Re: [ATM] new pits keep appearing

[Ken Hunter <atm_ken_hunter@yahoo.com>]

Move on to the next grit size if the curve is correct
and the glass is spherical. If the cause is random
grit variations you will see those pits go away and a
new, smaller size pit will take their place. If there
is a problem with the tiles shedding small pieces, you
will soon determine that (we can't) and you'll have to
bevel the tiles (use a Dremel tool) or re-make the
tool.

Ken Hunter


--- Paul Grapendaal <guardian@reaper.org> wrote:

> Folks,
> 
> I've been slogging away at my 13.5" f5.2 mirror,
> with what I think is a 
> grinding tool problem. I'm at about 320grit at the
> moment and new pits keep 
> appearing, they seem to be about the size of 150grit
> , I've spent a total of 
> about 90 minutes grinding on 320grit with still
> quite a few pits visible if 
> I hold the mirror up against a flourescent light (10
> or so pits) which 
> appear to be moving around. The tiles on the
> grinding tool have developed 
> sharpish edges and I suspect pieces of this may be
> breaking off and creating 
> these pits. The grinding tool has tiles littered
> randomly on the surface 
> which makes bevelling these virtually impossible.
> I've only ever made a 9" 
> f6 before and dont recall being overly observant as
> to how many pits the 
> mirror had. I've made another grinding tool plaster
> blank in the 
> anticipation of the tool being the cause.
> 
> I saw Bob Mays suggestion to apply less pressure but
> this hasnt helped in my 
> regards.
> 
> So, for my "how long is a piece of string" question.
> 
> How many pits are acceptable/normal on a quality
> mirror this size? (I'd 
> settle for 1 or 2) Is the more likely cause
> contamination or splintering off 
> of the main tile tool? Would offbreaks be more
> likely to create scratches 
> than pits?
> 
> Any feedback would be greatly appreciated!
> 
> Many Thanks,
> Paul Grapendaal (Cape Town, South Africa)
> http://elrond.reaper.org/telescope/
> http://elrond.reaper.org/13/ 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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