Re: [ATM] draper

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

Sounds like your suppliers of opticql stuff is about  a hundred
years behind the times for the USA.  Many a nice mirror has been
made from 19MM plate glass (3/4" ) so don't be afraid to use
that.  Pyrex didn't become popular for making mirrors with until
the 40's so it really isn't that big of a deal.  We are now
getting fused quartz in quantities which is supposedly better
than pyrex but that and other really low expansion glasses are
still a bit expensive.
As to abrasives, a kilo of the stuff is a fairly large amount but
if you are doing several mirrors with your friends (start a class
for grinding and polishing glass and you'll soon run out of the
coarser grades!) and the stuff isn't that expensive even if you
have to buy a 25 kilo sack of the stuff.  I'd be more interested
in making sure that the grit is pure than in the cost of it as a
large grain in some fine grit will make for a lot more work on
your glass.
For an example, one of the grinders in my grou0p is doing a 17"
mirror and it is of an old porthole which is 19mm thick.  The
f=ratio is 1.5 and will become a nice mirror when he is done.
Another guy is starting a 16" mirror of the same thickness so you
see that indeed you can do a nice mirror from something thin as
that glass you have available.  In addition, it isn't hard to
make disks from odd pieces of glass as the idea of trepanning is
how to do such.  One guy years ago did a fairly large mirror and
all he started with was a square of glass.  He built a support to
hole a trepanning disk and cut the disk out of the square by hand
with a chand0cranked machine.  He did say that the next time,
he'll buy an electric motor and power the machine with that but
he did pesevere and get through the job.
A Draper machine is a nice luxury for a small mirror of less than
400mm or so but for much larger mirrors, the job of pushing a
polishing tool around does become a bit wearying unless you keep
the tool to a smaller size thant the glass.
I'll also note that making tile tools with some cement (pure
cement, not the stuff with sand in it) will also save on glass if
that is important to you.  Even using a big rock can be done if
you realy put your mind to it.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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