From: John Murray 
To: James P Crombie , Atm List 
Reply-To: John Murray 


G'day Again,
        One thing I found with my hogging through to fine grinding was thatit
was
difficult to hit the f# right on the money. I was originally aiming for f6
but ended up at f5.1. With the #80 grit, I had a shallow centre with
progressively steeper curve as I went to the edge. That is, there was a
fair amount of variation in the spherometer readings. This I attributed to
sludge pooling under the centre of the blank (My setup was TOT, MOB) As I
moved up through the grits though, this difference slowly disappeared.
Obviously the allowable error range is greater the coarser the grit. As a
result my spherometer readings slowly standardised until I was at 1000 grit
with no measurable difference using my depth gauge.
        As I went through the grit sizes, I too noticed that the pits on the
outside took longer to disappear. I moved my overarm out slightly to stop
deepening the overall sphere and also to get the edge working a bit more.
As you know, this is my first mirror. It worked for me but I'm not saying
it's correct......

Cheers

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




> -----Original Message-----
> From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of James
> P Crombie
> Sent: Saturday, 26 July 2003 2:54 PM
> To: Atm List
> Subject: ATM Mirror grinder-MOT or TOT
>
>
>
> I have progressed to 120 grit and have most of the pits from the
> 60grit.  But the Sagitta is slowly getting deeper.  If I switch to TOT
> will this flatten the sagitta out a bit?  I am aiming for .012(on the
> spherometer) and I am currently at .0132".  This will give me a ROC of
> 116"  instead of the 128" I am looking for.
>
> Some new pictures up on the mirror grinder page below
>
> --
> --------------------------------------------------------------------------
>
> James P Crombie
>
> Summerside                 Machinist
> Prince Edward Island       Amateur Astronomer
> Canada                             3D Designer
>
> E-mail                             jpcrombie{at}pei.eastlink.ca
> Astronomy webpage          http://www.jamescrombie.com
> Rhinoceros 3D webpage      http://www.jamescrombie.com/rhino/
> Mirror Grinder page          http://www.jamescrombie.com/pics/
> ------------------------------------------------------------------
> --------
>
>
>

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