[ATM] Parabolization

[Jerry]

> -----Original Message-----
> From: Ken Hunter
> In reality,
> it's the edge that MUST be worked to reduce the APPARENT TDE that is
> caused by over-working the center of the disk. This is always the case
> when they complain that the mirror is finished and looks nice but there
> are pits around the edge because they didn't want to make the TDE worse...
> Wrong way to thnk!
> 
> Ken Hunter


On atm_free a question was asked about "unpolished edge" and why the edge of
his mirrors was not polishing like the center.  I thought that some here
would find my reply useful. This explanation is also valid for grinding with
a full size tool, but the rigidity of the grinding tool helps to speed edge
grinding somewhat in spite of the reduced contact time. This is also a
better explantation of the cause of the oblate spheroid than "rocking the
lap" at the beginning and end of forward and back strokes. And of course
this is why avoiding TDE does not mean reducing wear on the edge.
*****

I have more time to give a more complete answer than before.

Polishing by hand with a full size lap:
The mirror's edge polishes slower because at most it is only in contact with
the lap for half of each stroke. the center is in contact with the lap at
all times during the stroke. Both TOT and MOT this will be true.  

Lets look at what happens working TOT stroking center over center with a 2"
overhang forward and back.  With the full size lap centered over the mirror
the entire mirror is in contact with the lap.  But as soon as you push the
lap forward the mirror's edge near to you is uncovered.  The mirror's edge
away from you is in contact for the full 2" forward stroke and for 2" of the
4" back stroke. As soon as the center of the lap passes over the center of
the mirror on the 4" back stroke the mirrors edge away from you is uncovered
and no longer in contact with the lap.  The mirrors edge nearest you is now
covered by the lap and remains so for the last 2" of the 4" back stroke and
for 2" of the 4" forward stroke.

The center was in contact with the lap for the full 4" stroke but the edge
was only in contact for 2".  The edge of the mirror to the left and right
are in contact for even less of the stroke.

The size of the central area that is always in contact with the lap depends
on the length of the stroke and the size of the lap.

Do a little stroking while thinking about this and you will see what I mean.

It is the same working MOT and TOT.  But working TOT does add some pressure
on the edge during the forward or back part of the stroke where the lap is
in contact with the mirror's edge.

If you wish to make the edge polish more quickly you will have to add some
force to the lap over the mirrors edge. 

The longer strokes TOT as I suggested earlier are one way, but that still
leaves the edge polish lagging behind the center by a large margin.  Another
TOT method that works rather well is to offset the lap to the side of your
favored hand. If you are right handed move the lap to the right about 1 or
1.5 inches with your 12" and stroke forward and back (about 5" stroke) while
pressing down on the right side of the lap with your right hand with about 5
to 6 pounds (2.5 to 3 kg). The lap overhangs the edge of the mirror on the
side with pressure. Do about 4 or 5 forward and back strokes and step around
the barrel, rotate the lap in hand and repeat. Do about 12 to 15 steps
around the barrel (mirror). Do this in 20 to 30 minute sessions or until you
get to tired to push and pull the lap. Then let the lap press 15 minutes or
so and do it again and again and again.

Depending on the condition and hardness of your lap you might end up with
differernt shapes but that is always the case with any method, and in this
case you are trying to polish the edge faster. If the shape is ugly you can
fix the shape in a short time after it is polished.  I have actually ended
up with a good parabola from this with a not so bad edge.

You might think that this would turn the edge down.  This stroke (used in
shorter work sessions) is one method of fixing a turned down edge.

As always, and particularly with a method like this with added pressure,
when working TOT remember to rotate the mirror on its support frequently.

It may take some practice to learn to do this in uniform manner all the way
around the mirror.

On beveling the edge of the lap:
If beveling the edge of the lap makes it smaller than the mirror, don't
bevel it.
********* 

Jerry