In the process of product research and development, process specification, and manufacturing, various fields are involved, such as design engineers, process engineers, body engineers, quality engineers, measurement engineers, and so on. Each engineer has their own understanding of the product’s requirements and standards. In order to facilitate smoother communication during the product development process, a unique and authoritative (legally effective) standard is needed to bind everyone. This is where GD&T comes into play.
Terms related to GD&T
What is GD&T?
GD&T stands for Geometric Dimensioning and Tolerancing, which is a set of rules and GD&T symbols used on a drawing. Similar to linear dimensioning, geometric tolerancing is a comprehensive notation method that is not independent of traditional linear dimension tolerance notation. Instead, it is a member of the tolerance notation family. However, compared to traditional linear dimension notation, it has unique advantages that are gradually being embraced by more mechanical professionals.
After CNC machining, parts will have dimension tolerances, which result in differences between the actual shape or mutual position of the points, lines, and surfaces that constitute the geometric features of the part, and the shapes and mutual positions specified by ideal geometric bodies. These differences in shape are known as profile tolerances, while differences in mutual position are known as positional tolerances. Collectively, these differences are referred to as geometric tolerances.
Feature Control Frame-mark of GD&T
feature control frame
A feature control frame is a graphical symbol used in Geometric Dimensioning and Tolerancing (GD&T) to communicate the specific tolerances and geometric characteristics of a feature on an engineering drawing. It is a compact and standardized way to convey detailed information about the desired shape, size, orientation, and location of a feature on a part.
The feature control frame typically consists of several components:
- Feature Control Symbol: This is a letter or letters that represent the specific geometric tolerance or control needed for the feature. It is often accompanied by a modifier to further define the tolerance zone.
- Tolerance Zone Shape and Size: This portion of the frame indicates the desired geometric shape (such as a circle, square, or profile) and provides the dimensions of the allowable tolerance zone.
- Datum References: These are letters that correspond to datums or reference points on the part that are used to establish the tolerance zone. The datums define the origin for measurement and help ensure consistent interpretation of the feature’s position.
- Modifiers: These are optional symbols or letters that provide additional information about the feature’s tolerance, such as Maximum Material Condition (MMC), Least Material Condition (LMC), or Regardless of Feature Size (RFS).
Parts of the feature control frames
- Guide arrow, used to indicate the feature that needs to be controlled.
- Geometric tolerance symbol, used to express the category to be controlled, which can be either profile tolerance or positional tolerance.
- Diameter symbol, If the controlled tolerance zone is cylindrical or circular, the diameter symbol should be included. Otherwise, it is not necessary.
- Geometric tolerance value, the size of the controlled tolerance zone.
- Maximum material condition symbol, applicable only when the controlled feature is a slot or hole. If the datum is a plane, the maximum material condition symbol will not be used.
- First datum A, the primary reference point or plane selected on the part that serves as the initial basis for dimensional measurements and tolerances. It is a known feature or surface used to establish the starting point for evaluating the geometric relationship of other features. Datum A provides a fixed reference for other datums and subsequent geometric tolerances, ensuring accurate and consistent measurements.
- Second datum B, another essential reference point or plane on the part, selected to work in conjunction with Datum A. Together, Datum A and Datum B establish a coordinate system that forms the foundation for measuring the position, orientation, and other geometric characteristics of features. Datum B provides additional information to define the part’s three-dimensional orientation and alignment relative to Datum A.
- Third datum C. the final piece of the reference system, adding depth or height information to the measurement framework. It completes the three-dimensional alignment and orientation of the part’s features. Datum C, along with Datum A and Datum B, enables precise control over the part’s dimensional relationships, ensuring that features are correctly positioned and oriented within the specified tolerances.
Geometric Tolerance Standards of Various Countries
GD&T is a global engineering language that may vary slightly from country to country, but the vast majority of the content is the same.
