Re: ATM Beginner's questions

["Frank Q" <frank@katestone.com.au>]

Hi James

> 1) Are the grits and all still good after 25 years?
> I'm guessing they are, but the pitch probably isn't.

Grits last forever - go ahead and use them.
I've got the same pot of pitch I used when I made my first
6" - back in 1973/1974 and it's still OK. You might want
to add some turpentine to soften it if it's too brittle.

> 
> 2) Later in rough grinding, I understand that you use
> a stroke that is like VV, in other words you make some

In rough grinding, it doesn't matter what pattern you use
(within reason). The aim is to remove lots of glass quickly.
Some persons have used angle grinders armed with a diamond
blade to achieve this. I use a circular stroke. Here the centre
of the mirror traces a circular orbit around the edge of the tool.
This abrades the centre of the mirror and the edge of the
tool fairly quickly. You might find that this puts a hole in the
centre of the mirror and leave the edges rather untouched.
The cure for this is to reduce the radius of the orbit of the
mirror or revert to the more conventional strokes.

I use this circular stroke all through fine grinding and continually
keep track of and adjust the sagitta with a combination of
tool-on-top, mirror-on-top and varying the radius of the orbit.

You will find answers to more specific questions at the following
web site:
http://www.stellafane.com/atm/atm_main.htm

Cheers
Frank Q