Re: ATM 21" f/6 - full size tool report

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

Ken Bertapelle wrote:
>>
I'm now planning on finishing rough grinding with MOT and a COC stroke, as
described in Texereau's book.  This should get me close to a sphere and mate
the tool to the blank.  I was planning on doing most of fine grinding with
TOT.  But now that the tool has turned out so heavy, I'm wondering if there
are any dangers in doing so.  The mirror blank weighs 40 lb. and is 1.5"
thick.  Any comments on this question from the veterans out there?
<<

Ken

I hope my formula for estimating Hydrostone weight didn't mislead you for
Hydrocal.  My 2" thick, 20" Hydrostone tools weigh about 34 lbs.

Here is a Sharpie-Gram of an interesting test I did recently.   Like you, I
really wanted to minimize handling those heavy discs.  Prior to the Sharpie
test I had ground the pregenerated 20" f6 mirror exclusively TOT with 220
grit for about 1.5 to 2 hours.  The surface looked great.  About 0.030" of
tile surface had been removed, very evenly on all the tiles.  I had ground
with about 83 lbs total pressure at the surface.  I used classic W grinding
strokes.  My intention was to continue the whole grind TOT to see how it
would work out.  But good sense butted in.

http://www.kupercontrols.com/misc/sharpie2.jpg
http://www.kupercontrols.com/misc/sharpie1.jpg

The first photo was taken about 90 seconds after the first 25 micron wet,
with the Mirror On Top for the first time.  Sorry, parts of the mirror back
were wet and others dry, so it's a little hard to interpret the image.  The
photo understates the effect a bit.  The surprising thing was that the marks
were disappearing from the edge on in, opposite to the norm for a recent
change to MOT.  Also, the marks disappeared somewhat unevenly with regards
to radius.

This suggests that the mirror surface coming out of that long 220 grit TOT
grind was less than spherical, and probably had pronounced zones.  It took
about 10 more 25 micron wets, swapping MOT/TOT, before I started the get the
anticipated quick removal from center-to-edge just after swapping to MOT,
and edge-to-center just after swapping to TOT.  Ultimately the mirror
polished very nicely in about 3.5 hours, so I must have done something right
for a change.

An alternative explanation for the unusual Sharpie disappearance progression
is that the 17% weight difference between TOT and MOT was flexing the mirror
slightly differently.  To get right down to it, that 67lb tool riding on
your 40lb mirror bothers me.

I would have been ok had I alternated TOT/MOT during 220 grit, and then gone
exclusively one way or the other for the finer grits.  I have often done
this with good results.  I feel this is probably your best path.  If you
want to avoid swapping, I would encourage you to run the fine grits MOT
since the mirror is lighter, and can best resist flexing in that position.
But I would worry about  going "unswapped" through 220 grit, especially with
that heavy tool on top.

The bottom line is that if you develop problems at a particular grit, you
will certainly see them sometime during the next finer grit.  The only
really "tricky" grit is the last one, since it can take a lot of polishing
to show up any problems there.  For this reason, Sharpie test a lot,
especially during the final wets.

Bill T.

Tile tool page:

http://www.kupercontrols.com/tiletool.htm