GD&T (per ASME Y14.5M-1994) is an international language that is used on engineering drawings to accurately describe the size, form, orientation, and location of part features. It is also a design-dimensioning philosophy that encourages designers to define a part based on how it functions in the final product or assembly.

GD&T is an exact language that enables design engineers to "say what they mean" on a drawing, thus improving product designs and lowering cost. Process engineers and manufacturing use the language to interpret the design intent and to determine the best manufacturing approach. Quality control and inspection use the GD&T language to determine proper set-up and part verification.

By providing company-wide uniformity in the drawing specifications and interpretation, GD&T reduces controversy, guesswork, and assumptions throughout the design, manufacturing and inspection process.

Understanding
how to apply and interpret GD&T correctly will help you:

Create clear,
concise drawings

Improve product
design

Create drawings
that reduce controversy, guesswork, and assumptions throughout
the manufacturing process

Effectively
communicate or interpret design requirements for suppliers and
manufacturing

However,
because GD&T is such a precise language, it involves a great many symbols and terms. Here is a list of some of the topics involved in
geometric dimensioning and tolerancing fundamentals and a short
definition of each.

Actual
Local Size - The value of any individual distance at any cross
section of a feature of size.

Actual
Mating Envelope of an External Feature of Size - A similar
perfect feature counterpart of the smallest size that can be circumscribed
about the feature so that it just contacts the surfaces at the highest
points.

Actual Mating Envelope
of an Internal Feature of Size - A similar perfect feature
counterpart of the largest size that can be inscribed within the
feature so that it just contacts the surfaces at their highest points.

All-Around
Symbol - A circle placed on the bend of the leader line
of a profile control.

Angularity
- The condition of a surface, centerplane or axis being exactly
at a specified angle

Angularity
Control - A geometric tolerance that limits the amount a surface,
axis, or centerplane is permitted to vary from its specified angle.

ASME Y14.5M-1994
- The national standard for dimensioning and tolerancing in the United
States. ASME stands for American Society of Mechanical Engineers. The Y14.5
is the standard number. "M" is to indicate the standard is metric, and
1994 is the date the standard was officially approved.

Axis
Theory - The axis (or centerplane) of a feature of size must be within the
tolerance zone.

Basic
Dimension - A numerical value used to describe the theoretically
exact size, true profile, orientation, or location of a feature
or datum target.

Between
Symbol
- A double ended arrow that indicates the tolerance zone extends
to include multiple surfaces.

Bi-Directional
Control - Where the location of a hole is controlled to a different
tolerance value in two directions.

Bilateral
Tolerance - A tolerance that allows the dimension to vary in both the
plus and minus directions.

Bonus Tolerance
- An additional tolerance for a geometric control. Whenever a geometric
tolerance is applied to a feature of size, and it contains an MMC (or LMC) modifier
in the tolerance portion of the feature control frame, a bonus tolerance
is permissible.

Boundary - The
word "BOUNDARY" is placed beneath the feature control frames to
invoke a boundary control.

Cartoon
Gage - A sketch of a functional gage. A cartoon gage defines
the same part limits that a functional gage would, but it does not
represent the actual gage construction of a functional gage.

Circularity
- A condition where all points of a surface of revolution, at
any section perpendicular to a common axis, are equidistant
from that axis.

Circularity
Control - A geometric tolerance that limits the amount of circularity
on a part surface.

Circular
Runout - A composite control that affects the form, orientation, and
location of circular elements of a part feature relative to a datum axis.

Circular Runout
Control - A geometric tolerance that limits the amount of circular
runout of a part surface.

Coaxial
Datum Features - When coaxial diameters are used to establish
a datum axis.

Coaxial Diameters
- Two (or more) diameters that are shown on the drawing as being on the
same centerline (axis).

Composite Control
- Controls the form, location, and orientation of a part feature
simultaneously (in a single gage reading).

Concentricity
- The condition where the median points of all diametrically
opposed elements of a cylinder (or a surface of revolution)
are congruent with the axis of a datum feature.

Concentricity
Control - A geometric tolerance that limits the concentricity
error of a part feature.

Controlled
Radius - A radius with no flats or reversals allowed. The
symbol for a controlled radius is "CR."

Coordinate
Tolerancing - A dimensioning system where a part feature is
located (or defined) by means of rectangular dimensions with given
tolerances.

Coplanar
Datum Features - Two or more datum features that are on the same plane.

Coplanar
Surfaces - Two or more surfaces that are on the same plane.

Cylindrical
Feature of Size - Contains one feature: the cylindrical surface.

Cylindricity
- A condition of a surface of revolution in which all points
of the surface are equidistant from a common axis.

Cylindricity
Control - A geometric tolerance that limits the amount of cylindricity
error permitted on a part surface.

Datum
- A theoretically exact plane, point or axis from which a dimensional measurement
is made.

Datum
Feature - A part feature that contacts a datum.

Datum
Feature Simulator - The inspection equipment (or gage surfaces)
used to establish a datum.

