[ATM] Testing question

[melockwo at uiuc.edu (Mike Lockwood)]

Scott,

This is my first post to the ATM list.  I've been reading the list for a 
couple of years now, and in that time I have completed 9 mirrors to 
date, ranging in size from 4.25" to 16".  Your post describes a problem 
that I often have when polishing.  Here are a few things to think about. 
  I welcome others' comments.

I assume from your previous posts that you have an oblate spheriod.  If 
the nulled zone moves toward the center as you move the knife edge away 
from the mirror, then this is what you have.  (The edge of the mirror 
(outer 1/8") is often turned down, too, and the diffraction ring is not 
seen on the right side of the mirror.)

Concerning the flat substrate and the thickness of the lap, I would only 
worry about this if the lap is quite thin, i.e., 3/16" inch or less.  If 
it's 1/4" thick or greater at the edge then it should be fine.  (Thinner 
laps act harder than thick ones.)

I am most curious about the hardness of the lap.  I have often had 
oblate spheroids when the pitch was a bit hard, and they only seem to 
get worse!  The pitch at the edge of the tool does not flow and keeps 
digging in, carving a hole in the central part of the mirror.  You can 
actually feel this happening - the friction changes significantly during 
the stroke, with the most friction when the mirror is nearly centered 
over the tool.   Another contributor to this problem seems to be using 
too much Cerium Oxide in the polishing mixture, or using strokes that 
are too fast.  These cause the lap to "float" on the polishing mixture, 
preventing good contact.

My cure for this is to use softer pitch, or make the pitch "act" softer.
To make the pitch act softer, you can either 1) put more pressure on the 
mirror/tool when you are polishing, 2) warm the lap and mirror in warm 
water before polishing, 3) warm up the polishing room, or 4) pour a new, 
softer lap.

The easiest solution is usually 1), more pressure.  Try to apply the 
pressure in the center of the mirror/tool.  This can be difficult if 
your grinding stand is not the proper height.  I recently lowered my 
stand several inches and found obtaining a sphere to be easier, since I 
could put more easily (and with less fatigue) put consistent pressure on 
the work!

What type of pitch are you using?  What is the room temperature?  Have 
you done the fingernail test?  (If you want to make more mirrors in the 
future, a pitch tester is a great device to build.)  A good indicator of 
how the lap is working is how fast the channels close.  In my 
experience, I would expect to have to re-channel every 3 hours or so. 
If the channels are closing much more slowly than this, I usually try to 
do something about it.

Lastly, make sure your cold presses are long enough in duration and with 
enough weight that you get good contact.  Not getting the lap fully 
pressed into contact will cause unpredictable results!  Also, longer 
mirror-on-top strokes with some side swing may help reverse this 
condition, too, but be careful not to turn the edge.  Finally, be aware 
that you may have to press often while working back to a sphere.

Hope this helps.  I have done all these things in the past and found 
them to help.

	Mike Lockwood
	Champaign, IL

"When it comes to pitch, CONTACT IS EVERYTHING."


Scotto1234 wrote:
 > Based on your e-mail, I have an oblate shape.  Now I have to figure
 > out why.  I just did an hour of short W stroke with MOT, which should
 > head me towards spherical and I still am going the wrong way.  Based
 > on instinct, and responses to previous postings, I am seriously
 > suspicious of my lap.  I cast it on a flat blank and cut full depth
 > channels so the facets may be more rigid a the edge than the center,
 > because they are thinner at the edge.  Does this make sense?  The
 > channels close much faster at the edge of the lap than at the center.
 > Should I cast a plaster blank against the mirror (covered with plastic
 > wrap), epoxy coat and then pour a new lap on the plaster tool?  This
 > will give me even thickness facets on my lap.