From: "James Lerch" 
To: "ATM List" 
Reply-To: "James Lerch" 


----- Original Message -----
From: "Glenn Becker" 

> This explanation of the grabbing makes sense to me, but I've been
> faithfully rotating mirror and tool in opposite directions since #80 (I
> hogged out with #60). So I'm non-plussed at the possibility that I might
> be straying from a good sphere.

HI Glenn,

Question, When you were doing the 12 micron and had the 'vacuum' problem,
had you done any wets at a larger grit within the last few hours?

I ask as I have a hypothesis on this, which may be inaccurate, but its
something to think about.

Assuming the mirror blank and tile tool are made of different materials
(Pyrex/Plate for the blank, and Dental stone/Concrete/POP for the tool).

Lets say I've done several wets {at} 12 micron, and lets also Assume that the
mirror and tool are in near perfect contact, and everything is spherical.
At this point I call it a day and put everything aside to come read the ATM
list.

On the next day, I'm ready to get back to work on 12 micron.  ***
Hypothetical question: If the temperature or humidity have changed, could
this cause the Tile tool to subtly change shape?  If so, our mirror and
tool are no longer perfectly mated.  If this happens during larger grits, I
don't think its a problem, but at the real small grits could this cause the
"Grabby / Vacuum stiction effect?"

I believe the above is possible, towards that end I've made a rule for
myself.  When I move to the next grit smaller than 25 micron, I am
committed to finish fine grinding the mirror during that work session.  If
there is not enough time to finish fine grinding, I set everything aside
for the day. On the next day I'll start with several wets of 25 micron,
THEN move thru the rest of the grits until fine grinding is complete.

Just something to think about...

James Lerch
http://lerch.no-ip.com/atm (My ATM construction, testing, and coating site)

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