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

[Pete <lenses@adelphia.net>]

Hi Mutalib,

Not a dumb question but a very interesting problem.
The easy answer, depends on how round you want?
Pick the number the part roundness can be defined by.
I have seen parts measured on an interferometer optically to within .5 
microns of roundness.
If you look closely at a round part (metal) cut with an end mill you can 
see small flats.
You would have a very hard time trying to measure these flats even if 
you would need a microscope to determine their existence.
The other part of this easy answer is if you can measure with the 
available instruments to your satisfaction the part is round and it will 
work for the intended application you have established also the part 
roundness specification.
If I needed and wanted a part within 0.1 microns of roundness then even 
the optical measurement listed above won't satisfy my needs and I must 
look further or invent something to measure and also make what I want.

Hope this is helpful,

Peter Nance, Jr.
lenses@adelphia.net

Mutalib Abdallah 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?
>
>
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