Gage R&R studies studies through the full range of the gage?

cpine

Registered Visitor
#1
Once again I solicite the help of consensus

When conducting a GR&R on any particular gage (lets say a 0-1" micrometer)
does one have to conduct the studies through the full range of the gage?

For example 0-.250,250-.500 and so on. I have never heard of this requirement however a consultant was kind enough to make this statement to my superior.

Thanks guys, I appreachiate the help


Cpine
 
Elsmar Forum Sponsor

Helmut Jilling

Auditor / Consultant
#2
cpine said:
Once again I solicite the help of consensus

When conducting a GR&R on any particular gage (lets say a 0-1" micrometer)
does one have to conduct the studies through the full range of the gage?

For example 0-.250,250-.500 and so on. I have never heard of this requirement however a consultant was kind enough to make this statement to my superior.

Thanks guys, I appreachiate the help


Cpine
If you have verified the accuracy over the whole range when doing the calibration, then you don't have to do it when doing a gage R&R. However, you would have to do the whole range when doing a linearity study, because linearity measures the bias over the range of the gage. However, in rare cases where a gage is only used within a limited range (say a 0-1 mic in a sheet metal shop), then one has a basis for not using the whole range (though I would stil do it, rather than marking the reduced scope on the gage.
 

Jim Wynne

Staff member
Admin
#3
cpine said:
Once again I solicite the help of consensus

When conducting a GR&R on any particular gage (lets say a 0-1" micrometer)
does one have to conduct the studies through the full range of the gage?

For example 0-.250,250-.500 and so on. I have never heard of this requirement however a consultant was kind enough to make this statement to my superior.

Thanks guys, I appreachiate the help


Cpine
In general, there is no reason to do GR&R if you're not doing it on actual parts. A GR&R done on gage blocks doesn't serve much purpose. If you're doing the study on a given feature on production parts, it's best to use parts that include the entire operating range of the process (not the range of the gage) and this might be what has your consultant confused.
 

Miner

Forum Moderator
Staff member
Admin
#4
cpine said:
Once again I solicite the help of consensus

When conducting a GR&R on any particular gage (lets say a 0-1" micrometer)
does one have to conduct the studies through the full range of the gage?

For example 0-.250,250-.500 and so on. I have never heard of this requirement however a consultant was kind enough to make this statement to my superior.

Thanks guys, I appreachiate the help


Cpine
Gage R&R is highly dependent on both the gage and the feature to be measured. For example, using a 0-1" micrometer to measure thickness of sheet steel will probably give you a much different Gage R&R than using the same micrometer to measure the wall thickness of a casting even if the tolerances were equal.

The only rational reason that I can think of for checking the entire range for a given gage and feature is when the following criteria is met:
  • You have a line of products where the same feature ranges in size the full range of the gage
  • The difficulty in taking a measurement increases as the size increases (or decreases)
Given this scenario, a gage R&R over the full range makes sense.

The normal approach is to use samples that demonstrate the full range of normal process variation. Deliberately producing product beyond that range would add cost and would throw off your PV (Product Variation) component of the Gage R&R.

I would ask the consultant to justify his/her comment within this context.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
If the instrument is calibrated by an accredited laboratory and you have the measurement uncertainty, why do a gage R&R? You already have all the info you need regarding the instrument.....

However, if stuck with the R&R anyway, then certainly over the range the instrument is used, but maybe not over the full capability.

Hershal
 

Miner

Forum Moderator
Staff member
Admin
#6
Hershal said:
If the instrument is calibrated by an accredited laboratory and you have the measurement uncertainty, why do a gage R&R? You already have all the info you need regarding the instrument.....

Hershal
The measurement uncertainty provided by an accredited lab does not include the additional uncertainty of the gage in the application. The gage R&R does identify this uncertainty. The total uncertainty would be the sum of these (including the additional uncertainty of linearity and stability).
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#7
Miner,

I appreciate your response. Though do feel free to correct me if I am wrong here.

If I have the uncertainty from calibration, then I already know how it reacts, what its effects are, what its error is, and its total contribution to the method in discussion. Linearity and stability are components of MU, and are but two of an unlimited number of contributors, although they may be called by another name, and may be either Type A or Type B.

To determine the uncertainty of the gage in an application then the MU from calibration can simply be divided by 2 to reduce to standard uncertainty and becomes a plug-and-play number in the method MU.

Testing laboratories are an example of one type of organization that has the same issue of determining uncertainty of the test method. In their case, standard uncertainty (gage MU, divide by 2 because of normal distribution) is a drop in number as a Type B contributor. The method MU will also have many contributors, including the actual item(s) under test.

Perhaps a discussion of all the various contributions considered in a gage R&R, and in MU, for say a set of calipers.

Hershal
 

Miner

Forum Moderator
Staff member
Admin
#8
Hershal said:
Miner,

I appreciate your response. Though do feel free to correct me if I am wrong here.

If I have the uncertainty from calibration, then I already know how it reacts, what its effects are, what its error is, and its total contribution to the method in discussion. Linearity and stability are components of MU, and are but two of an unlimited number of contributors, although they may be called by another name, and may be either Type A or Type B.

To determine the uncertainty of the gage in an application then the MU from calibration can simply be divided by 2 to reduce to standard uncertainty and becomes a plug-and-play number in the method MU.

Testing laboratories are an example of one type of organization that has the same issue of determining uncertainty of the test method. In their case, standard uncertainty (gage MU, divide by 2 because of normal distribution) is a drop in number as a Type B contributor. The method MU will also have many contributors, including the actual item(s) under test.

Perhaps a discussion of all the various contributions considered in a gage R&R, and in MU, for say a set of calipers.

Hershal
Hershal,
My take on this is that the testing laboratory would probably provide the following elements of MU:
- Scale error,
- zero-point error,
- parallelism of anvils,
- temperature variation (possibly includes stability in a lab environment),
- temperature difference between caliper and workpiece, and possibly
- repeatability (as tested on gage blocks)

In the application, you would have additional sources of MU:
- repeatability (as tested on a workpiece),
- reproducibility (using actual operators and product), and
- stability (in factory environment)

The gage R&R would define the first two elements of repeatability and reproducibility above in a manner that the testing lab could not duplicate.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#9
Miner,

Thanks.....all the influences in your response are in fact accounted for in the MU calculations, plus

Uncertainty of the item being measured,
uncertainty of the calculations,
the difference in expansion/contraction due to temperature effects of the item being measured (different compounds used in calipers and item under measurement),
hysterisys (misspelled that I think),
humidity,
uncertainty of the resolution (typically .5 of least significant digit)
and other enironmental influences as appropriate

The method MU if I understand correctly takes the instrument MU and adds the other influences as you mentioned.....

Regards,

Hershal
 
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