6 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. Interchangeability of Parts Prior to the onset of mass production in manufacturing, mechanical parts and assemblies were designed and produced in one-by-one fashion. During the Industrial Revolution, manufacturing evolved and the need for interchangeability of parts gained importance. Interchangeability is the ability to manufacture parts to a precision that allows for the substitution of one part for another part of the same specification. In interchangeable manufacturing, parts are designed and produced to be within specified tolerances. Interchangeable manufacturing permits mass production because each correctly pro- duced part can fit into the product for which it is used and function after assembly. One part can be selected to replace another without the need for a custom fit. Interchange- ability allows for easier, faster, and less costly assembly and repair of products. Until interchangeable manufacturing became more common in industry, detailed drawings were not necessary. Today’s technological environment requires that prod- ucts do not need unnecessary individual fitting of parts. Geometric tolerancing helps ensure that interchangeability can be achieved. Development of GD&T Traditional dimensioning and tolerancing practices involved applying plus and minus tolerances or limits to dimensions to express allowable variations on part features. In addition, tolerance requirements for part features were traditionally communicated on drawings using notes. The transition from these practices to a more precise method of dimensioning and tolerancing occurred during the mid-1900s. The manufacturing requirements associated with military production during World War II called attention to the need for a uniform dimensioning and toleranc- ing system. Before World War II, the team involved in the design and production of a product worked together, often side-by-side. Designers discussed feature geometry and controls with manufacturing workers, and everyone involved with production understood engineering drawings approved for manufacture. This level of interaction between individuals within the organization helped to make sure that parts would fit together. In this environment, it was assumed that the drawings explained all neces- sary information about the form, fit, and function of the design. The demand for products such as military weapons, vehicles, ships, and planes dur- ing World War II required that different manufacturers reference the same drawings. The lack of direct interaction between engineers and contractors led to misinterpretation of drawings by manufacturing personnel. As a result, parts that previously fit together no longer would assemble. Some issues were corrected by altering manufacturing pro- cesses. However, organizations realized that the primary concern was the ambiguity of engineering drawings. A fresh form of dimensioning and tolerancing using symbols to communicate functional relationships and design intent was necessary. N O T E This textbook presents examples and print drawings using third-angle pro- jection. Third-angle projection is a method of 2D view projection used in the United States. First-angle projection is a method of 2D view projection used in Europe and other countries throughout the world. This textbook does not show first-angle projection on drawings and prints, but first-angle projec- tion can apply to your course requirements.