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Re: Micrometer responses

To: (Dave Williams)
Subject: Re: Micrometer responses
From: Berry Kercheval <>
Date: Tue, 17 Dec 1996 19:33:39 PST
>>>Dave Williams said:
 > -> They only measure at two points, and you can
 > -> miss an out-of round condition.
 >  How else are you going to find an out-of-round condition?  You turn the
 > guage 45 or 90 degrees and measure that way.  The difference tells you
 > out of round.

There are an infinite number of figures with constant width that are NOT 
The simplest to describe is a sort of rounded triangle: take an equilateral 
triangle, set a compass to the length of the side, place the pin in each 
vertex in turn and draw an arc connecting the other two vertices.  When you 
done, the three arcs thus drawn form a figure of constant diameter -- you can 
make rollers with this cross-section, place boards on them and roll along.  
boards will NOT go up and down but will roll along smoothly.

Other examples are left as an exercise.  Show your work for full credit.

The T-gauge will not detect this kind of out of roundness.

Whether this is of practial importance is another question, because I don't 
think worn cylinders achieve this sort of shape.  Certainly measuring at 
several angles will detect an OVAL cylinder, the most likely (I think) shape 
a worn engine, but it won't detect ALL (in the strict mathematical sense) 
out-of roundness.

I could be wrong; I haven't measured a lot of engines.


Berry Kercheval :: :: Xerox Palo Alto Research Center

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