Hello, I'm Jim Salsbury with Mitutoyo America Corporation
and welcome to the Metrology Training Lab.
In this episode, we're going to discuss the calibration of the high accuracy
Mitutoyo MDH outside micrometer.
In past episodes, we've discussed the general calibration
of outside micrometers, but the MDH is more than twice as accurate
as any other micrometer and therefore requires
some careful reconsideration of the calibration method.
So, step 1, let me show you what I'm talking about.
This is the common digital outside micrometer with a resolution
of 50 millionths of an inch or one micrometer.
As you can see here, we have five decimal places.
That's 50 millionths.
Now this is the MDH micrometer,
which may look the same as any other outside micrometer.
But there are a number of critical differences.
We have a much more enlarged and rigid frame here,
as well as improved measuring force.
You couple those with an improved resolution 10 times better.
This is a digital resolution of five millionths or point one micron.
Again, this is 50 millionths, five decimal places.
This is 5 millionths, six decimal places, so ten times better resolution.
And the MDH between all of those different design features leaves it
to have an improved accuracy of just 20 millionths
of an inch or half a micron.
The MDH is an outside micrometer,
can't you just calibrate it like any other outside micrometer?
In our calibration lab, our calibration technicians do use the same reference
standards to calibrate the MDH, just like any other micrometer,
and from a distance, the calibration method looks pretty identical.
But there is one critical little difference that’s
so very important to understand.
If you watched our episode on the calibration
of outside micrometers, you may recognize these gage blocks.
These are the same gage blocks that we use, and these are the ones
recommended in the American National Standard for micrometers, ASME B89.1.13.
For calibrating the MDH, we use the exact same 5 gage blocks.
In our micrometer episode,
we recommended using Grade 0 gage blocks for calibrating micrometers.
If you watched our episode on measurement quality requirements,
where we discussed T.A.R. and T.U.R., sometimes called Tar and Tur,
we discussed those rules, you learned the reason for using
Grade 0 gage blocks when calibrating micrometers
is to ensure sufficient calibration capability
when checking the tolerance of the micrometer.
For the MDH, being so much more accurate,
the capability of our calibration method is no longer good enough,
we need to change something with our reference standards.
This problem hit our calibration lab when the first MDH came in for calibration.
We found three possible solutions.
The first one’s easy, just get better reference standards.
You could just buy Grade 00 gage blocks, not Grage 0,
but Grade 00 gage blocks and the problem is solved.
So we thought about that, but we really didn't want to spend money
and we didn’t want to introduce new standards into our quality system.
So we looked for some alternative solutions.
If you look at the MDH, you will notice that the measuring faces
are about half the diameter of most other micrometers.
We realized we could take advantage of this,
and measure the gage blocks at the central gaging point.
This point has a calibrated value, given on the calibration certificate,
and using this calibrated value is generally much more accurate
than using the nominal value that’s marked on the gage block.
So that's the second solution, just use the calibrated value,
not the nominal value, for the gage blocks you already own.
We started implementing that process here at Mitutoyo America
in our calibration lab, when we realized something about our gage blocks,
which led to a third solution, which is the one we use today.
Now gage block tolerances apply
across the entire surface of the gage block.
When they are calibrated, if that’s done correctly,
the length is checked at multiple points, usually in the center
and in all the corners and all of those points must be in tolerance.
Now for that reason, Grade 00 gage blocks
are challenging to manufacture and can be quite expensive.
But what we noticed is that if we look at the center point alone,
our Grade 0 gage blocks actually meet the tolerance
for Grade 00 gage blocks at this point.
So if we measure with these smaller measuring faces on the MDH 25,
at only the center point, as you can see here, we can use the gage
block nominal value just like when calibrating any other micrometer.
Now, I can't promise this will work for you - our calibration history
of our gage blocks support this method, and so we are comfortable with it.
If you don't want to mess with worrying about these issues,
just get some Grade 00 gage blocks for calibrating the MDH micrometer.
And one last thing, when you start calibrating more accurate instruments,
like this MDH micrometer, you may need to look for a new calibration
provider for your gage blocks who can provide better measurement uncertainty.
Be careful with that.
Alright, that's everything for this episode.
Thank you, I'm Jim Salsbury, and I'll see you next time
from the Metrology Training Lab.