GD&T Symbols

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GD&T Symbols

gd&t symbols basic

GD&T is the abbreviation of Geometric Dimensioning and Tolerancing, a symbolic engineering language widely used in the world, which clearly and precisely defines the size, shape, direction and position of the geometric product function, so that the product can be On the premise of meeting its functional needs, obtain looser tolerance requirements as much as possible to effectively reduce manufacturing costs; the continuous improvement of GD&T popularization and application can effectively reduce or eliminate the drawings of the same product in different countries, or the same country but different departments Differences in understanding (R&D, technology, manufacturing, quality inspection); GD&T originated in the 1940s and has been widely used in many industries such as national defense, aerospace, automobiles, engineering machinery, electronic appliances, and medical equipment , has a decisive influence on a global scale.

GD&T generally refers to tolerance of form and position

Geometric tolerances are generally also called geometric tolerances, including shape tolerances and position tolerances. Any part is composed of points, lines, and surfaces, and these points, lines, and surfaces are called elements. The actual elements of the machined parts always have errors relative to the ideal elements, including shape errors and position errors. This type of error affects the function of mechanical products, and the corresponding tolerance should be specified during design and marked on the drawing according to the specified standard symbols. Around the 1950s, industrialized countries had form and position tolerance standards. The International Organization for Standardization (ISO) published the geometric tolerance standard in 1969, and recommended the geometric tolerance detection principle and method in 1978.

The processed parts will have dimensional tolerances, so the actual shape or mutual position of the points, lines, and surfaces that constitute the geometric features of the part are different from the shape and mutual position specified by the ideal geometry. This difference in shape is the shape tolerance, and The difference in mutual position is the position tolerance, and these differences are collectively called Geometric tolerances.

gdt symbols

Form Tolerance (GD&T):

1. Straightness The symbol is a short horizontal line (-), which is an index to limit the variation of the actual straight line to the ideal straight line. It is a requirement for straight lines that are not straight.
2. Flatness The symbol is a parallelogram, which is an index to limit the amount of variation between the actual plane and the ideal plane. It is a requirement for unevenness of the plane.
3. Roundness The symbol is a circle (○), which is an index to limit the variation of the actual circle to the ideal circle. It is a circular profile requirement in a normal section (a plane perpendicular to the axis) for parts with cylindrical surfaces (including conical surfaces and spherical surfaces).
4. Cylindricity The symbol is a circle (/○/) between two slashes, which is an index to limit the variation of the actual cylindrical surface to the ideal cylindrical surface. It controls various shape errors in the cross section of the cylinder and the shaft section, such as roundness, straightness of prime line, straightness of axis, etc. Cylindricity is a comprehensive index of various shape errors of a cylinder.
5. Line profile The symbol is a convex curve (⌒), which is an index to limit the amount of variation between the actual curve and the ideal curve. It is the shape accuracy requirement for non-circular curves.
6. Surface contour degree The symbol is a semicircle with a horizontal line on the bottom, which is an index that limits the amount of variation between the actual surface and the ideal surface, and it is the shape accuracy requirement for the surface.

Orientation Tolerance (GD&T):

1. Parallelism (∥) is used to control the requirement that the measured element (plane or line) on the part deviates from the direction of the reference element (plane or line) to 0°, that is, the measured element is required to be equidistant to the reference.
2. Perpendicularity (⊥) is used to control the requirement that the measured element (plane or line) on the part deviates from the direction of the reference element (plane or line) by 90°, that is, the measured element is required to be 90° to the reference.
3. Inclination (∠) is used to control the degree to which the measured element (plane or line) on the part deviates from a given angle (0°~90°) relative to the reference element (plane or line), that is, it is required to be The measured elements form a certain angle (except 90°) to the datum.

Positioning tolerance (GD&T):

1. Coaxiality (◎) is used to control the degree of misalignment between the measured axis and the reference axis that should be coaxial in theory.
2. The symbol of symmetry is the three horizontal lines in the middle, which are generally used to control the relationship between the measured element (center plane, center line or axis) and the reference element (center plane, center line or axis) that are theoretically required to be coplanar. degree of misalignment.
3. Position degree The symbol is a circle with two straight lines perpendicular to each other, which is used to control the variation of the measured actual element relative to its ideal position. The ideal position is determined by the datum and the theoretically correct size.

