RE: [ATM] figuring 10 inch mirror

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

I don't think anyone really feels comfortable advising on how to fix turned
edge. But I will stick my neck out.

But first the finger warmed edge turning down.  I don't believe that one.
Applying the theory in reverse, I tried curing TDE by cooling the edge with
ice on the mirror edge on a machine. No discernable difference with any
stroke I could imagine.

My opinion on turned edge from my experience is that the pitch lap is the
key. I'm talking about the last few millimeters at the edge. Much wider than
that is a turned down zone in my thinking. Sure there are things you can do
to turn the edge more with just about any lap. But for any given stroke the
degree of turned edge that will result is in the lap. Since in figuring you
need to have at least a small variety of strokes to make the shape you want,
I imagine that most mirror makers will settle into their favorites and end
up doing the same things on every mirror. But the same maker will have
differing amounts of turn down from mirror to mirror, at least until they
bring all the things that affect the condition of the lap under control. 
However the lap can be so bad that the turn down widens a bit or the excess
work trying to make it better turns the zone. But since the condition that
is at work at the edge is still there the edge is still turned down relative
to the turned down zone.

On Wayne's mirror I see over correction on the outer two zones. The zone can
be fixed relatively easy. Well there is a problem. It is close to the
parabola and it is hard to fix it without departing a bit from how close it
is now. But hopefully the larger error is moved inward and upward where it
is easier to fix.

Now it takes nerve to do work on the outer most zones and when someone gives
advice from a distance it is hard to convey exactly a technique.

Lets look the profile from the outside in. When I refer to over or under
correction of a zone I mean relative to the next zone inward. And proper
correction would be your target difference from your calculated ideal
readings. Proper correction would have no slope CHANGE from the slope of the
adjacent zone toward the center. Your zone 5 is an example. Zone 4 has a
down slope and zone 5 correction is just about right so the down-slope
continues rather straight. Over correction has a down-slope and under
correction an upslope relative to the adjacent zone toward the mirror's
center.

To fix the over corrected outer zones, and that is what I think you mean by
TDE in this case, you need to reduce the correction on the outer two zones,
7 and 6. The next zone in (5) is just about right on but it is on an upslope
from the well under corrected zone 4. And zone 3 is under corrected but is
still a down-slope from the steeper down-slope of well overcorrected zone 2.
Zone 1 has an upslope. It would be considered a hole except for the under
correction in zones 3 and 4 puts the bottom of the hole at the right place.

All that adds up to a raised zone at about the zone 2-3 boundary and another
extending across zone 4 to the edge of the mirror and centered about zone 6.


Fix the outer one first. Its upslope starts at zone 4. 

Now the scary part. Really working the edge to fix it. I can only say this
has worked for me and that if you have Texereau's book you will see this as
technique number 7 in figure 42. Full size lap, TOT 1/4 stroke, accented
pressure on overhang side of lap. In your case the side motion or width of
the "W" would be about the width from the boundary of zones 3 and 4 to the
edge of the mirror which would make the W about 1 1/2 inches wide. You
should not bring the pressure side of the lap inside the edge of the mirror.
If you put pressure with your right hand the overhang will be to the right.
The left side of the W will be at or just barely outside the edge. The right
side of the W will take the right edge of the lap to 1 1/2 inches of
overhang. Do about three forward and back strokes on the way to the right
side of the W. You could bring it straight back to centered or do a couple
of forward and back strokes back to center and take a step around the barrel
and rotate the lap in you hands a bit. Repeat until you have gone all the
way around. One trip around very well may be enough. That outer raised ring
is only 2 millionths of an inch or so high. 

This should reduce correction on the outer zones and you will probably see
an increase at zone 4 which could use some more. You don't want to over do
it. If one time around shows some movement in the right direction but not
much don't double the work the next time. Just do one trip around at a time
and test.
 
You have to concentrate when doing accented pressure, constant pressure, the
same treatment all the way around the mirror. Varying pressure through
different parts of the W can be useful at times after you get the hang of it
but you must vary the pressure the same at every step around the mirror. 

Accented pressure with the TOT will decrease correction of a zone relative
to the zone inside the W that the pressure side of the lap traces. It will
increase the correction of a zone outside of the W. In this case there is no
zone outside the W.

Let's imagine that zones 6 and 7 had proper correction and continued the
upslope of zones 4 and 5. You could use accented pressure to increase the
correction of zones 3 and 4 and reduce the over correction of zone 2 at the
same time by moving the pressure side of the lap and placing the W accros
zones 2 and 3.

I'm sure my suggestion sounds all wrong. You might say that it would be the
last thing you would try. I think it would be. Comments from others?


Jerry









-----Original Message----- trimmed
From: On Behalf Of Mark Holm
 Subject: Re: [ATM] figuring 10 inch mirror


I really can't claim to be expert at figuring, especially fixng turned
edges.  I'll defer to others on the list for the best advice under the
circumstances.


A recent bit of interferometric work posted on this list indicates that
hand warmth might be more a factor in edge turning than most think.  Try
not to curl your hands over the back edge of the mirror if you can help
it.  If you have to, only curl them over as little as you can get away
with.  If you have handled your mirror by the edge while putting it woen
on the lap, let it equilibrate at least 20 minutes before you start
working.  You might consider cotton flocked diswashing gloves.  They cut
down heat transfer quite noticibly.

Others on the list have their own favorite methods for fixing turned
edge.  They are mostly much more experienced at it than I and probably a
lot more successful, so I suggest taking their advice over mine if there
is a conflict.

-- 
Mark Holm



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