ABSTRACT
Geometric dimensioning and tolerancing (GD&T) is a symbolic language used to de¢ne part and assembly geometry and its allowable variation. As a symbolic language, GD&T transcends the borders and barriers of spoken language and is understood across borders, across continents and across the world. The two purposes of GD&T described in the ¢rst sentence above are distinct but overlapping, and are sometimes confused by casual users. One use of GD&T is to de¢ne the geometry of perfect, as-designed, as-modeled and as-drawn parts and assemblies-generally, this is the purpose of dimensioning. In most cases, dimensions represent the as-intended perfect state of part and assembly geometry. Exceptions to this include nominal sizes for screw threads and pipe sizes, which are often different than the stated value. Also common in ISO standards is the use of tolerance classes or grades, which sometimes designate size limits that do not encompass the nominal (or basic) size, such as a Ø20 mm shaft with size limits of Ø20.1 and Ø20.2. The other use of GD&T is to de¢ne the allowable geometric variation for as-produced parts and assemblies. This is the tolerancing portion of GD&T, and includes directly toleranced dimensions (±) and geometric tolerances. In addition to the tolerances themselves, other symbols and methods are needed to completely de¢ne the dimensioning and tolerancing schemes for parts and assemblies. So, if properly employed, GD&T is used to precisely de¢ne the perfect, as-intended state of part and assembly geometry, and GD&T is used to precisely de¢ne the variation allowed from that perfect ideal condition.
