Re: [ATM] Not tde or tue, important oblate spheroid, ellipsoid

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

I generally agree with the point Jim was making. COC makes the mirror
oblate.

I was a bit too absolute in my support of Jim's statement. I would delete
the word "further" from the sentence. The COC stroke pushes the curve to an
oblate but there are other forces that work the other way.

If you take up almost any stroke and continue with it long enough the mirror
will take on a shape and it will reach a certain degree of that general
shape and then progress no more. You might be able to find some stroke of
extreme overhang that would continue moving the shape in the same direction
until the lap wore out but the strokes you are likely to actually use will
take the curve so far and then the shape will stall.

The longer COC stroke tends to make a milder oblate and shorter strokes more
oblate. The overhang of the long stroke lessens the bump in the middle of
the oblate if the mirror is on top and if it is on the bottom the overhang
of the lap bearing on the edge takes the raised edge of the oblate down
some. You are fortunate if you can find something that balances the COC push
to oblate perfectly at every zone.

Jerry
P. S.   I will bet that within 6 weeks, here on this list someone will post
concerning their mirror and a link to a picture of a mirror that is terribly
oblate. They will ask what they have to do to fix that shape, that they have
been doing like they were told or read, using a short center over center
stroke to make a sphere and it is not getting better. They will say that
after some number of hours it might be getting worse.




-----Original Message-----
From: Thomas Janstrom [mailto:thomas@moiler.com] 
Sent: Wednesday, January 02, 2008 8:41 PM
To: 'Jerry'; 'atm_list'
Subject: RE: [ATM] Not tde or tue, important oblate spheroid, ellipsoid

Um, as I was the one who first posted that response, let me elaborate....

The CoC stroke I use is 1/3 D with no over hang on the back stroke (i.e.
there will be 2/3 of the mirror radius over hanging on the "side" away from
me at the end of the stroke, but on the return I stop before there is any
over hang) and it is dead centre over centre. This always gives me a great
sphere that is very smooth. I will say that the stroke length is varied a
little all the time (this is due to my not being a machine), but the average
is 1/3 of the mirror diameter.

Also I work seated on a bench with the grinding platform in from of me, I
rotate tool and mirror every 6-10 strokes in opposition to each other a
random amount....

Cheers, Thomas.

Sorry about the delay, but I had to check with my grinding/polishing (one
book per mirror) logs to make sure I wasn't spouting the proverbial.....



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