The Importance of GR&R in delivering high-precision measurement systems - Part 1.
In any industrial inline process, variation in the measurement system can cause problems such as rejected or defective products. One statistical tool that is used to test the variation of a measurement system and its ability to control the production process is called a Gauge Repeatability and Reproducibility (GR&R) study.
Measurement System Analysis using GR&R
A measurement system is a combination of instruments (such as a 3D sensor) and operators (people who set up the measurement, place the part, and manually measure the target with a gauge). GR&R measurement system analysis is an approach used to evaluate the ability of such a system to produce reliable measurements over time.
There are two main types of metrics in GR&R:
- 1. Repeatability – the ability of one device or operator to achieve the same results when measuring the same dimension after repeated trials.
- 2. Reproducibility – the ability of multiple devices or operators to achieve the same results when using the same gauge to measure the same dimension on the same parts after repeated trials.
A measurement system’s repeatability and reproducibility are assessed using GR&R calculated as:
- Precision to tolerance (P/T Ratio) – the ratio between the measurement variation (typically 3 or 6 standard deviations), and the tolerance band; and,
- Percentage of process variation – the ratio between the standard deviation of the measurement variation, and the standard deviation of the observed process, expressed as a percentage.
Why P/T Ratio is Important
"P/T ratio is the ratio of the precision of a measurement system to the (total) tolerance of the manufacturing process of which it is a part. If the P/T ratio is low, the impact on product quality of variation due to the measurement system is small. If the P/T ratio is larger, it means the measurement system is "eating up" a large fraction of the tolerance, in that the parts that do not have sufficient tolerance may be measured as acceptable by the measurement system.
Generally, a P/T ratio less than 0.1 indicates that the measurement system can reliably determine whether any given part meets the tolerance specification. A P/T ratio greater than 0.3 suggests that unacceptable parts will be measured as acceptable (or vice versa) by the measurement system, making the system inappropriate for the process for which it is being used." (Source: Wikipedia) (For a more technical definition of P/T ratio, visit NIST)
As you'll discover in Part 2, P/T ratio is critical in determining whether or not a measurement system—and the sensors used in that system—can provide adequately repeatable and reproducible data on a given part or feature.