![]() Drawing Indications of Datum Featuresĭatums are indicated using the following symbols (datum symbols). Therefore, a surface plate, gauge, or mandrel, which has a more precise surface, must come into contact as a simulated datum.Į Surface plate, gauge, mandrel, etc. The surface of a part indicated as a datum is not an exact shape. Datum System A group of datums created by combining more than one individual datum in order to use it as a reference for toleranced features. Simulated Datum Feature An actual surface with sufficiently precise shape that comes into contact with a datum feature and is used for establishing a datum (e.g. Datum Feature A real (non-ideal) integral feature of a target used for establishing a datum (e.g. Multiple datums can also be combined to make a datum system that specifies a feature. GD&T Tutorial – Learn How to Identify the Feature of Size Dimension:This GD&T tutorial will talk about the concept of feature, feature of size, non-feature of size and also about the application and identification of the geometric dimensioning and Tolerancing concept in engineering drawing.A datum is a plane, a straight line, or a point that is used as a reference when processing a material or measuring the dimensions of a target.Ī datum is defined as follows in the ISO standard: ISO 5459:2011 “One or more situation features of one or more features associated with one or more real integral features selected to define the location or orientation, or both, of a tolerance zone or an ideal feature representing for instance a virtual condition.” Types of Datumsĭatums are established using datum features and simulated datum features. 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. 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. Datum features are represented by capital letters enclosed by a square. 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).
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