Introduction to GD&T 301

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.

Profile Tolerance

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

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.

Orientation Tolerance

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

Location Tolerance

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

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.

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

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.

Median Point

A point that is exactly the same distance between two outer points. Concentricity finds the median point between two points opposite each other forming a diameter on a cylindrical feature.

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.

Cross Section

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

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.

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.

Cylindricity

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

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.

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.

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.

Concentricity

A three-dimensional locational tolerance that describes the location of opposing points in cylindrical features with respect to a datum reference. Concentricity 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.

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.

Parallelism

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

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.

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.

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.

Related Tolerance

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

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.

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.

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.

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.

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.

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.

American Society Of Mechanical Engineers

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

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.

Tolerance Zone

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.

The position tolerance, one of the location tolerances, is probably the most common type of geometric tolerance. The position tolerance is a three-dimensional related tolerance that uses one or more datums to determine the location of the feature's true position, or ideal, exact location.

An inspector often uses a functional gage to easily check a feature's position. If the feature is a round hole or shaft, the tolerance zone is usually an imaginary cylinder around the true position that must contain the axis of the actual feature. If the feature is rectangular, the tolerance zone is two imaginary planes spaced around the true position that must contain the imaginary center plane of the feature.

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.

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 (left) and total runout (right) restricting a part feature.

Circular runout is the two-dimensional runout tolerance, while total runout is three-dimensional. Both relate a cylindrical feature to a datum axis, simultaneously restricting the form, location, and orientation. Usually used on cylindrical parts, especially parts that rotate, runout tolerances can also apply to flat surfaces, like the end of a shaft. To inspect circular runout, the part is rotated, and an indicator placed against the part surface detects the highest and lowest points during rotation. The surface of the part must remain within two imaginary circles with centers located on the datum axis. The size of the tolerance indicates the space between these two circles. An inspector checks circular runout at a number of points along the part surface. Total runout is similar to circular runout, except the tolerance zone is between two imaginary cylinders and restricts the entire length of the cylindrical surface. By default, parts meeting a total runout tolerance also meet the corresponding circular runout tolerance.

The remaining form tolerances, circularity (sometimes simply referred to as roundness) and cylindricity, control the shape of round features.

Circularity is a two-dimensional tolerance. Though most often used on a cylinder, it may apply to other shapes, such as cones and spheres. Circularity requires any two-dimensional cross section of a feature to remain within two imaginary coaxial circles. An inspector will often check multiple cross sections, though each cross section must meet the specific tolerance independently. Cylindricity is the three-dimensional version of circularity. Now, the surface must remain within two imaginary cylinders, and all cross sections of the feature are inspected together. Consequently, the cylindricity tolerance is applied only to cylindrical features. To properly check circularity and cylindricity, the part must be measured with a roundness machine.

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.

The design or inspection of any part involves a comparison between the imaginary, geometrically perfect design and the actual, physically imperfect part. The design of a part consists of numerous datums. A datum is geometrically perfect. It can be a point, line, plane, axis, or a combination of these. A feature is the imperfect, physical characteristic of a part. The tolerances in the part design tell an inspector how much variation can exist between the dimensions of a feature and the part design. During inspection, a datum feature acts as an imperfect substitute for a theoretical datum. GD&T instructions indicate which part features act as the primary datum, secondary datum, and tertiary datum.

Each datum restricts the part's ability to move in space. The primary datum plane stops up-and-down movement and rotational movement in two directions. The secondary datum plane stops back-and-forth movement and rotational movement in one direction. The tertiary datum plane finally restricts side-to-side movement. These planes all lie perpendicular to each other.

Straightness is a two-dimensional tolerance. To meet its straightness tolerance, an edge must remain within two imaginary parallel lines. The distance between these parallel lines depends on the size of the tolerance. Rectangular parts may have a straightness tolerance, but the edge or center axis of a cylindrical part is the most common way that a straightness tolerance is used.

Flatness is the three-dimensional tolerance version of straightness used mostly on rectangular parts. Instead of two imaginary lines, flatness requires a surface to remain within two imaginary, perfectly flat planes. The size of the tolerance indicates the space between these planes. Any flat surface used as a primary datum feature will often include a flatness tolerance. Though most often applied to a surface, it is also possible to control the center plane with the flatness symbol.

For the angled feature on this part, the first compartment of the feature control frame indicates the feature's surface. The second compartment shows that the surface tolerance is plus or minus 0.02 inches (0.51 mm). The remaining compartments show that the tolerance uses A as its primary datum, B as its secondary datum, and C as its tertiary datum. An additional compartment shows that the surface must also reference datums J and K. The next frame controls the part's left side. This frame shows that the feature must be within 0.005 inches (0.127 mm) of perfectly perpendicular, or at a 90°angle, to the referenced datums. Its primary datum is datum A, and the second is datum B.

GD&T also has numerous modifier symbols, which convey additional specifications about part feature tolerances. Many of the GD&T modifiers deal with specific geometric tolerances, but the two material condition modifiers are limited to features that have a size dimension. The material condition of a feature establishes how much material that feature includes.

