GD&T Geometric Dimensioning and Tolerancing

What is the purpose of Geometric Dimensioning & Tolerancing?

Geometric dimensioning and tolerancing (GD&T) is a system of symbols used on engineering drawings to communicate information from the designer to the manufacturer through engineering drawings.

GD&T tells the manufacturer the degree of accuracy and precision needed for each controlled feature of the part. GD&T is used to define the nominal geometry of parts and assemblies and to define the allowable variation of features.

Common GD&T Symbols

	Symbol	Geometric Characteristic	Feature Modifier	Datums	Datum Modifier	Bonus Tolerance
Form		Straightness	✓	Datums Not Allowed Form tolerances are defined to limit the deviations of a geometric feature from its ideal form.	NA	✓ Only at MMC or LMC
		Flatness	X		NA	X
		Circularity	X		NA	X
		Cylindricity	X		NA	X
Profile		Profile of a line	X	Datums sometimes required	✓	X
Profile		Profile of a Surface	X	Datums sometimes required	✓	X
Orientation		Angularity	✓	Datums Required	✓	✓ Only at MMC or LMC
		Perpendicularity	✓		✓	✓ Only at MMC or LMC
		Parallelism	✓		✓	✓ Only at MMC or LMC
Runout		Circular Runout	X		X	X
Runout		Total Runout	X		X	X
Location		Position	✓		✓	✓ Only at MMC or LMC
		Concentricity	X		X	X
		Symmetry	X		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 treated geometrically as a single feature.
	Statistical Tolerance	Features identified as "statistical tolerance" should be produced with Statistical Process Control (SPC).
	Envelope requirement	The upper value of an external feature defines a virtual cylinder that the entire feature has to fit inside. Likewise an 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 must 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

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.

Feature Control Frames

In GD&T (Geometric dimensioning and tolerancing), a feature control frame is required to describe the conditions and tolerances of a geometric control on a part's feature. The feature control frame includes four parts:

GD&T symbol/control symbol
Tolerance zone type and dimensions
Tolerance zone modifiers - such as features of size
Datum references (if required by the GD&T Symbol)

Combined, the feature control frame provides all the information you need to measure the geometric tolerance of the features of the part and determine if the part is in spec.