Re: [ATM] Persistent turned-up-edge in mirror

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Good advice from Ken.  Personally I don't ever use the cheese cloth, 
but it may help in this case.  Gugulz 73 is HARD and I have only used 
it on flats.  It often needs warm pressing to get contact at all! 
This is why I stick with softer pitch for concave or convex mirrors - 
cold pressing is sufficient, and I don't have to wait very long for 
the mirror and tool to be in thermal equilibrium.

One thing to keep an eye on is your polishing mixture.  If the 
mirror/tool are kept wet, there is less of a likelihood for the outer 
part to dry out.  This causes either cooling, in which case the glass 
shrinks away during polishing and then pops back up causing a possible 
TUE, or extra friction, in which case it can cause an edge roll.  You 
might be experiencing the former.

The cure for TUE alone (after you're polished out, with the rest of 
the mirror spherical) is simple.  Put the mirror on top, CENTER it on 
the tool, and apply moderate pressure to the trailing edge of the 
mirror, the edge closest to you.  After about 1.5", release the 
pressure, rotate the mirror as usual, and step around the barrel. 
Repeat.  As you get used to it, you can take a few strokes (releasing 
the pressure on the return stroke) before you step around the barrel. 
  If you don't release the pressure you plow and turn the edge.

Do this for one or two turns around the barrel and test.  It is a most 
effective cure for TUE, and can even help with an oblate sphere.

	Mike Lockwood

Ken Hunter wrote:
> Try leaving the tool and mirror together (pressing without weights)
> overnight with some cheese cloth or "toole" material between them,
> Cover with a damp towel. Simply remove the cheesecloth or toole and
> rinse in room temp water then start polishing as before without
> heating or pressing again before the work session. I think that the
> warming period is changing the geometry of your lap, mirror or
> both. Polish tool on top using fairly long strokes to get the edge
> worked. Do this for only 10 minutes and look at the figure. You
> should not have to polish for hours to see that a change is
> happening.


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