Re: ATM Rough grinding

["Arthur" <bcjawsss24@verizon.net>]

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
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net



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