Gage R&R - Looking for help in deciphering this report

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Ron Rompen

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#2
I will leave it to others to interpret your analysis reports, however I will say that the ultimate summation (R&R) of .001 is, although an EXCELLENT score, highly suspect.

Looking at the data you provided, I would immediately suspect that the measuring equipment you are using does not have sufficient discrimination; the tolerance range is 0.003, and the reported values are in increments of 0.0005 (with only one operator reporting values to that increment).

On the good side, the parts DO seem to span the entire range of the accepted process (always useful).

Anyone else?


(BTW, don't be embarassed for asking.....most of us have to ask for help on a regular basis...that is the whole idea of the Cove; quality professionals (and me) helping each other :)
 
K

KenK - 2009

#3
The key metrics for this analysis are in the Summary in Excel cells V29 (%R&R) and X29 (%Tolerance).

%R&R is an index of measurement precision compared to the process variation. Acceptable %R&R is very important if the gage is used for SPC, process capability analysis, and process improvement (DOEs, etc...).

%Tolerance is an index of measurement precision compared to the tolerance. Acceptable %Tolerance is very important if the gage measurement is compared to a specification, such as in end-of-line testing.

For a capable process (Cp>1, the %R&R will always be larger (and less acceptable) than the %Tolerance. This is one reason many prefer to use the %Tolerance, since it provides a smaller value.

The acceptance criteria for both are:

<10% - Acceptable
10-30% - May be acceptable based upon importance of application, cost of measurement device, cost of repair, etc.
>30% - Not acceptable

In your case, the %R&R is 21.44% (MINITAB average & range method comes back with a value of 19.79%) which is in the "maybe be acceptable" range. Consider how vital the measurement precision is.

The %Tolerance is 35.87%, which is not acceptable, BUT the tolerance -/+ values and nominal value are entered into the spread sheet in a way that leads me to beleive that this is a one-sided lower spec. The nominal is 0.3 with a lower spec limit of 0.297. Your spreadsheet doesn't appear to be able to handle a one-side spec.

MINITAB can handle a one-sided spec, and returns a %Tolerance of 63.52% using a multiplier of 5.15 (as an FYI, the automotive industry has now started using a 6.00 multiplier, rather than 5.15). It uses a formula of 3s/(Xbar-LSL), where s is the standard deviation associated with R&R, and Xbar is the average of all measurements.

This is certainly not acceptable, but considering it is larger than the %R&R, I would suspect that either your process is not capable, or that your Gage R&R study did not produce a good estimate of the process variation. If the later is the case, you should replace the denominator of the %R&R calculation with 6s, where s is the overall standard deviation calculated from a good process capability study with sufficient sample size (n=100 or more).

Another metric that is utilized in GR&R studies, but not in your spreadsheet is the "number of distinct categories". This provides an assessment of your gage's discrimination or resolution. This value should be 5 or greater. The ndc value from MINITAB using your data with the average & range method is 6, which is acceptable.

The Rbar charts typically should be in control, but as you can see, all three opertors have out-of-control ranges. This means that the operators are having problems measuring those parts consistently - more variation that is expected.

The Xbar charts typically should be out-of-control. They show the observed variability of the parts measured, but the control limits are calculated using only the measurement error - as opposed to the part or total error. If the measurement error is small, the observed variation should be much larger than the width of the control limits. In your case the %R&R is 21.44%, which is marginally acceptable. This results in the Xbar chart being less out-of-control than might be expected.
 

Miner

Forum Moderator
Staff member
Admin
#4
KenK's comments are correct, but you should repeat the study under consistent rules.

If the entire study were done with measurements to .001, the gauge resolution is not adequate. Some of the measurements were .0001. This level of resolution is adequate. However, since the bulk of the study was at the .001 level of resolution, most of the ranges were zero. Many of the calculations are driven by the range, so it appears to be artificially low.

I question the validity of the GRR results, and recommend that you repeat the analysis using .0001 resolution.
 
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