Circular runout is the two-dimensional runout tolerance, while total runout is three-dimensional. Both relate a cylindrical feature to a datum axis, simultaneously restricting the form, location, and orientation. Usually used on cylindrical parts, especially parts that rotate, runout tolerances can also apply to flat surfaces, like the end of a shaft. To inspect circular runout, the part is rotated, and an indicator placed against the part surface detects the highest and lowest points during rotation. The surface of the part must remain within two imaginary circles with centers located on the datum axis. The size of the tolerance indicates the space between these two circles. An inspector checks circular runout at a number of points along the part surface. Total runout is similar to circular runout, except the tolerance zone is between two imaginary cylinders and restricts the entire length of the cylindrical surface. By default, parts meeting a total runout tolerance also meet the corresponding circular runout tolerance.

GD&T instructions offer significant improvements over traditional methods of part design specification. The typical blueprint often contains numerous notes to describe the part's design. However, GD&T is a compact language of symbols, and any GD&T print can be understood by anyone that has learned the ASME Y14.5 or ISO 1101 international standard. Because GD&T specifies datums, it relates part features to one another. An emphasis on how features relate best addresses how the part will actually fit and function during use. This emphasis ensures that a surface will have the correct angle after assembly or that a shaft and hole designed to fit together will fit properly. With GD&T, functional parts pass inspection and non-functional parts are caught and rejected.

Geometric Dimensioning And Tolerancing

GD&T. 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.

A basic dimension consists of a rectangular box placed around a number. This means that the dimension is theoretically perfect. It has no direct tolerance. Even a default tolerance in the title block does not apply to basic dimensions. GD&T frequently uses basic dimensions, because they establish the perfect size, form, orientation, or location that a geometric tolerance will be applied to. Basic dimensions are most commonly used in conjunction with position and profile tolerances. A reference dimension indicates information on the part drawing provided for reference only. A reference dimension is usually shown on a part drawing as a number enclosed in parentheses.

How coordinate grid squares appear in a blueprint. Tolerance zone vs. coordinate grid square. The traditional method for describing the location of features within a part design is coordinate tolerancing. Coordinate tolerancing uses a square grid of imaginary lines to describe the location of various part features. However, not all features easily fit within this square grid. For example, if a circular hole must be precisely located so that the shaft of another part will fit into it, the traditional coordinate tolerancing grid cannot take into consideration how closely these two parts will fit. It is the same as trying to fit a round peg into a square hole. GD&T is a feature-based system that uses a variety of geometric shapes to describe parts. Instead of the traditional coordinate grid, a GD&T design would specify that the hole must be located within an imaginary cylindrical tolerance zone tolerance zone 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. . This cylindrical shape provides a larger tolerance margin than a coordinate grid square. With GD&T, the difference between a good part that meets its specifications and a bad part is correctly based on the fit, form, and function of the part. GD&T emphasizes the relationship between part features instead of describing them separately.

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 standarization for GD&T in ISO 1101.

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.

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.

Angularity is a three-dimensional tolerance. The shape of the tolerance zone depends upon the feature. If angularity is applied to a flat surface, the tolerance zone is two imaginary planes spaced apart that are parallel to the ideal angle. If angularity is applied to a hole, the center axis of the hole must remain within an imaginary cylinder that exists around the ideal angle, or within two parallel planes.

Perpendicularity is the three-dimensional tolerance, similar to angularity, that specifies a 90-degree angle between features. Even though perpendicularity, and the other orientation tolerances, are controlling angles, notice that the tolerance value is not given in degrees, but linear units. Parallelism is a three-dimensional tolerance specifying two features that must remain parallel to each other. Sometimes, the flatness and parallelism tolerances are confused with each other. However, flatness is not related to a datum, while parallelism relates the feature to a specified datum. Whenever any orientation tolerance is applied to a flat surface, it also indirectly controls the flatness of the feature between the two imaginary planes.

The two versions of profile tolerance, profile of a line and profile of a surface, can be used to control features such as flat surfaces, cylinders, cones, curves, or even irregular surfaces. All of these features yield a profile, or outline of the part feature within a given plane. Both profile tolerances compare the actual profile to the true profile, or the ideal profile detailed in the part specifications. When used as related tolerances, they control the size, location, form, and orientation of a feature. Profile tolerances control the form of the feature when used as individual tolerances with no datum references.

Profile of a line is a two-dimensional tolerance that can be applied to any straight line or contour. It requires the actual profile of the feature to fall within two line elements that follow the true profile of the feature. The size of the tolerance indicates the distance between these two line elements. Profile of a surface is the three-dimensional profile tolerance commonly applied to contoured surfaces. This tolerance requires the surface to remain within two distinct three-dimensional shapes that follow the true profile of the part. Many aircraft and automobile parts require the profile of a surface tolerance because of their complex, curved surfaces.

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. RFS does not permit bonus tolerance.

Maximum material condition (MMC) is the most commonly used material condition modifier. Consequently, many holes, shafts, and tabs have an MMC modifier. MMC means that the tolerance applies when the feature contains the most amount of material possible within the stated limits. In other words, an MMC modifier reflects both the smallest possible hole and the largest possible shaft. The opposite is true for the least material condition (LMC) modifier. Now, the tolerance applies when the feature contains the least amount of material within the allowable limits. An LMC modifier reflects the largest possible hole and smallest possible shaft.

