To: atm{at}shore.net, DAVE-OESTREICH{at}ATTBI.COM
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Karlo,

Get a tile tool kit from www.gotgrit.com.  It is surprisingly easy to make
a new tool this way, and it will actually be easier, and less messy than
trying to clean off your old one.  Let me know if you choose to go this
way, and I will give you some tips.  (The directions from Got Grit are good
but miss one or two things. (Unless Tom has updated them.))

A scope that is too big, long, or heavy to use regularly isn't much good. 
I'd say, grind that mirror down to f/6.

The rough grinding certainly did make it all the way to the edge.  You
would have had unground area near the edge if it did not.  Further, the
fine grinding did too.  If it had not, you would have lots of pits near the
edge.  The Turned edge probably appeared in the last stage or two of fine
grinding, or in polishing.  I think you have to be fairly wild to get a
turned edge from fine grinding.  The hardness of the tool coupled with the
geometry of the situation helps prevent it.  Channeled tools are supposed
to be better than solid ones for preventing turned edge from fine grinding.

One can polish in a pretty bad turned edge.  I should know!

I am probably not the best one to give advice on preventing a turned edge. 
But I do know that even a fairly bad one can be polished out.  The trick is
that wear has to be concentrated just inside the edge, and blended right to
the edge as well as toward the center.  When that is done, the overall
figure and smoothness can again be addressed.  Concentrating wear near the
edge can be slow work because there is much more edge than center to a
mirror.  I presume, since you took a class with John Dobson, that you are
in the SF area.  There should be plenty of other ATM's in the area who
would be willing to help you.

Unless money is really tight, while you are ordering that tile tool kit
from gotgrit, get one extra, plus one just plain dental stone kit.  If you
have a screwup with the first tile tool, then you have one in reserve. 
Plus, it will be a good idea, after you have finished about 120 grit, to
make an all dental stone (no tile) disc as a lap support.  In fact it may
be wise to have two lap supports available.  That way, you can keep one
"normal" lap for polishing and smoothing, and have another whose
surface you mess with in order to help correct figuring problems.  (Of
course you need extra pitch too.)

Mark Holm
mdholm{at}telerama.com

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