2 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. A clear specifi cation of tolerance requirements is always good practice, and it is essential if parts are to be produced by more than one manufac- turer. In many industries, including the automotive and aerospace industries, parts are often made by supplier companies and shipped to a factory for assembly into a deliverable product. Detail part and assembly requirements must be clearly defi ned if the parts are to fi t together and properly function. Vague or incorrectly specifi ed dimensional requirements can result in parts that are incorrectly manufactured, and ultimately unable to be fi t into the assembly. The number of small tolerances on a part should be reduced as much as possible while still meeting the needed quality, and all unnecessarily small toler- ances should be eliminated. Calculating tolerances is an important part of ensuring that tolerance values are maximized. Maximized tolerances permit the use of less expensive machine processes and less restrictive controls during the fabrication of parts. It is common in industry for discussions to include colloquialisms in regard to dimensioning and tolerancing. However, to achieve an accurate and clear presentation of the facts, exacting terminology is needed. Common terminology used in dimensioning and tolerancing is referenced and explained through- out this text. Terminology is introduced as needed and appropriate. A list of technical terms is provided at the beginning of each chapter. Defi nitions for these terms are included in the Glossary and are given in an appropriate context when terms are introduced. Technological Developments in Product Documentation There are multiple aspects to the historical development of product documentation processes. One aspect is the establishment of a distance mea- surement standard. Another aspect is the establish- ment of dimensioning and tolerancing standards. The development of dimensioning and tolerancing standards occurred only recently when compared to the time period over which distance measurement standards developed. Advancements in technology have also had a major impact on product documen- tation processes. In recent decades, the emergence of computers and computer-aided design (CAD) is of major signifi cance. Distance measurement standards evolved over thousands of years, resulting in two com- mon systems of measurement used today— the US Customary system, generally used in the United States, and the International System of Units (SI), commonly known as the SI system (metric system). In the US Customary system, the basic unit of linear measurement is the inch. In the metric system, the basic unit of linear measurement is the meter. In product documentation, metric dimensions are most commonly specifi ed in millimeters. The cur- rent American Society of Mechanical Engineers (ASME) dimensioning and tolerancing standards are primarily illustrated using metric dimensions, but the standards are applicable to both inch and metric dimensions. The unit of measurement has no effect on the dimension and tolerance application methods it only affects how numerical values are shown in dimensions and tolerances. Dimensioning and tolerancing standards be- came relatively well defi ned during the past 100 years with most of the progress taking place after 1960. The 1966 and 1973 American standards introduced new far-reaching methods to clearly defi ne toler- ance requirements. Subsequent standards expanded capabilities, but the fundamental concepts changed very little from the standard issued in 1973. Although the fundamentals remained constant, the 1982, 1994, 2009, and 2018 standards introduced signifi cantly expanded capabilities. Because dimensions may be expressed in either inches or millimeters, a note must be included in the product defi nition to prevent any confusion as to which unit of measurement is applicable. When only a CAD model exists, the note is included in the model. If a drawing is created, the note is typically placed in the notes list. A note such as the following may be used: ALL DIMENSIONS ARE IN INCHES or ALL DIMENSIONS ARE IN MILLIMETERS Computers have been in existence for decades, but major advancements to current technological capabilities have largely taken place since 1980. CAD software was extremely expensive in 1980 and not nearly as powerful compared to today. The capa- bilities of CAD software dating to that time have been far surpassed. The CAD software now used in industry is available for a small percentage of the past cost. Today’s software technology is simply amazing when compared to the tools and methods traditionally associated with manual preparation. The tools used today make it possible to create designs and establish allowable tolerances such that all users of the design data can access the needed information effi ciently and reliably. Design require- ments are clearly communicated with only one possible interpretation. The precise communication of requirements fl ows from design to fabrication and fi nally to inspection in a manner that ensures products fi t together, function as designed, and meet quality expectations.