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Complete Guide for GD&T Symbols

Henry

Henry

15 years of experience in prototyping and manufacturing.

In the process of product development and manufacturing, Engineers from different departments are involved including design engineers, process engineers, body engineers, quality engineers, measurement engineers and more. Each engineer has their own understanding of the product’s requirements and standards. In order to have a smooth communication during the product development process between these departments, a unique and authoritative standard is needed for everyone. This is how GD&T comes to be a uniform standard. In this article, we will introduce everything about GD&T.

WHAT IS GD&T?

GD&T stands for Geometric Dimensioning and Tolerancing, which is a set of rules and symbols used on a drawing. Geometric tolerancing is a comprehensive notation method which is a member of the tolerance notation family. Compared to traditional linear dimension notation, it has unique advantages and be accepted by more mechanical professionals.

After CNC machining, parts will have dimension tolerances, which result in differences between the actual dimensions and dimensions specified by ideal geometric bodies. These differences in shape are known as profile tolerances, while differences in mutual position are known as positional tolerances. In short, these differences mean geometric tolerances.

WHAT IS THE PURPOSE OF GD&T?

As mentioned, the main purpose of GD&T is delivery all dimension information’s from design engineers to manufactures. the GD&T on the drawings can tell manufacturer how accuracy of each feature need to be controlled. this is to make sure the final parts can meet the desired function performance.

HOW DOES GD&T WORKS?

GD&T works by indicating a feature’s tolerance zone value. this zone value means the maximum and minimum of the feature dimension. with a Feature Control Frame, all the features to be controlled, tolerances, datums are in it.

TERMS RELATED TO GD&T

  • Datum:  Datum is a geometric ideal element related to the measured feature and used to define its geometric positional relationship (such as axis, straight line, plane, etc.), which can be composed of one or more features on the part.
  • Datum Feature: It is a feature used to establish a datum and be contacted during the machining and inspection process; it has a sufficiently accurate actual surface.
  • Datum Target: Taking manufacturing tolerance into account, only part of the surface of the datum feature (point, line, or area) is selected as the reference for the datum.
  • Dimension: Figures with a measurement unit used to specify the characteristics of a part, as well as the location elements of features on the part, such as points, lines, surfaces, holes, slots, protrusions and more.
  • Tolerance: The allowable total amount of variation in a dimension, which is the gap between the maximum and minimum limit dimensions.
  • Geometric Tolerance: Tolerance related to individual features of a part, such as shape, profile, orientation, location, runout and more.

FEATURE CONTROL FRAME – MARK OF GD&T

A feature control frame is used to express the specific tolerances and geometric characteristics of a features on an engineering drawing. It is a standardized way to explain detailed information about shape, size, orientation and location of a feature on a part. The feature control frame usually consists of several components as below:

  • Feature Control Symbol: this symbol means the type of geometric dimensions to be controlled.
  • Tolerance Zone: Figures stands of allowable total amount of variation in a dimension.
  • Datum References: these are letters of datums on the part which are used to establish the coordinate system to measure the dimensions.
  • 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) and Regardless of Feature Size (RFS).

HOW TO READ A FEATURE CONTROL FRAME?

Here, we use example to explain how to read a feature control frame to control GD&T. from the picture below, each of item explained as below:

feature control frames
  1. Guide arrow: used to indicate the feature that needs to be controlled.
  2. Feature Control Symbol: used to express the category to be controlled, which is position feature in the picture.
  3. Diameter symbol: When the controlled tolerance zone is circular, the diameter symbol should be included. Otherwise, it is not necessary.
  4. Geometric tolerance zone value: it means the value of the controlled tolerance zone is 0.03mm.
  5. 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.
  6. First datum A: the primary reference point or plane selected on the part that serves as the initial basis for dimensional measurements and tolerances.
  7. Second datum B: Another reference point or plane on the part, along with Datum A establish a coordinate system plane.
  8. Third datum C: The third reference point or plane to create a complete coordinate system.

BENEFITS OF GD&T IN PRACTICE

Here we use a comparison between GD&T and Linear Dimensional Tolerances to show the benefits of GD&T in practice and how it works.

Linear Dimensional Tolerances, also called linear tolerances, define the allowable range of dimensions for linear features such as lengths, widths, heights and depths on a part. These tolerances indicate the acceptable limits within which the actual dimensions of a feature still keep the functionality of the part.

Geometric Tolerances Have a Wider Tolerance Zone.

