The real validation of a measurement system is done by running a gage R&R study. The philosophy behind Gage R&R is that we must determine the uncertainty of our measurement systems before we can compare, control or optimize our manufacturing processes.
ANOVA Guage R&R (or ANOVA Gauge Repeatability & Reproducibility) is a Measurement Systems Analysis technique which uses Analysis of Variance (ANOVA) random effects model to assess a measurement system. The evaluation of a measurement system is not limited to gauges (or gages) but to all types of measuring instruments, test methods, and other measurement systems.
ANOVA Gauge R&R measures the amount of variability induced in measurements by the measurement system itself, and compares it to the total variability observed to determine the viability of the measurement system. There are several factors affecting a measurement system, including:
* Measuring instruments, the gauge or instrument itself and all mounting blocks, supports, fixtures, load cells, etc. The machine’s ease of use, sloppiness among mating parts, and, “zero” blocks are examples of sources of variation in the measurement system. In systems making electrical measurements, sources of variation include electrical noise and analog-to-digital converter resolution.
* Operators (people), the ability and/or discipline of a person to follow the written or verbal instructions.
* Test methods, how the devices are set up, how the parts are fixtured, how the data is recorded, etc.
* Specification, the measurement is reported against a specification or a reference value. The range or the engineering tolerance does not affect the measurement, but is an important factor in evaluating the viability of the measurement system.
* Parts (what is being measured), some items are easier to be measured than others. A measurement system may be good for measuring steel block length but not for measuring rubber pieces, for example.
There are two important aspects of a Gauge R&R:
* Repeatability: The variation in measurements taken by a single person or instrument on the same item and under the same conditions.
* Reproducibility: The variability induced by the operators. It is the variation induced when different operators (or different laboratories) measure the same part.
It is important to understand the difference between accuracy and precision to understand the purpose of Gauge R&R. Gauge R&R addresses only the precision of a measurement system.
Anova Gauge R&R is an important tool within the Six Sigma methodology, and it is also a requirement for a Production Part Approval Process? (PPAP) documentation package.
How to perform a Gage R&R
The Gage R&R (GRR) is performed by measuring parts using the established measurement system. The goal is to capture as many sources of measurement variation as possible, so they can be assessed and understood. Please note that the objective is not for the parts to “pass”. A small variation (a favorable result) might result from a GRR study because an important source of error was missed in the process.
To capture reproducibility errors, multiple operators are needed. Some (ASTM E691 Standard Practice for Conducting an Inter-laboratory Study to Determine the Precision of a Test Method) call for at least ten operators (or laboratories) but others use only two or three to measure the same parts.
To capture repeatability errors, the same part is usually measured several times by each operator. Each measurement cycle on an individual part must include the full set of operations required if the operator were testing multiple different parts, including the complete handling, loading, and unloading of the part from the measurement system.
To capture interactions of operators with parts (e.g. one part may be more difficult to measure than another), usually between five and ten parts are measured.
There is not a universal criterion of minimum sample requirements for the GRR matrix, it being a matter for the Quality Engineer to assess risks depending on how critical the measurement is and how costly they are. The “10×2×2″ (ten parts, two operators, two repetitions) is an acceptable sampling for some studies, although it has very few degrees of freedom for the operator component. Several methods of determining the sample size and degree of replication are used.
Common Misconceptions about GRR
* Only one GRR is needed per family of gauges. It is usual to say “There is an acceptable GRR for this caliper”. This statement is erroneous because a GRR applies to a complete measurement system, including the part, specification, operator, and method. For example, measuring a steel block with a caliper may achieve good precision, but the same caliper may not be suitable to measure soft rubber parts which may deform while they are being measured.
* The GRR will not pass using parts, so it has to be done with standard weights and blocks. A GRR done this way will assess the precision of the device or system only while it is measuring standard weights and blocks. The device or system might be unsuitable for measuring actual parts, however. If the part deforms or otherwise changes while being measured, this is regarded as a component of measurement system error.
* GRR results must be reported on PPAP (Production Part Approval Process) documentation for everything measured. This is not necessarily a requirement. The Quality Engineer usually makes an educated assessment. If the characteristic is critical to safety, a valid GRR is required. Instead, if there is enough understanding that some particular part is easy to measure with acceptable precision, a formal GRR may not be required. Customers may ask for additional GRRs during PPAP reviews. Knowing that a GRR is not good and still uses the measurement system does not make sense. This is like using bent calipers to take measurements: The measurements produce numbers, but they’re meaningless.
* Performing a GRR is very expensive. To perform a GRR usually a number of parts (sometimes between five to ten) is required to be measured by at least three operators (some suggest ten or more) two or three times each. The GRR measurement costs are the ones associated with the additional measurements. For simple devices, a GRR may be inexpensive, and the result is a known measurement error useful in assessing all subsequent measurements made using the system. The costs can be higher for destructive testing.
* GRRs must be within 10% to pass. There are AIAG guidelines for GRR errors relative to the specification, and what to report on a PPAP process. The final decision about the acceptable precision of any measurement system resides between the supplier and customer: It is a function of the criticality of the measured characteristic and the assessed measurement error. GRR is a tool useful in making this assessment, but GRR does not in itself provide insight as to what level of precision should be acceptable for a given measurement system and part.
The section above is excerpted from the Wikipedia website.
Testing the thread break torque of closure systems is a destructive method. When running gage R&R on torque testers, special considerations must be taken. The following document from Minitab gives some insight on how to run gage R&R on a destructive test device.
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