True Position does require a datum. In the context of Geometric Dimensioning and Tolerancing (GD&T), True Position is a control that specifies the allowable deviation of a feature’s location from its intended position. It is typically used to ensure that a feature, such as a hole or a pin, is located accurately relative to other features on a part or component.
The Feature Control Block (FCB) is a graphical symbol used in GD&T to specify the tolerances and control the dimensions and geometric features of a part. It consists of several elements, including the geometric characteristic symbol, the tolerance zone, and the datum references.
The datum references in the FCB are used to establish a coordinate system or a set of reference points from which the true position of a feature is measured. These datums are specific points, surfaces, or axes on the part that are considered to be theoretically perfect and form the basis for the dimensional and positional relationships specified in the GD&T callout.
When specifying True Position, the datum references are indicated by capital letters (A, B, C, etc.) in the FCB. These letters correspond to specific datums identified on the part drawing. The datums are typically identified by feature control frames or feature control frames with modifiers, which provide additional information about the datum features.
The True Position tolerance zone is defined by the size and shape of the tolerance zone specified in the FCB, as well as the location of the datums. The tolerance zone is usually represented by a cylindrical or spherical shape, centered on the theoretically perfect position of the feature. The size of the tolerance zone is determined by the specified tolerance value in the FCB.
By using datums in conjunction with True Position, the designer or engineer is able to define a set of reference points that are used to establish the intended position of the feature. The datums provide a basis for measurement and evaluation, allowing for the consistent and accurate inspection of the part’s dimensional and positional relationships.
In practical terms, the use of datums in True Position can be illustrated by considering a simple example. Let’s say we have a part with three holes that are intended to be positioned relative to each other. The part drawing specifies that the True Position of each hole must be within a certain tolerance, and it also identifies three datums on the part: Datum A, Datum B, and Datum C.
In this example, the True Position of each hole is measured relative to the datums. The datums establish a coordinate system or reference points from which the position of each hole is determined. The measurement of True Position takes into account the deviations of each hole from its intended position, as well as the deviations of the datums themselves.
By using datums, we can ensure that the positional relationships between the holes are maintained, even if there are variations in the size or location of the datums. The use of datums in True Position provides a robust and standardized method for specifying and evaluating the positional accuracy of features on a part.
True Position does require the use of datums. The datums in the Feature Control Block establish the reference points and coordinate system from which the True Position of a feature is measured. The use of datums ensures consistent and accurate measurement and evaluation of the part’s dimensional and positional relationships.