Re: [ATM] TDE (That Darned Edge)

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

Hello Robert.  

I did a poor job of stating my figuring comments earlier so I thought I
would clarify a little.

This is not for fixing the edge but the curve. I will leave the edge for
others to comment. 

The hole looks to have the rim at about the 30% zone. You can see the bowing
inward in that center 30%. Then the bands straighten somewhat outward to the
TDE.

If by overcorrected you mean that by measuring the center zone inside the 30
% and the edge zone just inside the TDE, then by pushing those outer areas
down, very little at the edge and increasing inward to the 30% zone the
overall correction, center to edge will decrease. It will likely not be
overcorrected when you do that.

Consider that for the same depth hole in the center, the larger the diameter
of the hole, the less slope the hole's floor will have. A very shallow small
diameter hole can have a very steep slope and hence a very short ROC (radius
of curvature). So the knife movement from center zone to edge zone does not
necessarily mean overcorrection. You have an over corrected center zone. But
it looks to me that you have under correction most of the rest of the way to
the edge. I think over all under correction. I can't be sure because I don't
know the knife readings.

For this explanation I will not consider the TDE.

If you draw a graceful curve from the bottom of the hole to the edge the
highest error is the rim of the central crater and error height decreases
steadily to the edge.  That is what you want to wear. More at the rim and
decrease to almost none at the edge. Most on the highest and least on the
lowest.

Therefore the accented pressure fix to the shape, there are other ways to
fix it, using your ronchi bands as a guide...


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.

That is a bit complicated in the beginning.  It is important to do it the
same way with the same pressure all the way around the mirror.

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. The pressure doing it this way would be
less than the max pressure you might use with the variable pressure method.
You do not want much action near the edge. You do want more inward near the
rim of the hole. You could do it by spacing the strokes in the W
differently. I like to double back on the W through the area I want the most
work.

You do this once around and test. If after you test you decide to it again
you would feather the work. Meaning don't start the W at exactly the same
place. Too much of that and you will make your zones into steps. You would
pay attention to what you see in the test to determine how far out and how
far in you carry the W.

The variable pressure is the superior method, but in the beginning there is
a fair chance that from the time you start a session to the finish you may
migrate to a different pressure or location on the mirror. You have to
concentrate and not let anything distract you. You want whole trips around
the mirror with the same action at each step around the mirror an applied to
the same zone as precisely as possible.

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.  The zone you actually work may sometimes be
half of one zone and part or all of the next. It does not have to be exactly
the same as the zones on the target. You choose where to work by what you
see in testing. The target is a guide. If your glass is not clear then you
find some other way of gauging.

This is not to say that you should abandon normal parabolizing strokes. This
is to fix what hasn't taken evenly across the curve. It can be used either
to force the curve to shape and then you use a stoke that will smooth the
curve or you have done a stroke that has made a smooth curve with a small
error that can be tweaked without so much work as to rough the curve.

Don't even think about doing this if you cannot limit yourself to one short
session between testing.
 
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. You can grope to a good parabola with a little
understanding. To get where you can use these things to drive directly to a
good parabola will take some time and more understanding of the curves.

I have told you enough that with some practice you could get there. But I
can't say that is what you should do.  You can probably get something
acceptable with only more normal spherizing and the normal parabolizing
strokes. Maybe faster than learning to use these methods well.

It might depend on how much you want to learn on one mirror.

Jerry
 

Original Message-----
From: atm- Meeks Robert-PT1784
Sent: Wednesday, April 26, 2006 12:31 PM
To: atm@atmlist.net
Subject: [ATM] TDE (That Darned Edge)

Hello - I've uploaded a not-so-good picture of a Ronchi pattern.
Details below, but it shows a TDE.  A few days ago, I had a seriously
deep center hole and a TDE.  I thought that fixing one would help the
other.  After about 5-6 hours, the center hole is mostly gone (you can
still barely see it when inside ROC), which means that I've taken out a
fair amount of glass to reduce the rest of the surface around the hole

http://www.atmlist.net/contrib/bob-dot-meeks-at-motorola-dot-com/IMG_3109.jp
g


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