Rough ground one side to a depth of about .02", but then decided I was
grinding the wrong side.  Re-flattened the ground side, turned the blank
over, and started grinding the other side.
4)  Rough grinding with a hydrocal tool, 5" thick, 14.5" diameter, faced
with metal washers.  Using 46-70 grit, heavy pressure, MOT, stroking the
center of the blank over the edge of the tool.  21 hours of grinding this
way.  Measuring sagitta with a machinist's ruler.  Stopped when reached
desired depth of .2165".
5)  Made a new full size tile tool.  Switched to 60-90 grit.  Grinding with
light pressure, MOT, 1/2 diameter W stroke.  5.5 hours of grinding this way.
By the end of that time the entire tool is making good contact, and the
entire mirror surface seems to be wearing evenly.  The grinding stroke are
smooth, with no grabbing.  I think the surface is developing a nice sphere.
The f ratio is still a little long compared to my f/6 target, but I prefer
it to be longer rather than shorter.
6)  Switched to #120 microgrit.  Same stroke as in 5.  Grinding for 5 hours.
Mirror wear appears to be uniform.
7)  Switched to #220 microgrit.  Same stroke as in 5.  Grinding for 4 hours.
Did a sharpie test and the ink wore away from the center in an even way,
starting at the center, and moving outward.
8)  Moved on to #320 microgrit, same stroke for 3 hours.  All looked good,
and then I had a setback.  Used a grinder to deepen the grooves between
tiles, and when I finished I noticed that a large number of tiles (about
50%) were loose.  Pulled then up and carefully re-glued them with gorilla
glue.  This loused up the fit between the tool and the blank.
9)  Back to #220 grit, same stroke for 3 hours.  Tool and blank are back
together.
10) Back to #320, same stroke for 3 hours.  Did another sharpie test, and
result looks good (I think) for a spherical surface.
11) Moved on to 20T microgrit for 3 hours.
12) Moved on to 9T microgrit for 2 hours.
13) Made hydrocal bases for 10.5" and 7" diameter figuring laps.

This is where I am now.  I figure on doing 1 more hour at 9T, and then will
begin polishing on the full size lap.

Thanks in advance for your suggestions.

- Ken Bertapelle