Metrology
According to ISO 8015, the principle of duality applies. This separates specification and verification from one another. Simply put: the tolerancing does not specify the means by which the measurement should be performed. Even with complete, standard-compliant tolerancing, there remains freedom in measurement that is not defined by the drawing.
Practice Meets Theory
Manufacturers of measurement machines and measurement software were forced to solve geometric problems before ISO GPS existed as a coherent set of rules. Different manufacturers have implemented different solutions in their products. This partly explains discrepancies between the theoretical rules of the ISO and common practice in measurement laboratories. Furthermore, the ISO rules do not yet cover all cases that make sense in practice. Compromises are necessary and require knowledge of standards and metrological practice.
Gauges
Gauges allow for fast and simple inspection of components. This can be carried out during production with up to 100 percent of the parts. In this way, important functions—typically assemblability—can be ensured cost-effectively. Especially when components are delivered to the customer for further assembly, assemblability should be given high priority. As a supplier, you don’t want to be responsible for a line stoppage. Such gauges cannot check individual dimensions; rather, they always inspect a combination of size dimensions, form, and positional tolerances. Designing a gauge requires in-depth knowledge of how these types of tolerances interact and how virtual limit states—and thus the dimensioning of the gauge—are defined.