[ATM] Figuring Methods

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

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
action is. That is where we see a difference made most easily. The part of
the mirror inside that edge is worn mostly right under the pressure and
falls off rapidly inward. If you do this you will see an increase in
correction from inside the lap edge to outside the lap edge. And inward from
the lap edge will be made flater. Except for farther in where the pressure
is reduced considerably in not changed much, resulting in that hole you are
speaking of. That is one reason for going farther in.  I could have told
Robert to work one zone at a time but it would have taken 4 or more sessions
to do what could be done in one.

*************

Jerry wrote:
> I make what looks like an archery target that represents the zones
> of my test mask and seal it with plastic spray. I put it under the
> mirror and use it as a guide as I work zones.


Mike wrote:
That's a very good technique.  Once you understand where the active
area of the lap is, using that to aid you, you can attack nearly any
defect.

Jerry's answer
I have seen that you also use this method.

******************
Jerry wrote:
> I can't say that it is wise to do unless you are willing to
> practice it a bit. That is what mirror making is, but you don't
> have to learn it all on one mirror. It is a lot.
Mike wrote:
I can't count the number of times I have been "practicing" on a mirror
until the early morning hours.....  :)  Took me about 20 mirrors 
before I got to the point where I didn't feel I was practicing.

	Mike Lockwood

Jerry's answer
I think more babies are born at night too. I don't think I have ever
finished a mirror in the daytime.  But I will there is a reason for that. I
spend much much much more time testing that working the glass. When the
shape is rough testing can be too. But when the shape starts getting good
the testing needs to be very careful so that you know the shape of the
glass. If you don't have the present shape accurately measured, you can't
expect to know properly where to work.


I should have used the 4 zone mask as in Texereau's book for the example of
zone changes that result from accented pressure. I will try to make this
short and anyone can test this and see if it is true. I hope this prints
correctly. The direction of the significant change is what I am trying to
show. I am not saying the resulting profile will be exactly as plotted.
I only have so many sloping characters on the keyboard.
 

Zone   2 4
Zone  1 3 
Slope   _/
      _/
     
If we work accented pressure across zone 3 the resulting change in
correction of the zones will be as follows

Zone   2 4
Zone  1 3 
Slope \___

The correction will increase from 3 to 4
The correction will decrease from 2 to 3
The correction will change little 1 to 2




Now let's add a 5th zone to clearly show what happens if you work accented
pressure across two zones. 

Now we have 5 zones we will start with a similar shape that has the same
slope through zones 3 and 4. Notice 3 and 4 have the same slope.

Zone   2 4
Zone  1 3 5 
Slope   __/   
      _/

This time the pressure will be applied across both zones 3 and 4.
The mirror doesn't know where you have drawn the boundaries of you zones.
This work is applied to zones 3 and 4 as if it were one zone. It is if you
figure it that way.
The result will be.

Zone   2 4
Zone  1 3 5 
Slope \____ 
      
The correction will increase from 4 to 5 
The correction will not change from 3 to 4 (not much)
The correction will decrease  from 2 to 3
The correction will change little 1 to 2

If a double back out from the inside of the W only went through zone 3 then
3 to 4 would increase also increase. You figure out what the resulting
profile would be. 

I will cover something about center work in a follow up. Someone will find
it interesting.  Don't make them dimples!  But if you do. Don't cry. There
is an easy way to make a dimple pretty.

Jerry







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