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Re: [ATM] Help me get back to sphere
Ken and other sphere-hunters,
There have been a lot of questions concerning pitch hardness lately,
and two have been prefaced by trouble getting a sphere, yours, Ken,
and Rich Ball, who started this thread.
I asked some questions about his lap after he sent out his original
question, but they didn't really get answered quantitatively. That's
OK - Hopefully he solved his problem and is making progress!
My motto is "Contact is everything." I'm not intending to be critical
of anyone here, I just want to help them understand what's going on.
It's a matter of contact more than just pitch hardness.
I think it's time for a little discussion of the "synergy" that needs
to exist between pitch hardness, polishing pressure, and the stroke
used. I've tried to organize this a bit, but it still is somewhat
"train of thought", so I apoligize if it's difficult to follow or too
long.
One important point before the discussion: The pressure exerted on
the mirror/tool must be downward through the center of the
mirror/tool. Exerting uneven pressure on either while polishing will
lead to big trouble, and none of the following info will apply!
Please read everything and think about it before applying corrective
action to your situation.
Oblate spheres and hyperbolas:
-----------------------------
I have found that problems with oblate spheroids (turned up outer
zones) and hyperboloids (turned down outer zones) are often caused by
bad contact.
Soft pitch requires less pressure during polishing to keep it in
contact. Too much pressure or too long of a stroke, and you dig a
hole in the center. To correct this, reduce pressure or reduce the
stroke length. My tendency is to go too short, and end up with a high
spot in the center (oblate sphere). I correct this by adding more
side swing (up to 1/2 D).
I prefer soft pitch because it is less fatiguing to polish because you
don't have to use as much downward pressure to maintain contact.
Hard pitch with light pressure usually leads to an oblate sphere for
me, since the lap and mirror do not stay in contact. The lap begins
to "float" above the pitch, and the leading edge digs in and creates a
turned-up outer area. I increase the pressure (and sometimes the
stroke length) to remedy this. (This is fatiguing and frustrating for
me, and I fought this problem for years.)
KEN: Your pitch is VERY hard if you have only trimmed it once in 50
hours. You need lots of pressure downward to maintain contact and get
near a sphere. You also need to press the lap under lots of weight
for a long time (see pressing section below.)
The important lesson here is that we can ADJUST the polishing pressure
and stroke depending on the pitch hardness.
Everyone seems to blame the pitch alone for many problems. In
reality, you can adapt your techinque to hard pitch and soft pitch by
adjusting the pressure you exert on the work. (This requires
practice!) Also, you can adjust the temperature of the work area.
I will assume that Rich's hyperboloid is really that, and not an
oblate sphere. It's easy to get them backwards!
If Rich has soft pitch, he should be able to get a sphere by using a
1/3-diameter (D) stroke, mirror on top (MOT), with a little side
swing. The stroke is slow and with moderate to light pressure. Press
frequently!
How do you tell if your pitch is too soft or hard? Here's an
important point of calibration:
*** For me, pitch that is soft needs to be rechannelled in 2-4 hours.
(I get good spheres with moderate pressure.) Hard pitch needs it
after 8 hours or more. (I get oblate spheres unless I apply heavy
pressure.) I have a pitch tester, but the channel closing time (CCT -
can I coin a term?) is the best indicator of the pitch
hardness/polishing pressure combination for a given person, and the
easiest to compare through email! ***
Purpose of proper pressing:
--------------------------
Changing the polishing pressure has unpredictable effects if the lap
is not in contact. I have been guilty of not pressing sufficiently
many times! It causes unpredictable changes (often the opposite of
what you want to happen) in the mirror.
Getting a soft lap into contact is easier. For a soft 8" lap, press
it under 10-15 pounds of weight for 15-30 mins. Remove the weight and
move it around a bit every 10 or 15 minutes.
A hard lap is more difficult. For an 8" lap, press under 20-30 pounds
for the same amount of time or longer. Ken, I suspect your problem
may be insufficient pressing. If you get only the edge in contact,
you make your problem worse.
With figure problems like hyperbolas and oblate spheres, frequent
pressing is needed to keep the lap in contact. Otherwise you end up
fixing the problem for the first 5-10 mins of polishing, and then
undoing your progress for the next 20 mins!!!!!
IMPORTANT: With significant asphericity (hyperbola or oblate
spheroid), you should FEEL more friction when you pass over the center
with the lap/mirror. This means you're in contact, and the shape of
the lap will naturally help get you back to a sphere. This will go
away after a bit of polishing as the lap flows. STOP and PRESS when
you notice it going away, and get the lap back in contact!
There are faster ways to fix particular shapes, but you should
understand how the mirror and the pitch are interacting before you try
them.
Hopefully this helps. Read it a few times, experiment, and watch what
happens to the surface. Send questions to the list.
Mike Lockwood
PS: Contact is everything!
Ken Reid wrote:
> Hi All,
>
> I've been tempted to ask this question, too, for some time. My pitch lap
> (made with a home made a Silicone II form: 3/4" squares, 5/16" channels) has
> been trimmed once in fifty plus hours. (Gugolz 64 originally:) Only recently
> has it warmed up above 65 F., but it has little affect on getting out of
> Hyperbola land. (my third trip) It isn't difficult to get a descent
> Parabola either. It's just that every stroke suggested for getting back to
> a sphere digs deeper. Short TOT, Short MOT, even TOT pabarolizing . I'm
> chicken to go above 1/3 for turning the edge, again. With the warmer
> temperature even chordal strokes on the 60% zone have little effect. So I'm
> currently using TOT 1/3 and slowly cutting down the edge. It's a 10" f/6
> plate glass, by the way. 1.75" thick. I've read Follet's treatise, so I
> appreciate the edge active, historical implications of pitch lap action.
> And, I press overnight using wetted Saran wrap between tool and mirror.
>
> I've been thinking it's hard pitch. Got to be too hard. Now the thread is
> soft pitch is the culprit. This topic 'hyperbola land ' seems to be avoided
> like the plague on the list. Only 1 good reference in the archive. So, no
> quick remedy? Simply, a long polish back with 1/3 TOT. Make a Tywann tester
> and avoid all the experimentation? I think once people learn "the way" they
> forget the numerous pitfalls they had to overcome, the details? Could the
> tywann tester be a bulwark against this? Sorry for the rant, I prefer to
> shut up.
>
> Thanks
> KR
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