Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances via a symbolic language on engineering drawings and computer-generated 3D models that describes a physical object's nominal geometry and the permissible variation thereof. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, the allowable variation in size, form, orientation, and location of individual features, and how features may vary in relation to one another such that a component is considered satisfactory for its intended use. Dimensional specifications define the nominal, as-modeled or as-intended geometry, while tolerance specifications define the allowable physical variation of individual features of a part or assembly. There are several standards available worldwide that describe the symbols and define the rules used in GD&T. One such standard is American Society of Mechanical Engineers (ASME) Y14.5. This article is based on that standard. Other standards, such as those from the International Organization for Standardization (ISO) describe a different system which has some nuanced differences in its interpretation and rules (see GPS&V). The Y14.5 standard provides a fairly complete set of rules for GD&T in one document. The ISO standards, in comparison, typically only address a single topic at a time. There are separate standards that provide the details for each of the major symbols and topics below (e.g. position, flatness, profile, etc.). BS 8888 provides a self-contained document taking into account a lot of GPS&V standards.
Origin The origin of GD&T is credited to Stanley Parker, who developed the concept of "true position". While little is known about Parker's life, it is known that he worked at the Royal Torpedo Factory in Alexandria, West Dunbartonshire, Scotland. His work increased production of naval weapons by new contractors. In 1940, Parker published Notes on Design and Inspection of Mass Production Engineering Work, the earliest work on geometric dimensioning and tolerancing. In 1956, Parker published Drawings and Dimensions, which became the basic reference in the field.
Fundamental concepts
Dimensions A dimension is defined in ASME Y14.5 as "a numerical value(s) or mathematical expression in appropriate units of measure used to define the form, size, orientation, or location, of a part or feature." Special types of dimensions include basic dimensions (theoretically exact dimensions) and reference dimensions (dimensions used to inform, not define a feature or part).
Units of measure The units of measure in a drawing that follows GD&T can be selected by the creator of the drawing. Most often drawings are standardized to either SI linear units, millimeters (denoted "mm"), or US customary linear units, decimal inches (denoted "IN"). Dimensions can contain only a number without units if all dimensions are the same units and there is a note on the drawing that clearly specifies what the units are. Angular dimensions can be expressed in decimal degrees or degrees, minutes, and seconds.
Tolerances Every feature on every manufactured part is subject to variation, therefore, the limits of allowable variation must be specified. Tolerances can be expressed directly on a dimension by limits, plus/minus tolerances, or geometric tolerances, or indirectly in tolerance blocks, notes, or tables. Geometric tolerances are described by feature control frames, which are rectangular boxes on a drawing that indicate the type of geometric control, tolerance value, modifier(s) and/or datum(s) relevant to the feature. The type of tolerances used with symbols in feature control frames can be:
equal bilateral unequal bilateral unilateral no particular distribution (a "floating" zone) Tolerances for the profile symbols are equal bilateral unless otherwise specified, and for the position symbol tolerances are always equal bilateral. For example, the position of a hole has a tolerance of .020 inches. This means the hole can move ±.010 inches, which is an equal bilateral tolerance. It does not mean the hole can move +.015/−.005 inches, which is an unequal bilateral tolerance. Unequal bilateral and unilateral tolerances for profile are specified by adding further information to clearly show this is what is required.
Datums and datum references
A datum is a theoretically exact plane, line, point, or axis. A datum feature is a physical feature of a part identified by a datum feature symbol and corresponding datum feature triangle, e.g.,
◻ A − − ◂ | {\displaystyle {\displaystyle \Box }\!\!\!\!{\scriptstyle {\mathsf {A}}}\!-\!\!\!-\!\!\!\blacktriangleleft \!\!\!|}
These are then referred to by one or more 'datum references' which indicate measurements that should be made with respect to the corresponding datum feature. The datum reference frame can describe how the part fits or functions.
Purpose and rules The purpose of GD&T is to describe the engineering intent of parts and assemblies. GD&T can more accurately define the dimensional requirements for a part, allowing over 50% more tolerance zone than coordinate (or linear) dimensioning in some cases. Proper application of GD&T will ensure that the part defined on the drawing has the desired form, fit (within limits) and function with the largest possible tolerances. GD&T can add quality and reduce cost at the same time through producibility. According to ASME Y14.5, the fundamental rules of GD&T are as follows,
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