ANOVA Analysis - Trying to understand MSA analysis- Please review my spreadsheet

Y

Yew Jin

#21
Hi guys,

If the process capability is adequate (Cp > 1.33) the lack of precision and inadequate GRR is not a immediate priority. As process improvements are made, the situation is not last. Determine the time line to improve the measurement improvement.

If the process capabiliy is between 1.00 and 1.33, unacceptable GRR variation may make difference in whether the observed process is report as capable.

If process capability is clearly inadequate, Cp <1.0, both measurement and process must be addressed. Measurement system should be in the priority.
 
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O

OceanDiver

#22
Now it looks right! I see that your decimal points were off by 2 in the results.

This gage is unacceptable. Your results are off somewhat from Minitab. I attached the Minitab Analysis for your review.

Your %GRR results of %Total Variation = 86.22% (Minitab's %SV = 89.39%) is much greater than the 30% max acceptable, so this gage is not acceptable for use in process control provided that the variation of the parts accurately reflects the actual process variation. An ndc = 1 is also unacceptable. Since the P/T Ratio is also unacceptable at 56%, you have a problem with the gage.
This leads into my problem with understanding how this all should work. My company, when we have done PPAPs in the past for customers would always take the Average and Range data Total Variation Percent of Tolerance to determine if it passes or fails. In this example this value is 10.36%, which is acceptable. However, the GRR is 78.60% but 8.15% of tolerance. The ANOVA GRR is 86.22%. If you look at this in regard to the tolerance then it is acceptable. But if you just look at the variation percent you have an unacceptable level. Which way are you suppose to look at this?


The % Contribution shows that the sources of measurement variation are large in all categories (Repeatability, Reproducibility and Operator x Part Interaction). Are your parts subject to variation in form (shape)?

The parts that we are doing are turned hot roll bars with a size of 1.1378 +0/-0.003 and are not subject to variation in form.

In response to Yew Jin, the Cp number is 2.69 and Cpk 2.41 for this part.

Thanks for all of the help, it is much appreciated.
 

Miner

Forum Moderator
Staff member
Admin
#23
This leads into my problem with understanding how this all should work. My company, when we have done PPAPs in the past for customers would always take the Average and Range data Total Variation Percent of Tolerance to determine if it passes or fails. In this example this value is 10.36%, which is acceptable. However, the GRR is 78.60% but 8.15% of tolerance. The ANOVA GRR is 86.22%. If you look at this in regard to the tolerance then it is acceptable. But if you just look at the variation percent you have an unacceptable level. Which way are you suppose to look at this?

The parts that we are doing are turned hot roll bars with a size of 1.1378 +0/-0.003 and are not subject to variation in form.
There is another error in your spreadsheet. When you calculated P/T Ratio (% Tol) you divided s by the tolerance. The correct formula is 6s/Tol. This changes your 10.36% to 62.16%, which is also unacceptable.

It is possible for one indicator to be acceptable and the other to be unacceptable. The one that is important is the one that is relevant to the gage usage. For inspection to tolerance, use P/T Ratio (a.k.a. % Tol). For process control (i.e., SPC) or any type of statistical study (e.g., DOE, t-test, etc.), use %GRR (a.k.a. %TV or %SV).

Regarding your turned parts, you may want to have a few samples measured on a roundness tester for roundness, taper, and hourglass/barrel-shape. You may have variation in form that your gage cannot see. My experience has been that when Repeatability, Reproducibility and the Operator x Part Interaction are all large, variation in forum is usually the reason. And, I have seen a lot of turned shafts with form variation.
 
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