Fundamentals of Geometric Dimensioning & Tolerancing


Geometric Dimensioning and Tolerancing, which is abbreviated GD&T, is a language used on different mechanical drawings that consists of a variety of symbols used to convey information relating to geometry. GD&T has its origins in the American and British munitions industries, who used to use plus minus tolerancing in product drawings. The inaccuracy of this method and the need to cut down on the waste that it created led to the establishment of GD&T, which is now used by the International Organization for Standardization or ISO as well as the American National Standards Institute or ANSI.

The Why Behind Geometric Dimensioning and Tolerancing

  • Looking at the less accurate method of plus minus tolerancing, also known as conventional tolerancing helps students to see the need for GD&T. While notes can be made on these drawings, the actual drawings themselves do not contain enough information to ensure that the the drawing will be properly interpreted. Once conventional tolerancing has been studied, students can progress to learning the basics of GD&T.

Position Tolerancing

  • Position tolerancing is the aspect of mechanical drawings using GD&T that indicates the size of different aspects of the drawing. Some features of a drawing whose size may be indicated using position tolerancing include holes, bosses, slots and tabs.

Product Plans and Drawings

  • For those studying GD&T, they will need to learn both how to create and how to interpret plans and drawings. In order to do so they need to familiarize themselves with the different GD&T symbols that are used in plans and drawings as well as the rules of application and interpretation that govern GD&T. Use of the unique symbols ensures that the proper information is being conveyed on a drawing. The rules further expand on the directions provided by the symbols, outlining concepts such as limits of size and the form of an object.

Datum Reference Frame

  • A datum reference frame can provide further details that help to make the size, shape and angles of an item in a plan or drawing clear. A datum reference plan uses a coordinate system such as a Cartesian coordinates to orient an object in space and within tolerance zones. A datum reference plan usually uses the other datum included in the drawing to orient and locate a tolerance zone.


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