GD&T is the abbreviation of Geometric Dimensioning and Tolerancing (Geometric Dimensioning and Tolerancing) is widely used internationally, and clearly and accurately defines the size, shape, direction, and location of geometric product functions. From the 1940s until today, it has been widely used in many industries such as national defense and military industry, aerospace, automobiles, engineering machinery, electronic appliances, and medical equipment. This article describes the basic knowledge, definitions and GD&T symbol table of GD&T.
What is GD&T?
The tolerance system used to define and communicate engineering design and computer-generated 3D solid models refers to the accuracy and precision requirements of CNC machining parts. GD&T usually refers to the American standard ASME Y14.5, the current 2018 edition.
Related standards and specifications of GD&T:
The term GD&T originated from the United States. The ASME Y14 series standards of the American Society of Mechanical Engineers are all related to GD&T. The following are commonly used:
1.ASME.Y14.2—2008 Line Conventions and Lettering
Y14.2 specifies the line type, line width and application used in computer-aided design (CAD) or manual drawing; at the same time, it specifies the font of the drawing and the minimum character height used in 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.
2.ASME Y14.8—2009 Castings, Forgings and Molded Parts
Generally, the structure of the molded part is more complicated, and the benchmark target is used to establish the benchmark, which is designed and manufactured by the mold. Y14.8 explains the design of molded parts and molds, the selection and application of reference targets for irregular rigid parts, the selection and application of reference targets for flexible (injection) parts, the detection and fixture design of flexible (injection) parts, and GD&T in the design of injection molds. Application and testing methods, etc.
3.ASME Y14.43—2011 Dimensioning and Tolerancing Principles for Gages and Fixtures
Inspection fixture is an important means to realize product inspection, and its structure and tolerance distribution have a vital influence on the judgment of product quality. The correct design of the gage structure and the reasonable allocation of gage tolerances not only help reduce the risk of measurement, but also help reduce costs. Y14.43 provides a design method for functional inspection tools to verify the contour and effective boundary under the maximum physical condition.
At the same time, it provides three principles for the tolerance allocation of inspection tools: absolute tolerance principle (pessimistic principle), optimistic tolerance principle and Tolerance tolerance principle: On the premise of satisfying the functional requirements of the workpiece, a reasonable selection of the tolerance distribution principle can not only reduce the manufacturing cost of the inspection tool, but also appropriately increase the proportion of qualified products.
4.ASME Y14.41—2012 Digital Product Definition Data Practices
Y14.41 defines the basic criteria for the definition of digital products based on 3D, laying a foundation for all-round applications of 3D data throughout design, manufacturing and testing. It is suitable for both 3D product development mode (Model Only) and 3D and 2D. Mixed research and development model (Model and Drawing). In addition to model data and revision history, 3D digital product definition 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.
5.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 defect. Y14.36 specifies the methods for controlling the surface texture of solid materials, including methods for controlling roughness, waviness and texture direction, and provides marking symbols used in engineering drawings and related documents; however, it does not specify methods for controlling or measuring waviness.
6.ASME Y14.5—2018 Dimensioning and Tolerancing
Y14.5 is the most important authoritative standard of GD&T. It determines the principles, definitions, requirements, defaults or recommended practices for the geometric dimensions and tolerances and related requirements in engineering drawings and related documents. The content covers basic rules, symbols, and dimensions. Tolerance, datum (system), geometric tolerance (shape, direction, position, profile, runout) and various combinations and applications, etc. Compared with the previous version, the current 2018 version is carried out in the concept of dimensional elements, the expression of the datum reference system and the degree of freedom, the composite position tolerance, the surface interpretation and axis interpretation of the position degree, the contour tolerance, symbols and modifiers, etc. Updated.