Why Datum References are Not Required for GDT's Straightness, Flatness, Circularity, and Cylindricity

In Geometric Dimensioning and Tolerancing (GDT), straightness, flatness, circularity, and cylindricity are form controls. These controls are used to define the ideal shape of a feature without reference to other features or the overall part. Here’s why datum references are not necessary for these GDT features:

Independent Characteristics

Independent Characteristics: Straightness, flatness, circularity, and cylindricity describe the ideal shape of a feature and do not need to relate to other features in order to be defined or measured.

Measurement Methods

Measurement Methods: These characteristics can be measured using specific tools like a straightedge for straightness or a roundness tester for circularity, which do not require a reference frame from other features. The measurement is based solely on the feature itself.

Functional Requirements

Functional Requirements: The primary purpose of these controls is to ensure that the feature meets its functional requirements on its own. For example, a flat surface must be flat regardless of its position relative to other surfaces.

Simplicity in Design

Simplicity in Design: By not requiring datums, designers can simplify the specification of parts. This makes it easier to communicate the desired characteristics to manufacturing and inspection.

Complementary to Datum References

Complementary to Datum References: While these form controls do not require datums, they can still be used in conjunction with datum references if necessary. For instance, a flatness control might be specified with a datum reference if the flat feature needs to be oriented in a specific way relative to other features.

In summary, straightness, flatness, circularity, and cylindricity are defined as form controls in GDT, allowing for straightforward measurement and ensuring that each feature meets its design intent without the need for additional references.

In Geometric Dimensioning and Tolerancing (GDT), straightness, flatness, circularity, and cylindricity are form tolerances. Form is the shape or configuration of an object. It does not pertain to any imaginary object, such as a datum. A circularity form can only be defined by maximum and minimum diameter circular profiles; it does not require an axis or point. However, the position on this circle does need a datum, such as an axis or a point. Note that the form of an object does not need a datum, but its position, location, or orientation does.