[ATM] Expanding Question

[Anthony Stillman <atmer@flash.net>]

In 1837 Lord Rosse had built a lighter weight ribbed back speculum mirror.
It was found to function without fault.  The ribs were made of a brass
alloy which unlike speculum bronze, is malleable.  The alloy was selected
to have the same coefficient of thermal expansion as speculum.  After an
extensive search of the internet and numerous books I found the coefficient
of thermal expansion (alpha) of Cu3Sn (speculum) and a couple of brass
alloys with alphas that bracket speculum's 10.0 per degrees C.  So far I've
yet to find the formulation Rosse used to match speculum's alpha.  An
interpolation of the found values as well as a percentage evaluation of the
alphas of copper and zinc implies a zinc content for the brass of 6.5 to
7.5 percent.  I made something close to this red brass.  It's pretty, looks
a little like rose gold.

Now here's the question.  I can't be sure of the exact alloys I have and
consequently their alphas.  So if I solder these two metals together and
their alphas are known to say +- 1 percent and differ in the worst way, how
bad is it going to be?  I don't expect anyone to figure this out for me,
though that would be nice, what I am hoping for is a description of how to
figure it out, and/or what software to use,  and what else I need know
about the metals and the physical arrangement to do it.  Anyone?


On other fronts. A few weeks ago I made a first attempt at casting the 90
mm blank.  About 1.2 kg of billet was melted in a kiln in a graphite
crucible.  The metal was then poured into a mold made of satin cast
investment.  The casting failed, the blank failed, and a little worse for
the wear, the mold survived.   Since then I've made a second casting.  The
blank isn't perfect, but it is usable.  I'm incline to try again.

The 62 mm speculum mirror has been polished, reworked and repolished
several times now.  I'm using it to try out different means.  Five micron
aluminum oxide on pitch leaves a smooth somewhat polished surface with a
haze of fine scratches.  Polishing with rouge on pitch, while slowing
removing the fine scratches, quickly turns the edge of every one of the
innumerable pits in the surface of the metal.  The problem of deep
scratches from the skin of metal that builds up on the lap, persists.
Soaping the lap helps, but it's not a cure.

Lastly, I won't be at RTMC this year.

Anthony

POSTEXTRAM IMPERAT
OUTBACK RULES


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