By Mark Boucher, CMM Quarterly http://www.cmmquarterly.com/
Measurement errors are unavoidable are inherent in any measurement process. By understanding what the variations are and how much variation exists will provide a clearer picture of just how your measurement system works. A Gauge R&R is the method to pinpoint measurement system variation.
ANOVA Gauge R&R measures the amount of variability induced in measurements that comes from the measurement system itself and compares it to the total variability observed to determine the viability of the measurement system. It is important to understand a Gauge R&R only address how precise a measurement system is. This is not a direct on the accuracy of the measuring instrument.
Terminology of R&R
Repeatability, the ability of the device to provide consistent results. It is a measure of the variability induced by the system if the same operator measured the same part using the same device repeatedly. The same operator measuring the same part using the same gage should get the same reading every time.
Reproducibility, the variability induced by the operators. It is the variation induced when different operators measure the same part. In other words, different operators, measuring the same thing, using the same gage, should get the same reading every time.
Measurement System, this is not to be confused solely with the instrument being used for the part measurement but the entire measurement system including the fixture, operators, etc…
Variability
A Gage R&R checks the variability of each component involved in inspection process including the measuring instruments, the gauge or instrument itself and all mounting blocks, supports, fixtures, load cells etc. A Gauge R&R will be influenced by several factors including the machine’s functionality, how the part fits in the fixture, software filter elimination. There is also the human element, operators (the people performing the Gage R&R) and their ability and/or discipline to follow the written or verbal instructions.
The Gage R&R measurement is reported against a specification or a reference value. The range of the specification does not affect the measurement, but is an important factor affecting the viability of the measurement system. The individual measurement might be in tolerance but how it relates to the entire Repeatability and Reproducibility is what is measured in the Gage R&R.
Parts (what is being measured), some items are easier to measure than others. A measurement system may be good for measuring steel block length but not for measuring rubber pieces.
Understanding the Goal
The goal is to capture as many sources of measurement variation as possible, so they can all be assessed and addressed. The purpose is not to "pass" but to look at the entire measurement system and to expose any variation reported on a GRR that may be because an important source of error was missed.
Gage R&R Method
In order to capture reproducibility errors, multiple operators are needed. Typically only 2 or 3 operators are needed to measure the same parts. To capture repeatability errors, the same part is usually measured several times per operator. In order to capture interactions of operators with parts usually between 5 and 10 parts are measured.
Gage R&R criteria is usually established by the customer but a !0x3x2 (10 parts, 3 operators, 2 repetitions) is typically acceptable for minimum requirements for the Gage R&R.
The 10% Rule
A variation of 10% is the general rule. This tells us that if a measurement instrument exceeds 10% of the part tolerance, the measurement instrument has no ability to measure the process variation. If the percent of tolerance consumed by the R&R does not exceed 10%, the measurement system is excellent.
Calculating Gage R&R
Run a 10x3x2 Gage R&R. Each operator has now measured 10 parts twice.
For each inspector, count the number of times his or her two readings agree. Divide this number with the total inspected to obtain the percentage of agreement. This will show how consistent with himself and may indicate that he needs retraining. To obtain the overall repeatability, obtain the average of all individual repeatability percentages for all inspectors. This will go a long way in eliminating one controllable variable.
Now compute the percentage of times all the inspectors’ assessments agree for the first and second measurement for each sample item. This percentage is the reproducibility of the measurement system. This may indicate that the same measurement process is being used between operators.
Compute the percentage of the time all the inspectors’ assessments agree with each other and with the standard. This percentage gives the overall effectiveness of the measurement system. It is the percent of time all inspectors agree and their agreement matches with the standard.
This is the ratio of variation in the measurement system for the variation in product measurements. i.e. R&R = 20% means that 20% of the variation in product measurements is due to the measurement system.
So What If the Gauge R&R Is Not Good?
The key in all measurement systems is having a clear test method and clear criteria for what to accept and what to reject. The steps are as follows:
Identify what is to be measured.
Select the measurement instrument.
Develop the test method and criteria for pass or fail.
Test the test method and criteria (the operational definition) with some test samples (perform a gage R&R study).
Confirm that the gage R&R in the study is close to 100 percent.
Document the test method and criteria.
Train all inspectors on the test method and criteria.
Pilot run the new test method and criteria and perform periodic gage R&Rs to check if the measurement system is good.
Launch the new test method and criteria.
This article is copyrighted. Please contact Mark Boucher at info@cmmquarterly.com for permission to reprint.