As shown in the picture 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 ToleranceGeometric Tolerance
Tolerance Zone ShapeSquare or rectangular for hole tolerancesCircular, can use diameter 
symbol
Smaller Hole ToleranceIncreased by 75%
Higher Manufacturing CostLower Manufacturing Cost
Tolerance Zone 
Flexibility
Fixed dimension tolerance zoneCan increase tolerance zone
under certain conditions using
 MMC
Good Parts ScrappedGood parts used
Higher Production CostLower Production Cost
Inspection ConvenienceDifferent inspection results possibleUnified inspection setup using 
datum system
Good Parts ScrappedClear guidance for inspection
Bad parts AcceptedEliminates disputes over part 
acceptance
Linear Tolerance compare with Geometric Tolerance

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, but geometric tolerances can specify datums, accurately stating the tolerance of a specific feature.

Application Of Linear Tolerance And Geometric Tolerance

Tolerance TypeLinear ToleranceGeometric Tolerance
DimensionBetter
ChamferBetter
RadiusBetter
Wall ThicknessBetter
Step FaceBetter
Position ControlBetter
Orientation ControlBetter
Shape ControlBetter

GD&T SYMBOLS CHART

Straightness

GD&T is a feature-based system for defining the size, shape, and location of features on parts, each feature has one symbol and can be classified by categories. The commonly used categories are form, profile, orientation, location, and runout. We explain the definitions of each GD&T symbol as below:

Straightness  

Straightness means the degree of straightness of linear elements on a component, reflecting the quality of not bending or curving or not sloping to either side.

ExampleWithin a specified plane, the tolerance zone of Straightness must lie between two parallel lines separated by a distance of 0.1mm.

GD&T Symbol-Straightness

Flatness 

Flatness, sometime called evenness, means the degree of flatness of planar elements on a component, reflecting the quality of being level and without curved.

Example: Within a specified area, the tolerance zone of flatness must lie between two parallel planes separated by a distance of 0.08mm.

GD&T Symbol-Flatness

Circularity  

Circularity, also called roundness, means the degree of circularity of circular elements on a part, it is used to describe how close an object should be to an actual circle. 

Example: The actual circle size must lie between two concentric circles with a radius difference of the tolerance value, 0.03mm.

GD&T Symbol-Circularity

Cylindricity 

Cylindricity specifies the roundness and straightness of a cylinder.

Example: the tolerance zone of Cylindricity must lie between two coaxial cylindrical surfaces with a radius difference of the tolerance value, 0.1mm.

GD&T Symbol-Cylindricity

Line profile 

Line profile describes a tolerance zone to which any curve on a given plane of a component maintains its ideal shape.

Example: 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.

GD&T Symbol-Line profile

Surface profile 

Surface profile tolerance describes a three-Dimensional tolerance zone around a surface, usually which is an advanced curve or shape.

Example: The tolerance zone is defined between the two-envelope plane 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.

GD&T Symbol-Surface profile

Parallelism

Parallelism is an orientation tolerance that maintains that two features (faces or axes) are parallel to each other.

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.

GD&T Symbol-Parallelism

Perpendicularity 

Perpendicularity is a specific form of Angularity at 90°between two features.

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.

GD&T Symbol-Perpendicularity

Angularity  

Angularity is a measure of maintaining any specified angle between two features on a component.

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.

GD&T Symbol-Angularity

Position 

Position is used to control the accurate condition of elements such as points, lines, and surfaces on a component relative to their ideal positions.

ExampleWhen 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.

GD&T Symbol-Position

Concentricity  

Concentricity control the condition of measured axis on a component being in the same straight line relative to the datum axis.

Example: the tolerance zone is the area between two cylinders of diameter 0.08mm. The axis of the cylindrical tolerance zone coincides with the datum.

GD&T Symbol-Concentricity

Symmetry 

Symmetry is a tolerance used to ensure that two symmetric features on a part are uniform across a datum plane.

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.

GD&T Symbol-Symmetry

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.

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.

GD&T Symbol-Circular runout

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. it is a composite tolerance that controls the location, orientation and cylindricity of the entire surface simultaneously.

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.

GD&T Symbol-Total runout

MODIFIER SYMBOLS CHART

Continuous Feature

GEOMETRIC TOLERANCE STANDARDS OF VARIOUS COUNTRIES

Although the most of content of GD&T standards are same for different countries, some countries have their own standard versions as below:

American Standard: ASME Y14.5—2018 Dimensioning and Tolerancing

Y14.5 is the most authoritative standard which establishing principles, definitions, requirements, defaults, tolerances and related specifications in engineering drawings and associated documents. you can buy ASME Y14.5 from ASME website.

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)

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Get in touch with our professional team today, and we will respond to you within 24 hours.

Employees of Kusla.

Gang Liu

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Employees of Kusla.

Hua Liu

Project Manager

Employees of Kusla.

Yiyang Zhao

Project Manager

Employees of Kusla.

Lu Wang

Technical Sales

Employees of Kusla.

Anming Li

Technical Sales

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Xiaoli Wu

Technical Sales

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