GD&T Tutorial on Bonus Tolerance:This geometric dimensioning and Tolerancing tutorial will discuss about the concept of bonus tolerance and the example of the GD&T. Datum targets are specified regions of the datum feature used for locating the part in a fixture. A datum feature, on the other hand, is the physical representation of datum. The datum is an imaginary but theoretically perfect surface, line, or points. The datum feature, datum, and datum target are the important geometric dimensioning and Tolerancing (GD&T) concept for creating and manufacturing a model. In the below example, the primary datum targets “A1”, “A2” and “A3” are circular surfaces of diameter 2, the secondary datum targets “B1” and “ B2” are the surfaces of diameter 4 and the tertiary datum targets “C1” is of diameter 5. 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. 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. 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.
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