Jim B.,
Thanks for the background. I was particularly interested in those smaller
caliper and the fact that the legs were round. I assume you are talking
about the tips. In the version of the article I just put up, I added a
section about the error incurred by using squared off end inside calipers. I
do see a few on eBay.
Rick
-----Original Message-----
From: gingery_machines@yahoogroups.com
[mailto:gingery_machines@yahoogroups.com] On Behalf Of Jim Barnes
Sent: Tuesday, February 01, 2011 11:05 AM
To: gingery_machines@yahoogroups.com
Subject: [gingery_machines] Re: new article: The Intersection of Art and
Precision
Rick,
Thanks for posting the article. I recently picked up a set of Starrett
Toolmaker inside and outside calipers--these are much smaller than the ones
in your article. They are probably no more than 4" in overall length. The
small size is one of the main reasons I got them. They are alot easier to
use than 6" dial calipers. I also have a set of spacer blocks to practice
with and to set the calipers to a specific width. For practice the drill is
to put the outside calipers on the blocks, then transfer the measurement to
the inside calipers, and then measure the result with a micrometer. Or, in
reverse from mic to inside to outside to blocks. It is a bit of an
art--getting the alignment right, just the right amount of feel, and
measuring with the mic without actually compressing the calipers. And with
all due respect to Dave Patterson, it's my understanding that using calipers
with a ruler died out at the turn of the last century. I won't claim any
personal knowledge but I was told that they were usually used with gauge
blocks, set to the desired dimension and then used to check the progress of
machining. Certainly makers sense to me. Since I'm a hobbyist I don't have
to work to any greater precision than .001" +/- some tolerance. When I
started using dial calipers (Helios, with very sharp needle points) about 25
years ago to measure dental pathologies, I was working to hundredths of a
millimeter (0.00039"). We did cross comparisons for repeatability. Once you
get to a certain point, the inter- and intraobserver error starts to wash
out any alleged gain in precision. So, I guess what I'm trying to say is
that unless you have a CMM, you may not get all that much precision from
instruments that allegedly deliver it. The person using the instrument is
part of the instrument.
Again, thanks for the article and if you run across a set of the dainty,
round-legged Starrett calipers, try 'em out.
Jim B.
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