The Importance of GR&R in Delivering High-Precision Measurement Systems – Part 2
GR&R Compliance and The Rule of Ten
Acceptance of a measurement system to inspect a part at a specified tolerance is determined based on the following GR&R criteria:
- If less than 10% of the total measurement tolerance, the gauge is acceptable. A P/T ration less than 0.1 indicates that the measurement system can reliably determine whether any given part meets the tolerance specification.
- If between 11-30% of the total measurement tolerance, the gauge is acceptable for this application only, and must be closely monitored.
- If greater than 31% of the total measurement tolerance, the gauge is unacceptable for use in this application. 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 insufficient for the process in which it is being used.
Simply stated, the “Rule of Ten”* (or “one to ten”) is that the resolution of the measuring instrument should be ten times as precise as the tolerance to be measured. For example, if the tolerance is 0.1 mm, then the total of GR&R for the measurement system should be 0.01 mm or better. The combination of measurement variation introduced by both instrument and human error should not total more than 10% of the tolerance being measured.
Many companies experience difficulties regarding product conformance merely because they fail to apply this rule when choosing a measurement instrument to determine adherence to specifications.
3D Sensors and GR&R
GR&R can be used in many ways, but system integrators typically use it to test their measurement systems as if they were being used by a technician in a plant. Most of today’s measurement systems are driven by 2D machine vision, with a growing number leveraging 3D machine vision.
The sensor is only a part of the overall measurement system; each sensor can only take up a fraction of the total measurement tolerance. This means that if the acceptable system tolerance is +/- 0.01 mm, the sensor resolution should be 0.001 mm (i.e. 1% of the total measurement tolerance of 0.1 mm).
Advantages of 3D Over 2D for GR&R Compliance
In general, 3D sensors are superior to 2D machine vision solutions because, in a real-world factory environment, parts aren’t always positioned correctly on the conveyor. 2D does not provide the necessary geometric data to correct for out-of-position targets in the sensor’s field of view.
With 3D data and smart measurement tools, technicians can set an “anchor” off a reference feature on a 3D height map of a part to generate high measurement repeatability despite part position variation.
In general, part recognition, alignment, and measurement anchoring are more robust when you have height-map information as opposed to just 2D images.
GR&R – A Requisite Test for Optimal Factory Production
GR&R studies are invaluable for understanding and analyzing variation in a measurement system. Since GR&R can generate continuous and attributive data, it is useful in both manufacturing and lab metrology processes.
In general, 3D smart sensors are the best solution to provide the necessary level of reproducibility and repeatability to satisfy strict GR&R standards––making them a reliable alternative to traditional 2D solutions and the right organizational choice to deliver robust industrial inspection.