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GD&T Definitions
From ASME Y14.5M-1994
40
Engineering
Not Applicable
10/03/2016

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Term
Inner Boundry
Definition
A worst case boundry (that is, locus) generated by the smallest feature (MMC for an internal feature and LMC for an external feature) minus the stated geometric tolerance and any additional goemetric tolerance (if applicable) from the features departure from its specified material condition.
Term
Outer Boundry
Definition
A worst case boundry (that is, locus) generated by the largest feature (LMC for an internal feature and MMC for an external feature) plus the stated geometric tolerance and any additional goemetric tolerance (if applicable) from the features departure from its specified material condition.
Term
Datum
Definition
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.
Term
Datum Feature
Definition
An actual feature of a part that is used to establish a datum.
Term
True Geometric Counterpart
Definition
The theoretically perfect boundry (virtual condition or actual mating envelope) or best-fit (tangent) plane of a specified datum feature.
Term
Datum feature simulator
Definition
A surface of adequately precise form (such as a surface plate, a gage surface, or mandrel) contacting the datum feature(s) and used to establish the simulated datums.
Term
Simulated datum
Definition
A point, axis, ot plane established by processing or inspection equipment, such as the following simulators: surface plate, a gage surface, or mandrel.
Term
Datum target
Definition
A specified point, line, or area on a part used to establish a datum.
Term
Dimension
Definition
A numerical value expressed in appropriate units of measure and used to define the size, location, and geometric characteristis, or surface texture of a part or part feature.
Term
Basic dimension
Definition
A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or a datum target. It is the basis from which permissible variations are established by tolerances on other dimensions, in notes, or in feature control frames.
Term
Reference Dimension
Definition
A dimension, usually without tolerance, used for information purposes only. A reference dimension is a repeat of a dimension or is derived from other values shown on the drawing or on related drawings. It is considered auxiliary information and does not govern production or inspection opertions.
Term
Feature
Definition
A general term appiled to a physical portion of a part, such as a surface, pin, tab, hole, or slot.
Term
Axis of a feature
Definition
A straight line that coincides with the axis of the true geometric counterpart of the specified feature.
Term
Center plane of a feature
Definition
A plane that coincides with the center plane of the true geometric counterpart of the specified feature.
Term
Derived median line of a feature
Definition
An imperfect line (abstract) that passes through the center points of all cross sections of the feature. These cross sections are normal to the axis of the actual mating envelope. The cross section center points are determined as per ANSI B89.3.1.
Term
Derived median plane of a feature
Definition
An imperfect plane (abstract) that passes through the center points of all line segments of the feature. These line segments are normal to the actual mating envelope.
Term
Feature of size (FOS)
Definition
One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension.
Term
Individual features of size (Rule #1)
Definition
Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its geometric form, as well as size, are allowed
Term
Maximum Material Contition (MMC)
Definition
The condition in which a feature of size contains the maximum amount of material within the stated limits if size -- for example, minimum hole diameter, maximum shaft diameter.
Term
Least Material Contition (LMC)
Definition
The condition in which a feature of size contains the least amount of material within the stated limits if size -- for example, maximum hole diameter, minimum shaft diameter.
Term
Regardless of Featue Size (RFS)
Definition
The term used to indicate that a geometric tolerance or datum reference applies at any increment of size of the feature within its tolerance.
Term
Resultant Contition
Definition
The variable boundry generated by the collective effects of a size feature's specified MMC or LMC material condition, the geometric tolerance for that material condition, the size tolerance, and the additional geometric tolerance derived from the feature's departure from its specified material condition.
Term
Virtual Condition
Definition
A constant boundry generated by the collective effects of a size feature's specified MMC or LMC material condition and the geometric tolerance for that material condition.
Term
Actual Size
Definition
The general term for the size of a produced feature. This term includes the actual mating size and the actual local sizes.
Term
Actual Local Size
Definition
The value of any indiviual distance at any cross section of a feature.
Term
Actual Mating Size
Definition
The dimensional value of the actual mating envelope.
Term
Actual Mating Envelope
Definition
This term is defined according to the type of feature, as follows"

a) For an external feature. A similar perfect counterpart of smallest size that can be circumscribed about the feature so that it just contacts the surface at the highest points. For example, a smallest cylinder of perfect form or two parallel planes of perfect form at minimum separation that just contact(s) the highest points of the surface(s).

b) For an internal feature. A similar perfect counterpart of largest size that can be inscribed within the feature so that it just contacts the surface at the highest points. For example, a largest cylinder of perfect form or two parallel planes of perfect form at maximum separation that just contact(s) the highest points of the surface(s).

For features controled by orientation or positional tolerances, the actual mating envelope is oriented relative to the appropriate datum(s), for example, perpendicular to a primary datum plane.
Term
Limits of Size
Definition
The specified maximum and minimum sizes.
Term
Nominal Size
Definition
The designation used for purposes of general identification.
Term
Resultant Contition Size
Definition
The actual value of a resultant contition boundry.
Term
Virtual Contition Size
Definition
The dimensional value of a virtual contition boundry.
Term
Tangent Plane
Definition
A theoretically exact plane derived from the true geometric counterpart of the specified feature surface.
Term
Tolerance
Definition
The total amount a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits.
Term
Bilateral Tolerance
Definition
A tolerance in which variation is permitted in both directions from the specified dimension.
Term
Geometric Tolerance
Definition
The general term applied to the category of tolerances used to control form, profile, orientation, location, and runout.
Term
True Position
Definition
The theoretically exact location of a feature established by Basic dimensions.
Term
Unilateral Tolerance
Definition
A tolerance in which variation is permitted in one direction from the specified dimension.
Term
Full Indicator Movement (FIM)
Definition
The total movement of an indicator where appropriately applied to a surface to measure its variations.
Term
FRTZF
Definition
Feature-Relating Tolerance Zone Framework
Term
PLTZF
Definition
Pattern-Locating Tolerance Zone Frame
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