Re: [ATM] Figuring Methods

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

As others have already said, if you are digging the center you are making it
worse. 

It is the intermediate zones between the edge and the center that need to be
worn to make the hole less deep. 

This is where you should find out how those guys make those 1.7 oblates.
Well. Maybe just a little of that, say .7 will get you back where you want
to be.

We really know they do that by the 1/3 COC with almost no side. And they are
alternating MOT and TOT. 

The TOT accented pressure method would be to work across the 70% zone,
pushing the crest of the doughnut down. That will reduce the correction but
you will likely have some work to do smoothing the curve again to the edge,
because while it will reduce the correction inward it will most likely leave
a zone at the edge that is overcorrected. Don't use this yet. It' tricky.

You gotta get it moving back so if it gets zoney and under corrected you can
smooth it again and creep up on the parabola next time.


Just do the 1/3 COC with a narrow "W" about 1/6 the diameter of the mirror.
Aalternate MOT and TOT and work long enough for the lap to warm well. 


When you are in a hole stop digging down, dig out.
Jerry






-----Original Message-----
From: Kevin MIchael Zabbo [mailto:chaosopher23@yahoo.com] 
Sent: Saturday, April 29, 2006 1:43 PM
To: Jerry; 'Mike Lockwood'; atm@atmlist.net
Subject: Re: [ATM] Figuring Methods

I'm watching the figuring postings and I can figure
(no pun intended) that the best way to start figuring
is to start from a sphere and turn the edge down
evenly from where the Raleigh criterion for that f/D
mirror starts, and turn it down carefully.

Just my distillation, mostly because I have a
hyperboloid of about 1.7 or so indicating that I have
a really huge TDE and I have to dig a hole in the
center that's a little more than a micron deep.  So
much for giving my arms a break!

Kevin of (rainy) Eastern Iowa
Seeker of the Darkness

--- Jerry <wa4guu@verizon.net> wrote:

> Hello again Mike. I'm still practicing.
> 
> I have changed subject title...to better fit the
> discussion since it is
> settled and agreed that I don't know much about
> edges.
> 
> 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. Just because I 
> 
> 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. There are
> different methods and some might be faster and some
> slower. Sometimes slower
> might be better and sometimes faster is nice.  Some
> methods may not be as
> predictable as others. The reader has the option of
> taking or discarding
> advice. I think in most cases they do not take much
> of it anyway. They might
> remember it when faced with some particular figuring
> challenge they can't
> find their own solution to.
> 
> So one maker may find some methods more sutiable to
> his style and another
> maker finds others more favorable. And some have
> only learned one way.  I am
> one of those because I learned in isolation from
> other mirror makers. If it
> isn't in Texereau's book, or I didn't make it up
> myself, I probably don't
> know it.
> 
>  
> I am speaking of TOT.  There are MOT analogies to
> the TOT. The operating
> factor is the pressure of the lap on the glass. If
> you can duplicate that
> MOT as it would be with the TOT, the results will be
> the same.
> 
> I will answer your concerns..
> 
> -----Original Messages
> From: Mike Lockwood 
> And From: Jerry
> Subject was:  Re: [ATM] TDE (That Darned Edge)
> Subject now" Figuring Methods
> 
> ********************************
> 
> Jerry wrote:
> > If you were experienced at using accented pressure
> and using the
> > right pressure, I would suggest starting with very
> little pressure
> > just a little inside the edge, and as you stroke a
> W inward to the
> > 30% you increase pressure steadily as you go
> reaching max at 30%
> > and then reverse back outward while reducing
> pressure.
> 
> Mike wrote:
> 
> I would like to clarify that a little more.
> 
> If I were to work the lap inward (I think you were
> talking about TOT
> work) using that "pressure profile" (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, with knowledge of what
> can go wrong.
> 
> 
> 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.
> Just as the edge you want minimum and increasing
> wear to the rim, from the
> rim inward you still want some removed but
> decreasing to near nothing in the
> center.  In this case the 30% is closer to the
> center than the edge but
> there is still some there.  If you do this until the
> crater is gone you will
> actually have a flat center to the 30 percent zone.
> Until one is well
> practiced at this. One time around is enough at any
> one work session. In the
> case of Robert's mirror the defect should be history
> after one session. That
> is why I stress the short sessions.  There is no
> reason to take hours to
> remove a hole in the center.
> 
> 
> *******************
> 
> Jerry wrote:
> > A bit easier is to use the same pressure for the
> entire W. and some
> > method of increasing wear in the inner zones such
> as working the W
> > from the edge inward but when you reverse to go
> back outward only
> > go 1/3 or 1/2 way from the 30% to the where you
> started at the
> > outside of the W.  That will give more work near
> the rim of the
> > hole.
> 
> 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.
> 
> ***************
> 
> Jerry wrote:
> > The variable pressure is the superior method
> 
> I must disagree - there are few absolutes in mirror
> making, and I 
> think this depends on the pitch hardness and the
> operator.  With soft 
> pitch, using lots of pressure will make it flow
> quickly, and the 
> "active area" of the lap will move inward rapidly. 
> I have seen this 
> cause deepening of the center when de-correction of
> outer zones was 
> the intended effect.
> 
> I semi-agree. Any method well done is as superior.
> Variable pressure is more
> easily done to a superior result. There are patterns
> of interlacing from one
> step around to the next going a different distance
> inward that is
> particularly useful in the center, where if you went
> that far inward on
> every position around the mirror you would dig a
> hole or dimple.
> 
> The deepening you are speaking of is because by
> keeping the pressure too far
> out you have altered the radius out side the central
> area to a longer radius
> making the center comparatively deeper even with no
> wear in the center.
> 
> Lots of pressure is relative.  If I remember
> correctly about 5 pounds is the
> max pressure I use on a 12.5 inch. I know the feel
> and I pressed a scale one
> time to see but just don't remember for sure. I
> think 3 pounds or so would
> be adequate for an 8 inch. One will learn that
> rather fast if they look at
> the surface frequently and see how fast it moves
> glass
> 
> The active area is the lap and the inactive area is
> where the lap isn't.
> With the pressure on the side of the lap inside the
> edge, a difference of
> radius if formed between what is worked and what is
> not. That boundary is at
> the edge of the lap. That is why we think of the
> edge as being where the
> 
=== message truncated ===


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