On 2010-02-20, Christopher Tidy wrote:
Hi folks,

Can anyone tell me what the typical accuracy of a decent pair of vernier
calipers is (when used carefully and correctly)?

How new are the calipers? I would expect 0.001" accuracy, if
zeroed properly, if the gibs are set right so the head does not rotate,
and if you have sufficient light and magnification to read the vernier
properly.

I'm assuming that you are not calling dial calipers "vernier".
Just to be sure, I'll cover them too.

Digital calipers typically have a resolution of 0.0005", but
check the actual accuracy against gauge blocks -- in particular at the
size you want to set.

Dial calipers can have cyclic errors a function of the gear
pinion which engages the rack.

I just bought a dial
bore gauge and am wondering if I can reasonably set the zero point with
vernier calipers instead of having to buy a 3" to 4" micrometer. The
calipers are Etalon brand, heavy and made in Switzerland. The bore gauge
dial is graduated down to 0.0005".

Ideally, it should be set to a ring gauge, not a micrometer.
These will typically be accurate to 0.000050". You really want the
standard to be more accurate than what is being set.

If you use a 4-5" micrometer, you will need a 4" standard to
zero it before you use it. (And I'm assuming that the micrometer has a
tenths vernier around the barrel so you can set it to 0.0001".

Also -- you really don't want to hold the micrometer's C-frame,
as it can expand with heat from the hand. Put it in a micrometer bench
clamp so you can set it and not worry about it drifting as it cools off.

I'm not reboring an engine, I just
want an idea of the wear, so I don't need the highest accuracy. From
using these calipers, my own guess is that they're probably good to
0.002", but some of that might be my errors in reading them. I'd be
interested to hear what other people think.

A lot of the variation in calipers comes from a loose fit of the
head to the bar, allowing it to tilt when pressure is put on the jaw
tips. If you have it pretty snug, you can keep this from being a
problem. And the closer to the bar you do the measurement, the less the
problem from a tilting head and jaws.

Enjoy,
DoN.

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