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Geometric Dimensioning and Tolerancing (GD&T)

symbolic-language
GD&T Symbols on Engineering Drawings

Geometric dimensioning and tolerancing (GD&T) is a symbolic language on engineering drawings that explicitly describes nominal geometry and its allowable variation. Geometric tolerancing encourages a dimensioning philosophy called functional dimensioning. It defines a part based on how it functions in the final product, to insure the proper assembly of mating parts, to improve quality, and to reduce cost. Proper application of GD&T will ensure that the allowable part and assembly geometry defined on the drawing leads to parts that have the desired form and fit and function as intended. Coordinate dimensioning was the sole best option until the post-World War II. The need for precise communications developed the engineering drawings to the geometric dimensioning and tolerancing, which departs from the limitations of coordinate dimensioning.

When applying GD&T the first consideration is to establish a datum reference frame based on the function of the part in the assembly with its mating parts. After the datum reference frame is established, the form of the primary datum feature is controlled, followed by the location of the secondary and tertiary datum features. After the datum features are related relative to each other, the remaining features are controlled for orientation and location relative to the datum reference framework.

Since many major industries are becoming more global, resulting in the decentralization of design and manufacturing, it is even more important that the design more precisely state the functional requirements. To accomplish this it is becoming increasingly important that the use of geometric dimensioning and tolerancing replace the former limit dimensioning for form, orientation, location, and profile of part features.