Re: [ATM] Commercially available spherometer with MitutoyoDigimatic Indicator

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Terin Lyr D'Amico wrote:

>I wanted to write a little about "why" I want a good, precision spherometer,
>and due to length, separated it from the previous email, for those
>interested in reading such a long diatribe.
>
>Anyway, I like taking multiple readings of the surface with a spherometer
>between wets.  I also like the thought of a HIGHLY accurate spherometer
>because the added precision will give me a better feeling about the readings
>I'll obtain.  I may not need precision down to 50 millionths during fine
>grinding, but it would be nice to know I'm very accurately reading +/-
>.0001.  I can then get concerned about getting down to that 50 millionths of
>an inch as I get to the finer grits.  Finally, the appeal of the digital
>indicator is in the fact that you can zero digitally with the touch of
>button; as long as you have a good flat reference.
>
I think I should recommend that, when choosing your shop, be sure
that you find one that has the ability to measure the results.
Even if the positioning of the posts is only accurate to 0.001",
you can still get them measured more accurately.  While the
symmetry of the tool is gone, you can remove several sources of
error by going from the measurements rather than the design;
e.g.: use the inscribed circle radius formula in the final
equation, even if it isn't, say, 3.0000".  Of course, a lot of
this depends on the reproducibility of the positioning of the
feet.

My $0.02...
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