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Richard,
Should I continue with the #80 grit or start with #220? I do have a
curve, how spherical it is I can't tell,but it does have a curve. The
sigatta I measure with a straight edge and filler gage this method is
about .081
Arthur
Patience?, I've waited years to do this, and I'm addicted to
this hobby.
-------Original Message-------
Date: Monday, August
25, 2003 11:29:40 AM
Subject: Re: ATM Rough
grinding
To get back to a spherical surface and away from a flat with a hole
in the
center, do center-over-center short strokes, maximum displacement
about 1/3
the mirror radius.
Concepts such as "turned down edge" are reserved for final polishing
and
figuring and are meaningless during rough grinding.
You seem to suffer from a lack of patience. You should try growing
sago
palms for a hobby.
.. . . Richard
----- Original Message -----
Sent: Sunday, August 24, 2003 8:53 AM
Subject: Re: ATM Rough grinding
As far as remembering what I had done? I thought I was rough grinding
using
the cordial stroke and I understood it in the books I read. But
after
reading other literature (on-line) I believe I know now what cause
the
problem. I was rotating my work surface, as well as the tool in
different
directions.However, I believe I was over rotating and grinding as if
I wasn
t.
Ok, so I reversed the process with TOT, and working around the
mirror. I
could feel the raised edge of the mirror and depression of the hole;
I
concentrated my grinding process in these areas. I was able to
correct the
problem of the crescent at the edge and the sigatta is several
thousandths
less that the desired depth.
I'm sure at this point, I've most likely turned down the edge,so,
should I
continue with TOT and bring up the sigatta a little more? Then
continue
again with MOT and cordial stroke to achieve the depth I need.
I am using the straight edge and feeler gage to measure, and have
tried the
flashlight test with the error.
Arthur
-------Original Message-------
From: Bob May
Date: Saturday, August 23, 2003 11:10:22 PM
To: atmlist
Subject: Re: ATM Rough grinding
Ah, the non-fatal over curvature!
First set back with a beer and reflect on what you have done. You
have
gotten a hole in the mirror without getting that hole out to the
edge. If
you are measuring ROC by the light method or by a small spherometer,
you are
in a lot of luck. If you are measuring it by the edge to the center
method,
you have your depth but you now just need to get the curve out to the
edge
without getting in any deeper.
After you get finished with the beer, pick up the mirror, put it on
the
bottom and start grinding with some somewhat smaller strokes than
what it
took to get the deep hole in the center. If you tested by the
flashlight
method, you still have a bit deeper to go so just start working. If
the
depth is near full, you will want longer strokes and more chordal
ones to
get the edge curved to meet where the center is now.
I usually get the guys to do about 80% of the depth with the 80 grit
and
finish up with the finer grits so that the mirror stays on the
top,
continually and slowly deepening the center. Once they get to the ROC
that
they desire, it becomes a continual flipping of the mirror and tool
as you
go through the grits to insure that the ROC doesn't get any
shorter.
In addition, I'd take the slurry from around the mirror and wash the
glass
out of it along with the broken down grits and that way, you will
have a lot
more grit to work with as you are probably wasting a good 50% plus of
the 80
grit right now. Remember that the 80 grit is the hogging grit and you
don't
have anything coarser than that around and dirt won't be any bigger
so don't
feel the need to keep it clean.
Bob May
..
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