#Datum in drawing download#

The unit of measure is the same for the whole drawing and will be defined in the title block. The numerical value defines the tolerance zone. There are a variety of tolerance zones, see them all here.

If the geometric control is a diametrical tolerance then the diameter symbol will be in front of the tolerance.

#Datum in drawing download#

Download our free GD&T Symbol Reference Guide and Font for more information.ĭiameter Control / Cylindrical Tolerance Zone (if required) This is where geometric control is specified. The arrow is optional and may not be present on some drawings. If the arrow points to a surface, then the surface is controlled by GD&T. This points to the feature that the geometric control is placed on. If the arrow points to a diametric dimension, then the axis is controlled by GD&T.

Datum references (if required by the GD&T Symbol)Ĭombined, the feature control frame provides all the information you need to measure the geometric tolerance of the features of the part and determine if the part is in spec.

Tolerance zone modifiers - such as features of size.

The feature control frame includes four parts: 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.In GD&T (Geometric dimensioning and tolerancing), a feature control frame is required to describe the conditions and tolerances of a geometric control on a part's feature. 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).