From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


John, the first rule with polishing and figuring is that you have to turn
the mirror a lot slower than with grinding.  Fast turning in polishing will
creat rings of zones because the friction of the glass-pitch interface will
heat the glass and pitch to some degree.  This causes a problem as the
heated glass and pitch will expand.  This really causes problems as the
expanded glass will then support the lap from the rest of the glass and
thus you will then plane that part of the glass down until it is the same
height as the rest of the glass (which isn't warmed from the friction!) and
the friction evens out and this means that the heated part of the glass is
no longer getting the heat input that it has been getting and guess what
happens - that part of the surface then cools and the glass retreats from
the lap surface, leaving the rest of the glass to take the friction,
heating the glass and so forth and it keeps oscillating until you stop the
polishing.  I'll note that heavy weights on the lap will also do the same
thing at a lower speed.
Thus, if you are wanting to get the pits out of a surface, attach heavy
weights to the lap and turn fast and they will go away quickly.  Then you
have to slow down and remove the weights to get the artifacts from the
rapid polishing removed.  I'll note that many of the Ronchi tests show that
fast heavy polishing action where the rings have started oscillating with
the heat of polishing.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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