Re: ATM Aluminum Mirror

[Kreig McBride <kmcbride@az.com>]

Hello Jack,

I also worked on a 5" aluminum mirror several years ago and everything 
went fine using an aluminum tool and standard grinding techniques. The 
use of a pitch lap and polishing compound proved to be disastrous with 
rather large gouges being cut from the surface similar to a loaded file 
when filing aluminum.

After fine grinding these mistakes away I polished the surface using a 
diamond paste. The results were excellent and when the surface is viewed 
from the side of the mirror one can see a polished surface. At this point 
my project ended, however the next step is to  have the mirror nickel 
plated, then polish and figure, and then have the mirror coated with, 
yes, aluminum just like a glass mirror.

It would be interesting to hear from anyone who has succesfully 
completed a metal mirror. How about a speculum mirror with an aluminum 
coating to prevent corrosion?

Kreig McBride
Bellingham, Wa.

On Tue, 20 Aug 1996, Jack Schmidling wrote:

> Well, they said it couldn't be done.  And.... they were right!
> 
> My 4.25" cast aluminum mirror project has been put aside for greater
> minds to work on.  All of the predicted problems and then some,
> did indeed rear their ugly heads.
> 
> I started with an aluminum blank and the pre-curved tool from the glass
> one just completed.  Many hours of rough grinding failed to produce the
> desired curve in the mirror and removed glass from the tool at a faster
> rate than from the aluminum mirror.
> 
> Not wanting to see the pyrex blank vanish, I cast another aluminum 
> blank and used this for the tool.  I used a belt sander, lathe and
> grinder to bring it to shape but the free abrasive grinding was
> pitifully slow.  I finally got something close to overall contact at
> the right sagitta and went on to fine grinding.
> 
> What surprised me the most is the way the ground surface reacted to the
> tumbling operation.  My intent was to tumble the mirror between each
> grit to peen the surface into submission and hopefully reduce the
> air pits.  We tumble all aluminum parts for our MALTMILL with stainless
> sheet metal screws, soap and vinegar and they come out sparkling bright
> as did the rough cast surfaces of the mirror but the ground surface
> just stayed a muddy grey and showed no signs of the tumbling action.
> 
> Next problem I ran into was when I got to 5 micron prepolish.  As soon
> as the surplus was worked out, it locked up like glue.  I could not take
> a single stroke of any length.  It was hopeless and I quit.
> 
> After pressing on the old pitch lap left from the glass version,
> polishing felt like rubbing it on the driveway.  The only way it 
> felt right was if I left the faceting netting between the pitch and
> mirror after pressing.  After several hours, all I achieved was a dull
> grey sheen and nothing resembling a polish.
> 
> I quit!  So much for fun at Astrofest.  Guess I will just annoy 
> folks by bringing along an EASYTESTER.
> 
> js
> 
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