What is a basic dimension?

What is a basic dimension?

• A. An exact, theoretically perfect size or location used to define the true position; it carries no tolerance.
• B. A tolerance zone around a feature size.
• C. A measured dimension with allowance.
• D. A feature size symbol marking material.

Answer: (A)

A basic dimension is an exact, theoretically perfect value used to define the true geometry of a part—such as the true location of a feature or the true size of a dimension—and it carries no tolerance of its own. In GD&T, these ideal values establish the reference from which allowable variation is defined by geometric tolerances, so the tolerance zone is specified separately rather than on the basic dimension itself. The other phrases describe tolerancing concepts (a tolerance zone around a feature size), a measured dimension with allowance, or a symbol related to material, none of which define the precise, ideal value that basic dimensions provide. On drawings, basic dimensions are used to pin down the exact intent, while tolerances tell you how much deviation from those ideals is acceptable.

A basic dimension is an exact, theoretically perfect value used to define the true geometry of a part—such as the true location of a feature or the true size of a dimension—and it carries no tolerance of its own. In GD&T, these ideal values establish the reference from which allowable variation is defined by geometric tolerances, so the tolerance zone is specified separately rather than on the basic dimension itself. The other phrases describe tolerancing concepts (a tolerance zone around a feature size), a measured dimension with allowance, or a symbol related to material, none of which define the precise, ideal value that basic dimensions provide. On drawings, basic dimensions are used to pin down the exact intent, while tolerances tell you how much deviation from those ideals is acceptable.