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Subject: Re: Aluminum finishing on Lathe?
From: (Jonathan M. Elson)
Date: 6 Apr 1995 18:48:02 GMT

Steven Marcotte ( wrote:
: I'm trying to turn some automotive parts made out of cast aluminum on a 
: Monarch Lathe.

: I need a mirror like finish.  

I've made aluminum optical mirrors on a lathe.  I used a carbide indexable
cutting tool, as normally used on a lathe.  I had a computer crank out the
curve I needed at .010" steps on the compound, with the compound parallel
to the bed.  The computer made a listing of the actual dial readings on
the cross slide to be used for each .010 setting of the compound.  So, I
became a "carbon-based" CNC controller.

After cutting the mirror to approximate shape with the cutting tool, I
put a coarse Cratex wheel (small, rubberized abrasive) in my homemade
tool post grinder, and followed the original curve with it.  I then
went to finer and finer cratex wheels until the grooves and ridges left
by the original cutting were removed.  I then switched the lathe to very
high speed, ~1000 RPM, and used a rag and jeweler's tripoli to start to
bring up the finish.  I then used the grinder with a tiny felt polishing
wheel (as used with a Dremel) and more Tripoli, holding it across from
the marks left by the previous polishing step, to remove scratches.  there
is always some grit left over from the Cratex, and it causes scratches
when polishing.  You will eventually remove all of that grit.  Then you
go back to high speed, and use the rag and Jeweler's Rouge for final
finish.  It is slow work, but I got a true mirror finish, and the mirror
is still working in an optical instrument, as the light collector for a
200 Watt mercury arc lamp.

If this is too convoluted, let me know where I lost you and I will try
to clarify the process.

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