6 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. USASI Y14.5-1966 ANSI Y14.5-1973 ANSI Y14.5M-1982 ASME Y14.5M-1994 ASME Y14.5-2009 ASME Y14.5-2018 The ASME Y14.5 dimensioning and tolerancing standard and other related standards are periodically revised to add capabilities needed by industry and to clarify existing practices. There is no fi xed period of time between revisions of the standards. Longstanding practices are not changed unless there is a compelling reason to do so. As an example, the transition from manually produced drawings to model-based documentation has resulted in the need to make changes in how dimensions and tolerances are shown. The latest revisions of ASME Y14.5 and ASME Y14.41 made substantial advancements in the ability to defi ne tolerances while maintaining the pri- mary principles of previous standards. It is important for companies to adopt the lat- est issue of the standard to take advantage of the improvements and to stay current in industry. There are situations where it may be necessary to know the content of previous standards. This may be the case when working with an existing product defi nition that was created in compliance with a previous revi- sion, or when creating a new product defi nition in fulfi llment of an old contract that invokes one of the older standards. Usually, only the methods included in the invoked standard may be used in the product defi - nition. Generally, methods that are not included in the invoked standard may not be used. As an exam- ple, if a document invokes ASME Y14.5-2009, then the dynamic profi le symbol fi rst introduced in ASME Y14.5-2018 would not be allowed. To overcome this restriction, a company may invoke new capabilities by note. Generally, it is unnecessary to know the require- ments of all previous standards because it is unlikely that many people are still working with much older standards. In the rare event where an old standard is a contractual requirement, a copy can be obtained from the contracting agency. This textbook includes descriptions based on the current dimensioning and tolerancing standards and highlights areas of signifi cant change from the past issues of the standards. The most recent issue of each applicable standard should be used when there is an option regarding which issue (year of standard) to use. If there is any doubt about whether a new issue of the standard has been released, the American Society of Mechanical Engineers (ASME) should be contacted. Almost all fi gures in the current standard show metric dimensions. See Figure 1-3. The only excep- tions are fi gures that show how tolerances and limit dimensions are applied using inch values. The ASME Y14.5 standard does indicate that either inch or millimeter dimensions are permitted. A signifi cant part of the industrial world uses inch dimensions, especially in the United States. Most of the illustrations in this book show inch values. Changing the inch values to millimeters would have no effect on how the dimensions and tolerances are applied it would only change the dis- played numerical values. Standards are established to provide consis- tency in how things are done. However, a standard cannot show every possible situation that may arise it can only show the principles and how they apply to various situations. Principles covered by the current dimensioning and tolerancing standard may be applied to many situations that are not shown in the standard. If a complete understanding of the principles is gained, then the principles are adequate for meeting most design documentation needs. It is up to the individual to apply the principles to spe- cifi c situations as they are encountered. Unique situations occur where the principles of the standard do not meet the needs for a specifi c dimension or tolerance application. In these cases, it is possible to use notes to explain the special requirement. Although ASME Y14.5-2018 permits the use of notes to specify geometric tolerances, that practice is not recommended as the general means of specify- ing tolerances on individual features. Excessive use of notes to explain tolerance requirements should be avoided because one of the purposes of the current standard is to provide a means for clear defi nition of dimensional requirements through symbology. Metric Goodheart-Willcox Publisher Figure 1-3. Illustrations in the current standard include metric dimensions.