What does the entire process range mean? (MSA manual page 73)

[darkafar]

MSA manual 3rd page 73 states, “For Process Control situations where the measurement result and decision criteria determine, ‘process stability, direction and compliance with the natural process variation’, the availability of samples over the entire operating range becomes very important. An independent estimate of process variation (process capability study) is recommended when assessing the adequacy of the measurement system for process control. ”

Does this mean I should use different samples to compute GRR (repeatability & Reproducibility) and process variation, and use them to compute %GRR?

Page 74, “When an independent estimate of process variation is not available, OR to determine process direction and continued suitability of the measurement system for process control, the sample parts must be selected from the process and represent the entire production operating range.”

My colleagues and I have some disagreement on how to select samples to perform GRR. And we have the following three methods:

Assumption: We have a cup manufacturing process. The cup height’s spec is 10(+/-1) mm. Take 50 consecutive samples, and the standard deviation of the samples is 0.4.

Method1: Select 10 samples, the 10 samples need to be between 9 mm to 11 mm. We are advised to have a sample as close to 9 mm as possible, and a sample as close to 11 mm as possible. Use these 10 samples to perform GRR.

Method2: Selected 10 samples, the 10 samples need to be between 9.6 mm and 10.4 mm. We are advised to have a sample as close to 9.6 mm as possible, and a sample as close to 10.4mm as possible. Use these 10 samples to perform GRR.

Method3: Randomly select 10 samples from the production line, and use these 10 samples to perform GRR (my prefer).

Which sampling method do you think is correct? Or you have another method?

 

[martin elliott]

Re: What does “the entire process range” mean? (MSA manual page 73)

As no one else more competent has come forward, I will offer my opinion. Remember it may be worth exactly what you are paying for it. I don't use any of your methods and there is good and bad aspects to them all.

Many say you should do MSA before Capability but I try to sort of combine them and although it is not strictly “right”, and I can be sure others might have better approaches, so far this has worked OK for me – no auditor has questioned it deeply.

Sorry but some of my decisions below can not be accurately quantified, sometimes you have to go by instinct.


Initial Gage selection

I use “experience” to determine which gage to use during APQP. If I expect the process capability to be adequate to good, or I am looking at P/T ratio applications, then I will try to use a gage at a minimum of resolution:tolerance 1:10. If however I expect process capability to be to good to very good, then I will try to use a gage at a minimum of resolution:tolerance 1:20.


Parts selection

I then take the 125 parts segregated and measured from preliminary capability study applications and try to select the 10 parts for MSA from them in a manner to very nearly reflect actual distribution.

I list, sort the parts, and very loosely divide the distribution into six equal sections - in effect near equivalent to +/- 3 sigma’s sections.

Select the absolute min and absolute max from the population, these are the only two parts taken from the outer sections i.e. between +/- 2 to 3 sigma.

I then select 1 part each from next sections inwards i.e. between +/- 1 to 2 sigma. Wherever possible these parts should be mid point or very slightly closer to mean within the section.

From the inner two sections, i.e. between +/- 0 to 1 sigma I select 3 parts either side of the mean, fairly equal distributed within the sections. Again some minor bais towards mean is acceptable.


Issues

I accept this doesn’t exactly give you a sample population exactly reflecting the overall process, but its good enough in my experience.

Again if the preliminary capability study turns out to not be a true reflection of ongoing capability you may need to repeat this exercise but so far that has not been a great issue to me.

Hope this helps.

 

[roder - 2012]

Re: What does “the entire process range” mean? (MSA manual page 73)

This is also our dillema in selection of parts to be used in MSA.
I think Method 2 is more appropriate for your existing specification, if you're processing only 10 mm. Suggest to use your standard deviation as your control limit for the min and max values.
9.6 mm min and 10.6 mm max.

Given:
Assumption: We have a cup manufacturing process. The cup height’s spec is 10(+/-1) mm. Take 50 consecutive samples, and the standard deviation of the samples is 0.4.


Method2: Selected 10 samples, the 10 samples need to be between 9.6 mm and 10.4 mm. We are advised to have a sample as close to 9.6 mm as possible, and a sample as close to 10.4mm as possible. Use these 10 samples to perform GRR.

 

[Miner]

Re: What does “the entire process range” mean? (MSA manual page 73)

The method that you should use to select samples depends on the type of MSA that you intend to perform.

Decide in advance whether the gage will be used for inspection to the tolerance or for process control (SPC).

If the gage is to be used for inspection, part selection is less critical. Many recommend selecting parts from the full range of the tolerance. There is nothing wrong with doing this, but it is not critical that this be done. When the study is evaluated, use the P/T Ratio (% of Tolerance) to assess the gage.

If the gage is to be used for process control (SPC), part selection is extremely critical or the results are invalid. The parts selected MUST have a standard deviation that is equivalent to the standard deviation of the process from which they came. This is why it is important to have some benchmark capability data with which to compare the MSA samples. If the standard deviations are equivalent, use the %GRR to assess the gage.

 

[vallesj]

Re: What does “the entire process range” mean? (MSA manual page 73)

When talking about the entire production operation range, as established on the MSA, I understand that if my test equipment tests different part numbers from the same family, I should select the different part numbers that are from the minimal to the high range tested by the equipment.
For example, my test equipment test like 5 different part numbers from the same family, the p/n with lowest tolerance are 90 to 100 and the p/n with the highest is 150 to 100. So the samples I took are 2 pcs from each part number to cover the complete range that the test equipment is measuring.

That is my interpretation about how to select the range, and of course I will have a big part to part variation. Because in the other way, if I select parts from just one part number, how can I be sure that the test equipment measuring variations will not be affected on the other part numbers?

Any thoughts about this?

 

[Miner]

Re: What does “the entire process range” mean? (MSA manual page 73)

When talking about the entire production operation range, as established on the MSA, I understand that if my test equipment tests different part numbers from the same family, I should select the different part numbers that are from the minimal to the high range tested by the equipment.
For example, my test equipment test like 5 different part numbers from the same family, the p/n with lowest tolerance are 90 to 100 and the p/n with the highest is 150 to 100. So the samples I took are 2 pcs from each part number to cover the complete range that the test equipment is measuring.

That is my interpretation about how to select the range, and of course I will have a big part to part variation. Because in the other way, if I select parts from just one part number, how can I be sure that the test equipment measuring variations will not be affected on the other part numbers?

Any thoughts about this?

You should perform a Linearity study to cover the full operating range of your gauge. If you are concerned that the variability will change with size, you could perform SEPARATE R&R studies at the high and low ends, but should not combine the entire operating range into a single R&R study unless the gauge is ONLY used for inspection. If the gauge is used for SPC, using the entire range in a single study will artificially inflate the process variation and will provide artificially good results.

 

[Eradmoor]

Re: What does “the entire process range” mean? (MSA manual page 73)

I know this thread is old, but thank you Miner for backing up a point I have been arguing with others for years now. I phrased it almost exactly the same when you said one artificially inflates the process variation. I despise that area of the MSA manual that says to select parts from the entire operating range. That is certainly a good way to make my part-to-part variation huge, and thus give me a small GR&R percentage. Who writes this stuff, anyways?

 

[Urajan]

Re: What does “the entire process range” mean? (MSA manual page 73)

From AIAG MSA Manual 3rd ed pg.74 stated as " the sample parts must be selected from the process and represent the entire production operating range"

Until today many of us has this dilemma on sample selection. The definition is not well defined by AIAG. Many of the people in this forum fail to address the definition precisely and quite vague. Hope someone can really help to clarify this. I would like to know what does it mean "represent the entire production operating range"?