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GD & T Symbols, Terms and Definitions

Geometric Dimensioning & Tolerancing (GD & T) (sometimes refered to as GDT) Is a set of standard symbols which are used to define parts and assembly features and their tolerance zones in dimensioning engineering drawings. Also, it defines a part based on how it functions. Geometric Dimensioning & Tolerancing helps individuals to understand the design intent by allowing better tools for describing the drawings.
Currently, ASME Y14.5M - 1994 is the accepted geometric dimensioning and tolerancing standard.



COMMON TERMS AND DEFINITIONS

Symbol
Name
Description
basic
Basic Dimension 
A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target. It is the basis from which permissible variations are established by tolerances on other dimensions, in notes, or in feature control frames.
datum
Datum
A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometric characteristics of features of a part are established.
Datum Target
Datum Target
A specific line, or area on a part used to establish a datum.
datum point
Datum Point
mmc
Maximum Material Condition (MMC)
The condition in which a feature of size contains the maximum amount of material within the stated limits of size-for example, minimum hole diameter, maximum shaft diameter.
lmc
Least Material Condition (LMC)
The condition in which a feature of size contains the least amount of material within the stated limits of size-for example, maximum hole diameter, minimum shaft diameter.
rfs
Regardless of Feature Size (RFS)
The term used to indicate that a geometric tolerance or datum reference applies at any increment of size of the feature within its size tolerance.
Full Indicator Movement (FIM)
The total movement of an indicator when appropriately applied to a surface to measure its variations (formerly called total indicator reading-TIR).
Virtual Condition
The boundary generated by the collective effects of the specified MMC limit of size of a feature and any applicable geometric tolerances.


Symbol
Name
Description
Feature Control Frame
Feature Control Frame
The feature control frame consists of: A) type of control (geometric characteristic), B) tolerance zone, C) tolerance zone modifiers (i.e., MMC or RFS), D) datum references if applicable and any datum reference modifiers.


TOLERANCES

Symbol
Name
Description
Tolerance
flatness
Flatness
A two dimensional tolerance zone defined by two parallel planes within which the entire surface must lie.
Form
straightness
Straightness
A condition where an element of a surface or an axis is a straight line.
circularity
Circularity
A condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface intersected by any plane perpendicular to a common axis (cylinder, cone) or passing through a common center (sphere) are equidistant from the axis of the center.
cylindricity
Cylindricity
A condition on a surface of revolution in which all points of the surface are equidistant from a common axis.
perpendicular
Perpendicularity (squareness)
 The condition of a surface, axis, median plane, or line which is exactly at 90 degrees with respect to a datum plane or axis.
Orientation
angularity
Angularity
The distance between two parallel planes, inclined at a specified basic angle in which the surface, axis, or center plane of the feature must lie.
parallel
Parallelism
The condition of a surface or axis which is equidistant at all points from a datum of reference.
position
True Position
A zone within which the center, axis, or center plane of a feature of size is permitted to vary from its true (theoretically exact) position.
Location
concentricity
Concentricity
A cylindrical tolerance zone whose axis coincides with the datum axis and within which all cross-sectional axes of the feature being controlled must lie. (note: this is very expensive and time consuming to measure. Recommended that you try position or runout as an alternative tolerance.)
line profile
Profile of a Line
A uniform two dimensional zone limited by two parallel zone lines extending along the length of a feature.
Profile
surface point
Profile of a Surface
A uniform three dimensional zone contained between two envelope surfaces separated by the tolerance zone across the entire length of a surface.
runout
Runout
A composite tolerance used to control the relationship of one or more features of a part to a datum axis during a full 360 degree rotation about the datum axis.
Each circular element of the feature/part must be within the runout tolerance.
Runout
total runout
Total Runout
A composite tolerance used to control the relationship of one or more features of a part to a datum axis during a full 360 degree rotation about the datum axis.

References
Anonymous, 1994, Dimensioning and Tolerancing, ANSI Y14.5M-1994, ASME International, New York