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1 inch = 1 inch = 1 inch

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With a conventional mechanical micrometer, like the ones shown, while the zero point with the anvil closed can be set up by the operator using the small 'C' spanners usually provided with the instrument,  the ultimate accuracy along it's operational measuring length is determined by the pitch accuracy of the lead screw in the barrel, there is usually means of tightening the nut to lead screw pressure with an adjustable collet arrangement on the nut. If you are attempting to work to tenths of a thousandths of an inch, ambient temperature will make a small difference to accuracy.

I have this fairly ancient 'Draper' 0-4" caliper mic. set that has the replaceable anvils and the set up test pieces, which specify length accuracy at 20 degrees C.

Draper micrometer.jpg

Edited by JohnD

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Just did some tests on my Roebuck digital caliper, using the Draper test pieces....

For each test I closed the caliper first and checked the 0.000" reading then opened it up and placed the test piece in and closed it till it was lightly clamping it...

1" Test.... spot on


2" Test....minus one thou


3" Test....minus 5 tenths of a thou

Ambient temperature 28C

Quite pleased with this :thumbsu:



Roebuck caliper 1 inch test.jpg

Roebuck caliper 2 inch test.jpg

Roebuck caliper 3 inch test.jpg

Edited by JohnD

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Great thread.  Reminds me of the "What's heavier?" thing.  Pound of feathers or gold. 

FWIW, I've always been partial to analog instruments.  It's probably personal, but I feel more in control, and know when it's zeroed. 

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I literally just watched this comparison video the other day between watching videos on casemaking.. I'd have liked to have seen a comparison done on both sets of jaws as internal measurement is often less nice than external. The cheap measuring tools are good enough for their purpose.

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