Re: [ATM] Figuring Methods

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Jerry wrote:
> I enjoy the discussion. This is why I usually answer requests for
> figuring advice.  I know there are different methods. I just don't
> know what they are, so I want to hear from you and others. Mike,
> you are one of the few who will step up with advice, and of a more
> thorough nature than most others will.  I thank you for that.

You are welcome, and thanks for the compliment, and for understanding
that the list is just a discussion.

> The ones asking advice should not take every disagreement on
> figuring technique to mean one is right and the other is wrong. I
> may well try some of the things I have argued against the next time
> I make a mirror.

Yes, there is no "correct" way to figure a good mirror - it can be
done a variety of ways.  It all depends on the operator and the pitch
lap.  (But some methods are easier to express over email, and some are
more suitable to those making their first mirror.)

The new mirror maker is advised to pick one technique and stick with
it.  This can be hard to do amongst a myriad of list messages
containing seemingly contradictory advice!

>> Mike wrote: If I were to work the lap inward (increasing the
>> pressure while moving inward), I suspect I would find that it
>> very effectively dug a hole near the center.  Using accented
>> pressure near the center of the mirror will change the shape much
>> more quickly than near the edge, and it must be done carefully

> Jerry's answer: If you stop the inward travel at the rim of the
> crater this should not happen. Keep in min the rim of the crater is
> not the end of the high glass.

It shouldn't happen, but it does, because pitch flows while you are 
figuring.  And that moves the effective active lap area
inward, and removes glass well away from the edge of the lap.

>> Mike wrote: Again, one must be careful because the center can
>> deepen very quickly.
> 
> Jerry's answer: The difference is that I am going for a different
> curve than you are. I am going for a parabola of a slightly shorter
> focus. There is less glass to remove. By adjusting the target curve
> to the one that touches the very center and the very edge.

I guess I am going for the parabola that requires the least work
on the outer zones, which I would already hope to have most of the
correction in at this point.  The less work you have to do on the
outer zones, the better, and the less chance of screwing up the edge.

> Lots of pressure is relative.  If I remember correctly about 5
> pounds is the max pressure I use on a 12.5 inch.

Sounds about right.  You can use more on soft pitch.

> Except for farther in where the pressure is reduced considerably is
> not changed much, resulting in that hole you are speaking of.

The hole I spoke of was caused by pitch flowing due to using lots of 
pressure on a softer lap.  The active area moves inward well away from 
the edge of the lap in this case.  This is not usually a problem with 
harder pitch, unless the figuring session is quite long.  I use lots 
of softer pitch because I work with subdiameter laps (on faster 
mirrors) quite a bit.

This is getting long, and I'm out of time.  Off on vacation very soon, 
and away from email.

	Mike L.

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