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What is GD&T？

Due to variations in the manufacturing process, the size and dimensions of the manufactured product may differ from the original CAD model. To better control and communicate these variations, engineers and manufacturers use a symbolic language called GD&T (Geometric Dimensioning and Tolerancing).

GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, the allowable variation in the form and size of individual features, and the permissible variation between features.
- Dimensioning specifications define the nominal, as-modeled, or intended geometry. For example, a basic dimension.
- Tolerancing specifications define the allowable variation in the form and, possibly, the size of individual features, as well as the permissible variation in the orientation and location between features. Examples include linear dimensions and feature control frames using a datum reference.

Common GD&T Symbols

	Symbol	Geometric Characteristic	Feature Modifier	Datums	Datum Modifier
Form		Straightness	✓	Datums Not Allowed Form tolerances are defined to limit the deviations of a geometric feature from its ideal form.	NA
		Flatness	X		NA
		Circularity	X		NA
		Cylndricity	X		NA
Profile		Profile of a line	X	Datums sometimes required	✓
Profile		Profile of a Surface	X	Datums sometimes required	✓
Orientation		Angularity	✓	Datums Required	✓
		Perpendicularity	✓		✓
		Parallelism	✓		✓
Runout		Circular Runout	X		X
Runout		Total Runout	X		X
Location		Position	✓		✓
		Concentricity	X		X
		Symmetry	X		X

Modifiers

If no modifier follows a datum feature size, the datum feature applies regardless of material boundary.

Symbol	Modifier	Notes
	Continuous Feature	This symbol is used to identify a group of features when there is a requirement that they be teated geometrically as a single feature.
	Statistical Tolerance	Features identified as statistical toleranced should be produced with Statistical Process Control (SPC).
	Envelope requirement	The upper value of a external feature defines a virtual cylinder that the entire feature has to fit inside. Likewise a internal features minimum value defines a cylinder that has to fit inside the hole.
	Free State	Applies only when part is otherwise restrained.
	Independency requirement	This principle sets no limits to the number of errors of form possessed by individual features of a work piece. is the default for ISO tolerancing. For ANSI/ASME tolerancing this has to be specified.
	Least Material Condition (LMC)	Useful to maintain minimum wall thickness.
	Maximum Material Condition	Provides bonus tolerance only for a feature of size.
	Projected Tolerance Zone	Useful on threaded holes for long studs.
	Tangent Plane	Useful for interfaces where form is not required.
	Unequal Bilateral	Appears in the 2009 version of the standard, and refers to unequal profile distribution.

Tolerance Zones

Specifies the type of zone applied to the tolerance.

Symbol	Tolerance zone	Notes
	Controlled radius	Creates a tolerance zone defined by two arcs that are tangential to the adjacent surfaces
	Spherical diameter tolerance zone	Creates a spherical tolerance zone defined by a specified diameter
	Spherical radius tolerance zone	Creates a tolerance zone defined by two spherical segments with the minimum and maximum radii mentioned
	Diameter	This prefix can be used to signify that a tolerance zone exists of a diameter specified by the numerical value after this prefix.

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