Geometric Dimensioning and Tolerancing (GD&T)

GD&T-

• GD&T is a symbolic language for defining and communicating engineering tolereances.
• Used to specify the size, shape, form, orientation, and location of features on a part.
• GD&T was created to insure the proper assembly of mating parts, to improve quality, and to reduce cost.
• Drawings with properly applied geometric tolerancing provide the best opportunity for uniform interpretation and cost-effective assembly.

History-
DuringWorldWar II, the United States manufactured and shipped spare parts, Many of these parts were made to specifications but would not assemble.They recognized that producing parts that do not properly fit or function is a serious problem since lives depend on equipment that functions properly. To find a way to insure that parts would properly fit and function every time. The result was the development of GD&T.
GD&T has been credited to a man named "Stanley Parker", who developed the concept of "true position".
Ultimately, the USASI Y14.5–1966 document was produced on the basis of earlier standards and industry practices. The following are revisions to the standard:
ANSI Y14.5–1973 (American National Standards Institute)
ANSI Y14.5M–1982
ASME Y14.5M–1994 (American Society of Mechanical Engineers)
The 1994 revision is the current, authoritative reference document that specifies the proper application of GD&T.

Three important advantages over the coordinate tolerancing system:

• The cylindrical tolerance zone
• The maximum material condition
• Datums specified in order of precedence

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