Metrology is the science of measurement and its applications. In manufacturing, it covers dimensional inspection: checking, with suitable calibrated equipment, that the part meets the dimensions, tolerances and surface finish on the drawing. Measurement, which gives a value, is distinguished from inspection, which decides whether the part is compliant.
The equipment ranges from simple to sophisticated: calliper, micrometer, dial indicator, plug or ring gauges, profile projector, roughness tester, coordinate measuring machine, 3D scanner and, for internal shapes, tomography. Alongside dimensional measurement there are non-destructive tests (dye penetrant, magnetic particle, ultrasonic, radiography), which look for defects without damaging the part.
Three notions underpin the quality of a measurement. Calibration links each instrument to reference standards at regular intervals; laboratories accredited under ISO/IEC 17025 offer the strongest guarantee. Uncertainty expresses the doubt about the measured value: the equipment must be markedly more precise than the tolerance being checked, otherwise parts that are not compliant may be declared compliant, or the reverse. Finally temperature: the reference is 20 °C (ISO 1), and fine measurements are made in an air-conditioned room, because parts expand. An aluminium or plastic part coming warm off the machine cannot be measured like a stabilised part.
Workshops organise inspection at several levels: operator self-inspection during production, first-part approval, final inspection by sample or 100%, with a report. The most complete start-up inspection is the first article.
In a quote request
List the characteristics to inspect (ideally on a marked-up drawing), the frequency (first part, sampling, 100%), the measuring method imposed if any, and the documents expected: dimensional report, certificate of conformity. Consult inspection and metrology workshops and the guide on dimensional inspection and first article.