Re: [ATM] Scratches keep forming on edge of mirror

[Dominic-Luc Webb <dlwebb@canit.se>]

On Thu, 16 Aug 2007, Adam Krause wrote:

> I have made my tile tool and it now has good contact
> with the mirror surface (8" f/6.2)
SNIP
> knife sharpening oil stone.  When I get scratches, I
> have gone back to rebevel the tile tool.
SNIP
> 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.


To add something new, I got chips on my first tile tool, which
was concrete. In my case, I definitely had small rocks coming off
the concrete. Sealing solved this problem. The sides are important
to seal as well. Small rocks can fall from the sides onto the
glass. I think some of the tiles were also a bit loose and broke
chips off the mirror as well. Tiles were better embedded in my
later tools.

As for the the bevel, I have tried a few angles without any
effort to quanitiy this. Suffice it to say this is much more
problematic with the plate glass that I generally work, and the
angle may be much more critical. The pic below shows a surface
I have baffled on top of another piece that I have not bevelled
at all. Notice the way the lower plate glass chips on the corner.
I have quantified this and these are a lot bigger and more
frequent than what I get with optical borosilicate. For scale,
consider that the glass is 6 mm thick (Schmidt corrector plates).

http://www.canit.se/~dlwebb/catadioptric/310schmidt/corr_col.jpg

Looking at the upper plate glass, you can sense the angle
used, and this has worked very well for me. In terms of the grit
size for this bevel (notice, I also work the sides of plate glass
because these can also chip, BTW), I use a roughly 200 mm long
combination coarse/fine sharpening stone, like this one:

http://www.soonerstateknives.com/NT-JB6.JPG

I have never found specs for the grit size for my stone, but
similar ones I have encountered specified that they are silicon
carbon, just like we use for grinding mirrors. My guess is that
the fine side is probably finer than #600, and probably about
#1000. I have had/used this tool for many years and it has worked
very well for bevelling. My bevels end up nearly polished, and this
brings up my final point. Sharpening stones can be found with
grits of any sizes, so it is nice to see that when we discuss this,
there is mention of the grit size. Look here for instance.

http://www.fine-tools.com/japwas.htm


Dominic

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