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Doublereed Archive - Posting 000078.txt from 2008/02

From: David Morley <dmorley@-----.net>
Subj: [DR-L] micrometers
Date: Thu, 14 Feb 2008 13:56:23 -0500

Well, it's a day for Davids. Mr. Crispin is correct in asking for
more information about what you've got. If it is a sliding jaw device,
it is a caliper (either Vernier, dial, or digital - dependent upon how
it displays its motion). A Dial Indicator has a sensing barb that moves
in and out of the dial body, and the whole thing is usually mounted on a
base block with a rod running vertically. These too can be mechanical
dials, or digital. No idea what a brass jobs with a screw doo-hicky
is. A micrometer will have a fixed jaw, and a movable jaw that is
driven by turning the end of the spindle held in a c-shaped frame.
If your two micrometers are showing a constant 0.1 mm error when
measuring a variety of objects, I'd suspect one (or both)of them is not
zero'ed properly. Screw them down to a closed position (they should have
a smaller knob at the end of the spindle that ratchets - clicks - so you
don't over-tighten the micrometer. They should indicate zero. Zeroing a
digital is simple - there will be a button to press. Zeroing a
mechanical (Vernier) is a little trickier, and usually involves
unlocking the spindle with a small wrench that rather looks like a
distorted tuning fork - should be in the box that the mike came in.
For a very quick reference, a US penny is measuring 19.00 to 19.02
diameter, and 1.39-1.40 thick at Mr Lincoln's ear. Feeler gauges are
okay, but thin, and stacking them up can introduce error. Gauge blocks
are probably the best reference, but may be hard to find without
visiting a Tool and Die shop. Dowel pins are easy to find, and usually
very accurate, and would do for a reference. Hope this helps.
David Morley / Adam Shaper Tips Ltd

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