# Is this GR&R (Gage R&R) acceptable on the surface

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#### Creech

Hello -
Attached are results from a gage R&R. I'm not sure if this is acceptable. My total tolerance is .002. My readings were from a micrometer measuring a shaft, 3 places, 120 degrees apart. I then took the average. The categories are 1. This comes up from time time and typically gives us a bad R&R. I'm not sure this is the case this time. Any thoughts on this would be appreciated. - Rich

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#### lee01

Re: Is this GR&R acceptable on the surface

Welcome to the forums,

First of all I'm not sure you should be using a micrometer to measure your shaft (sorry personal joke).

Assumptions: we are talking about inches, you conduct GRR regularly.

Reading your post it appears you are regularly conducting a GRR on your equipment, and you say you sometimes find an error. This leads me to ask if you have found an error in the process and not an abnormality in your readings?

How are you deciding you 120 degree apart? is it a case of there you go its roughly a 3rd way round the shaft? if so is your shaft not perfectly round and your building the ovallity into the calculation every now and then.

As for acceptable, only you and your customer can determine what acceptable GRR result your operating with. (And not the typical 10% rule that people think it is).

C

#### Creech

Re: Is this GR&R acceptable on the surface

Leeo1

Thanks for the reply. Yes the assumptions are true. And yes my company does gage R&Rs from time to time. The reason the 3 times at 120 degrees is because the parts are not perfectly round, say .0005. I found that measuring the parts in one place I had to much part variation. I would measure part 1 at say .1820 then the same part .1815 then .1818 and so on. We found the parts oval. So I averaged the 3 readings and hence the data. I'm told that I have to have less than 30% GRR and less than 10% PT and 5 categories. But then I look at the chart and it shows the operators very close in their measurements. I'm just a little confused. Thanks

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#### Frank T.

Re: Is this GR&R acceptable on the surface

My first thoughts are:

Are both operators checking the part at the same 3 places, same 120 degree location?

Variation in the circularity of the shaft are factors if not measuring the same locations.

The operators touch/feel, when turning the spindle on the micrometer, plays a factor as well.

Just my

Bye the way, welcome to the cove.

J

#### justncredible

Re: Is this GR&R acceptable on the surface

Mark the parts where they need to measure them, measure them in one spot and use 3 operators, you need to be able to have a system that can identify 5 parts, that is the 1 NDC, yours will only identify 1 part out of 10 right now.

Why are you useing anova? Why does it show one operator, and then 2 on the reports?

Just crashed this cmm 2 times writing this......LOL

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#### Creech

Re: Is this GR&R acceptable on the surface

Thank you for the welcome.

The operators had used (just so happens) 3 , 120 degree features to visually turn the parts 120 degrees apart to measure the parts. They used 3 clicks on a digital 1 inch mic. We could probably improve the measurement. I'm thinking the distinct categories are effecting the GRR. Does that make any sense? Thanks

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#### Creech

Re: Is this GR&R acceptable on the surface

justncredible - Not really sure why I'm using ANOVA. It is just by default. As for the operators, I have no idea how the data is displayed. As you can tell I am very wet behind the ears to this subject, unfortunately I have no internal support. Thanks

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#### Frank T.

Re: Is this GR&R acceptable on the surface

As you can tell I am very wet behind the ears to this subject
Creech,

Might I suggest using this file or the one I have attached below. If neither of these are to your liking you might try doing a search in the post attachments list.

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• 38.5 KB Views: 325

#### Miner

##### Forum Moderator
I'll try to adress several issues and misconceptions that I noted in the above posts.

1) Averaging the measurements as you are doing is an excellent method of reducing the impact of the within part variation (ovality). Variation is reduced by the square root of the sample size (SQRT of 3 = ~1.7), so you have reduced within part variation by almost half.

2) Your operators are very close in their measurements. The Operator portion of Reproducibility is 0. The issue is the interaction between part and operator that was caused by part number 2. Have both operators remeasure part number 2 and reanalyze the study with the remeasured part data.

3) justncredible, There were 2 operators. The 1 that you saw was the degrees of freedom for two operators (df = n - 1 = 2 - 1 = 1)

4) The number of distinct categories is correlated to the %GRR, but does not cause it. The two are different metrics measuring the same thing.

5) ANOVA is a better method than the Range method used in most spreadsheets. It provides the operator x part interaction, which the Range method does not provide. In addition, the Range method uses the range to estimate the variation versus the direct calculation of variation by ANOVA. Stick with Minitab over the spreadsheets.

6) Regarding the ndc and %GRR: These metrics are highly dependent on the variation of the 10 parts used in the study. They must accurately reflect the actual process variation. 6 standard deviations of the parts used in this study were 0.0003576. How does this compare to your process capability measured the same way (average of three)? If it is smaller, your ndc/%GRR will be worse than they actually are.

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#### Creech

I did a little experiment and modified the data by adding "made up size" to a few of the readings and I got an incredibly good gage R&R

%Contribution
Source VarComp (of VarComp)
Total Gage R&R 0.0000000 0.14
Repeatability 0.0000000 0.14
Reproducibility 0.0000000 0.00
Part-To-Part 0.0000008 99.86
Total Variation 0.0000008 100.00

Process tolerance = 0.002

Study Var %Study Var %Tolerance
Source StdDev (SD) (6 * SD) (%SV) (SV/Toler)
Total Gage R&R 0.0000335 0.0002011 3.72 10.06
Repeatability 0.0000335 0.0002009 3.71 10.05
Reproducibility 0.0000015 0.0000088 0.16 0.44
Part-To-Part 0.0009015 0.0054091 99.93 270.46
Total Variation 0.0009021 0.0054129 100.00 270.64

To me, it seems if I have more part variation I'll get a better GR&R. I understand that if I don't span the range of the tolerance then I can't call the gage good. But in this instance (using a mic) can't I assume my measurements will be linear? The machinery we use is very accurate and repeatable. Dare I say "intentionally make parts that span range".

Thank you for taking the time to respond.

#### Attachments

• 98.1 KB Views: 144
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