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

["Richard Ozer" <rozer@pacbell.net>]

Very interesting question...

One way to approach the problem is to use a compass and a straight-edge. Do 
this on the blank itself.

"If you have a straight edge (unmarked ruler) and a compass, it is easy
to locate the center of a circle. Draw any line L through the circle,
so it intersects the circle at A and B.  Then with the compass, find
the perpendicular bisector of AB, which will intersect the circle at C
and D. Clearly, CD is a diameter, so if you bisect CD at point O, O is
the center of the circle."

Then you can make a series of measurements from O to the edge and see if 
they are identical.

If the jig is way off, it will be immediately apparent.  If you are 
measuring microns, then this method
is too rough I think...

RO

----- Original Message ----- 
From: "Mutalib Abdallah" <omegatroid@hotmail.com>
To: <atm@atmlist.net>
Sent: Tuesday, May 30, 2006 1:00 PM
Subject: [ATM] Dumb Question: how to measure a...


> 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?
>
>
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