From: Guy Brandenburg 
To:  atm{at}shore.net,  novac{at}his.com,  capitalastronomers{at}yahoogroups.com
Reply-To: Guy Brandenburg 


We at NCA were given tonight a home made polishing grinding machine made by
Dave Strout, who decided he didn't want to make his own mirrors after all.

I would like to thank Dave for his donation. It is a well-put-together
machine. (All the work that he put into making it he probably could have
used easily to make a mirror by hand, but that is another story entirely.)

Michael Mills brought it over to our weekly workshop, and we put it back
together and played with it for a while - with no glass on it. I personally
have no experience with using these machines. We tried changing the length
of the stroke and so on, and noticed that the center of the bolt that holds
the tool always goes over the center of the mirror. We had heard that there
was a fixed ratio between the number of revolutions of the mirror and the
number of strokes that the tool does back and forth, but we didn't know
what the ratio was.

We learned, by playing around with it, that you can change whether the
stroke goes to the left of center or to the right of center. Then we
decided to attach a pencil to the bolt in order to see what sort of pattern
we would get when we placed things so that the pencil would draw patterns
on paper placed where the mirror would go. We saw that after a few cycles,
the pencil would follow almost precisely the same path. The mathematically
inclined among us said - ooh, cardioids - a nice application of precalculus
mathematics.

We really had a major question about this, however. If there is a
simple-integer ratio between revolutions and strokes, and the center of the
tool is always follwing precisely the same path across the mirror, then how
can one possibly avoid having zones on a grinding/polishing machine? When
we make mirrors by hand, we do the best that we can to AVOID repeating
anything exactly the same way. It looks like this machine will repeat
EVERYTHING exactly the same way, which is what we have always tried to
avoid. It seemed to me that an improvement would be to change the gear
ratios so that the number of strokes was not, say, 1:3 but rather 1:3.14 or
1.24:4 or something else, whatever was easiest to make and was farthest
from a simple whole-number ratio ...

I know that the tool will rotate somewhat randomly since it is not powered
by anything other than the turntable, but ....

Any comments from those with experience?

Guy Brandenburg

--- BBBS/NT v4.00 MP