[ATM] fun with a lathe

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Nagesh Kanvindeh and I (and several other onlookers)
had a good lesson on lathe use from Jerry Schnall this
evening at the NCA mirror-making class at the Chevy
Chase COmmunity Center in Northwest Washington DC.

Nagesh and I are attempting to make a Lurie-Houghton
telescope, which requires two corrector plates that
have equal and opposite curvatures on identical glass.
Getting the radii of the curves correct is extremely
important, according to the ray-tracing software that
we (mostly Nagesh) are using  [OSLO LT}. Our current
set of spherometers [made in the past mostly by Bob
Bolster and Jerry Schnall] only measure to within
about +/- 0.001" on the sagitta (or depth of curve on
the glass) which can often mean changes of several
inches in the radius of curvature, which is
unacceptable because it's not nearly accurate enough.

We happened to have on hand some linear micrometers
that were part of some surplus optical equipment we
'inherited' from the physics department at American
University. We disconnected one of the micrometers
from the rest of the stuff, and tried to figure out
how to make the rest of the spherometer. I was
planning to make a brand-new base, but Jerry suggested
that we simply remove one of the dial indicators in
another spherometer, and then use our lathe to make a
sleeve that would enable the micrometer to fit into
the hole where the dial indicator would go. He told us
what to do, and we did it, with plenty of fumbling
around. But we got a valuable lesson in the use of a
centering tool, boring a hole of the right size, using
the cutoff tool, and in the use of the clutch and the
gearbox. After all of that, we got a really nice
spherometer that measures to the nearest
ten-thousandth of an inch. An experienced machinist
could probably have done it in about 10 minutes. We
got it done in about 2 hours, but it works great!
Reading the micrometer accurately is the tricky part;
you always have to read what the number is for a flat
surface, and then read the curved surface, then
subtract the two, and then calculate what that means.

It was great having somebody (i.e. Jerry) telling us
what to do and letting us do it. I'd previously tried
making things entirely on my own, and had always
screwed up big-time, not able to produce anything
useful to speak of. Watching somebody else do it is
OK, but it's not nearly as good for learning how to do
it yourself.

(Also, our wedge-o-meter works great, too - for this
we merely used an old instrument stand and drilled a
couple of holes in it.)

Guy Brandenburg

=====
Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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