On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

This is for a searchlight application where the light source shines
backwards into the reflector which then reflects the light forwards
producing a narrow beam.
The reflector needs to be about 2 to 3 inches in diameter with a
focal point about 2 to 3 inches in front.

Presumably this was possible in the days before CNC machinery, so how
did they do it?


Mike, since I've DONE it before, using NO machine tools whatsoever, I'd
say, "Yes, there was a way before CNC." G

In fact, about the only 'tool' it requires is a hand saw, but you don't
use that to make the reflector, you use it to make one of the polishing
tools for the reflector.

Look up "hand-grinding telescope mirrors". What you're doing requires a
MUCH deeper curve than most Newtonian telescope mirrors have, but it's
completely do-able.

The only 'special' device you'll need is a Focault knife-edge tester to
check the curvature of the mirror during final figuring; even that you
can make with just 'stuff' lying around the shop, and it, too, requires
no tools except ordinary screwdrivers and knives and razor blades to
make.

A lot of years ago, I spent three weeks of evenings walking around a
barrel, grinding a 12" mirror for my telescope. It's not hard, just a
slow process. In fact, I'd say anyone who can stand up is capable of it,
and if you can't stand, a rolling chair would probably serve to allow it,
too.

You'll need two thick glass blanks and an assortment of carborundum dry
grit abrasives. The abrasives are available through lapidary supply
houses. You used to be able to buy them and the blanks from Edmund
Scientific, but now that finished mirrors are so inexpensive, they may
not sell them anymore. Since this won't be a precision mirror for
telescopy, you can get away with any-ol' glass, instead of the Pyrex we
used for telescopes. Some ordinary flint glass about 1/2" thick should
do.

Lloyd

Lloyd