Re: [ATM] Why not metal mirrors? (was Beryllium)

[George Anderson <georgea@cam.org>]

You don't want to use Cerium on an aluminum mirror. From experience I
can tell you that you will end up with a very shiny dark grey surface.
The cerium reacts with the aluminum.
I have a 5" ~f/6.2 1 inch thick aluminum mirror (6061 T-6 IIRC Al round
stock extrusion) that I work on every now and then. I hogged it uot with
a router on a pendulum lowering the router 1/32" at time. The mirror was
then ground to 800 grit on previously made 6" tile tool (that is what
determine the fl). Fine grinding was done with a piece of shower curtain
on the tool.
Polishing has been challenging. The shower curtain and CeO gave me the
grey mirror. Shower curtain and rouge only progressed so far. Pitch
stuck to the aluminum no matter what I tried.
Felt pulled down over the tile tool was looking good until everything
came to a halt when summer landscaping around the house became a
priority.

George Anderson
Montreal Canada

Montreal Canada

vorblesnak@peak.org wrote:
> 
> On 14 Sep 2004 at 23:10, Ellen Mackenzie wrote:
> 
> The real trick here is to polish out the aluminum.  NASA did this by diamond
> turning the surface and then polishing it out with india ink.   I think the best
> idea is to cast in the rough curve,  as I don't have a precision lathe.  The old
> timers talk about using blue stone, cut to small squares and pitched up on a
> positive to grind out the metal mirror.  Any soft coarse stone should work.  Or
> perhaps a lap of silicon carbide cut off disks of decreasing grit size.  The key
> seems to be no loose grit like one would use on glass.  Also shape the lap
> well and maintain it.  Polish on hard pitch, cerium oxide then india ink.
> 
> Once I get the last two pie plates done and figure the 16, (almost done), I will
> try a metal mirror and let you all know what I find.
> 
> What word lately from the speculum caster back east?
> 
> David Davis
> Toledo, OR
> 
> You know Mel,  that nine inch may never get finished.
> 
> > Hate to tell you Mel but nickel and aluminium are a really bad cobination -
> > - it makes a nice thermet bomb. It' really hard to find an inexpensive over
> > coating for aluminium. The surface prep is done with fluorides, very toxic,
> > and only a few materials are suitable. Copper is a good intermediate for
> > subsequent finish reflecting surfaces like chromium.
> >
> > -----Original Message-----
> > From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
> > Mel Bartels
> > Sent: Tuesday, September 14, 2004 9:53 PM
> > To: ATM Mailing List
> > Subject: Re: [ATM] Why not metal mirrors? (was Beryllium)
> >
> >
> > The late Tom Waineo sent me an aluminum substrate and grinding/polishing
> > instructions.  Basically, as I survey the literature, the biggest issue is
> > extreme warpage thanks to the differential expansion between the coated
> > nickel and the aluminum backing.
> >
> > Mel Bartels
> >
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