Gage R&R (Reproducibility and Repeatability) - Automotive Requirements Basics

Courier

another day closer
#1
Gage R&R

I Download the file PPAPALL.xls,(the newer file was called somthing like Partcertall.xls) It's Delphi,s Version 6.06 February 2000. I wanted to use the Gage R&R from it. But I had to update the newer K3 numbers from page 114 in the MSA 3rd Edition into it. Anyway the Question and problem I have is it uses two methods to choose from "Part to Part Variation" & Tolerance Method". The Part to Part method is in the MSA book but I was woundering where the Tolerance method came from i dont see it in the book, but it is in Delghi's file and they are used for Chrysler, Ford, & General Motors so it must be good to use right? and i think the upper limit allowed for GR&R is 10% not like the 30% used for Part to Part method. Is this Tolerance method good to use in a ISO/QS company for Gage R&R studys? and what are the allowable limits then to use? :frust:
 
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Atul Khandekar

Quite Involved in Discussions
#2
Courier said:
... Anyway the Question and problem I have is it uses two methods to choose from "Part to Part Variation" & Tolerance Method". The Part to Part method is in the MSA book but I was woundering where the Tolerance method came from i dont see it in the book, but it is in Delghi's file and they are used for Chrysler, Ford, & General Motors so it must be good to use right? and i think the upper limit allowed for GR&R is 10% not like the 30% used for Part to Part method. Is this Tolerance method good to use in a ISO/QS company for Gage R&R studys? and what are the allowable limits then to use? :frust:
Courier,
Welcome to the Cove Forums.

R&R is usually compared with the Total variation observed during R&R study (TV) and the acceptance criteria is 10-30%

You can also use:
1. Process variation as seen from your SPC studies and
2. Tolerance on the characteristic being measured

3rd edition MSA manual, on page 116 (Chapter III, Section B) states that "Either or both approaches can be taken depending on the use of measurement system and the desires of the customer"

In the book, "Concepts for R&R Studies", 2nd Ed, (ASQ Quality Press), Larry Barrentine describes these two ratios as Measurement Capability Indices (MCI-1, based on process variation and MCI-2, based on tolerance.)

As a general rule, comparing GRR with process variation is the preferred approach. The ratio of (GRR/ProcVar) indicates the 'distortion' in the process variation estimates due to measurement error. Since the process variation is the measure of what the process actually delivers on the shop floor, this ratio should be used if you use the measurement system to control the process. If GRR is high as compared to process variation, the measurement system does not have adequate 'discrimination power' to detect shifts and assignable causes in the process. This ratio can also be used in case of one-sided specifications.

The downside of using (GRR/PV) is that when the process is already very capable (say, Cp>1.67), it may indicate that the measurement system is inadequate when in fact improving the measurement system is not the topmost priority.

The other ratio (GRR/Tolerance) can be used when the measurement system is used for product acceptance decisions or inspection. This ratio indicates if you have sufficient 'space' to center the process within the specifications. If this ratio is high, the measurement system may classify good parts as bad ( and vice versa), esp. when operating near the limits of tolerance zone. Customer may accept a higher %of tolerance if the process Cpk is high. The problem with this approach is that specifications (in many cases) may be set arbitrarily.

Larry Barrentine prescribes acceptance criteria as under: (for both, MCI-1 as well as MCI-2):
<=20% :Good
>20 - <=30% :Marginal
>30% :Unacceptable

Hope this helps.

- Atul.

PS: Also see the graphs for impact of GRR on actual and observed Cp - MSA manual, Appendix B
 

Atul Khandekar

Quite Involved in Discussions
#3
From the manual:

Some addition to my above post:

From Page 73 (Chapter II, Section C): MSA Manual

For Product Control situations where the measurement result and decision criteria determine, "conformance or nonconformance to the feature specification", (i.e. 100% inspection or sampling), samples (or standards) must be selected, but need not cover the entire process range. The assessment of the measurement system is based on the feature tolerance (i.i. %GRR to TOLERANCE)

For Process Control situations where the measurement result and decision criteria determine, "process stability, direction and compliance with the natural process variation", (i.e. SPC, process monitoring, capability and process improvement), 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 (i.e. %GRR to process variation)
 
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