American Standard: ASME Y14.5—2018 Dimensioning and Tolerancing
Y14.5 is the most authoritative standard in Geometric Dimensioning and Tolerancing (GD&T), establishing principles, definitions, requirements, defaults, or recommended practices for geometric dimensions, tolerances, and related specifications in engineering drawings and associated documents. Its contents cover fundamental rules, symbols, size tolerances, datums (datum systems), geometric tolerances (form, orientation, position, profile, runout), and various combinations and applications. Compared to the previous version, the current 2018 edition of Y14.5 has undergone updates in concepts of dimension elements, expression of datum references and degrees of freedom, composite positional tolerance, surface and axis interpretations of positional tolerance, profile tolerance, symbols, and modifiers.
International standard: ISO 1101-2017
Chinese standard: GB/T 1182-2008 (equivalent to ISO1101)
German standard: DIN ISO 1101 (equivalent to ISO1101)
Japanese Standard: JIS B0021 (equivalent to ISO1101)
British Standard: BS ISO 1101 (equivalent to ISO1101)
Comparison between GD&T and Linear Dimensional Tolerances
Linear Dimensional Tolerances, often referred to as “linear tolerances,” are specifications that define the allowable variation or range of dimensions for linear features (such as lengths, widths, heights, and depths) on a part or component. These tolerances indicate the acceptable limits within which the actual dimensions of a feature can vary while still ensuring the functionality, fit, and interchangeability of parts.
Geometric tolerances have a wider tolerance zone.
As shown in the diagram below, the linear dimensional tolerance annotates the axis line in the hole compared to the axis line annotated by the geometric tolerance in the hole.
| Linear Tolerance | Geometric Tolerance | |
| Tolerance Zone Shape | Square or rectangular for hole tolerances | Circular, can use diameter symbol |
| Smaller Hole Tolerance | Increased by 75% | |
| Higher Manufacturing Cost | Lower Manufacturing Cost | |
| Tolerance Zone Flexibility | Fixed dimension tolerance zone | Can increase tolerance zone under certain conditions using MMC |
| Good Parts Scrapped | Good parts used | |
| Higher Production Cost | Lower Production Cost | |
| Inspection Convenience | Different inspection results possible | Unified inspection setup using datum system |
| Good Parts Scrapped | Clear guidance for inspection | |
| Bad parts Accepted | Eliminates disputes over part acceptance |
Clear controlled feature by Geometric Tolerance
Geometric tolerances have datums, which can indicate the controlled feature. Linear dimensions indicate the size tolerance between two features, whereas geometric tolerances can specify datums, explicitly stating the tolerance of a specific feature. Unified inspection setup.”
Application of Linear Tolerance and Geometric Tolerance
| Tolerance Type | Linear Tolerance | Geometric Tolerance |
|---|---|---|
| Dimension | Better | |
| Chamfer | Better | |
| Radius | Better | |
| Wall Thickness | Better | |
| Step Face | Better | |
| Inclined Plane | Better | |
| Countersink | Better | |
| Position Control | Better | |
| Orientation Control | Better | |
| Shape Control | Better |
Types Of GD&T Symbols
GD&T is a feature-based system for defining the size, shape, and location of features on parts. Geometric tolerances are applied to features by feature control frames. The commonly used tolerance categories are form, profile, orientation, location, and runout.
Straightness 
Straightness, commonly referred to as flatness, signifies the degree of straightness of linear elements on a component, reflecting their actual shape conforming to an ideal straight line. Straightness tolerance denotes the maximum permissible deviation of an actual line from an ideal straight line.
Illustrative Example: Within a specified plane, the tolerance zone must lie between two parallel lines separated by a distance of 0.1mm.
Flatness 
Flatness, commonly referred to as evenness, signifies the degree of flatness of planar elements on a component, reflecting their actual shape conforming to an ideal plane. Flatness tolerance denotes the maximum permissible deviation of an actual surface from an ideal plane.
