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


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.

John Murray wrote:

>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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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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