Re: [ATM] Open Question Concerning Pitch Laps

[Mike Lockwood <melockwo@uiuc.edu>]

Jonathan,

Jonathan Bishop wrote:
> What technique problem do you think Bryan had? Is there any lap which 
> you would consider too soft or too hard? I know many have build pitch 
> hardness testers to measure this. Or is what you are say is that there 
> is a large range of pitch hardness which is workable?

If it wasn't a crusty lap, like several suggested, then I think it is 
likely the pitch was contacting the outer part of the mirror and not
the center.  Then the outer part gets polished, but the hole in the
center remains and can actually get worse if the edge is polished
down.  A good press and some short to medium length strokes MOT
should have removed it, but it may have taken several sessions of
this.  Focault measurements could have quantified its depth.  The 
principle I have found to almost always be true is that if the lap is 
not in contact, either nothing will happen or something totally 
unexpected will happen.  Contact is everything (my motto).

Lap hardness depends on application.  For figuring 8" or 10" F/5ish
parabolas, I consider a lap that you need to channel every hour quite
soft, and one that needs it every 5 hours fairly hard.  The range
between is okay.  Again, that's my OPINION, and everyone uses
different techniques.  Hard pitch can be made to work by warming it
(but working time is limited) and soft pitch can be worked with less
pressure.

On my Texereau-style pitch tester (1 kg mass), I find that between
0.040" (medium-hard) and 0.120" (soft) of fall in 5 minutes is a nice
range, and approximates the range of hardness described in terms of
channel closing above.

For a reality check, look at what the pros do.  Figuring laps for
Zambuto Optical and OMI Torus typically use Gugulz 64 or mixtures
between G64 and G55.  They've come out and said this - check out the
discussions on the "zambutomirrorgroup" and "atm_free" Yahoo groups.
I've used this mixture and it falls somewhere in the middle of the
range (channels close in a few hours), and is nice for medium sized
figuring laps (6" - 12") for hand work at a temperature of around 70
degrees.  These companies use machines, but the figuring strokes are
slow and similar in speed to hand work.

Gugolz 73 is quite hard - I just made two flats using it and it worked
well for that.  Pressing takes a LONG time at typical winter work
temperatures (65 F).  I would avoid it for figuring moderate F/ratio
paraboloids if the working temperature is below 75 degrees.  It would
probably be okay for longer spheres, though.

As you gain experience, you learn to feel when the lap is not doing
what it should, just by how the strokes feel.  You also learn to
adjust your stroke speed and pressure to produce the the type of feel
that you know is modifying the glass in a smooth, controlled manner.

Hopefully Bryan is still there.  Bryan, when that mirror is polished 
again, let us know.  I should have my page about parabolizing up by 
the time you finish.  Practice testing while you're polishing, but 
don't worry about the figure too much until you're polished out.

> Great to know. I have gotten out of the ATM mood for a while (new job, 
> family, etc...). A nice 16" blank would be pretty tempting! F/4 would be 
> pretty challenging. You probably chose that to make it shorter. What 
> type of glass did you use? Thickness?

Pyrex, 1.625" thick, pregenerated.  F/4 or F/4.5 is challenging, and
requires no ladder.  F/5 or F/6 is less demanding, but a short ladder
is required.

	Mike Lockwood




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