# Choosing Samples for Gage R&R - Randomly picked samples show very little variation

J

#### jimbowen

This one again!! Ihave followed MSA 3rd edition and taken 10 random samples from our process. The problem is we have a stable process so the randomly picked samples show very little variation. Our part has a diameter of 26.32mm and the parts produced do not really vary due to the stable nature of the process.
R & R is a ratio of variation in measurement system divided by variation between the parts. By reducing part to part variation, you are reducing the denominator which will result poor ratio.
This basically means I end up with a poor %GRR.
How can I get around this?
Thanks

#### Miner

##### Forum Moderator
Re: Choosing Samples for R&R

There is no easy way around this. If the standard deviation of your samples is equal to the short term standard deviation of your process, and you get a poor %GRR, your only option is to improve the gaging system. It is an iterative process. That is you improve the process until the gage becomes inadequate, then you improve the gage and repeat the cycle of improvement.

Can you attach the results of your study, including the graphs? We might be able to point you in the right direction. Did you verify that the gage has adequate resolution? This can kill an MSA before you start.

J

#### jimbowen

Re: Choosing Samples for R&R

i'll have to post it tommorrow as I'm sat in my front room but thanks anyway. What I did find though is that if you enter 6.85 into all of the inputs apart from one where you enter 6.86 then the %GRR was 59.76%. I don't understand how if the process is so tight and the variation is so low that you would get such a high result.

#### Miner

##### Forum Moderator
Re: Choosing Samples for Gage R&R

That sounds like a gage resolution issue. I'll be able to confirm when I see the data.

A

#### Atul Khandekar

Re: Choosing Samples for R&R

jimbowen said:
i'll have to post it tommorrow as I'm sat in my front room but thanks anyway. What I did find though is that if you enter 6.85 into all of the inputs apart from one where you enter 6.86 then the %GRR was 59.76%. I don't understand how if the process is so tight and the variation is so low that you would get such a high result.
To add to what Miner has said...Are you doing SPC on this characteristic? What is the 6-sigma process width, Tolerance and Cp? Are you using this gage for process improvement or final inspection? Over what period of time did you collect your samples?

J

#### jimbowen

Re: Choosing Samples for Gage R&R

The tesing we carry out is final inspection. I think on reading a couple of other threads that the answer to our problem is to use tolerence rather than variation for the %GRR. We are assessing the gage and not the process and believe that using the tolerence range gives us a better indication of the gage %GRR. The only thing I would ask is do I replace the TV or PV with the tolerence range. Please refer to page 114 MSA 3rd edition. Thanks...

A

#### Atul Khandekar

Re: Choosing Samples for Gage R&R

Are you refering to the spreadsheet you attached in the other thread? You need to divide the values of EV,AV and GRR by (Tolerance/6). Refer to the para at the end of page 116 MSA manual.

You will have to look at the parts selection and gage resolution as well.

J

#### jimbowen

Ok here goes. The gauge is 0.001mm resolution. The tolerence is +-0.02mm. The 10 parts were randomly picked from process and there are 3 operators who carry out 3 trials. The actual data collected is entered into the attached spreadsheet which is taken from MSA 3rd Edition, page 113-114. Incendently we put the same data into a Ford spreadsheet and came up with 19.32%. The MSA version using the same data came up with 51.29%

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#### Wesley Richardson

##### Wes R
Trusted Information Resource
Hi Jim,

I see two immediate problems with your spreadsheet. Your range is showing 0.00 in several cases, when it is > 0. Also your averages do not reflect the data you have shown. Both may be due to the number of digits you are displaying. You did not include the equations for the calculations, so I cannot tell.

Wes R.

A

#### Atul Khandekar

Look at the values.
I think Ford doc uses K constants from MSA 2nd edition. The GRR VALUE is already multiplied by a facor of 5.15. Also the values in Ford doc are shown rounded off (ex. GRR=0.01 which I think is 0.0115), the tolerance it uses is 0.06! and the GRR % is a % of Tolerance.

Your calculations seem to be correct as per MSA 3rd edition. The GRR% (51.29) is a % of TV, not a % of tolerance.

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