Re: ATM Question re: polishing a 21" f/6 blank

[Aplanatic@aol.com]

Hi Ken:

I've seen this problem (or its inverse, where the center polishes first) 
quite a few times at the mirror making class where I help out.  It is almost 
always due to a nonuniform lap.  Do the following and see if it helps:

1) If the pitch is thin, make a new lap.  If your pitch is less than 1/8" 
thick anywhere on the lap, then you will probably have to remake the lap.  I 
like to start with a lap that has about 1/4" thick pitch.  I'm assuming that 
the lap substrate is convex spherical and reasonably well mated to the 
mirror.  If not then you have other, bigger problems.  

2) Re-channel the lap so that the channels are uniform in thickness from 
center to edge, at least on the average.

3) Microchannel the lap using wedding veil material.  The stuff that works 
the best for me has a 1 mm hex pattern.

4) After hot pressing in the microchannels, press for a few minutes more 
without the wedding veil material, and then immediately go to work with the 
mirror on top for five minutes.  Then let the lap completely cool while in 
contact with the mirror, no additional weight.  

5)  After the lap and mirror have equilibrated, use 1/3 center over center 
strokes with just a bit of side to side to get a smoother surface.

This procedure always seems to produce a very beautiful, uniformly polished 
sphere as long as fine grinding was done correctly.  One other possible 
problem with a mirror of this size is flexing of the lap.  How thick is it, 
and what is it made of?

Best luck,

Dave Rowe.

> I've come to the conclusion that there's something wrong with my polishing
>  procedure, but I'm not sure what to change.  Here's the story:
>  
>  I've done 14.5 hours of polishing using a full size pitch lap.  From about
>  the 8 hour time to now, examination of the surface using a bright 
flashlight
>  in a dimly lit room seems to show that the outer 3" or so is completely
>  polished, but the inner area is not.  That is, I cannot find the surface of
>  the glass in the outer area, but in the inner area the light beam shows a
>  haze on the glass.  This situation has not changed in the past 6 hours of
>  polishing.
>  
>  Examination of the mirror in a test stand using a knife edge edge seems to
>  indicate that the mirror is pretty close to spherical.  The surface nulls
>  out pretty quickly and uniformly.
>  
>  Here's my polishing procedure.  I start by warming up the mirror in a hot
>  bathtub.  While that's happening I warm up the lap surface under two 500
>  watt light bulbs.  After several minutes the pitch squares become quite
>  soft.  Then I place the mirror on the lap, and put my grinding tool on top
>  of the mirror.  The tool weighs 65 pounds.  I let them sit like that for
>  about 7  minutes or so, and then polish for at least an hour, sometimes as
>  long as 1.5 hours.  The stroke is a 1/3 diameter W type stroke.  The
>  polishing seems to be effective.  There is heavy, even drag between the
>  mirror and the tool.  I've been polishing about 75% of the total time with
>  MOT, 25% with TOT.
>  
>  As I look through the glass during polishing with MOT, it appears that the
>  entire pitch lap is making good contact.  However, the appearence of the
>  pitch lap might be telling a diffent story.  The outer squares of the lap
>  are flattening out much more than the inner squares.  Also, the outer
>  squares have a uniform (flat) surface, but the inner squares have a curious
>  pock marked appearence (numerous small depressions in the surface).
>  
>  So, I think the inner part of the lap is not making the same close contact
>  that the outer part is making.
>  
>  Can anyone out there see an error in the way I'm doing this?  As always,
>  thanks in advance for your suggestions.