posted 12 October 2000 09:24 AM               

We have developed excel spreadsheets to document all studies except stability. For stability we do an X-R chart using 5 measurements from a known standard (master?)on a weekly basis. The resulting Cp and CpK's are used for evaluation.

We have found this method painless and acceptable to three Lead Auditors we have dealt with.

I have found that Gage R&R alone will not pass the muster these days and if an auditor accepts them I would wonder about the knowledge base of the auditor.

posted 22 October 2000 06:57 AM               

Once the charracteristics have been decided e.g. stability, bias , linearity, R&R , etc.
the study has to be conducted for the same.
The phase 1 study may be helpful in taking such decisions.

However only R&R MAY NOT be sufficient.

posted 02 November 2000 10:09 AM            

1. Gage R&R required for ALL gages/test equipment.

2. All 5 MSA studies required for tests of special characteristics.

With that said, we have also recently been discussing "what is appropriate?" (Reference the first sentence in 4.11.4) There are cases for particular tests on electronics that a bias study is not appropriate and it becomes our obligation to demonstrate through sound statistics and reasoning that it is an "inappropriate" statistical study.

posted 06 November 2000 08:04 PM               

STABILITY:

When considering the subject of stability in connection with measurement system, it becomes extremely important to differentiate between what is generally referred to as measurement system stability ----

a ) The amount of total variation in the systemâs bias over time on a given part or master part : Known as "Stability over Time"

and

statistical stability, the more general term which is applied to not only stability ,but to repeatability, bias, process in general., etc.

to understand the difference between the two stability, let us consider that there can be 2-measurement systems, measuring exactly the same master part, both of which demonstrate statistical stability, yet one system may have significantly higher variation in its bias over time than the other. From a statistical standpoint, they are equally stable. From a traditional gage stability stand point, the system with greater bias variations over time is considered less ã STABLE ã than the one with lower bias variation. ----------------- (page-21,chapter-2-section-2 )

Statistical properties of measurement systems :

The measurement system must be in statistical control. This means that variation in the measurement system is due to common causes only and not due to special causes. This can be referred to as statistical stability. ---------- (page-5, chapter ö1 , section-2 )

Quality of measurement data
The statistical properties most commonly used to characterize the quality of data are

Bias and Variance.

The property called bias refers to the location of the data relative to the master value and the property called variance refers to the spread of the data.

---------- ( page-3 , chapter-1,section-1)

Specifically, the procedures assess the following statistical properties;

Repeatability, reproducibility, bias, stability, and linearity.

Collectively, the procedures are sometimes referred to as ãgage R&Rä procedures. ------------ (page-15,chapter-15,-section-1)

DOES THE GAGE R&R ASSESS THE SAID STATISTICAL PROPERTIES OTHER THAN REPEATABILITY & REPRODUCIBILITY. YES/NO.

If yes -------------- how

Analysis of results ---graphical analysis

Stability

From range chart stability is determined by: a point or points beyond the control limit ; within operator or within part patterns, ------- ( page-46 , chapter 2- section 4 )

linearity

the averages of the multiple readings by each appraiser on each part are plotted with the reference value or overall part average as the index. This plot can assist in determining :

linearity (if the reference value is used ) ------- (page-52, chapter 2-section4 )

CONCLUSION

GAGE R&R study is an apt exercise for statistical stability and if used, as said above all statistical properties are revealed. Like stability, linearity, repeatability & reproducibility. GRR should be done first and GRR values should be brought below 10% . This can be achieved by understanding the graphical representation and taking appropriate steps. By doing so we are achieving the "Statistical stability".

Without data öbased knowledge of the state of control of a measuring process, R&R figures are only descriptions of the data obtained during study. They have no meaning for future performance. Assessing the repeatability ,reproducibility, etc, of a measurement system for which the state of stability is unknown may cause more harm than good. When talking of measurement system statistical stability, the length of time a system is stable is often a major point of discussion. However by means of TIME STABILITY, the length of time a system is stable can be found by using x-bar r-bar control chart. This time stability is to be done after statistical stability in other words called gage R&R. Incase if time stability is performed prior to gage R&R the bias readings will not be exact as readings are contaminated with repeatability and, reproducibility errors.

Above all, any manufacturing process is supposed to be statistically stable if CP & CPK are controlled as they are representing spread & bias (centrality. ).The normal practice to control SPREAD first then to CPK the bias or centering OPN.

Similarly, we have to look at measurement system.

First control repeatability and reproducibility errors of SPREAD by doing gage R&R STUDY and then go to TIME STABILITY to know the extent of DRIFT OR BIAS the CPK.

posted 01 December 2000 03:38 AM               

quote:

---

Originally posted by Frank45:
Hello:

Our Mfg. Facility has thousands of measuring devices which are in several types. They are used in the manufacturing process, tool room, gage lab, and final testing. My question is what have others done in meeting the MSA requirements of bias, linearity, stability, repeatability, reproducibility? We have done GR&R's on each type of gage on initial certification for measurement systems used for in process control and final testing. Also GR&R's have been performed on the measuring systems used in our Gage Lab. Also key Control Characteristics have had GR&R's performed as per customer requirements

---

As per PPAP Third Edition, Appropriate MSA studies need to be conducted to all the measuring system available.

Organisation will be get benifited if we understand the intent of MSA Requirement mentioned in QS 9000 Standard.

MSA has Phase I and Phase II.

In Phase I, Bias and Linearity need to conducted before accepting the new gauge.

For old gauges, Bias and Linearity study is conducted by measuring the master value for 12 times and compute the bias as per MSA manual.

As a Phase II, GR&R study is conducted to quality the gauge to use in appropriate measuring system (system compraises part, appraiser, environment).
As ongoing stability of gauge is done to confirm the gauge variation statistically stable.

Hence GR&R alone is not sufficient.

As you are having plenty of measuring gauges, you can group the type of gauge and part tolerance for which the gauge is used and find out the Close tolerance for which the gague is used. Do GR&R study for that type of instrument alone.