From: James P Crombie 
To: daveg{at}tpg.com.au
Cc: ATM 
Reply-To: James P Crombie 


Dave
  The Sagitta reading were for my 3.5000" spherometer and not the
overall depth of the mirror.  The actual total sagitta for the 8"
mirror was .0625.
  A lot of the grit probably ended up on the platter instead of the
glass :-)  Better to err on the side of caution.  Plus I have a couple of
pounds of 60grit SiC that I retrieved from work (30gallon drum of it)
  I have also done a lot of lapping, 25 years in a machine shop will get
you around to it :-)  Mostly using diamond products for lapping seals in
aircraft engine.  Stuff does a nice job on hardened tool steel as well
though :-)  I have a really shiney bench block with a mirror finish.
  I like the enclosure on your machine, you could take a hose to it to
wash everything down. Or was it mainly as a noise barrier?

Dave Gault wrote:

>G'day James,
>
>James P Crombie 
>
>
>>I finally got around to using my mirror grinding machine. 
>>I did three sessions or about 1 hour each.......left a sagita of about
>>
>>
>.001.
>
>
>>After the final session I have a sagita of .004.  May have used 2oz of
>>
>>
>60grit
>
>
>>SiC over the three sessions. Does this sound about right for a mirror
>>
>>
>grinder
>
>Nice looking and well made machine......
>
>Sounds like an awful lot of SiC for only 4 thou of glass in the
>bucket.......
>
>I cleaned up my 10 inch, both sides and hogged to f5 with a little over
>one 35mm film container of 80 grit.
>
>FWIW - What I do for cleaning up the mould marks and unevenness off the
>blank is to make a steel tool a little larger than the blank.   The tool
>is made up from some 3mm (but1.6mm would do) thick steel sheet cut
>roundish (16 sided) on the guillo.  This is epoxied to some 1 inch ply
>and a couple of coats of paint/varnish to seal the ply.   The tool is
>used MOT and I wheel my machine outside onto the grass as the noise is
>incredible. For hogging I use another steel tool, this time only 50 or
>60% mirror size and made the same fashion as the previous tool but is
>used TOT.   Don't worry about "wets", just squirt on more water to wash
>off the mud and then flick on more Sic to make more noise without
>stopping the machine.   BTW pictures of my machine on my website, url
>below.    Once near depth cast a POP/dental stone and tile tool and your
>into smoothing.
>
>My reasoning behind the steel tool dates from my toolmaking days where
>we lapped high speed steel and carbide tooling with copper and wood
>tools loaded with lapping paste of various grades.   The theory is that
>the soft tool "loads up" with the abrasive which cuts the work while it
>is embedded in the tool.   The soft tools were not worn any where as
>fast as the much harder work piece and these steel tools last for ages.
>I previously cleaned up three  6 inch blanks, both sides and then used
>the same tool for hogging the 10 inch and have barley removed 1/2mm from
>the face. The goal for smoothing and finishing is to match the radiuses
>of the tool and mirror and both tool and mirror need to be of the same
>hardness and now the abrasives grains roll between each surface, taking
>chunks out of each equally.
>
>--
>Regards
>Dave
>http://www4.tpgi.com.au/users/daveg/index.html
>
>
>
>
>

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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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