Hello, I'm Jim Salsbury with Mitutoyo America Corporation and welcome to the Metrology Training
Lab
In this episode, we're going to talk about the accuracy and calibration of mechanical
outside micrometers.
In past episodes, we have covered general calibration concepts and specifically discussed
the calibration of outside micrometers.
However, we used a digital micrometer for all the examples and some people have been
asking about mechanical mics.
In particular, I want to discuss mechanical mics with no vernier scale.
First let's be clear about some of the differences in products.
This is the most common mic you'll generally see.
This digital model allows you to easily measure in either inches or millimeters.
The physical design of this model is in inches and the analog scales on the thimble and sleeve
can optionally be used.
The intent is to use the digital readout, which most people do, but on some of the newer
models, like this faster QuantuMike, you can only use the digital readout.
As you can see, the analog scale is not designed to be used for measurement.
The traditional mechanical micrometer, which has been around for over 100 years, offers
the option of a vernier scale.
In inches, the graduations on the thimble are in thousandths of an inch.
This model includes the vernier scale on the sleeve, which allows direct measurements to
a tenth thousandths or a tenth.
A metric micrometer is similar, with graduations of 10 microns on the thimble, and 1 micron
with the vernier.
The specified accuracy of all these micrometers is about the same.
In particular, on the mechanical micrometers, whether or not there is a vernier, the accuracy
is ± 0.0001” or ± 2 µm.
Let me repeat that - on this tool, with a graduation of 0.001”, the accuracy is one-tenth
of that.
Is that right?
Now for digital instruments, you can make the general statement that the accuracy can
never be smaller than the resolution - you can't measure more accurately than the smallest
deviation the instrument can see.
So what is with this mechanical mic - are we saying that you can read between the lines
to a tenth of a graduation?
Let me read you something.
Now I've been able to find many answers to perplexing problems by digging into the history,
and I like metrology history.
So I was reading an old US military standard on gage inspection, called MIL-STD-120, which
was published in 1950 - back before digital tools were commonly used, and I want to share
a sentence from that.
Here it is:
Most micrometer calipers - that's what these were called in 1950 - micrometer calipers
- that name has disappeared from use these days.
Let me continue…
Most micrometer calipers are graduated to read to thousandths of an inch - ok, right,
that hasn't changed in a century - ok let me finish.
Most micrometer calipers are graduated to read to thousandths of an inch and may be
read to ten-thousandths by estimating tenths of the thimble graduations.
So there you have it.
According to best practice in the US military in 1950, a practice which is still used in
industry today, on a micrometer, go ahead and subdivide a graduation into tenths.
Read to a tenth of a graduation.
You need good eyes, and if you question this, it's pretty easy to run a test to see how
good you are.
Let me show you how.
We have a mechanical mic here, in inches, with the vernier.
Again, the only difference between these two mics
is the vernier.
We can use the one with the vernier to see how well we can
read between the lines on the one without the vernier.
We just put the mic in any position, read between the lines, and then check ourselves
with the vernier.
We can do this same experiment with a digital micrometer, if you happen to have the type
that has both analog and digital readout, like this common style here.
Since the digital mic can read to 50 millionths you can do this test with even greater accuracy.
The only hitch is that you have to carefully set the digital zero.
Let me show you how.
When we're calibrating micrometers, and trying to decide if they are in or out of tolerance,
there is another very powerful trick you need to know.
I honestly don't know if this works for all micrometers, but it
does for Mitutoyo models.
The width of the scale lines on the sleeve and thimble are specifically designed to help
you make higher accuracy measurements without having to read between the lines.
As shown in this figure, on the metric models, the width of the scales lines are equivalent
to 2 microns when measuring.
You can measure errors during calibration of 1 or 2 microns by just looking at the amount
of the overlap between the index line on the sleeve and scale line on the thimble.
When they overlap about 50%, that is 1 micron.
When they are right at the edge of other, that is 2 microns.
And when you start to see a gap forming, you know the error is larger than 2 microns, which
happens to be the Mitutoyo tolerance for many of the smaller size mics.
So in calibration, in many cases, you don't have to read between the lines.
You can just look to see if a gap is forming or not.
No gap, it's in tolerance.
That's a pretty useful trick when trying to check a tolerance of 2 microns of 0.0001”.
Not unlike the old MIL-STD-120, this little trick has been published in Mitutoyo literature
for decades, but with the popularity of digital tools, it seems many people have forgotten
about it.
I find it to be pretty cool that documents published in 1950 are still teaching us things
today.
Thank you, I'm Jim Salsbury, and I'll see you next time from the Metrology Training
Lab.