Gauge Repeatability and Reproducibility

Variance Source

Root cause investigations identify whether a lack of precision stems from the mechanical tool or the human technician. This variance source is the main output of a gauge repeatability and reproducibility study and guides the subsequent improvement efforts. Repeatability refers to the variation observed when the same person measures the same part multiple times with the same tool.

Reproducibility refers to the variation observed when different people use the same tool to measure the same part. If the repeatability is poor, the tool may be worn or poorly calibrated. If the reproducibility is poor, the workers may need better training or more detailed instructions.

Equipment Precision

Tooling capability assessments measure the inherent limitations of the physical measuring device under controlled conditions. This equipment precision is a fundamental part of gauge repeatability and reproducibility and is affected by environmental factors like temperature and vibration. High resolution digital calipers or coordinate measuring machines offer greater precision than manual scales.

Wear on the contact points of a micrometer can lead to inconsistent readings over time. The cost of calling for a tool replacement too early is the unnecessary capital expenditure on new hardware. However, using an imprecise tool leads to the shipment of defective parts or the rejection of perfectly good ones.

Maintenance schedules must include regular checks of the gauge’s accuracy against a known standard.

Operator Consistency

Human performance analysis evaluates the skill and attention to detail of the individuals performing the measurements. This operator consistency is the final component of gauge repeatability and reproducibility and is often the most difficult to control. Variations in how a technician holds a part or applies pressure to a gauge can change the result.

Standard operating procedures must be followed exactly to ensure that every measurement is taken the same way. A pilot result from a highly skilled technician might not represent the actual performance of the entire production crew. The study involves multiple operators measuring a set of parts in a random order to eliminate bias.

Results are analyzed using an analysis of variance to determine if the measurement system is acceptable for use. An acceptable system typically has a variance of less than ten percent of the total process variation. Continuous training ensures that the staff maintains the required level of proficiency.

Precise measurements are the only way to prove that a manufacturing process is under control.