Datum Reference
Frame - A set of three mutually perpendicular datum planes.

Datum Shift
- The allowable movement, or looseness, between the part datum feature
and the gage.

Datum System
- A set of symbols and rules that communicate to the drawing user how dimensional
measurements are to be made.

Datum
Target - A symbol that describes the shape, size, and location
of gage elements that are used to establish datum planes or
axes.

Dimension
- A numerical value expressed in appropriate units of measure and
used to define the size, location, orientation, form, or other geometric
characteristics of a part.

Engineering
Drawing - A document that communicates a precise description of a part.
This description consists of pictures, words, numbers, and symbols.

Equal Bilateral
Tolerance - A tolerance where the allowable variation from the
nominal value is the same in both directions.

Feature
- A general term applied to a physical portion of a part, such as
a surface, hole, or slot.

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Feature
Control Frame - A rectangular box that is divided into compartments
within which the geometric characteristic symbol, tolerance
value, modifiers, and datum references are placed.

Feature
of Size - One cylindrical or spherical surface, or a set
of two opposed elements or opposed parallel surfaces, associated
with a size dimension.

Feature of
Size Dimension - A dimension that is associated with a feature of size.

Fixed Fastener
Assembly - Where the fastener is held in place (restrained) into one
of the components of the assembly.

Fixed
Fastener Formula - H = F + 2T or T =
Where:
T = position
tolerance diameter
H = MMC
of the clearance hole
F = MMC
of the fastener

Flatness
- The condition of a surface having all of its elements in one
plane.

Flatness
Control - A geometric tolerance that limits the amount of flatness
error a surface is allowed.

Floating
Fastener Assembly - Where two (or more) components are held together
with fasteners (such as bolts and nuts), and both components have clearance
holes for the fasteners.

Floating
Fastener Formula - T = H - F
Where:
T = position
tolerance diameter (for each part)
H = MMC of
the clearance hole
F= MMC of the
fastener

Functional
Dimensioning - A dimensioning philosophy that defines a part based
on how it functions in the final product.

Functional
Gage - A gage that verifies functional requirements of part features
as defined by the geometric tolerances.

Fundamental Dimensioning
Rules - A set of general rules defined by ASME for dimensioning
and interpreting drawings.

Geometric
Characteristic Symbols (14) - The symbols are divided into five categories:
form, profile, orientation, location, and runout.

Geometric
Dimensioning and Tolerancing (GD&T) - A set of fourteen symbols
used in the language of GD&T. It consists of well-defined of symbols,
rules, definitions and conventions, used on engineering drawings to accurately
describe a part. GD&T is a precise mathematical language that can be
used to describe the size, form, orientation, and location of part features.
GD&T is also a design philosophy on how to design and dimension parts.

Go Gage
- A gage that is intended to fit into (for an internal feature of size) or fit over
(for an external feature of size) the feature of size.

Great Myth
of GD&T - The misconception that geometric tolerancing raises product
costs.

Implied Basic
90° Angles -A 90° basic angle applies where centerlines of
features in a pattern (or surfaces shown at right angles on a drawing)
are located and defined by basic dimensions, and no angle is specified.

Implied Basic
Zero Dimension - Where a centerline or centerplane of a feature of size is shown
in line with a datum axis or centerplane, the distance between the centerlines
or centerplanes is an implied basic zero.

Implied Datum
- An assumed plane, axis, or point from which a dimensional measurement
is made.

Inclined
Datum Feature - A datum feature that is at an angle other than 90o,
relative to the other datum features.

Inner Boundary
(IB) - A worst-case boundary generated by the smallest feature of size
minus the stated geometric tolerance (and any additional tolerance, if
applicable).

International Standards
Organization (ISO) - The organization that published an associated
series of standards on dimensioning and tolerancing.

Least
Material Condition - The condition in which a feature of
size contains the least amount of material everywhere within
the stated limits of size.

Limit
Tolerance - When a dimension has its high and low limits stated.
In a limit tolerance, the high value is placed on top, and the low
value is placed on the bottom.

Maximum
Material Condition - The condition in which a feature of
size contains the maximum amount of material everywhere within
the stated limits of size.

Median
Point - The mid-point of a two-point measurement.

Modifiers
(8) - Communicate additional information about the drawing or tolerancing
of a part.

Multiple
Single-Segment Profile Control - When two or more profile controls
are tolerancing a surface relative to different datums.

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Multiple
Single-Segment Tolerance of Position Control - When two (or more) single segment tolerance of position
callouts are used to define the location, spacing, and orientation of a
pattern of feature of size.

No-Go Gage
- A gage that is not intended to fit into or over a feature of size. A No-Go gage is
made to the LMC limit of the feature of size.

Non-Feature
of Size Dimension - A dimension that is not associated with a feature of size.

Outer Boundary (OB)
- A worst-case boundary generated by the largest feature of size
plus the stated geometric tolerance (and any additional tolerance,
if applicable).

Parallelism
- The condition that results when a surface, axis or centerplane
is exactly parallel to a datum.

