Re: [ATM] Help me get back to sphere

[Jonathan Bishop <jonathan_bishop@sbcglobal.net>]

Hi,

Must be something in the air - I just struggled with
the same problem last few weeks (seems like months).

I am making an 8" F5 and had a narrow turned down edge
that would not go away. Last night I finally resolved
the issue and very quickly (less than 1 hour) got back
to spherical and cured the turned edge.

The solution was to use a harder lap. I guess as warm
weather crept up on me I did not realize how big an
effect that has on lap hardness. Well what had been a
good lap at aroud 60F was a very soft lap at 80F!

My solution was to boil my pitch (outdoors) to harden
it. I heated it on an electric hot pad until it
started to smoke and make a gentle rolling boil. I
emphasize to do this outdoors on an electric hot pad
as there is a danger of catching the whole thing on
fire, and the fumes are pretty bad.

Of course you can buy harder pitch, like burgundy hard
from Newport Glass, or Gugolz 73.

After I did this I made a new lap, made sure the edge
was trimmed back a little, microfacetted, and then
proceeded to polish using short COC strokes TOT. In my
case this meant strokes less than 2" long.

This immediately caused a hill to start rising in the
center. Better yet, the turned down edge started to
disappear. I continued until I started to get a bright
difraction ring on the dark side of the mirror under
test. I am told this is a good indicator that there is
no turned edge. Overall I did about 1 full hour of
polishing, broken into several spells.

Then I switched to a normal W stroke, again TOT. After
about 15 minutes I tested and had a nice spherical
surface. In fact, more spherical than I have ever
gotten before.

So I am now a big fan of hard laps, at least for
getting to a sphere. If you find that the lap is too
hard and too "grabby" then I suggest that
microfacetting will help a great deal to help bring it
into full contact, along with some warm/cold pressing.
I would try this before destroying the lap and trying
softer pitch. In general, use the hardest pitch which
will still alow you to maintain full contact.

Another good idea is to get a thermometer for your
work  place. I did and I was surprised at the amount
of variation (I work in my garage) in temperature was
really going on. Now I limit myself to working when
the  temperature is 75 +/- a few degrees, which seems
to be just about right for the pitch I am using.

Hope this helps.

Jon Bishop


--- Rich Ball <ShooterBall@hotmail.com> wrote:
> I have been attempting to bring my 8" f6 mirror back
> to sphere after finding
> the middle too deep and the edge turned down.  I
> first tried a simple short
> COC stroke MOT.  Things got only worse.  The numbers
> all got farther apart.
> The edge didn't improve.   Then I tried, with great
> hope, Bob May's little
> half stroke.  The idea of it appealed to me.  But
> alas, the numbers
> continued to spread.  The hole is embarrassingly
> deep.  With stationary
> light source movement of the KE is about one inch!
> from center to edge.
> 
> I have pressed a great deal, hot and cold.  Am I to
> suspect (after hours of
> the above procedures) that my lap is at fault? 
> Would it be a reasonable
> gamble to pour a new lap and start over?   Can this
> mirror be saved?
> 
> I don't know what to do.
> 
> Rich Ball
> Oak Park
> 
> PS.  Note on my lap:  The deep lines that I cast
> into the pitch are long
> gone.  Several times I have restored them with a
> saw.  But they don't last
> long.  I always use a net when I press and I don't
> start pushing glass until
> the net is taken up into the pitch.  I chamfered the
> edge of the pitch about
> 45*.  Temperatures in my work room have been near
> ideal lately.  Not too hot
> or too cold.  (Cool spring in Chicago)
> I have not chamfered the edge of the mirror.
> 
> _______________________________________________
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> 

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