Basic dimension

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In a technical drawing, a basic dimension is a theoretically exact dimension, given from a datum to a feature of interest. In Geometric dimensioning and tolerancing, basic dimensions are defined as a numerical value used to describe the theoretically exact size, profile, orientation or location of a feature or datum target. [1]

Allowable variations from the theoretically exact geometry are indicated by feature control, notes, and tolerances on other non-basic dimensions. [1]

Basic dimensions are currently denoted by enclosing the number of the dimension in a rectangle.

In earlier times, they were denoted by appending "BASIC" or "BSC" to the dimension [2]

When features are located using BASIC dimensions by chain dimensioning, there is no accumulation of tolerance between features, because the dimensions refer to the theoretically perfect position of the feature, not the actual location of the feature within the range permitted by tolerances. [3]

Basic dimensions, identified by the engineer, locate tolerance zones. Because that’s true, basic dimensions identify tolerance information located in feature control frames that state geometric tolerances. When an engineer makes a statement of tolerance, that statement is made in the form of a feature control frame.

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Geometric dimensioning and tolerancing

Geometric Dimensioning and Tolerancing (GD&T) 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.

Engineering tolerance

Engineering tolerance is the permissible limit or limits of variation in:

  1. a physical dimension;
  2. a measured value or physical property of a material, manufactured object, system, or service;
  3. other measured values ;
  4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel ;
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ASME Y14.5 is a standard published by the American Society of Mechanical Engineers (ASME) to establish rules, symbols, definitions, requirements, defaults, and recommended practices for stating and interpreting Geometric Dimensions and Tolerances (GD&T). ASME/ANSI issued the first version of this Y-series standard in 1973.

References

  1. 1 2 ASME Y14.5M-1994 Dimensioning and Tolerancing
  2. Tool and Manufacturing Engineers Handbook Desk Edition, Volume 5, 9-1
  3. Tool and Manufacturing Engineers Handbook Desk Edition, Volume 5, 9-1