Re: [ATM] mirror grinding

["Bob May" <bobmay@nethere.com>]

Dave, maybe I'm using the word "hole" a bit off from what some understand
it.  What I mean is that I'm digging the majority of the glass out with an
iron (or other material that doesn't abrade fast like a glass or tile piece
would) tool so that you end up with a shallow hole which is approximately
the shape that you want the finished surface to be.  You can also make a
nice hill if you want a convex surface by grinding the edges of the glass.
Remember that a hole is a very relative term as testing a polished surface
with the Foucault tester can show a "hole" in the middle of the surface
which may be less than 1/4 of a wave deeper than what it should be.
My whole approach to tile tools is to make them cheaply.  Why make a tool
that is going to be as expensive as a piece of glass and takes a fair bit of
time to make in addition?  For a small lens or mirror, the difference in
cost isn't much but when you get to a larger piece of glass, the cost
difference becomes quite important.  Why buy another $100 piece of plate
glass when the tile tool of equivalent size is more like $10?  Right now,
one of the grinders in the group is doing a 17+" F4.5 mirror and his sost
right now for the stuff other than the mirror glass has been $10 for a
diamond saw that he has been using to grind the hole into the mirror's
surfface.  He's ready for his tile tool now and that is going to cost about
$6 for the materials (12" tool) and that plus about $3 in grit is going to
be the cost in finishing the mirror down to the point where it will be ready
for polishing.   If he didn't have the availability of the tile tool method,
he would have had to pay about $80 or more for a piece of glass to grind his
mirror against.  Stacking tiles on edge and doing the coarse grinding
(digging the hole) is a bit more expensive than doing the other way as you
need a lot more tiles, probably about 5 times the cost for the tiles alone.
Epoxying tiles to a tool is also time consuming (got to let that eoxy cure
for a start) and you end up with big channels where the grit that you put on
the tool gets wasted in.  I will note that some shallow groves at the last
stages of grinding do make the job easier as you tend not to vacuum lock the
tool and mirror together but those are just a few lines that you dig into
the tool that are a few thousandths of an inch deep and they can be dug in
the plaster matrix rather than the hard tiles.
A terra cotta tile would make a nice substrate for a tile tool but I'd worry
about the flaking off of the terra cotta with my hands (contamination of the
grinding surface problem here, especially when you get to the polishing
operation unless yo expend more time and money for more expensive epoxy to
cover it) and that would be bad for a good surface.  I'll also note that 40
grit really has no use in an optical shop unless you are doing very large
surfaces and don't want to get the hole dug out with diamond tooling.
Something that coarse digs very big pits and the cracks that end up with
such large grit also go deep into the surface and you then need to dig them
out for a very significant distance or they will come back to bite you later
on.
Basically, the tile tool process is part of a search for making mirrors
cheaper than the old process that has been in usage for centuries and, when
done right, ends up being a lot cheaper to do.  I'll note that I'm so cheap
that I wash the 80 grit from the coarse grinding and find that abou 50% of
the grit never gets used by many of the grinders because it gets washed off
of the grinding area before being used.  Using channeled tools probably ends
up with more than 80% of the grit never being used.
My first mirror (8" F7.5) ended up using so little grit because I reused the
grit for the larger sizes that I've been doing all kinds of other optics
with the same stock of grit and still have about half of the 80 grit that I
got in that kit. - I am cheap!
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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