GD&T Symbols and Terms

In plain English

Size tolerances say how big a feature may be, but not whether it is straight, flat, square or in the right place. Geometric Dimensioning and Tolerancing (GD&T) adds those controls with a set of standard symbols placed in a box on the drawing. It lets the designer say exactly what matters for the part to work, and lets inspection check it in a consistent way.

Technical definition

GD&T is the system of symbols, rules and definitions for specifying the allowed variation of form, orientation, location and run-out of features, defined in ASME Y14.5 (USA and many international companies) and in the ISO GPS standards, mainly ISO 1101. A geometric tolerance defines a tolerance zone within which the feature must lie. A datum is a theoretically exact reference (point, axis or plane) derived from a real datum feature on the part.

How it works and where it is used

A requirement is written in a feature control frame: the symbol, the tolerance value (with Ø if the zone is cylindrical, and modifiers such as Ⓜ for maximum material condition), then the datum letters in order of precedence. For example “⌖ | Ø0.1 Ⓜ | A | B | C” means the axis of the hole must lie in a cylinder of 0.1 mm diameter at the exact position relative to datums A, B and C, with extra tolerance allowed as the hole departs from its smallest size.

* ASME Y14.5-2018 removed concentricity and symmetry (position, profile or run-out are used instead); ISO 1101 still defines coaxiality/concentricity and symmetry.

In the workshop and in practice

On a drawing, GD&T tells the machinist which surfaces to set up from (the datums) and which features need the most care. Flatness and perpendicularity often decide how a part is clamped; position tolerances on bolt holes, often with MMC, allow more variation where it does not affect assembly, which can reduce cost. Inspection uses surface plates, height gauges, dial indicators and coordinate measuring machines to check the tolerance zones.

Common mistakes and misconceptions

• Mixing ASME and ISO rules. The two systems share symbols but differ in some defaults and interpretations (for example the default relationship between size and form). Check which standard the drawing title block calls up.
• Measuring from the wrong datum. Datums must be used in the order given in the frame.
• Treating geometric tolerances as size tolerances. A feature can be within its size limits and still fail flatness or position.
• Ignoring modifiers. Ⓜ and Ⓛ change the allowed tolerance as the feature size changes; without them the tolerance is fixed.

Related calculators

• Tolerance Stack-Up Calculator (Worst Case) — Worst-case maximum and minimum gap of a 1-D dimension chain, with an RSS comparison.
• Limits of Size & Tolerance Calculator — Maximum and minimum limits, tolerance and mean size from nominal size and deviations.

Related topics

• Limits of Size and Tolerance — The largest and smallest acceptable sizes of a feature, and the tolerance between them.
• Clearance, Transition and Interference Fits — How the limits of a hole and a shaft decide whether they slide, locate or press together.
• Measurement Uncertainty — The quantified doubt about a measurement result, combined from all its sources.

References

• ASME Y14.5-2018 Dimensioning and Tolerancing
• ISO 1101:2017 GPS — Geometrical tolerancing — Tolerances of form, orientation, location and run-out
• ISO 5459: GPS — Geometrical tolerancing — Datums and datum systems