Re: [ATM] Dumb Question: how to measure a...

[Kevin MIchael Zabbo <chaosopher23@yahoo.com>]

Perfect circularity doesn't exist, especially in
glass.  If you can get a hunk of glass circular within
1/64" you are doing well.  The exact center isn't
needed.  It's blocked by a secondary mirror.  The
telescope adjustments themselves offset this
centerline quite a bit, and if you can get the
centerline within 1/32" you're doing something
excellent.

But to find the center of a round thing, you need a
dividing square.  This is a square with a rule
extending at the 45 degree mark.  Use a permanent
marker first on several points, then scribe a line
with a long straight pin on those marks.  Straight
pins are usually nickel plated brass and won't scratch
the glass.  You will discover how out-of-round the
glass is when you have a crisscross pattern in the
center of the glass that's about a millimeter or two
across.  There's your center, somewhere in that
circle.

Kevin's Law of Precision:
Measure with a micrometer.  Mark it with chalk or
permanent marker.  Cut it with an axe.

Kevin of Eastern Iowa
Seeker of the Darkness

--- Mutalib Abdallah <omegatroid@hotmail.com> wrote:

> Circle.
> 
> Given a circle of glass, how can I measure it to
> characterize its 
> circularity?   Assume I do not know where on the
> glass is the "center", if, 
> in fact a center exists.
> 
> The only way I can thing of is to measure the
> coordinates of a large number 
> of edge points, copy the data into a computer file,
> and then do a curve fit 
> to find the circle of best fit (either rectangular
> or polar coordinates).   
> Or maybe a truncated fourier series (which is a
> least squares best fit).
> 
> Another way might be to measure the diameter at many
> points, but I know that 
> there are non-circular forms with constant diameter.
> 
> Suppose I was writing a specification-- how would I
> specify roundness?  Is 
> there some way to just measure it directly?
> 
> I ask this because a friend is making round looking
> mirror blanks on a jig 
> that geometrically should not work.  I want to
> measure the error of his 
> ways.
> 
> Ideas?
> 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


I published my first novel!  You can get it here, from the publisher!  
http://www.publishamerica.com/greetingcardpro/pickup.asp?CardSentID=126045&rID=48283



Their search stringbox is on the right side about halfway down.

search using my name:  kevin michael zabbo
and it will take you right to my book page.

The title is anna's blood, which also takes you there.

Please order your copy soon!  I love you!

__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 
_______________________________________________
ATM mailing list http://www.atmlist.net/