Parallelism
Control - A geometric tolerance that limits the amount a surface,
axis, or centerplane is permitted to vary from being parallel to
the datum.

Perpendicularity
- The condition that results when a surface, axis, or centerplane
is exactly 90° to a datum.

Perpendicularity
Control - A geometric tolerance that limits the amount a surface,
axis, or centerplane is permitted to vary from being perpendicular
to the datum.

Planar Datum
- The true geometric counterpart of a planar datum feature.

Planar Feature of Size
- A feature of size that contains two features: the two parallel plane surfaces.

Plus-minus
Tolerance - The nominal or target value of the dimension is given first,
followed by a plus-minus expression of a tolerance.

Primary Datum
- The first datum plane that the part contacts in a dimensional measurement.

Profile-
The outline of a part feature in a given plane.

Profile
Control - A geometric tolerance that specifies a uniform boundary
along the true profile that the elements of the surface must lie
within.

Profile
of a Line Control - A geometric tolerance that limits the
amount of error for line elements relative to their true profile.

Profile
of a Surface Control - A geometric tolerance that limits
the amount of error a surface can have relative to its true
profile.

Projected
Tolerance Zone - A tolerance zone that is projected above
the part surface.

Radius
- A straight line extending from the center of an arc or circle
to its surface.

Regardless
of Feature Size - The term that indicates a geometric tolerance applies
at any increment of size of the feature, within its size tolerance.

Rule #1
- Where only a tolerance of size is specified, the limits of size of an
individual feature prescribe the extent to which variations in its form--as
well as in its size--are allowed.

Rule #2 - RFS
applies, with respect to the individual tolerance, datum reference,
or both, where no modifying symbol is specified.

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Secondary
Datum - The second datum plane that the part contacts in a dimensional
measurement

Simulated
Datum - The plane (or axis) established by the datum feature simulator.

Simultaneous
Engineering - A process where design is a result of input from marketing,
engineering, manufacturing, inspection, assembly, and service.

Special-Case Feature of Size
Datum - When a feature of size datum feature is referenced at MMC, but simulated
in the gage at a boundary other than MMC.

Straightness
(Axis or Centerplane) - The condition where an axis is a
straight line (or, in the case of a centerplane, each line element
is a straight line).

Straightness
of a Line Element - The condition where each line element (or
axis or centerplane) is a straight line.

Straightness
Control (Feature of Size) - A geometric tolerance that, when applied to a feature of size,
limits the amount of straightness error allowed in the axis or centerplane.

Straightness Control
(Surface) - A geometric tolerance that, when directed to a surface,
limits the amount of straightness error allowed in each surface
line element.

Symmetry
- The condition where the median points of all opposed elements
of two or more feature surfaces are congruent with the axis
or centerplane of a datum feature.

Symmetry
Control - A geometric tolerance that limits the symmetry error
of a part feature.

Tertiary
Datum - The third datum plane that the part contacts in a dimensional
measurement.

3-2-1 Rule
- Defines the minimum number of points of contact required for a part datum
feature with its primary, secondary, and tertiary datum planes.

Tolerance
- The total amount that features of the part are permitted to vary from
the specified dimension.

Tolerance
Analysis Chart - A means of graphically displaying the limits of a
part as defined by the print specifications

Tolerance
of Position Control - A geometric tolerance that defines the
location tolerance of a feature of size from its true position.

Tolerance Stack
- A calculation used to find the extreme max. or min. distance on
a part.

Total
Runout - A composite control affecting the form, orientation,
and location of all surface elements of a diameter (or surface)
relative to a datum axis.

Total
Runout Control - A geometric tolerance that limits the amount
of total runout of a surface.

True Geometric
Counterpart - The theoretical perfect boundary or best fit tangent
plane of a specified datum feature.

True Position
- The theoretically exact location of a feature of size as defined by basic dimensions.

True Profile -
The exact profile of a part feature as described by basic dimensions.

Unequal
Bilateral Tolerance - A tolerance where the allowable variation
is from the target value, and the variation is not the same in both
directions.

Unilateral
Tolerance - A tolerance where the allowable variation from the target
value is all in one direction and zero in the other direction.

Variable
Gage - A gage capable of providing a numerical reading of a part parameter.

Virtual Condition
(VC) - A worst-case boundary generated by the collective effects of
a feature of size specified at MMC or at LMC and the geometric tolerance
for that material condition.

Virtual Condition
Boundary Theory - A theoretical boundary limits the location of the
surfaces of a feature of size.

Worst-case
Boundary (WCB) - A general term to refer to the extreme boundary of
a feature of size that is the worst-case for assembly. Depending upon the part dimensioning,
a worst-case boundary can be a virtual condition, inner boundary, or outer
boundary.

Zero Tolerance at
MMC - A method of tolerancing part features that includes the
tolerance geometric value with the feature of size tolerance and states a zero
at MMC in the feature control frame.

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The Ultimate
Pocket Guide on Geometric Dimensioning and Tolerancing is a handy
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at your fingertips. More
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