8 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. ASME and ISO standards are distributed and used worldwide. Adoption of specific stan- dards is determined by a variety of operational and organizational considerations. As discussed earlier in this chapter, the ASME Y14.5 Dimensioning and Tolerancing standard specifies approved dimensioning and tolerancing practices for engineering drawings. Previous editions of the Y14.5 standard include standards designated by the American National Standards Institute (ANSI) and the organization that predated ANSI, the United States of America Standards Institute (USASI). The USASI Y14.5-1966 Dimen- sioning and Tolerancing for Engineering Drawings standard was based on earlier standards developed by different organizations, including the ASA Y14.5-1957 standard. The ANSI Y14.5-1973 standard was revised from the USASI Y14.5-1966 standard. ASME revises the Y14.5 standard approximately every 10 years. The following lists the editions of the Y14.5 standard dating to 1973: • ANSI Y14.5-1973 • ANSI Y14.5M-1982 • ASME Y14.5M-1994 • ASME Y14.5-2009 • ASME Y14.5-2018 There are many other standards in the ASME Y14 series. The Y14 series includes standards related to drawing applications such as drawing formats, line conventions, dimensioning and tolerancing, and revisions, and specialty applications such as rep- resenting screw threads, gears and splines, and castings and forgings. Example 1-3 shows some of the standards in the Y14 series. ISO provides an extensive number of standards applicable to design and drafting. ISO standards are prepared by ISO technical committees. ISO technical committees are designated with the abbreviation ISO/TC followed by a number reference. For example, the ISO/TC 213 Dimensional and geometrical product specifications and veri- fication committee develops geometrical product specifications (GPS) standards for dimensioning and tolerancing. ISO standards use multiple documents, many of which are grouped into several parts, to define engineering design and drafting practices and theory. Example 1-4 shows some of the many published ISO standards related to dimensioning and tolerancing. Benefits of GD&T A design concept, drawing, model, or document can represent the perfect size and shape of geometry. But, product design and manufacturing tools, equipment, materials, and pro- cesses are not perfect, and so end product geometry is not perfect. The purpose and relation- ship of a particular feature of a part dictates dimensioning and tolerancing practices. GD&T is a design tool that offers a number of benefits in design and production. It allows for com- munication of the functional design intent of parts and assemblies to everyone involved in N O T E This textbook focuses on the GD&T practices and theory provided in the ASME Y14.5-2018 Dimensioning and Tolerancing standard. Important differ- ences between ASME Y14.5-2018 and earlier editions, such as ASME Y14.5- 2009, are explained when necessary. When necessary, differences between ASME Y14.5-2018 and related design and drafting standards are explained. The primary differences between ASME and ISO standards related to GD&T are associated with the appearance and meaning of certain symbols.