[ATM] Scratches keep forming on edge of mirror

[Adam Krause]

Dear ATMers,

I have made my tile tool and it now has good contact
with the mirror surface (8" f/6.2) after grinding with
120 grit, then 220.  The tile tool is made from dental
plaster- not dental stone, and is not sealed.  I am
having a recurring problem with scratches.  These
scratches are forming on the edge of the mirror only. 
I don't seem to have any trouble with scratches in the
middle of the mirror.  They start happening, then go
away, then start happening again.  With 120 and 220, I
can get by with it for now, because they grind out. 
But there is no way I'll be able to get these
scratches out when I start going to finer grits.  I
must stop the scratches from happening.

I have fully bevelled the mirror and tile tool with a
knife sharpening oil stone.  When I get scratches, I
have gone back to rebevel the tile tool.

As far as grinding history, I started with 120, got
tile contact, and then went to 220.  The center of the
mirror never would polish as well as the outside, so I
figured it was a hyperbolic surface and used short
strokes (about 1-1.5" total stroke) alternating with
MOT and TOT.  After much grinding, I went back to 120
grit to speed up the process.  This brought me to an
oblate spheroid.  Then I started with 1/2 length
stroke (4" total stroke length) with MOT to try to
bring the surface to spherical.  That is when I
noticed the scratches coming up again.  I will be
grinding along fine, and then I will feel resistance
from the edge of the mirror.  When I look, then I find
scratches.  I don't know why I am having problems
since I've beveled the edges of the mirror and tool. 
I took these pictures to show you the scratches,
mirror, and tile tool.  Please help me to figure out
how to stop these recurring scratches.

http://bellsouthpwp2.net/a/d/adamlisakrause/Mirror%20Scratch%20Pictures%2008-16-07/Mirror%20Scratch%20Pictures%2008-16-07.htm

BTW: The black marks on the mirror bevel are from the
sharpy test.

Thanks,

Adam Krause


       
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