Geometric dimensioning and tolerancing


Geometric Dimensioning and Tolerancing
"GD&T" Is like a common language "English" so that the different language people can convey their words in common. Here the different language people are designer, manufacturer, quality inspector and the language is "GD&T". The quality of the product is attained by these three contributors with the perfect agreement.
It is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describe nominal geometry and its allowable variation. It tells the manufacturing staff and machines what degree of accuracy and precision is needed on each controlled feature of the part. GD&T is used to define the nominal geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the allowable variation between features.
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 Y14.5. This article is based on that standard, but other standards, such as those from the International Organization for Standardization, may vary slightly. The Y14.5 standard has the advantage of providing a fairly complete set of standards 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.

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.

Dimensioning and tolerancing philosophy

According to the ASME Y14.5-2009 standard, the purpose of geometric dimensioning and tolerancing is to describe the engineering intent of parts and assemblies. The datum reference frame can describe how the part fits or functions. GD&T can more accurately define the dimensional requirements for a part, allowing over 50% more tolerance zone than coordinate dimensioning in some cases. Proper application of GD&T will ensure that the part defined on the drawing has the desired form, fit and function with the largest possible tolerances. GD&T can add quality and reduce cost at the same time through producibility.
There are some fundamental rules that need to be applied :
Tolerances: Type of tolerances used with symbols in feature control frames can be 1) equal bilateral 2) unequal bilateral 3) unilateral 4) no particular distribution
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.
SymbolUnicode
character		ModifierNotes
Ⓕ
U+24BB
Free stateApplies only when part is otherwise restrained
Ⓛ
U+24C1
Least material condition Useful to maintain minimum wall thickness
Ⓜ
U+24C2
Maximum material condition Provides bonus tolerance only for a feature of size
Ⓟ
U+24C5
Projected tolerance zoneUseful on threaded holes for long studs
Ⓢ
U+24C8
Regardless of feature size t part of the 1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3-8.
Ⓣ
U+24C9
Tangent planeUseful for interfaces where form is not required
Continuous featureIdentifies a group of features that should be treated geometrically as a single feature
Statistical toleranceAppears in the 1994 version of the standard, assumes appropriate statistical process control.
Ⓤ
U+24CA
Unequal bilateralAdded in the 2009 version of the standard, and refers to unequal profile distribution. Number after this symbol indicates tolerance in the "plus material" direction.

Datums and datum references

A datum is a virtual ideal 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.,
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.

Data exchange

Exchange of geometric dimensioning and tolerancing information between CAD systems is available on different levels of fidelity for different purposes:

ISO TC 10 ''Technical product documentation''

In ISO/TR 14638 GPS – Masterplan the distinction between fundamental, global, general and complementary GPS standards is made.
ASME is also working on a Spanish translation for the ASME Y14.5 – Dimensioning and Tolerancing Standard.

GD&T standards for data exchange and integration