From: "John Murray" 
To: "James P Crombie" ,
"Amateur Telescope Makers"
        ,
        "Mirror Matic" 
Reply-To: "John Murray" 


James,
        One thing that I would be wary about is swapping the positions of the
tool
and the blank. I'm not saying it can't be done but I would be concerned. My
concerns are as follows.
1)      I am doing an 8" blank with an 80% diameter tool. I had the tool on top
and at the midpoint of its stroke the tool overhaung the edge by about 10%
of the TOOL diameter. At one extreme of the stroke though, I made sure that
that the tool at all times covered the edge of the blank and at the other
extreme the tool had an overhang of around an inch an a half. This put more
grinding action at the centre regions of the blank (on the bottom). Once I
was in the target area for the sagitta, I then moved the overarm out
slightly to slow down the centre action and start doing more work on the
edge. Doing a few checks with the depth gauge for the sagitta will show the
effect of this. Eventually as you go through the grits and are adjusting
the position of the arm outwards you will reach a point where the action of
the tool is spread evenly over the entire blank. Dennis Rech calls this the
Neutral Position. No matter how much grinding is done afterwards in this
position, you will not change the sagitta. 2)      Now visualise this setup
after you have established all the positions
you
need to get to the neutral position. If you then swap mirror and tool top
to bottom you will then find that the positions you established are no
longer relevant to the new setup. The overhang will be out etc etc. You
will then need to find all the gory details out again and then make the
adjustments. ie the 10% overhang at the midpoint of the sweep would change
to a different amount and have a different grinding action. I hope I
explained that ok because I lost myself there;-)

Cheers

John Murray
Whyalla
South Australia
Check our web page at http://astronomy.sa86net.com ATM and M-O-M stuff.




> -----Original Message-----
> From: James P Crombie [mailto:jpcrombie{at}pei.eastlink.ca]
> Sent: Saturday, 26 July 2003 4:27 PM
> To: John Murray
> Cc: Atm List
> Subject: Re: ATM Mirror grinder-MOT or TOT
>
>
> John
>   I am not getting a lot of variation in the readings, maybe .0002",
> with most of that being at the very edge where it is the roughest.   I
> will try switching around to TOT for a couple of sessions.  I am
> currently using about a 25% overhang ( 2" on an 8" mirror) , does this
> sound about right for flattening it out a bit or should I increase it to
> a 1/3 stroke.
>

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