Geometric Dimensioning and Tolerancing

Geometric DImensioning and Tolerancing (GD&T) is a system of rules and symbols used for describing the geometry of parts. Common GD&T practices provide accurate communication of design intent between manufacturers, engineers, and machinists. {|

Fundamentals
GD&T is a compact language for describing a part that replaces notes with symbols. It is used worldwide under the ISO 1101 and ASME Y14.5M standards. This system is defined by the tenants of clarity, improved inspection methods and realistic tolerances manufacturers have better control of fit, function and design.

Dimensioning
To build an object as precise as possible, complete information about shape and size of the object must be available. Orthographic drawings, developed following standard drawing practices, are used to express the exact shape of an object. Dimensioning a drawing can be described as the process of adding size information to the drawing [1].

The science of geometrics is applied to specify and tolerance the shapes and locations of features on objects. Then, once an orthographic drawing has enough information to fully define the shape, the size specifications can be added in the form of dimensions, as well. Note that by dimensioning a drawing, the tolerance for each dimension can be identified. Graphics, dimensions, and words are the three elements for a fully defined part that make the intent of the designer clear to the person building the part as well as the inspector inspecting the part. Moreover, it's important to take into consideration that the necessary information to make the object only without excessive dimensions, such as duplicating a dimension or dimensioning the same feature in multiple ways [1].

The dimensioning of parts is based on:
 * "Basic size and locations of the features"
 * "Details of a part's construction and for manufacturing" [1].

Formats


Some important terminologies to specify size and location of features illustrated on the drawings by numbers are:


 * 1) Dimension: "is the numerical value that defines the size or geometric characteristic of a feature"


 * 1) Basic Dimension: "is the numerical value defining the theoretically exact size of a feature"


 * 1) Reference Dimension: "is the numerical value enclosed in parentheses provided for information only and is not used in the fabrication of the part"


 * 1) Dimension Line: "is the thin solid line which shows the extent and direction of a dimension"


 * 1) Arrows: "are placed at the ends of dimension lines to show the limits of the dimension"


 * 1) Extension Line: "is the thin solid line perpendicular to a dimension line indicating which feature is associated with the dimension"


 * 1) Leader Line: "is the thin solid line used to indicate the feature with which a dimension, note, or symbol is associated"


 * 1) Tolerance: " is the amount a particular dimension is allowed to vary"


 * 1) Plus and Minus Dimensioning: "is the allowable positive and negative variance from the dimension specified"


 * 1) Limits of Size: "is the largest acceptable size and the minimum acceptable size of a feature"


 * 1) * The largest acceptable size is expressed as the Maximum Material Condition (MMC)


 * 1) * The smallest acceptable size is expressed as the Least Material Condition (LMC)


 * 1) Diameter Symbol: "is the symbol which is placed preceding a numerical value indicating that the associated dimension shows the diameter of a circle. The symbol used is the Greek letter phi, ∅"


 * 1) Radius Symbol: "is  the symbol which is placed preceding a numerical value indicating that the associated dimension shows the radius of a circle. The radius symbol used is the capital letter R"


 * 1) Datum: "is the theoretically exact point used as a reference for tabular dimensioning" [1]

Feature Control Frame


A features tolerance information can be described in a box, which reads from left to right, called a feature control frame.
 * 1) The first space describes the characteristic of the feature (position, flatness, etc).
 * 2) The second space defines the tolerance of the feature, tolerance zone, and any material condition modifiers.
 * 3) The following spaces contain the primary datum and any tertiary datums and material condition modifiers that apply.

Symbols

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