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Monday, July 7, 2014

Gauge R & R Study: The TV Report (Part 3 of 5)

The total variation (TV) report analyzes the gauge study data as a percentage of total variation. The report is divided into sections presenting the equipment variation, appraiser variation, part variation and the gauge R&R.
Equipment variation (EV) represents the repeatability of the equipment or measurement device. On the TV chart, it is presented as a percentage of the total variation of the system (see below). The equipment variation percentage is 17.6% of the total variation of the system. While the result in the example is low, a high percentage, greater than 30%, would tell operators that they have issues with the measurement equipment itself that must be resolved. The gauge may need maintenance or perhaps the fixture holding the part for measurement is not adequate.

Appraiser variation (AV) represents the reproducibility of the system. The TV chart also reports the AV as a percentage of the total variation of the system. A high percentage here, greater than 30%, indicates a large operator-to-operator difference. A possible cause could be operators not following proper measurement procedure, not trained properly or perhaps trained in different methods.

Part variation (PV) represents the variation of the products or parts used to conduct the gauge study. In the example, the PV% is 96.4%. One would expect to see a high percentage of the total variation from the parts on an adequate system where the parts truly represent the range of the process variation. If the PV% is low, less than 30%, the parts selected do not represent the full variation of the process.

Gage repeatability and reproducibility (GR&R) represent both the equipment variation and the appraiser variation. The GR&R% returned in the TV report is the percentage of the total variation of the system used by the measurement system. In Figure 1, GR&R% is 26.7%. This means that 26.7% of the total variation is due to the measurement system itself. Rule of thumb for GR&R is that a result of 10% or less means that the system is acceptable. Most of the variation is from the parts and not the measurement system.

If the GR&R is less than 30%, the system may be accepted, but there should be some plan to review the system for improvement. A GR&R result of greater than 30% shows that the system must be improved as the appraisers and equipment contribute to more than 30% of the system variation.
Another critical calculation in determining if the gauge study is valid is the number of distinct categories (NDC). The NDC is the calculation for the number of non-overlapping 97% confidence intervals that span the product variation. In other words, it is basically the part variation divided by the GR&R result and multiplied by a constant. The NDC should be five or greater for the study to be considered valid.

As discussed earlier, a common problem with gauge studies is that the part variation does not represent the expected process variation. If the part variation is too small, then the NDC will be below five and the study should not be considered valid. In this case, operators may sample again to obtain parts that do represent the full range of process variation. Remember to evaluate both part variation and gauge R&R. If the gauge R&R itself is too large, then the NDC will be low as well. If this occurs, operators could have an issue with AV, EV or the measurement system itself.

When evaluating the TV report results, remember that these results are calculated against the process variation only. In the example, the GR&R% is 26.7% of the total variation. This report shows which section of the measurement system may need improvement, but it does not show how the system works within the actual tolerance of the parts. To evaluate the measurement system against the tolerance, one must use the total tolerance report.

This series of blogs will include:
  • What does it measure?
  • Conducting a Study
  • Evaluating the Gauge Study: The TT Report
  • Evaluating the Gauge Study: The Average and Mean Report
REFERENCE: Quality Magazine, Sept. 1, 2009