Illustrative Example: Within a specified area, the tolerance zone must lie between two parallel planes separated by a distance of 0.08mm.
Circularity 
Circularity, commonly referred to as roundness, signifies the degree of circularity of circular elements on a component, reflecting their actual shape maintaining equidistance from the center. Circularity tolerance denotes the maximum permissible deviation of an actual circle from an ideal circle on the same cross-section.
Illustrative Example: Within a specified cross-section, the tolerance zone must lie between two concentric circles with a radius difference of the tolerance value, 0.03mm.
Cylindricity 
Cylindricity signifies the degree to which the points along the outer profile of a cylindrical surface on a component maintain equidistance from its axis. Cylindricity tolerance denotes the maximum permissible deviation of an actual cylindrical surface from an ideal cylindrical surface.
Illustrative Example: Within a specified area, the tolerance zone must lie between two coaxial cylindrical surfaces with a radius difference of the tolerance value, 0.1mm.
Line profile 
Line profile tolerance signifies the degree to which any arbitrary curve on a given plane of a component maintains its ideal shape. Line profile tolerance denotes the allowable variation of the actual profile of a non-circular curve.
Illustrative Example: Within a specified area, the tolerance zone is defined between the two envelope lines of a series of circles with diameters differing by the tolerance value of 0.04mm. The centers of these circles lie on a line with the theoretically correct geometric shape.
Surface profile 
Surface profile tolerance signifies the degree to which any arbitrary-shaped surface on a component maintains its ideal form. Surface profile tolerance denotes the permissible variation of the actual contour of a non-circular surface from the ideal profile.
Illustrative Example: Within a specified area, the tolerance zone is defined between the two envelope lines of a series of spheres with diameters differing by 0.02mm. The centers of these spheres should theoretically lie on a surface with the correct geometric shape.
Parallelism
Parallelism, commonly referred to as maintaining parallelism, signifies the degree to which the actual feature on a component, when measured, maintains equidistance from a reference. Parallelism tolerance denotes the maximum permissible deviation between the actual direction of the measured feature and the ideal direction parallel to the reference.
Illustrative Example: If the symbol φ is added before the tolerance value, then the tolerance zone lies within the cylindrical surface of a parallel diameter φ0.03mm to the reference.
Perpendicularity 
Perpendicularity, commonly referred to as maintaining orthogonality between two features, signifies the degree to which the measured feature on a component, relative to a reference feature, maintains the correct 90° angle condition. Perpendicularity tolerance denotes the maximum permissible deviation between the actual direction of the measured feature and the ideal direction perpendicular to the reference.
Illustrative Example: If the symbol φ is added before the tolerance value, then the tolerance zone lies within the cylindrical surface of a diameter φ0.1mm perpendicular to the reference plane.
Angularity 
Angularity, known as the representation of maintaining any specified angle between two features on a component, signifies the correct condition. Angularity tolerance refers to the maximum allowable deviation between the actual direction of the measured feature and the ideal direction at any specified angle relative to the reference.
Illustrative Example: If the symbol φ is added before the tolerance value, then the tolerance zone must lie within the cylindrical surface of a diameter of 0.1mm. This tolerance zone should be parallel to plane B, which is perpendicular to reference A, and should form an ideal angle of 60° with reference A.
Position 
Position, known as the representation of the accurate condition of elements such as points, lines, and surfaces on a component relative to their ideal positions. Position tolerance refers to the maximum allowable deviation between the actual position of the measured feature and its ideal position.
Illustrative Example: When the symbol Sφ is added before the tolerance value, the tolerance zone is the area within the sphere of diameter 0.3mm. The center point of the sphere’s tolerance zone is positioned relative to references A, B, and C according to their theoretical correct dimensions.
Concentricity 
Concentricity, commonly referred to as the degree of coaxially, signifies the condition of the measured axis on a component being in the same straight line relative to the datum axis. Concentricity tolerance indicates the allowable variation between the actual measured axis and the datum axis.
