Subject: Calipers vs. Micrometers

From: [email-masked] (Rod Barman)
Date: 13 Dec 1993 00:16:20 GMT

I have few questions regarding micrometers and calipers.  I understand
that micrometers are more accurate than calipers, but I wondering if
the electronics now used has changed this to some extent.  From what
I've seen micrometers are accurate to about one ten-thou and the new
digital calipers are rated about the same.  Is this true or marketing
hype?  I interested mostly in automotive rebuilding and I'm wondering
if I should buy fancy digital calipers or a set of micrometers.  The
calipers would be cheaper and I would perfer to have only one tool.

Thanks --rod.
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Rod Barman, IRIS NCE @ Laboratory for Computational Intelligence, 
University of British Columbia
[email-masked]

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From: [email-masked] (Tony Poole)
Date: 12 Dec 1993 21:00:50 -0500

>calipers would be cheaper and I would perfer to have only one tool.

While digital calipers may, by their readouts solely, have one thou
increments, you shouldn't judge the instruments accuracy solely on that.
In a perfect world, where surfaces are always square and parallel, your
hands don't shake, you apply the same amount of pressure to the caliper, etc
etc - a caliper would be as accurate as a micrometer.

However, nothing works like this.  A micrometer, just because of the nature
of what it is, is more repeatedly accurate.  It's a more solid beast.  Most
have a mechanism for constant pressure (the thimble).  Micrometers have to
absolutely solid parallel surfaces, while calipers have the same -
theoretically.  In reality, calipers flex - the contact surfaces become
unparallel, etc...

Spend the money for a decent set of 0-6 mics....you'll be happy you did, as
long as you want repeated accurate measurements.  Save the calipers for
taking reference dimensions....

-- 

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Tony Poole  Traverse City, MI USA  [email-masked]  [email-masked]

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From: Doug White <[email-masked]>
Date: Mon, 13 Dec 93 09:23:01 -0500

> calipers would be cheaper and I would perfer to have only one tool.

This mainly depends on what sort of tolerances you need to measure to.
The better modern electronic dial calipers are rated to .0005".  When
used with care, I would trust them to read reliably to .001".  I just
bought a Mitutoyo electronic micrometer that is rated to 0.00005" (That's
50 millionths!), but you can't expect to get that without a micrometer stand
and some practice.  The caliper is faster, easier to use (within its
accuracy limitations), and can cover a wide range with one tool.  I'm
not that familiar with auto-rebuilding, so I don't know what sort of
tolerances you will run into.  Anything involving precision 'fits'
(press fits, sliding fits, shrink fits) will probably require more accuracy
than you can get with a caliper.

Whatever you get, I would avoid the cheap imports.  A measuring tool like
this will last a lifetime.  Twenty years down the road, you'll never
miss the extra $20 it costs to buy a quality tool, but if you buy less
than the best, you'll still be living with its quirks and inaccuracy.
Brown and Sharp, Starrett, and Mitutoyo measuring tools go on sale all
the time in a dozen different mail order catalogs, so you can usually
get a good deal if you shop around.

Doug White
MIT Lincoln Laboratory