In 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 used to communicate the critical design dimensions of a part. Basic dimensions represent an ideal case and as such, have no tolerance. To facilitate manufactureability, a feature control frame is often used to assign a dimensional tolerance to the feature that is referenced in by the basic dimension. Chained basic dimensions do not create tolerance stack up. Proper tolerance must be inferred by Datums referenced in the feature control frame, and not by dimension arrows or start points.
In other words, A numerical value used to describe the theoretically exact size, profile, orientation or location of a feature or datum target...It is the basis from which permissible variations are established by tolerances on other dimensions, in notes, or in feature control frames.
Basic dimension are denoted by enclosing the number of the dimension in a rectangle.
- ASME Y14.5M-1994 Dimensioning and Tolerancing
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