ANOVA Gauge R&R

Gauge Repeatability and Reproducibility (ANOVA Gauge R&R) is a technique of Measurement Systems Analysis. It evaluates the measurement system using the ANOVA (Analysis of Variance) Random Effects model.

This evaluation is not only limited to gauges, but also applicable to measurement systems like test methods, measuring instruments, and others.

ANOVA Gauge R&R determines the viability of a measurement system by measuring the amount of variability in the measurements and comparing it with the total variability. A measurement system may be affected by several factors like:

• Measuring instruments – The gauge or the instrument, and all supports, mounting blocks, load cells, fixtures, etc. Examples of variation sources are sloppiness in mating parts, ‘zero’ blocks, machine’s ease of use, etc. Sources of variation in systems making electrical measurements include analog-to-digital converter resolution and electrical noise.
• Operators – Efficiency of the people to carry out the verbal/written instructions.
• Test methods – Include how the devices are set up, parts are fixed, data is recorded, etc.
• Specification – Based on which the measurement is being reported. Though engineering tolerance does not affect measurement, it is vital in the evaluation of the measurement system‘s viability.
• Parts – What are being measured. While a measurement system may hold good for measuring steel block length, it may not be suitable for measuring rubber pieces.

Gauge R&R consists of the following two important aspects:

1. Repeatability – The variation in measurements taken on the same item, under the same conditions, by a single person or instrument
2. Reproducibility – The variability induced when different operators (or laboratories) measure the same item

Gauge R&R is used only for the precision aspect of a measurement system. It is an important Six Sigma methodology tool, and is also a PPAP (Production Part Approval Process) documentation requirement. GRR (Gauge R&R) measures parts under the established measurement system, and aims to report all possible variation sources in measurement, for understanding and assessment.

Multiple operators are needed for getting report on reproducibility errors. The ASTM E691 Standard Practice requires at least 10 operators or laboratories. Others demand only 2 or 3 for measuring the same parts. For accounting repeatability errors, one operator measures the same part several times. In case of multiple testing of different parts, full set of operations should be included in each measurement cycle. For accounting operator interaction with different parts, usually five to ten parts are measure. The GRR matrix enables the Quality Engineer to assess risks based on the vitality of the measurement and its cost. There are several methods for determining the degree of replication and sample sizes. The ’10x2x2′ (ten parts, two operators, two repetitions) is a common sampling for some studies.

The following are some common misconceptions regarding GRR:

• Single GRR is sufficient per family of gauges – GRR relates to a complete measurement system – including the specification, part, operator, and method – rather than one caliper.
• GRR will not pass using parts – GRR can assess precision of a device of system. only if that system is measuring standard weights and blocks. In case of changes in part like deformity occur in measurement, it is taken as a component of measurement system error.
• It is necessary to report GRR results on PPAP documentation for each measurement – It is not necessary, as usually, the Quality Engineer himself makes a good assessment. Only if the characteristic is vital to safety, it requires valid GRR. Otherwise, it is not require for easy measurement of parts with acceptable precision. During PPAP reviews, customers may require additional GRRs.
• GRR performance is very expensive – GRR performance is inexpensive for simple devices, and the result can be utilized for assessment of subsequent measurements using the system. Costs occur for additional measurements only. Only in case of destructive testing, the costs are higher.
• GRR must be within 10% to pass – The final decision to accept the precision of measurement system is taken between the supplier and the customer. GRR is just a tool for assessment, and not a deciding factor whether to accept the level of precision of a measurement system or not.

Example:

Let’s see an example of ANOVA gauge R&R to understand this easily.

For example, a manufacturing unit producing small parts that use in the heat exchanger components. This small component is accurately and precisely measurements are requiring ensuring the quality.

The quality control team uses a digital micrometer to measure the thickness of the components, to ensure that the measurements are accurate, precise and consist of. The team follows the ANOVA Guage R&R study for that.

Conditions:

1. Three operators will use the micrometer to measure the thickness of 10 components.
2. Each operator measures the components – 3 Times.
3. The study will conduct for two days.

On the completion of the process, the team had obtained the data from the study as below:

The ANOVA Gauge R&R study involves 4 key components. Let’s understand what these components are:

• Total variation. This is sum of the squares of the differences between the mean value and individual values.
• Operator Variation. This is variability in the measurement system, usually cause by the operator. The calculation comes by comparing the measurements taken by each operator and the average value for each part.
• Part variation. This variability in the measurement system is cause by the part. The calculation comes by comparing the measurements taken for each part by all operators.
• Measurement System variation. The variability in the measurements system cause by the measurement tools and the environment. It is calculated by comparing the variation in the measurements taken over the two days.

Results:

From the ANOVA Gauge R&R study, the operators have achieve following results:

As you can see results as on above table, the ANOVA Guage R&R study shows that the operator variation has a significant impact on the measurements system. The table describing the evidence, there is F-Value for the operator component. Same way as you can see, the measurement system & part variation has a relatively low impact on the measurements system. There is F-values showing low in numbers.

This result is very important for the improvement in the measurement system. The management can provide additional training to the operators or identify and address problems with the measurement tools.

Let us include some more analysis. There you can see the results of the ANOVA Gauge R&R study; a calculation of the % Total variation is also includes. The metric measures the proportion of the total variation in the measurements that can be attribute to each component. The results are available as below:

The results show that the majority of the variation in the measurements can be attributes to the operator. 45% of the total variation being cause by this component. The part variation accounts for 24% of the total variation, while the measurement system variation accounts for 31%.

Conclusion

To accurately identify the issues in the measurement system, the ANOVA Guage R&R study is really a valuable statistical tool. It is easy to evaluate the variability in a measurement system. The outcome from this study is helpful to management for identifying the source of variation that impacting on the system. As result the management can improve their measurement system’s accuracy and reliability.

As you see above example is demonstrates – How ANOVA Guage R&R can help the management to evaluate the accuracy and repeatability of a measuring device in the manufacturing setup.