GD&T CONCEPTS

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).

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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 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.

or slot.

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.

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

GD&T Definitions

Class Vocabulary

Term Definition

angle plate

A precise measurement device used to establish an accurate 90° vertical surface.

angularity

A three-dimensional geometric tolerance that controls how much a surface, axis, or plane can deviate from the angle described in the design specifications.

ASME

The American Society of Mechanical Engineers. ASME is an organization that publishes technical materials and sets industrial and manufacturing standards.

bonus tolerance

Additional tolerance that applies to a feature as its size shifts from a stated material condition. Both MMC and LMC allow bonus tolerance.

circular runout

A two-dimensional geometric tolerance that controls the form, orientation, and location of multiple cross sections of a cylindrical part as it rotates.

circularity

A two-dimensional geometric tolerance that controls how much a feature can deviate from a perfect circle.

concentric

Sharing the same center.

concentricity

A three-dimensional geometric tolerance that controls how much the median points of multiple diameters may deviate from the specified datum axis.

coordinate tolerancing

A system for describing the design of a part that compares its features to distances along three linear axes. These axes create an imaginary rectangular grid.

cross section

A section of a feature that is formed by an intersecting imaginary plane.

cylindricity

A three-dimensional geometric tolerance that controls how much a feature can deviate from a perfect cylinder.

datum

An imaginary, perfect geometric shape or form. A perfect point, line, flat plane, circle, or cylinder are all examples of possible datums.

datum feature

A physical feature that acts as an acceptable substitute for a datum. Datum features relate the various features of the part to each other.

datum reference frame

Three imaginary planes perpendicular to one another that are mapped onto the part to relate features to each other.

feature

A physical feature of a part that naturally contains variation and imperfections. A corner, edge, flat surface, or hole are all examples of possible features.

feature control frame

A series of compartments containing symbols and values that describe the tolerance of a feature. The order and purpose of these compartments follow a consistent standard.

fixture

A dedicated work holding device used to locate and hold a part during machining or inspection.

flatness

A three-dimensional geometric tolerance that controls how much a feature can deviate from a flat plane.

form tolerance

A group of geometric tolerances that limit the amount of error in the shape of a feature. Form tolerances are independent tolerances.

functional gage

A gage for a specific part that quickly checks its form and fit in a manner similar to its intended use.

geometric dimensioning and tolerancing

An international standard for communicating instructions about the design and manufacturing of parts. GD&T uses universal symbols and emphasizes the function of the part.

granite surface plate

A precise, flat plate made of granite that is used to establish a datum plane for inspection. Granite surface plates are available in standardized grades.

individual tolerance

A tolerance that does not require a specified datum.

ISO

The International Organization for Standardization. ISO is an organization based in Switzerland that develops and publishes standards for its international membership base.

least material condition

The point at which a feature contains the least amount of material within its acceptable size limit. The largest acceptable hole and the smallest acceptable shaft are examples of LMC.

location tolerance

A group of geometric tolerances that limit the location or placement of features. Location tolerances are related tolerances.

material condition modifier

One of three modifiers that further define the tolerance of a feature in relation to its acceptable size limits.

maximum material condition

The point at which a feature contains the greatest amount of material within its acceptable size limit. The smallest acceptable hole and the largest acceptable shaft are examples of MMC.

median point

A point that is exactly the same distance between two outer points.

orientation tolerance

A group of geometric tolerances that limit the direction, or orientation, of a feature in relation to other features. Orientation tolerances are related tolerances.

parallelism

A three-dimensional geometric tolerance that controls how much a surface, axis, or plane can deviate from an orientation parallel to the specified datum.

perpendicularity

A three-dimensional geometric tolerance that controls how much a surface, axis, or plane can deviate from a 90 degree angle.

position

A three-dimensional geometric tolerance that controls how much the location of a feature can deviate from its true position.

primary datum

The datum feature that first situates the part within the datum reference frame. The primary datum is the first feature to contact a fixture or surface during assembly.

profile

The outline of a part feature within a given plane.

profile of a line

A two-dimensional geometric tolerance that controls how much the outline of a feature can deviate from the true profile.

profile of a surface

A three-dimensional geometric tolerance that controls how much a surface can deviate from the true profile.

profile tolerance

A group of powerful geometric tolerances that control the size, location, orientation, and form of a feature. Profile tolerances can be either independent or related.

regardless of feature size

A modifier indicating that the stated tolerance for a feature applies regardless of its actual size within an acceptable size limit. RFS does not permit bonus tolerance.

related tolerance

A tolerance that requires a specified datum.

