Re: [ATM] First mail to atm list

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

Grit sizes is always a problem.  BTW, welcome to the group!
Water grading the grits you have as another poster geve will help with the
grit size problem.  Another help is to not grind hard for the first few
strokes.  Slightly hold the mirror up a bit (not lifting the mirror off of
the tool but rather not applying as much pressure on the tool) will allow
the large grits to break up before you really start grinding.
Next, a good source of "inbetween" grits is to wash the glass out of the
grit that you've been using.  The glass is a lot lighter than the grit so
you will quickly end up with a nicely sized grit (providing you don't
overgrit in the grinding process) which will be the right size for the next
round of grinding.  People have done a mirror just by using the original 1/4
lb. of grit that they started with by using thiis process.
As to the problem of pits not going away, , you have to persist in the
grinding with the smaller grit as eventually the large pits will go away.
Everytime that you grind, the grit is cracking the glass peaks off of the
surface of the mirror and there is usually a little bit of the crack that
propagates deeper into the glass than the surface.  This means that you have
to keep going with the finer grit to get that crack removed in addition to
smoothing the basic surface itself.  With the grinding process, the size of
the pits will vary by a lot because of the nature of the chipping process.
You will always have some pits that will be larger than the average and
these larger pit sized holes are one indicatior of how well you have ground
the surface.  After you have done about 5 wets, go and find the largest pits
(1 ninute with the eye loupe is long enough) and mark them on a map.  Grind
2 more wets and see if you can find those pits again.  When Those all go
away, do the search again for more large pits and do the process again.  At
that point, you're ready to go to the next smaller grit size.  I find that
it takes about 2 hours of grinding for the grits after the first hogging
grit size to be ready for the next size of grits.
Grinding with more force on the mirror makes the chipping of the glass go
faster and deeper.  This means that the materail is removed faster and the
chipping goes deeper into the glass and this isn't wanted so just let the
force of the mirror and your hands be the weight after the hogging is done.
As to speed of grinding, you can move the mirror over the tool as fast as
you want to, the pros use a lot higher surface speeds than we amateures can
even begin to do when grinding.
As to finding other amateurs around, find the astronomy groups and attend a
star party or so and ask about and talk to people.  Anybody that sets up a
telescope will usually have knowledge of others that have telescopes and, if
the group is like many here in the states, they are more informal most of
the time than many other societies are.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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