In engineering and manufacturing, the Geometric Dimensioning and Tolerancing (GD&T) tolerance symbols or system defines and communicate the design intent about a part’s geometry and tolerances. On engineering drawings, GD&T uses symbols to express these specifications.
Table of Contents
GD&T Tolerance Symbols are a set of fourteen Symbols used in the language of geometric tolerancing.
The symbols are divided into 5 categories:
Understanding the basic definitions of geometric dimensioning and tolerancing is most important for better clarity.
Some important GD&T definitions based on ASME Y14.5M are:
Feature: A general term applied to a physical portion of a part, such as a surface, hole, or slot.
Feature of Size (FOS): It is one cylindrical or spherical surface, or set of two opposed elements, or opposed parallel surfaces, associated with size dimension.
Datum: A theoretically exact plane, point, or axis from which a dimensional measurement is made.
Datum symbol is attached to a plane or size feature that must be contacted for machining and Inspection
The Datum plane, datum axis, and datum centre plane are used to make measurements of a part.
Datum Feature: A part feature that contacts a datum.
Feature Control Frame: It is a rectangular box that is divided into compartments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.
Free State: The parts must not be restricted during the Inspection. In other words, the free state symbol applies only when the part is otherwise restrained.
LMC- Least Material Condition: That condition where a feature of size contains the least amount of material within the stated limits of size. For example – Maximum hole size and minimum shaft size.
MMC- Maximum Material Condition: That condition where a feature of size contains the maximum amount of material within the stated limits of size. For example – Minimum hole size and maximum shaft size.
RFS- Regardless of Feature Size: This is the default condition for all geometric tolerances. No bonus tolerances are allowed. Functional gages may not be used.
Datum Tangent: A symbol that describes the shape, size, and location of gage elements that are used to establish datum planes or axes.
Here are a few typical GD&T tolerance symbols and their short definitions:
The straightness symbol is used to control the form of a feature so that it remains within two parallel lines.
Flatness is used to control the form of a surface, ensuring that it remains within a specified tolerance zone.
Circularity is used to control the roundness of a feature, constraining it within a specified tolerance zone.
Cylindricity is used to control the overall form of a cylindrical feature, ensuring that it remains within a specified tolerance zone.
The profile of a Line is used to control the form of a feature along a straight path.
The profile of a Surface is used to control the form of a surface within a specified tolerance zone.
Surface Profile tolerance specifies a uniform boundary that the surface must lie within. It controls the shape, rather than the size or location, of a surface.
Surface Profile is often used when the surface needs to fit with another surface or when a specific form is critical for functionality.
It can be applied independently to a surface or in conjunction with other GD&T symbols to specify more complex geometric requirements.
Angularity refers to the permissible deviation of a surface or feature from a specified angular orientation. It quantifies the allowable amount of tilt or inclination that a surface can have concerning a datum reference plane or axis. Angularity can be applied to various features, including surfaces, holes, slots, and axes.
A standardized technique for defining and managing feature orientation is provided by angularity in GD&T, which aids in achieving exact alignment and functioning in production and assembly processes.
Perpendicularity defines the allowable deviation of a surface, axis, or line from a true perpendicular orientation relative to a specified reference datum. In other words, it ensures that a feature is oriented at a 90-degree angle (i.e. perpendicular) to a reference plane or axis within a specified tolerance zone.
Perpendicularity is normally applied to features like holes, surfaces, or edges where a 90-degree orientation is critical for proper functionality or assembly. For example, correct brake alignment and performance in automotive manufacture depend on the mounting holes of the brake callipers being perpendicular to the mounting surface of the wheel hub.
Parallelism defines the allowable deviation of a surface, axis, or line from a true parallel orientation relative to a specified reference datum. In simple words, it ensures that two features or surfaces maintain a constant separation distance, remaining parallel to each other within a specified tolerance zone.
Parallelism is normally applied to features like surfaces, edges, or axes where maintaining parallel orientation is critical for proper functionality, assembly, or mating with other components. For example, precise cuts and dimensional consistency in machining applications depend on a workpiece’s surfaces staying parallel to the machine bed or tooling.
Runout is used to control the location of a circular part features relatives to its axis.
There are two types of runout tolerance:
Circular runout is a three-dimensional tolerance and can apply only to cylindrical parts. Circular runout of part can only be inspected when it is rotating.
The Instrument is placed on the surface of the rotating parts to detect the maximum and minimum readings.
Circular Runout is also known as Runout of a surface, focuses on controlling the circular or rotational variation of the feature relative to a specified datum axis or centerline. Unlike Total Runout, which considers all geometric variations, Circular Runout only evaluates the circular movement of the feature.
Total runout is similar to circular runout except that involves tolerance control along the entire length.
Total Runout is also known as Total Runout of a surface, is a comprehensive control of all geometric variations of a feature relative to a datum axis or centerline. It considers both radial/linear and axial/vertical variations along the entire length or circumference of the feature. Total Runout ensures that the feature rotates smoothly and consistently without significant deviation from its true axis of rotation.
| Total Runout | Circular Runout |
| Total Runout provides a more comprehensive control of all geometric variations, including radial and axial deviations, along the entire length or circumference of the feature. | Circular Runout focuses specifically on controlling the circular or rotational movement of the feature relative to a specified datum axis or centerline. |
| Total Runout is typically used for features with complex shapes or multiple surfaces, where controlling all variations is critical. | Circular Runout is commonly applied to cylindrical features such as shafts, spindles, or bearing surfaces, where ensuring smooth rotation and alignment is essential. |
GD&T True Position tolerance is used to control the location of a feature’s center or axis within a specified tolerance zone.
Concentricity is used to control the location of a feature’s axis relative to another feature’s axis. It is a three-dimensional tolerance similar to the position.
The shaft is measured in multiple diameters to ensure that they share a common center axis.
Symmetry tolerance is used to ensure that a feature is symmetrically disposed about a datum or a specified axis. It is much similar to concentricity. The only difference is that it controls the rectangular features and involves two imaginary flat planes.
In GD&T (Geometric dimensioning and tolerancing), feature control frames are required to describe the conditions and tolerances of a geometric control on a part’s feature.
The feature control frame has four parts:
Envelope requirement: The upper value of a external feature defines a virtual cylinder that the entire
feature has to fit inside. Likewise a internal features minimumvalue defines a
cylinder that has to fit inside the hole.
Free State: The parts must not be restricted during the Inspection. In other words, free
state symbol applies only when part is otherwise restrained.
Independency requirement: This principle sets no limits to the number of errors of form possessed by
individual features of a work piece. is the default for ISO tolerancing.
LMC- Least Material Condition: That condition where a feature of size contains the least amount of material
within the stated limits of size. E.g. Max. hole size and Min. shaft size.
MMC- Maximum Material Condition: That condition where a feature of size contains the maximum amount of
material within the stated limits of size. E.g. Min. hole size and Max. shaft size.
Projected Tolerance Zone: When the symbol is shown, it means the stated tolerance zone is extended
beyond the surface of the part, not within the part.
RFS- Regardless of Feature Size: This is the default condition for all geometric tolerances. No bonus tolerances
are allowed. Functional gages may not be used.
Tangent Plane: Useful for interfaces where form is not required.
Unequal Bilateral: Refers to unequal profile distribution.
Statistical Tolerance: Features identified as statistical toleranced should be produced with Statistical
Process Control.
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