roundness measuring machine

A sophisticated inspection device with a precision spindle that measures various circular or cylindrical features.

runout tolerance

A group of geometric tolerances that simultaneously limit the form, location, and orientation of cylindrical parts. Runout tolerances are related tolerances requiring a datum axis.

secondary datum

The datum feature that situates the part within the datum reference frame after the primary datum. The secondary datum is the second feature to contact a fixture or surface during assembly.

straightness

A two-dimensional geometric tolerance that controls how much a feature can deviate from a straight line.

symmetry

A three-dimensional geometric tolerance that controls how much the median points between two features may deviate from a specified axis or center plane.

tertiary datum

The datum feature that situates the part within the datum reference frame after the secondary datum.

three-dimensional tolerance

A tolerance that controls a shape having a length, width, and depth.

tolerance

An unwanted but acceptable deviation from a given dimension. Tolerances indicate the allowable difference between a physical feature and its intended design.

tolerance zone

An imaginary zone in which a part feature must be completely contained for the part to pass inspection.

total runout

A three-dimensional geometric tolerance that controls the form, orientation, and location of the entire length of a cylindrical part as it rotates.

true position

The imaginary perfect position of a feature described by the design specifications.

true profile

The perfect, imaginary profile described by the design specifications.

two-dimensional tolerance

A tolerance that controls a shape having only a length and width.

Interpreting GD&T Concepts

Class Vocabulary

Term Definition

3-2-1 rule

A rule that defines the minimum number of contact points necessary to properly locate a part within the datum reference frame. The primary datum requires three points, the secondary datum two points, and the tertiary datum one point.

actual mating envelope

A geometrically perfect shape that is a "best fit" around a feature. The AME is the smallest possible cylinder contacting a shaft at its highest points or the largest possible cylinder contacting a hole at its highest points.

ASME Y14.5M

The standard for geometric dimensioning and tolerancing published by the American Society of Mechanical Engineers (ASME). The 1994 revision is the most recent version of the standard.

axis

An imaginary, perfectly straight line used to indicate measurements or positions. An axis runs through the center of a cylindrical object.

basic dimension

A measurement that describes the theoretical and perfect size, shape, or location of a feature.

bolt-hole pattern

A series of holes with centers equally positioned around the circumference of a larger imaginary circle. Bolt-hole patterns can be controlled with composite tolerancing.

bonus tolerance

Additional tolerance that applies to a feature as its size shifts from a stated material condition. Both MMC and LMC allow bonus tolerance.

Cartesian coordinate system

A positioning system that uses three linear axes perpendicular to each other to locate positions in three-dimensional space. The three axes in the Cartesian coordinate system are the X-axis, Y-axis, and Z-axis.

casting

A part that has been made by melting the material, pouring it into a mold or cavity, and removing the part after it has cooled and solidified. Metal castings tend to have rough surfaces.

center point

A single imaginary point located in the center and equally distant from the exterior of a circular feature.

circularity

A two-dimensional geometric tolerance that controls how much a feature can deviate from a perfect circle.

composite tolerancing

A combination of more than one geometric tolerance applied to the same feature. Composite tolerancing is used with position or profile tolerances and is applied with a multi-segment feature control frame.

conical taper

A cylindrical feature that gradually changes from a larger diameter to a smaller diameter at a constant ratio.

conicity

A three-dimensional geometric tolerance that controls how much a feature can deviate from a perfect tapered cone.

controlled radius

A radius that yields a circle, arc, or sphere with no flat sections or reversals.

cylindricity

A three-dimensional geometric tolerance that controls how much a feature can deviate from a perfect cylinder.

datum feature symbol

A GD&T symbol in a print indicating a part feature that acts as a datum feature and that contacts a datum reference frame simulator.

datum reference frame

Three imaginary planes perpendicular to one another that are mapped onto the part and provide an anchor for relating part features to each other.

datum reference frame simulator

A gaging surface, machine component, or other device that establishes a datum plane in the datum reference frame. These devices adhere to gaging tolerances and act as a reasonable substitute for the datum reference frame.

datum target

A GD&T symbol in a print indicating the size, shape, and location for a matching gaging point, line, or surface that is used to position the part in the datum reference frame. Datum targets are most often used with rough or irregular parts.

feature control frame

A series of compartments containing symbols and values that describe the geometric tolerance of a feature. The order and purpose of these compartments follow a consistent standard.

feature of size

A cylindrical surface, spherical surface, or two opposed parallel elements or surfaces that can be associated with a size dimension.

feature without size

A feature that cannot be associated with a size dimension. A single flat surface is a feature without size.

flat taper

A flat surface that gradually change from a larger height to a smaller height at a constant slope or incline.

