Re: Are Aluminum Mirrors in reach of the ATMs?

["Malcolm" <fangb@aztec-net.com>]

Hi All,

I have to admit when posting the original message I was spurred on by a
previous message about using India Ink as a final polishing material.    I
still think although defiantly different and possibly more involved it my
still have advantages.

>From a ATM point of view its "eeeek sounds like a nightmare :)P"

But I think I was coming from the Astronomy idea of " hmm bet we could make
a really BIGGER cheaper mirror from this"

I think though, it sound like the mechanical tools required are not that
easy to amass.  I seem to have as much an affinity to metal as I do wood and
glass.. :)

Regards
----- Original Message -----
From: "John Lynch" <john_p_lynch@hotmail.com>
To: <fangb@aztec-net.com>; <atm@shore.net>
Sent: Saturday, March 24, 2001 5:49 AM
Subject: Re: Are Aluminum Mirrors in reach of the ATMs?


> Malcolm,
>
> Out of curiosity after seeing numerous other posts regarding the
> aluminum mirror question, I went back and more carefully read the
> chapter of the book that I referred to in my last E-mail message.
> Oddly enough, it reinforced many points that some of the folks on
> this listserv have already made.  Summarizing this article, the
> author points out that:
>
> 1.  It is not suggested that better mirrors can be made of metal
>     than of glass, and a comparison shows that glass is the
> preferred material.
> 2.  Glass held a decisive advantage over metal in the days when
>     mirrors were silvered, and a tarnished silver mirror coating
>     can be removed and replaced at home in a single evening
>     (remember this was written in 1963!).  Now aluminum has replaced
>     silver, but requires a protective film of oxide.  However,
> applying an aluminum coating requires an apparatus beyond the
reach
> of most amateurs, who must send their mirrors away to be        coated.  A
> small solid mirror can be repolished at home in an         evening.
> 3.  The polished aluminum still requires the protective oxide
>     coating (I thought this also required the vacuum chamber
>     process, which kind of defeats the argument).
> 4.  Rough shaping of the mirror and tool can be done by hand
>     using either a scraper or file, or a lathe can be used.
> 5.  Progressive fine grinding is done in the usual manner with
>     only 220 and 320 carborundum, and polishing is accomplished
>     using a HARD pitch lap with, initially, 320 carborundum,
>     then pumice (the author claims that AJAX powered cleaner
>     works wonderfully for this purpose), and then a new lap is
>     made and rouge is used for the final polishing process.  He
> strongly indicates that this process can be successful only
>     if HARD pitch is used for the faceted polishing lap.
> 6.  Once polishing with rouge begins, the mirror should never be
> removed from the lap for extended periods.
> 7.  The last point refers to the thermal properties of both
>     substrates (glass and aluminum).  He states that glass retains
>     its shape except during periods of changing temperature, but a
solid
> metal mirror retains its optical figure during changes
>     of temperature that would put a glass mirror out of business.
>
> That sort of sums up the points of the article.  In my opinion,
> sounds like a neat project if only for the novelty.  Otherwise,
> it seems as if it's too much trouble for the work you have to
> put into it, and, like I said before, he limits the discussion
> to small solid metal mirrors with long focal lengths, i.e.,
> spherical.  He also stated that he's made plenty of mirrors using
stainless
> steel.
>
> Regards,
>
> John Lynch
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