Hello, I’m Jim Salsbury with Mitutoyo America Corporation
and welcome to the Metrology Training Lab.
In past episodes, we have defined the concept of calibration
and also discussed the purpose of calibration.
In this episode, we’re going to talk about what makes a good calibration method.
Calibration plays an important role in establishing traceability of measurements
back to international definitions, like the meter or the inch.
But when it comes to measuring instruments, like this micrometer,
or this caliper,
or this indicator,
or these gage blocks, calibration is really about a search for errors.
We want to develop a calibration method that efficiently hunts down
and finds any potential errors in the measuring instrument.
A good calibration mitigates the risk of the measuring instruments
not performing as accurately as expected or required,
but calibration takes time and costs money.
And since different organizations have different thresholds
of acceptable risk, there’s really no perfect or best calibration method.
A good calibration method will quickly search for errors
and give a sufficient likelihood that the measuring equipment
is either good or bad for its intended purpose.
A good calibration method is a well-engineered test
that finds the optimal balance between cost and risk.
So what does a good calibration method look like?
For this caliper – micrometer – indicator – gage block – even this surface plate?
We get these questions a lot here at the metrology training lab,
and we have students come here all the time to learn good methods.
Now, I’m a big fan of not reinventing the wheel
and a lot of smart people have already thought about these calibration methods.
In fact, there are American National Standards
that address the calibration of all these items.
For not too much money,
you can get some pretty awesome guidance documents in this area.
We know that here at Mitutoyo America, because we use these standards,
and we also help contribute to developing them.
In the United States, the American Society of Mechanical Engineers
or ASME, has the responsibility
for developing American National Standards in the dimensional measurement area.
That work is managed by a committee called ASME B89,
which is comprised of experts from a variety of organizations,
including end-users, equipment manufacturers, calibration
providers, distributors, and government agencies.
In later episodes of the metrology training lab,
we will cover some calibration methods in detail,
but let me give you some insights, based on the appropriate ASME B89 standards.
The following are often done wrong in calibration
but are covered quite nicely in the standards.
For dial indicators,
there is ASME B89.1.10,
and one of the topics in that standard
is about properly checking the hysteresis of the indication,
which is the error that occurs when you approach the same position
from different directions.
As we just showed here at the 9 o’clock position.
For gage blocks,
the ASME B89.1.9 standard
discusses the importance of checking the variation in length
by calibrating the gage block at the gage point
and at all four corners.
All five points must be in tolerance
to state conformance to the grade.
For outside micrometers, the ASME B89.1.13 standard
recommends five specific size gage blocks for calibration,
and these gage blocks are not commonly available sizes in a standard set of gage blocks.
The reason for these specific sizes is that they will force the micrometer thimble
to be in different orientations for each test point.
The goal is to have an efficient test
that looks for errors across the measuring range
simultaneously with errors within a rotation of the thimble.
For calipers, the ASME B89.1.14 standard
discusses the need to check the internal measuring jaws
with a small ring gage around 0.2 inches in diameter.
This test is specifically designed to test for the additional error
that can occur due to the crossed knife-edge measuring faces.
Lastly, on surface plates, the ASME B89.3.7 standard
discusses the importance of the two different flatness
specifications for surface plates, one being a local flatness,
often called the repeat reading, and the other being the overall flatness,
which covers the much larger area of the entire surface plate.
A proper calibration needs to address both flatness specifications.
Unfortunately, there is confusion in what makes a good calibration method.
You really need to understand the technology inside
the measuring equipment to develop the calibration method.
Or you can take advantage of the appropriate documented standard,
which has already done all that work for you.
In future episodes of the metrology training lab, we will go into more detail
on the calibration of these specific measuring instruments, but for now,
I hope you’ve gained some insight into what makes a good calibration method
and the power of the available standards.
Thank you, I’m Jim Salsbury, and I’ll see you next time
from the Metrology Training Lab.