GD&T (Geometric dimensioning and tolerancing)

GD&T (geometric dimensioning and tolerancing; within the ISO standards also referred to as geometrical product specifications, GPS) is a standardized system of symbols and rules for expressing the required accuracy of geometry on engineering drawings. It makes it possible to define unambiguously the permitted deviations of form, orientation, location and runout of geometric features.

Unlike plain size tolerances, geometric tolerances express functional requirements better, that is, what actually decides whether the part works correctly in the assembly. They include, among others, tolerances of flatness, circularity (called roundness in ISO), parallelism, perpendicularity and position. Datums are an integral part of the system: they are the references to which orientation and location tolerances are related.

GD&T is standardized by ASME Y14.5 in the United States and internationally by the ISO GPS system of standards, with ISO 1101 as the core standard (adopted in Europe as EN ISO 1101). The two systems are similar but differ in some default principles and in interpretation. For example, ASME applies the envelope requirement (Rule #1) by default, whereas ISO applies the independency principle of ISO 8015. Design engineers and quality inspectors both need a working knowledge of geometric tolerances.

Correct use of geometric tolerances reduces costs: it defines precisely what is and is not acceptable, which prevents good parts from being rejected and bad ones from being accepted. It is the language in which the design engineer tells manufacturing and metrology exactly what is needed.

Specify geometric tolerances wherever function depends on the relative position or form of surfaces, not just on their size: hole patterns for bolts and dowel pins, coaxiality of bearing seats, perpendicularity of mating faces, flatness of sealing faces or runout of rotating parts.

They are especially useful for parts that are assembled with mating parts from another manufacturer, for production parts inspected on a coordinate measuring machine (CMM), and wherever a size tolerance alone leaves room for interpretation (for example, how far a hole may be tilted).

The most common mistake is missing or poorly chosen datums. A position tolerance without a clearly defined datum reference frame cannot be measured unambiguously. Datums should reflect how the part actually seats in the assembly and how it will be fixtured during machining and inspection.

Another mistake is overtolerancing: unnecessarily tight geometric tolerances on non-functional surfaces drive up the cost of both manufacturing and inspection. State on the drawing which standard (ISO or ASME) the tolerancing follows, because the interpretation can differ. Check that your supplier has the measuring equipment to actually verify the specified tolerances and whether they will provide an inspection report.