Measuring customer parts on a CMM - How many decimals to report to the customer?

[dwperron]

We use ISO 17025 calibrated glass scales in the metrology business, so this is not a crazy situation.

As for reporting the results, you can't just report 50.0007 when your instrument has a 0.002 resolution. You will need to state that the results are an average of 3 readings. You will also need to include the resolution of the CMM and the repeatability / reproducibility of your results in your uncertainty budget, so these numbers will get accounted for.

 

[Eredhel]

I think I see the miscommunication on what I was saying. I didn’t mean he fell under that specific API requirement. I just used it as an example of why some of us might be asking seemingly irrelevant questions before we give advice. Especially when the requirements someone does fall under may not be clear.

 

[Jim Wynne]

We use ISO 17025 calibrated glass scales in the metrology business, so this is not a crazy situation.

In the situation of your glass scales, is the precise location of the graduations unimportant? It apparently is in the OP's situation.

 

[greif]

I think we can agree that what NIST did to my master scale was calibration. They did the following:
1)Placed it on a video measuring machine (Moore M48)
2) Measured it multiple times
3) Reported the results to me

What I plan to do for my customer;
1)Place scale on my CMM and measure with a microscope.
2) Measured it multiple times (maybe not multiple?)
3) Reported the results to customer.

Why is one different from the other?

I point out that the Moore M48 used by NIST is not a "standard". It is measuring equipment.
I use a "standard" to calibrate my CMM.
NIST uses several standards to calibrate their MOORE M48.

By the way- thanks for a lively discussion!

 

[Ninja]

I think we can agree that what NIST did to my master scale was calibration.

We cannot agree with this statement...assuming that I am included in "we".

You had a thing with spacings.
Someone measured the spacings.
The thing was measured.
The thing was certified as having XXX spacings.

Your customer wants you to measure something else.
Go for it.

Your customer wants you to have ISO 17025. Great, go for it.
Your customer wants a certificate stating that a company with ISO 17025 cert measured it. Great go for it.
Your customer can then claim whatever they want. You should stop right there.

Things do get rather murky with things like a glass scale, since "adjustment" is a non-starter...so I guess you could call it "calibration" if you wanted... but it is a stretch of the term....but I've seen way worse.
Each time you use a master to center equipment, then use that equipment to measure another master, then use that secondary master to center equipment............on and on.....
I haven't seen anything on this thread that references the compounded uncertainty... just sayin'

If the customer wants a cal cert...there is no way you can reference more decimals than you measure....period.

 

[greif]

greif says: I think we can agree that what NIST did to my master scale was calibration.
Ninja says: We cannot agree with this statement...assuming that I am included in "we".

I have a document from NIST , titled "Report of Calibration" for the scale measurement that they did. Your opinions differs from this "proof" I have from NIST that the measuring of a scale and reporting the measurement is a calibration. On what do you base your opinion?



This document has a precise definition of calibration as used for metrology:
International vocabulary of
metrology – Basic and general
concepts and associated terms
(VIM

https://www.bipm.org/utils/common/documents/jcgm/JCGM_200_2012.pdf

2.39 (6.11)
calibration
operation that, under specified conditions, in a first
step, establishes a relation between the quantity
values with measurement uncertainties provided
by measurement standards and corresponding
indications with associated measurement uncertainties and, in a second step, uses this information
to establish a relation for obtaining a measurement
result from an indication
NOTE 1 A calibration may be expressed by a statement, calibration function, calibration diagram, calibration curve, or calibration table. In some cases, it may
consist of an additive or multiplicative correction of the
indication with associated measurement uncertainty.
NOTE 2 Calibration should not be confused with
adjustment of a measuring system, often mistakenly
called “self-calibration”, nor with verification of calibration
NOTE 3 Often, the first step alone in the above definition is perceived as being calibration

 

[dwperron]

First, let's define what a "calibration" is according to ISO 17025 - not the masses.

17025 references ISO Guide 99 International vocabulary of metrology, lovingly referred to as the VIM, for definition of terms.
The VIM defines calibration as :
"operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication"
Nothing about adjustments, nothing about moving devices, nothing about non-adjustable equipment....

The definition is followed by an explanatory note:
"NOTE 1 A calibration may be expressed by a statement, calibration function, calibration diagram, calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of the indication with associated measurement uncertainty."
You would be making measurements to establish a relationship between the quantity values and measurement standards - measuring your devices with a calibrated instrument - and providing a calibration statement, with an additive correction of the indication with associated measurement uncertainty. That's a calibration according to the definition.

You are establishing traceability to NIST, or another national lab, by using equipment that has received 17025 accredited calibrations with calibration uncertainties. You would be providing your "quantity values" of the customer device along with your measurement uncertainty. Accredited calibration labs do this all of the time.

Making a pass / fail determination is optional for 17025, that is between you and your customer. You can just supply the results and uncertainties.

 

[greif]

We agree on the definition of calibration.

 

[dwperron]

We agree on the definition of calibration.

Absolutely.
Amazing, I was writing my response while you posted yours and we came to the same conclusion.