Gauge R&R Studies Bad, but Process has a Cmk of over 7

C

Covenant

#1
I've got a process with a Cmk of over 7 but my gauge R&R (range 0.002mm) tells me that my gauging process is rubbish! Do highly capable processes mess with the R&R calculations. I can't believe the spreadsheet's results.:mad:
Can anyone help, please?
 
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P

Pudge 72

#2
Re: Gauge Studies

Hi there. As I believe I am the first to respond, let me be the one to ask if it is possible to post the data - it is a common request and helps to see the numbers.
Without the numbers, it seems you have a suspiciously high CPK value. Without seeing the data - a couple of questions come to mind, what is your tolerance on the feature that is being measured? We need to take into account that as far as gage is concerned, your results may indicate that while your process is alright, the accuracy and precision with which you are measuring the feature is not good. It may indicate that Operators cannot repeat the repeat the results within their own datasets or amongst themselves. Are we talking about the Tolerance% of the R&R or the Total Variation?
I would post the data and wait for answers from some of our more expierienced R&R guys / gals - Bev, Miner, Steve & Jim come to mind.
 
D

David DeLong

#3
Re: Gauge Studies

I've got a process with a Cmk of over 7 but my gauge R&R (range 0.002mm) tells me that my gauging process is rubbish! Do highly capable processes mess with the R&R calculations. I can't believe the spreadsheet's results.:mad:
Can anyone help, please?
Yes, they sure do.

Here is what that automotive suppliers have found out in North America. Make sure that when you select the samples that they reflect a range of some sort. Otherwise, you will obtain a very high percentage of the process and your ndc will be low.

I was performing an R & R with a group of people during a seminar and we found that the GRR % was really high with a very low ndc. I just changed 1 value in the sample and then everything was great. I kept the same range with the 3 Operators and 2 samples but moved it up about 0.3 mm. What a difference.
 

Miner

Forum Moderator
Staff member
Admin
#4
Re: Gauge Studies

Yes, they sure do.

Here is what that automotive suppliers have found out in North America. Make sure that when you select the samples that they reflect a range of some sort. Otherwise, you will obtain a very high percentage of the process and your ndc will be low.

I was performing an R & R with a group of people during a seminar and we found that the GRR % was really high with a very low ndc. I just changed 1 value in the sample and then everything was great. I kept the same range with the 3 Operators and 2 samples but moved it up about 0.3 mm. What a difference.
This is very bad advice!

Covenant,
What do you use your gage for, inspection or process control?

If you use the gage for inspection, your selection of samples to measure does not matter, because you should calculate the P/T Ratio, which is the percentage of your tolerance taken by measurement variation. Part variation is not included in this calculation.

If you use the gage for process control, your selection of samples is critical. The sample variation MUST reflect the actual process variation. You should calculate the %GRR and ndc, both of which use the process variation in the calculation. %GRR is the percentage of process variation taken by the measurement variation. So, to answer your question, a highly capable process does require a much better gage than a less capable process.

David,
Deliberately selecting parts with a wider range of variation than the actual process results in an invalid %GRR and ndc. Doing this with the knowledge that you are influencing the results in your favor is falsification of the MSA results.
 
D

David DeLong

#5
Re: Gauge Studies

David,
Deliberately selecting parts with a wider range of variation than the actual process results in an invalid %GRR and ndc. Doing this with the knowledge that you are influencing the results in your favor is falsification of the MSA results.
I did not say that one should select parts with a wider range than the actual process results, I said select the sample carefully.

Let's say we are going to perform an R & R study on 10 parts. How do we select the parts? Randomly? How about consecutively off the process? I did not say we were going to manufacture nonconforming parts. I said select them carefully making sure that reflect the width of the process. Let's say that we select 10 parts that were in the first sigma limit. How would that affect the result? We would end up with a high percentage. I see this all the time.

Even if we selected the samples carefully, we still could achieve a high R &R percentage of the process and that happens all the time. MSA 2nd edition allowed either the percentage relative to the tolerance range or process abut MSA 3rd edition is the process unless the tolerance range is smaller.

Miner - please let me know how one should be selecting the samples for this study? There is nothing in the AIAG MSA 3rd edition on this subject.
 

Miner

Forum Moderator
Staff member
Admin
#6
Re: Gauge Studies

Let's say we are going to perform an R & R study on 10 parts. How do we select the parts? Randomly? How about consecutively off the process? I did not say we were going to manufacture nonconforming parts. I said select them carefully making sure that reflect the width of the process. Let's say that we select 10 parts that were in the first sigma limit. How would that affect the result? We would end up with a high percentage. I see this all the time.

Even if we selected the samples carefully, we still could achieve a high R &R percentage of the process and that happens all the time. MSA 2nd edition allowed either the percentage relative to the tolerance range or process abut MSA 3rd edition is the process unless the tolerance range is smaller.

Miner - please let me know how one should be selecting the samples for this study? There is nothing in the AIAG MSA 3rd edition on this subject.
First, the decision to assess the gage to tolerance or process should be based on the application of the gage (i.e., inspection or process control). This tells you whether to use P/T Ratio or %GRR/ndc. MSA 3rd edition does not contradict this, but does put a lot of emphasis on the process as you said, so it is easy to miss this distinction.

If the gage is to be used for process control, the parts selected must have a variation equivalent to the process variation. The gage to be studied should be used to perform a preliminary process capability study retaining the part/measurement traceability. I say preliminary because the gage must be determined to be acceptable before performing the actual capability study. However, in this situation the observed variation will contain the same measurement variation as the MSA. This allows you to quantify the combined process/measurement variation. Select a subset of these parts that will provide the same combined process/measurement variation as the preliminary capability study, and perform the MSA.

This will give you an accurate %GRR/ndc. If these are acceptable, your preliminary capability study may be assessed for control and capability as is. If they are not acceptable, the measurement system must be improved and the cycle above repeated.
 
S

shahshahani

#7
I think it's usual because your process has a small variation (Cpk>7) and the %R&R is equal R&R/TV, because u have a small TV the fraction is going to increase and u will have a big %R&R.

Mansoor
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#8
Covenant - if you post your data and answer some basic questions about your gage and process and what you are attempting to do (is the gage used for final acceptance inspection, in process SPC, incoming inspection, etc) we can provide specific advice to help you

otherwise we get to debate each other about hypotheticals...
 

Stijloor

Staff member
Super Moderator
#9
Covenant - if you post your data and answer some basic questions about your gage and process and what you are attempting to do (is the gage used for final acceptance inspection, in process SPC, incoming inspection, etc) we can provide specific advice to help you

otherwise we get to debate each other about hypotheticals...
Bev,

Covenant's post is from 20th December 2007. I hope is still interested. ;)

Stijloor.
 
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