Runout Tolerance (GD&T):

1. The circular runout symbol is a slash with an arrow. The circular runout is the maximum and minimum readings measured in a given direction by an indicator with a fixed position during the actual element being measured around the reference axis without axial movement and one revolution. Difference.
2. The symbol of full runout is two oblique lines with arrows. Full runout is the continuous rotation of the measured actual element around the reference axis without axial movement. At the same time, the indicator moves continuously along the ideal prime line, and the indicator moves in a given direction. The difference between the maximum and minimum readings measured.

GD&T symbols related standards and specifications

The term GD&T originated from the United States. The ASME Y14 series of standards of the American Society of Mechanical Engineers are all related to GD&T. The following are commonly used GD&T related standards and specifications:

1: ASME Y14.5—2018 Dimensioning and Tolerancing

Y14.5 is the most important authoritative standard of GD&T. It determines the principles, definitions, requirements, default or recommended practices for geometric dimensions and tolerances and related requirements in engineering drawings and related documents, covering basic rules, symbols, dimensions Tolerance, datum (system), geometric tolerance (shape, direction, position, profile, runout) and its various combinations and applications, etc. Compared with the previous version, the current 2018 edition has improved in the concept of dimension elements, the expression of datum reference system and degree of freedom, the tolerance of compound position degree, the surface interpretation and axis interpretation of position degree, the tolerance of contour degree, symbols and modifiers, etc. updated.

2: ASME Y14.36—2018 Surface Texture Symbols

The surface of the part formed by processing is in a non-ideal state, and its characteristics can usually be separated into surface roughness, surface waviness, surface shape error, and surface defects. Y14.36 regulates the method of controlling the surface texture of solid materials, including the method of controlling roughness, waviness and grain direction, and provides marking symbols used in engineering drawings and related documents; but does not specify the method of controlling or measuring waviness .

3: ASME Y14.41—2012 Digital Product Definition Data Practices

Y14.41 defines the basic criteria for 3D-based digital product definition, and lays the foundation for all-round application of 3D data throughout design, manufacturing and testing. It is applicable to both 3D product development mode (Model Only) and 3D and 2D Mixed R&D mode (Model and Drawing). 3D digital product definition, in addition to model data and revision history, also includes related data such as materials, processes, analysis data, and test requirements. For the 3D R&D mode (Model Only), drawings are no longer needed; the direct application of geometric dimensions and tolerances to 3D digital models is the development trend of GD&T applications.

4: ASME Y14.43—2011 Dimensioning and Tolerancing Principles for Gages and Fixtures

Gauge is an important means to realize product inspection, and its structure and tolerance allocation have a crucial impact on the judgment of product quality. The correct design of the gage structure and the reasonable distribution of the gage tolerance not only help to reduce the risk of measurement, but also help to reduce the cost. Y14.43 provides a design method for functional checking fixtures, which is used to verify the contour and effective boundary under the maximum physical condition, and provides three principles for the tolerance allocation of checking fixtures: absolute tolerance principle (pessimistic principle), optimistic tolerance principle and The principle of inclusive tolerance; on the premise of meeting the functional requirements of the workpiece, the reasonable selection of the tolerance distribution principle can not only reduce the manufacturing cost of the inspection tool, but also increase the proportion of qualified products appropriately.

5: ASME Y14.8—2009 Castings, Forgings and Molded Parts

Generally, the structure of the molded part is relatively complex, and the datum target is used to establish the datum, which is designed and manufactured by the mold. Y14.8 explains the design of molded parts and molds, the selection and application of datum targets for irregular rigid parts, the selection and application of datum targets for flexible (injection molding) parts, the inspection and fixture design of flexible (injection molding) parts, GD&T in mold design of injection molding parts Applications and detection methods, etc.

6: ASME.Y14.2—2008 Line Conventions and Lettering


Y14.2 stipulates the line type, line width and application used in computer-aided design (CAD) or manual drawing; at the same time, it stipulates the minimum font height for drawing fonts and different drawing frames. In addition, standards related to drawing engineering drawings include Y14.1, Y14.3, Y14.24, Y14.34, Y14.35, Y14.38, Y14.100, etc.

GD&T usually refers to the American standard ASME Y14.5, the current 2018 edition.

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Author: Mose Li

Author: Mose Li

Director of Project Engineering at 3Q Machining

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