MSA Study Types - How to make an informed decision about a gauge

A

aproddutoor

#11
Hi,

Since everyone is talking about Gage R&R, I have a question. I heard that the parts need to represent the entire range of possible parts. So should I manually select the parts by measuring it to make sure it represents the entire range of the process? or should I randomly select parts from the process which is collected over different days and shifts? What is the right approach? Why?

Thanks
 
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bobdoering

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#12
Re: Gage R&R (GR&R) Brain Teaser

Manually select the parts by measuring it to make sure it represents the entire range of the process. It is also handy to have some parts just beyond the process limits.

Why?

Because it tests the ability of the gage to detect correctly the measured value throughout its intended use. The concept is not a process capability study - it is a gage study. Randomness is not the point - difference is (to capture "between" variation) If the samples measured are narrower than the process variation, you will not know for sure that any product that might occur outside those specimen values will be detected correctly. Also, as products reach dimensional limits, there may be physical characteristics that affect the gage or user (e.g. more cantilevered load from a 'larger' part)

Now, you might have to run several shifts or days to collect those samples - or force the process to make them for the study.
 
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Hershal

Metrologist-Auditor
Staff member
Super Moderator
#13
One other point, you mention that the gage is calibrated regularly.

What is the measurement uncertainty of its last calibration for the readings you need? Without that information you do not have sufficient input regarding the error contributed by the gage itself.
 

Miner

Forum Moderator
Staff member
Admin
#14
Hi,

Since everyone is talking about Gage R&R, I have a question. I heard that the parts need to represent the entire range of possible parts. So should I manually select the parts by measuring it to make sure it represents the entire range of the process? or should I randomly select parts from the process which is collected over different days and shifts? What is the right approach? Why?

Thanks
What is the purpose of the gage (inspection or process control)?

For inspection, selection of samples is not critical. Many will recommend that you select from the entire range of tolerance or the entire range of measurement. If you calibrate the entire range (thus checking linearity), this is not really necessary, but doe not hurt if you want to do it.

For process control, the variation (StdDev) of the parts must represent the variation (StdDev) of the process. The best way to do this is to randomly select parts collected over different days and shifts. The other method may artificially inflate the variation.
 

Miner

Forum Moderator
Staff member
Admin
#15
Re: Gage R&R (GR&R) Brain Teaser

Manually select the parts by measuring it to make sure it represents the entire range of the process. It is also handy to have some parts just beyond the process limits.

Why?

Because it tests the ability of the gage to detect correctly the measured value throughout its intended use. The concept is not a process capability study - it is a gage study. Randomness is not the point - difference is (to capture "between" variation) If the samples measured are narrower than the process variation, you will not know for sure that any product that might occur outside those specimen values will be detected correctly. Also, as products reach dimensional limits, there may be physical characteristics that affect the gage or user (e.g. more cantilevered load from a 'larger' part)

Now, you might have to run several shifts or days to collect those samples - or force the process to make them for the study.
This is okay for gages used for inspection only. However, this is the wrong approach for gages used for process control. It could inflate the variation and make the gage look better than it truly is.

See my post immediately above.
 
A

aproddutoor

#16
Thanks Miner for your reply. Can you provide a reference to support your answer? I'm just asking you because it's not written anywhere. I was confused between the two approaches.

The reason why I asked this question was because I wasn't getting a right answer from the books. The gage is used for process control and by forcing the variation by selecting the parts a good approach? Can we do that? But collecting parts from different days and shifts seems right but it would take you ages to get the right parts.

Suppose you randomly collected parts from different days and shifts and you do your gage R&R for process control, what if your %Tolerance is like more 10% or more than 20%? What would you conclude from such a study? Is the gage capable?

Suppose that for the same process you have forced the variation and have collected the parts which represents the entire range of possible parts and you do your gage R&R and the results look good which is less than 10%. What would you do? Which one should I accept?

How will I have confidence in the Gage?

One more thing

Suppose there is an ISO Audit, will they even accept such a methodology?
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#17
Re: Gage R&R (GR&R) Brain Teaser

This is okay for gages used for inspection only. However, this is the wrong approach for gages used for process control. It could inflate the variation and make the gage look better than it truly is.

See my post immediately above.
Good point. For process control (SPC) the other thing to look at is an ndc (calculated using the control limits rather than the tolerance) >10.
 

Miner

Forum Moderator
Staff member
Admin
#18
Thanks Miner for your reply. Can you provide a reference to support your answer? I'm just asking you because it's not written anywhere. I was confused between the two approaches.

The reason why I asked this question was because I wasn't getting a right answer from the books. The gage is used for process control and by forcing the variation by selecting the parts a good approach? Can we do that? But collecting parts from different days and shifts seems right but it would take you ages to get the right parts.
You said that your gage is used for process control (e.g., SPC). The answer is very clear (refer to AIAG's MSA manual). The variation of the parts selected for the study MUST match the variation of the process.

If you are comfortable with doing some basic manual calculations, you can substitute the PV (Part Variation) from the parts used in the MSA study with a more accurate Part Variation obtained from a capability study or from SPC charts.

Suppose you randomly collected parts from different days and shifts and you do your gage R&R for process control, what if your %Tolerance is like more 10% or more than 20%? What would you conclude from such a study? Is the gage capable?
Since your gage is used for process control, not inspection, the %Tolerance is irrelevant. Only %Total Variation (or ndc) is relevant.

Suppose that for the same process you have forced the variation and have collected the parts which represents the entire range of possible parts and you do your gage R&R and the results look good which is less than 10%. What would you do? Which one should I accept?

How will I have confidence in the Gage?
I would have no confidence in the gage unless the part variation matched the actual process variation.

One more thing

Suppose there is an ISO Audit, will they even accept such a methodology?
If you follow the AIAG MSA manual, ISO will accept it. If you substitute the study PV with PV from a capability study or from SPC charts, this will depend on the auditor. Most reasonable auditors should accept it if you can rationally explain and defend it statistically. However, there is always that one auditor that will not accept any deviations.
 
A

aproddutoor

#19
If you are comfortable with doing some basic manual calculations, you can substitute the PV (Part Variation) from the parts used in the MSA study with a more accurate Part Variation obtained from a capability study or from SPC charts.
Miner
Can you please explain the difference between the PV from parts and PV from Capability study or SPC charts?

Thanks
 
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