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

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

There is something called a "Reuleaux triangle". See
http://mathworld.wolfram.com/ReuleauxTriangle.html
for a nice demo of that.
Guy

Woodchuck <djv@bedford.net> wrote: On Tue, 30 May 2006, Dale Eason wrote:

>
> The definition of a circle is all points equal radius
> from a point.  Thus any diameter should be the same
> within a tolerance you pick and can measure.
>
> I know of no other geometry that has that attribute.
> Can you give me one?

There are many such figures.  A sort of fat, inflated triangle
is one that pops to mind.  They do not have "centers" but they
roll smoothly.

This became a practical matter in WW-II when making submarine
pressure hulls.  The wrong cross-sections do not have the same happy
properties of strength as a circle.  Instead of measuring diameters,
the shipbuilders turned to using large circular templates.

To specify/test a circle, mount it on a turntable.  Center it
by reducing runout to a minimum, conveniently measured with a dial gauge.
Record the runout (the minimum of (Max-min) of the dial gauge) as
the circularity.  This is the same drill as mounting work in a 4-jaw
chuck on a lathe or milling machine

Dave

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Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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