Re: [ATM] Updated pictures of 8"f6 mirror............

["Jerry" <wa4guu@verizon.net>]

G'day... (I'm trying to learn a second language)

ON PITCH:

	Which came first, temperature or pitch hardness? Which has a lever
attached for easy adjustment?

	Once the lap is made, in most cases it will be easier to control
hardness by either controlling the temperature or controlling when or where
you work based on temperature.

	Channel closing time by itself is not a good indicator. There are
many factors involved in that time. Depth and width and angle of the
channels, stroke length and speed, weight, slurry quantity and dilution are
some. All those variables mean that until you have settled into a personal
standard of methods, channel closure time won't mean much. 

	The results on the mirror surface are a good indicator. Results are
an indicator of the effectiveness of all those other things too. That is
good and that is bad. You have so much control. But there are so many things
to control.


ON SHORT (1/4D) STROKE:

	The short stroke is a cause of roughness because it is "the nature
of the beast" to be too regular. It is more difficult to add variation to
short strokes. You just can't vary much shorter and if your variation is
only towards longer, the average is longer. The 1/4 stroke is probably not
extending beyond the width of the facets. Each facet would only work one
zone. Can't get much more regular than that.

	It is also the nature of short strokes that a larger portion of the
stroke is in the rougher action of starting and stopping of the motion.


JOHN'S MIRROR AND LAP:

	John is in the experimenting phase. He wants to play a bit to see
what happens. That is a good thing to do what with all those previously
mentioned variables and more. He tried 1/4 stroke. It happened that the work
space was warmer than with previous work. He considered that the lap may be
too soft. It might be. I was letting him know I think that is not the
problem.

	Looking at John's results for the specified actions, I have seen no
reason at this time to question the lap beyond a possible minor temperature
tweak. That does not mean that it was or was not used at the ideal
temperature. It may work better at 25 or maybe 19 than at 21 or 22 C.

	Someday the lap might be just plain worn out. Then a new one will be
called for if the mirror is not finished before then. Until then I suggest
finding out how well this one works. 

	 I try to NOT explain in total detail every suggestion I make. Even
so, I'm still too long winded on partial explanations. You can see I need to
get better at that. Heee HEEEE! I have trimmed half of my comments out of
this post. The message gets lost in the details.


Jerry


	
 
-----Original Message-----
From: Mike Lockwood

Hi,

temperature is not a problem if you 
have the right pitch hardness.  Adjust the factor that you can control 
easily - for me that's pitch hardness.  My basement work area is ~62F 
in the winter and ~72F in the summer, and I just adapt the pitch 
hardness to the working conditions.

Hard pitch in poor contact causes dog biscuit in my experience.  I 
don't have experience with too warm - I've worked up to 78F in my 
kitchen and, so long as the pitch is the right hardness, it is not a 
problem.

I have no idea how hard the pitch is.

John, how long does it take the channels to close under normal 
polishing?  (If you said this in an earlier email, then I missed it.)

I know you said you are using Gugulz 64 which should be OK, but we 
have to verify that it is indeed working properly.  Should take a few 
hours at ~22C (72F).

What I see in the series 7 photos is a classic oblate condition in 
which the outer 1" - 1.5" of the mirror turns up.  Why does this 
happen?  Well, the transition zone between the raised outer zones and 
the "flat" central region is a low spot, and that's where the "active 
region" of the lap stops on the forward and back stroke, and where it 
causes the most wear.  So, if it is indeed a 1/4D stroke, that's 2" 
long, so the active region of the lap is ~1/2" in from the edge of the 
lap for the particular conditions you are working under.

The stroke you are using is likely too regular.  One thing you can try 
is to move the active region around by doing strokes of different 
lengths. 

This moves the wear pattern around.
I recommend working both MOT and TOT with strokes of more random length.

Scratching the facets of the lap can help lap contact and may really 
help in getting things more spherical.  I recommend ~10-15lbs of 
downward force to the mirror as you make the polishing strokes.  This 
helps keep the pitch in contact.  As for speed, I recommend that one 
forward and back movement takes about 2 seconds.  Keep the lap/mirror 
fairly wet - don't let it dry out.

	Mike Lockwood

_______________________________________________
ATM mailing list http://www.atmlist.net/

_______________________________________________
ATM mailing list http://www.atmlist.net/