Re: [ATM] figure stroke ten inch

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

Hello Tom... Here is what I see from your numbers that would matter to me if
I were making the mirror.

Percentage of correction from zone to zone...

Previous session results
0
    >  67%
.49
    >  58%
.84
    >  23%
.98
    >  0%
.98


Latest session results
0
     > 127%  almost doubled
 .91
     >  75%  about 1 1/4 more
1.36
     >  96%  4 times more
1.93
     >  37%  improved but not fast enough to catch up.
2.15



I would recommend smoothing the correction before adding anymore. If you add
any more correction to the center while correcting the edge zone you will
have a hyperbola. Time to think a little and plan what you want to see
happen and how to make it happen that way. Figuring is always time to plan
and think but you are at a fork in the road and you don't want to take the
wrong one.

This is probably to complex for you to execute this time but it is a good
time to try to understand it, so maybe sometime you will be able to use it.
Usually in learning to figure it is best to fix one problem at a time. You
want to see the effect of a stroke or technique isolated from other strokes.
If you do two things you may confuse the results of one with the other.
After you have learned the results of various things you can mix them to fix
more than on thing at a time. You have a situation where the same stroke can
fix two things at the same time. Well it is really two sides of the same
defect but it is a wide defect that you could fix one side at a time.

If you wear the wall of the crater, zone 2, you would reduce the 127% from
zone 1 to 2 to a smaller percentage. If you wear zone 3 you will make the
37% from zone 3 to zone 4 larger. If you do both by wearing zone 2 and 4 you
would kill two birds with one stone. But then that other bird, the 96% which
is mighty close to 100%, would get away and you already have that one in the
bag. So don't change that one. Wear zone 2, 3 and 4. TOT accented pressure
about 2 pounds on the edge of the lap stroking a W as per Texereau across
the zones you want to wear. Two times around the barrel would probably be
enough to see if that takes you in the right direction. Three times around
might do better but could be too much. Don't do three without testing in
between. A little too much may be much worse than no work at all. 

I could elaborate but you would still have to do it to learn it. This is
probably too complicated. There are other easier ways. Not necessarily
better.

If I were to do it MOT I would be more careful and only work to make the
outer zones 4 and 5 better and then work farther inward on a later session.
To get more correction out near the edge without adding to the correction
inward keep the W narrow and put a little pressure on the overhang side of
the mirror. 

The reason correction does not go on the edge easily with the parabolizing
strokes is that there is hardly any pressure on the edge of the lap with the
mirror so slightly overhanging. Add some pressure out there on the overhang
and reduce the pressure as you increase overhang. With increased overhang
the weight of the mirror puts the pressure on the lap edge. That is why the
center correction moves out ahead of the edge zones.

One reason I would prefer the TOT method in your case is that while
increasing correction of the edge zone I would be reducing correction
farther inward. Most of the center is already fully corrected. If while
correcting the edge you get any increase of correction in from 80% zone
inward you will have a hyperbola. And that is also why I would only try to
improve the edge correction and not work farther in.

So the MOT has a risk to it, and that makes me favor complicated. I will
guess that you have probably already done some more MOT and have that
hyperbola I'm talking about. Hope you haven't.

But if you did...
Simple is hoping that a long stroke parabolizing like W except narrow, maybe
about 1/4 to 1/3 diameter wide would blend the zones and get the correction
percentages closer to the same number while increasing the outer zone to
near 100%. All that without the central 80 percent being more than 100%.
That can happen but there is a large element of luck involved. The other
methods are a more direct action approach but you have to learn to execute
the direct action. That would require some luck to do the first time.

Whatever you do...
You should have a lap of the proper hardness and must have good contact. I
can't tell from what you have posted how your mirror got to its present
shape. It is not so unusual and you could have gotten there with a proper
lap. I would prefer a well pressed lap of the wrong hardness to one that has
been "hot dipped" to get it warm but has not been pressed to shape well. If
I thought the lap needed to be warmed to work right I would warm the work
space and keep it warm until the mirror is finished. Or I would make a lap
of pitch mixed to be more suitable at the work space temperature. There is a
little tolerance on hardness but you cannot compromise on contact. Don't
work with poor contact. <That is a period there.


That ought to be enough to think about and keep you from getting any sleep
for a few nights.

Jerry



Hi all,

I wanted to add a quick note of results from warming the mirror and lap
to soften the pitch. I set them in water at about 80 degrees for 1 1/2
hours. did W stroke as before for 3 turns around the mirror result in
over corrected center and high edge. also more roughness and ripple as
contact was still not good.

three more turns after a long press COC MOT 1/3 wave edge still high.
so problem was hard pitch. now it is contact. need to smooth the over
all roughness.

new readings in mm 0, .91, 1.36, 1.93, 2.15 

-- Tom
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