Illustrative Example: When the tolerance value is accompanied by the symbol, the tolerance zone is the area between two cylinders of diameter 0.08mm. The axis of the cylindrical tolerance zone coincides with the datum.
Symmetry 
Symmetry is the representation of the condition where two symmetric center elements on a component are maintained within the same central plane. Symmetry tolerance indicates the allowable variation between the actual center plane (or centerline, axis) of the elements and the ideal symmetry plane.
Illustrative Example: The tolerance zone is the area between two parallel planes or lines that are at a distance of 0.08mm from each other and symmetrically arranged relative to the datum center plane or centerline.
Circular runout 
Circular runout represents the condition where the rotating surface of a component, within a defined measurement plane, maintains a fixed position relative to the datum axis. Circular runout tolerance allows for the maximum allowable variation when the actual element being measured rotates around the datum axis without any axial movement for a complete revolution within the specified measurement range.
Illustrative Example: The tolerance zone is the area between two concentric circles with a radius difference of 0.1mm, and both circles have their centers located on the same datum axis. The tolerance zone is perpendicular to any measurement plane.
Total runout 
Total runout refers to the amount of runout along the entire measured surface of a component as it undergoes continuous rotation around the datum axis. Total runout tolerance allows for the maximum allowable runout when the actual element being measured rotates continuously around the datum axis, while the indicator moves relative to its ideal profile.
Illustrative Example: The tolerance zone is the area between two coaxial cylindrical surfaces with a radius difference of 0.1mm and aligned with the datum axis.
Tools for GD&T measurements
Geometric Dimensioning and Tolerancing (GD&T) measurements involve a variety of tools and instruments to ensure accurate assessment and control of geometric features on a part. Some common tools used for GD&T measurements include:
- Calipers: Digital or analog calipers are versatile tools for measuring distances, lengths, and diameters accurately.
- Micrometers: These precision instruments provide highly accurate measurements for dimensions such as thickness, outer diameter, and depth.
- Height Gauges: Used to measure and mark vertical distances, heights, and step dimensions accurately.
- CMM (Coordinate Measuring Machine): A highly precise machine that measures complex geometries and features using a computer-controlled probe.
- Gauge Blocks: Precision blocks of known lengths used for calibration and comparison to measure linear dimensions.
- Profile Projectors: These optical instruments project an enlarged image of a part’s profile onto a screen for measurement.
- Surface Roughness Tester: Measures the texture or roughness of a surface, which is essential for specific GD&T requirements.
- Thread Gauges: Used to measure and verify thread dimensions, pitch, and angle accurately.
- Optical Comparators: Utilize optical magnification and illumination to compare a part’s features with a standard template.
- Contour Measuring Instruments: Used for measuring complex contours and shapes on a part’s surface.
- Roundness Tester: Measures the roundness of cylindrical parts and helps ensure circularity requirements.
- Profile Gauge: Used to trace the outline of a surface, ensuring it conforms to the specified profile.
- Shadowgraph: An optical instrument that measures the silhouette or shadow of a part’s profile.
- Sine Bar: Used to measure angles and set up parts at precise angles for inspection.
- Dial Indicators: These instruments provide accurate measurements of linear displacement or deviation from a reference point.
- Profilometers: Measures the surface roughness and texture of a part’s surface.
- Thread Measuring Wires: Precisely measures thread pitch diameter and angle.
- Go/No-Go Gauges: Verifies whether a part’s dimensions conform to tolerance limits.
- Microscopes: Used for magnified inspection of small and intricate features.
- Digital Height Gauges: Provide quick and accurate measurements of heights and step dimensions.
Conclusion
GD&T offers an effective approach for articulating design dimensions and tolerances, providing a precise portrayal of a component’s intended function and functional prerequisites in a clear and comprehensible manner. If your aim is to ensure impeccable fit and performance alignment of your designed parts, GD&T is an essential tool.
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