As per the 3-2-1 principle, we typically define three primary datum targets, two secondary datum targets, and one tertiary datum targets. The sole purpose of the datum target is for locating or constraining the part to a fixture. But, for specifying the datum targets we use some portions of the datum features. Datum Targetĭatum targets also are generated from the datum features. Nevertheless, it is often side-by-side with its chain counterpart in a design. Datum dimensioning overcomes the tolerance buildup shortcoming of the chain/continuous method. The above drawing has two datum features and hence two datum: an imaginary plane at the left hand surface of the part and the axis of the hole. Datum dimensioning is another method that enjoys wide adoption in the industry. If you have axis and plane, then one axis and one plane will suffice for the need. In the case of planes, you need three mutually perpendicular ones. If you have a point datum, then only one datum is sufficient to constrain the model. Datums are established from the datum features. This reference frame is used for dimensioning for the drawing and model creation. Datumĭatums are theoretically perfect but imaginary planes, axises, and points used for creating reference frame in a drawing. In the above example, the left hand vertical surface and the cylindrical feature of the right are taken as datum features. See the following image for an example of how typically the datum features A and B are used in the drawings:

The datum features are used to restrict the transitional and rotational movement of the part when it placed in fixtures. Note: If you cannot find a satisfactory position for the datum dimension by. Datum features are represented by capital letters enclosed by a square. Datum measurements are the horizontal and vertical distances measured from a. Datum Featureĭatum feature is a feature of a part from which datum is marked. Out of the five we will comparatively discuss three terms: datum, datum feature, and datum target. There are five different terms starting with the word “datum” in ASME Y 14.5 (the Bible of GD&T).