flatness

A three-dimensional geometric tolerance that controls how much a feature can deviate from a flat plane.

flexible datum reference frame simulator

A datum reference frame simulator that can reposition the datum reference frame without losing any datum relationships.

functional gage

A gage representing a "worst case" mating part that provides a simple pass/fail assessment of the inspected part. Functional gages often can quickly inspect several features at once.

gaging tolerances

Tight or strict tolerances that ensure an inspection gage provides a reasonable amount of measurement certainty. Gaging tolerances are typically ten times more accurate than the part that the gage inspects.

geometric dimensioning and tolerancing

An international standard for communicating instructions about the design and manufacturing of parts. GD&T uses universal symbols and emphasizes the function of the part.

go gage

A gage on or in which a good part should fit easily. A go gage quickly checks a part's features without providing a measurement value.

hard datum reference frame simulator

A datum reference frame simulator that is rigidly fixed in place in one or more planes. The combination of a surface plate and angle plate is a hard DRF simulator.

helix angle

The angle formed by the helix or spiral of the thread at its pitch point.

least material condition

The point at which a feature contains the least amount of material within its acceptable size limit. The largest acceptable hole and the smallest acceptable shaft are examples of LMC.

major diameter

On a thread, the diameter from crest to crest of an external thread or the diameter from root to root of an internal thread. On a gear, the widest diameter formed by the gear.

maximum material condition

The point at which a feature contains the greatest amount of material within its acceptable size limit. The smallest acceptable hole and the largest acceptable shaft are examples of MMC.

median plane

An imaginary, perfectly flat plane positioned in the middle between two opposing flat surfaces.

minor diameter

On a thread, the diameter from root to root of an external thread or the diameter from crest to crest of an internal thread. On a gear, the base diameter matching the root of the teeth.

perpendicular tolerance

A three-dimensional geometric tolerance that controls how much a surface, axis, or plane can deviate from a 90° angle.

phantom line

A line drawn by alternating a long dash, followed by two short dashes. A phantom line can indicate a datum target line in a GD&T print.

pitch diameter

On a thread, the measured diameter between the pitch points taken in the groove between the threads. On a gear, the diameter of the circle passing through the pitch points on the gear teeth.

position tolerance

A three-dimensional geometric tolerance that controls how much the location of a feature can deviate from its true position.

primary datum

The datum feature that first situates the part within the datum reference frame. The primary datum is the first feature to contact a fixture or surface during assembly.

profile tolerance

A geometric tolerance that controls either how much the two-dimensional outline or three-dimensional surface of a feature can deviate from its true profile.

projected tolerance zone

A tolerance zone that extends beyond a feature by a specified distance. Projected tolerance zones help ensure that mating parts fit during assembly.

radius

A straight line extending from the center to the periphery of a circle, arc, or sphere.

regardless of feature size

A modifier indicating that the stated tolerance for a feature applies regardless of its actual size within an acceptable size limit. RFS does not permit bonus tolerance.

resultant condition

A variable "worst case" boundary opposite the virtual condition that equals the feature's actual mating envelope and its location tolerance.

Rule #1

The GD&T rule stating that, when a tolerance for a feature of size is specified, the surfaces of that feature cannot extend beyond its boundary of perfect form at its maximum material condition (MMC).

Rule #2

The GD&T rule stating that, for all applicable geometric tolerances, the regardless of feature size (RFS) modifier applies to the individual tolerance, datum reference, or both where no modifier symbol is specified.

secondary datum

The datum feature that situates the part within the datum reference frame after the primary datum. The secondary datum is the second feature to contact a fixture or surface during assembly.

section lines

A series of diagonal lines drawn close together. Section lines can identify a datum target area in a GD&T print.

six degrees of freedom

The six basic possible linear and rotational movements that a part can have if left unrestricted. Positioning a part in the datum reference frame restricts all six degrees of freedom.

sphericity

A three-dimensional geometric tolerance that controls how much a feature can deviate from a perfectly round sphere.

statistical process control

The use of statistics and control charts to measure key quality characteristics and control how the related process behaves. With SPC, operators typically track the variation of part measurements over time.

statistical tolerancing

The assigning of tolerances for mating parts of an assembly that applies the principles of statistics. Statistical tolerancing is used in conjunction with SPC.

straightness

A two-dimensional geometric tolerance that controls how much a feature can deviate from a straight line.

tertiary datum

The datum feature that situates the part within the datum reference frame after the secondary datum.

virtual condition

A constant " worst case & quot; imaginary boundary defined by the collective effects of a feature's size, geometric tolerance, and material condition.

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