Regardless of feature size (RFS) means that the stated geometric tolerance applies to the feature regardless of its material condition. If no symbol is present in the GD&T specifications, assume that the specified tolerance applies regardless of feature size.

Coaxial

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

Every manufactured part should meet specific design requirements. To ensure accurate designs, many manufacturers use geometric dimensioning and tolerancing (GD&T) to describe part specifications. GD&T describes a part using 14 standard international symbols. These symbols reduce the number of written notes that traditionally appear in part designs and act as a universal language anyone can understand. Though prints should contain specifications for the units of measurement, this class will not specify English or Metric units in its examples.

The American Society of Mechanical Engineers (ASME) and the International Organization for Standardization (ISO) maintain the actual written standards describing GD&T, designated as ASME Y14.5 and ISO 1101. Both versions of GD&T contain slight differences. GD&T provides a standard for the dimensioning, tolerancing, and documentation of engineering drawings, as well as a clear understanding of the designer's intent and the function of the part. This information can help an inspector determine the best approach for measuring the part.

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.

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.

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.

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.

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.

GD&T instructions contain a significant amount of information. A feature control frame organizes this information as a series of symbols within standardized compartments. The feature control frame reads left to right, like a sentence.

The first compartment contains one of the standard geometric tolerance symbols. If a second geometric tolerance applies, it appears in a second feature control frame. The second compartment contains the total tolerance for the feature. If a diameter symbol is present, it indicates a cylindrical tolerance zone. If they apply, material condition modifiers also appear in the second compartment. The third compartment contains the primary datum. Independent geometric tolerances, such as the form tolerances, are not related to any datum. The fourth and fifth compartments contain the secondary datum and tertiary datum for the feature, which may or may not be necessary. The best way to keep track of GD&T symbols in a part print involves marking the datums with a highlighter in one color and any GD&T symbol in a second color. This makes very clear which symbol references which datum.

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.

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.

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.

Those modifiers that do not pertain to material condition specify restrictions upon the particular feature being modified.

The projected tolerance zone symbol transfers the tolerance zone of a hole beyond its part feature. The unequal tolerance zone symbol is used only with the profile tolerances and designates that the given tolerance is not equally disposed around the true profile. It may be unequal bilaterally or unilaterally to one side. The tangent plane symbol specifies that the tolerance zone applies to a plane defined by the high points on a surface. The diameter symbol either acts as an abbreviation for the word "diameter" within the part drawing itself or indicates the shape of a particular tolerance zone. The radius symbol is used to provide dimensional measurement for an arc in a part drawing. It creates a tolerance zone defined by a minimum and maximum arc between which the part feature must occur. The controlled radius symbol functions much like the radius symbol but further limits the tolerance, excluding allowance for arcs that contain imperfections affecting roundness.

The three orientation tolerances are angularity, perpendicularity, and parallelism. These tolerances compare one feature to other features to define the angle that they must form. Consequently, the three orientation tolerances are related tolerances that specify one or more datums.

The three orientation tolerances are angularity, perpendicularity, and parallelism. These tolerances compare one feature to other features to define the angle that they must form. Consequently, the three orientation tolerances are related tolerances that specify one or more datums.

GD&T uses a range of tolerance types to define part features. These tolerance types are divided into five groups: form tolerance, profile tolerance, orientation tolerance, location tolerance, and runout tolerance. Form tolerances include straightness, flatness, circularity, and cylindricity. Form tolerances are relative to themselves and look at a feature individually for shape. Profile tolerances include profile of a line and profile of a surface and control multiple aspects of a feature. Profile tolerances are often used on contours. Orientation tolerances include angularity, perpendicularity, and parallelism. Orientation tolerances describe how a feature must be oriented in relation to other features. Location tolerances include the commonly used position and the infrequently used concentricity and symmetry. Runout tolerances include circular runout and total runout and typically control wobble on rotating parts. Each tolerance is defined as an individual tolerance, a related tolerance, or both. An individual tolerance is not held with respect to a datum, but a related tolerance must be related to one or more datums.

There are four form tolerances. As individual tolerances, they exist independent of datum locations. The form tolerances straightness and flatness control how much a feature conforms to a linear or planar shape.

Concentricity restricting a part feature.

Unlike position, location tolerance concentricity is uncommon. This three-dimensional tolerance compares two or more cylinders and ensures that they closely share a center axis. An inspection of concentricity finds the median point between two points opposite each other forming a diameter on the cylindrical feature. This process is repeated multiple times at different cross sections. Every median point must fall within a cylindrical tolerance zone, centered around the datum axis. Concentricity is difficult to inspect but useful when balance is a primary concern for the part. Symmetry functions like concentricity, except the features are rectangular. Instead of a cylindrical tolerance zone, every median point between two flat surfaces must fall between two imaginary planes that represent the tolerance zone. Symmetry is rarely used. The less costly position, parallelism, or straightness tolerances are used instead.