Introduction to GD&T 301

American Society of Mechanical Engineers

ASME. An organization that publishes technical materials and sets industrial and manufacturing standards. Along with ISO, ASME provides written standardization for GD&T in ASME Y14.5M.

angularity

A three-dimensional orientation tolerance that describes the allowable variability in the angular relationship between a surface and a datum. Angularity is a related tolerance.

ASME Y14.5

The standard for geometric dimensioning and tolerancing published by the American Society of Mechanical Engineers (ASME). The 2018 revision is the most recent version of the standard.

basic dimension

A dimension that is theoretically perfect. A basic dimension has no direct tolerance and is denoted on a GD&T blueprint as a number enclosed in a rectangular box.

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. Circular runout is a related tolerance.

Circularity

A two-dimensional form tolerance that describes the allowable variability in the shape and appearance of a circle. Also known as roundness, circularity is an individual tolerance.

Coaxial

Sharing a common center between two cylindrical features. Circularity requires any two-dimensional cross section of a feature to remain within two theoretical coaxial circles.

controlled radius

A radius that yields a circle, arc, or sphere with no flat sections or reversals. In GD&T, controlled radius occurs on a blueprint with the symbol CR.

coordinate tolerancing

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

Cross section

A section of a feature that is formed by an intersecting theoretical plane. Circularity requires any two-dimensional cross section of a feature to remain within two theoretical coaxial circles.

cylindricity

A three-dimensional form tolerance that describes the allowable variability in the shape and appearance of a cylinder. Cylindricity is an individual tolerance.

datum

A point of reference for machine tools, programs, and fixtures from which measurements are taken. A datum can be a hole, line, or any three-dimensional shape.

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

DRF. Three theoretical planes perpendicular to one another that are mapped onto the part. The datum reference frame provides an anchor for relating part features to each other.

dial indicator

A measuring instrument with a contact point attached to a spindle and gears that move a pointer on the dial. Dial indicators have graduations that show different measurement values.

diameter

The distance from one edge of a circle to the opposite edge that passes through the center. Round or cylindrical features require diameter measurements.

engineering drawing

A two-dimensional print that details the size and shape of a part before it is produced. Engineering drawings also specify the materials to be used in part creation.

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.

features

A defining physical characteristic. Features include shapes, lines, and other elements machined into a workpiece.

flatness

A three-dimensional form tolerance that describes the allowable variability in the shape and appearance of a surface that lies in a plane. Flatness is an individual tolerance.

form tolerance

A geometric tolerance that limits the amount of error in the shape of a feature. The form tolerances include straightness, flatness, circularity, and cylindricity.

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.

geometric dimensioning and tolerancing

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

individual tolerance

A tolerance that does not require a specified datum. Form tolerances are always individual tolerances, while profile tolerances can sometimes be individual tolerances.

International Organization for Standardization

ISO. An organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. Along with ASME, ISO provides written standardization for GD&T in ISO 1101.

ISO 1101

The standard that provides the foundation, rules and definitions of symbol language for geometric specification as published by the International Organization for Standardization (ISO). The 2017 revision is the most recent version of this standard.

least material condition

LMC. 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 geometric tolerance that limits the location or placement of features. Location tolerances are related tolerances.

material condition modifiers

Defines the tolerance of a feature in relation to its acceptable size limits. There are three material condition modifiers in GD&T, maximum material condition, least material condition, and regardless of feature size.

maximum material condition

MMC. 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.

modifier

An element that communicates information about the tolerance of a feature. In GD&T, each modifier has a symbol that distinguishes it with other modfiers on a blueprint.

orientation tolerance

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

parallelism

A three-dimensional orientation tolerance that describes the equal distance between pairs of points, lines, or planes. Parallelism is a related tolerance.

perpendicularity

A three-dimensional orientation tolerance that describes the allowable variability in the 90-degree angular relationship between a surface and a datum. Perpendicularity is a related tolerance.

position tolerance

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

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.

print

A document that contains the instructions necessary to manufacture and/or assemble a part. Prints include the degree of tolerance necessary for the part to function as intended.

profile

The outline of the part feature within a given plane. Two-dimensional profiles are toleranced using profile of a line, and three-dimensional profiles are toleranced using profile of a surface.

profile of a line

A two-dimensional profile tolerance that describes the allowable variability in the contour of the edge seen in the section view. Profile of a line can be either an individual or a related tolerance.

profile of a surface

A three-dimensional profile tolerance that describes the allowable variability in the contour of a surface. Profile of a surface can be either an individual or a related tolerance.

profile tolerance

A geometric tolerance that controls the size, location, orientation, and form of a feature. Profile tolerances can be either individual or related.

projected tolerance zone

A tolerance zone that extends beyond a feature by a specified distance. Symbolized by a P enclosed in a circle, projected tolerance zones help ensure that mating parts fit during assembly.

radius

The distance from a center point to a point on a circle or arc. In GD&T, radius forms a curved feature formed by identifying a uniform distance from a center point to the edge of a circle or arc.

reference dimension

A dimension that is provided for informational purposes only. A feature is on a GD&T blueprint for reference, rather than for inspection or production use, if its dimensions are enclosed in parentheses.

regardless of feature size

RFS. A modifier indicating that the stated tolerance for a feature applies regardless of its actual size within an acceptable size limit. Regardless of feature size is no longer listed on drawings since it is the assumed default material condition modifier.

related tolerance

A tolerance that requires a specified datum. Orientation, location, and runout are always related tolerances.

runout tolerance

A geometric tolerance that simultaneously limits 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.

specifications

A description of the essential physical and technical properties of a part. Specifications, sometimes called specs, include information on the shape and tolerance of a part as well as required mechanical and physical properties.

straightness

A two-dimensional form tolerance that describes allowable variability in the shape and appearance of a line in a section view. Straightness is an individual tolerance.

tangent plane

A modifier specifying that the tolerance zone applies to a plane defined by the high points on a surface. In GD&T, tangent plane is symbolized by a capital T enclosed in a circle.

tertiary datum

The datum feature that situates the part within the datum reference frame after the secondary datum. This plane must be perpendicular to both the primary and secondary planes and is usually the smallest surface of the workpiece.

three-dimensional tolerance

A tolerance that controls a shape having a length, width, and depth. Flatness, profile of a surface, and angularity are all examples of three-dimensional tolerances.

title block

The area of a blueprint containing information such as company name, part name, part number, designer, scale, and material. Title blocks are unique to each manufacturer.

tolerance zones

An imaginary zone in which a part feature must be completely contained for the part to pass inspection. This zone contains the dimensions between the maximum and minimum limits of a feature's location.

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. Total runout is a related tolerance.

true position

The imaginary perfect position of a feature described by the design specifications. The location of a feature's true position is determined by the positional tolerance.

true profile

The perfect, imaginary profile described by the design specifications. The profile tolerances compare the actual profile of a feature to the true profile.

two-dimensional tolerance

A tolerance that controls a shape having only a length and width. Straightness, circularity, and circular runout are all examples of two-dimensional tolerances.

unequal tolerance zone

A tolerance zone used for the profile tolerances that designates if a given tolerance is not equally disposed around the true profile. A profile may be unequal on two sides or one side.

X axis

The linear axis representing side-to-side movement or coordinate position in a feature, relative to the origin. Coordinate tolerancing uses the x and y axes to describe location.

Y axis

The linear axis representing back-and-forth movement or coordinate position in a feature, relative to the origin. Coordinate tolerancing uses the x and y axes to describe location.