Information related to the construction, form, and proportion of individual symbols described herein is contained in Appendix C.
3.3.1 Geometric Characteristic Symbols. The symbolic means of indicating geometric characteristics are shown in Fig. 3-1.
3.3.2 Datum Feature Symbol. The symbolic means of indicating a datum feature consists of a capital letter enclosed in a square frame and a leader line extending from the frame to the concerned feature, terminating with a triangle. The triangle may be filled or not filled. See Fig. 3-2. Letters of the alphabet (except I, O, and Q) are used as datum identifying letters. Each datum feature of a part requiring identification shall be assigned a different letter. When datum features requiring identification on a drawing are so numerous as to exhaust the single alpha series, the double alpha series (AA through AZ, BA through BZ, etc.) shall be used and enclosed in a rectangular frame. Where the same datum feature symbol is repeated to identify the same feature in other locations of a drawing, it need not be identified as reference. The datum feature symbol is applied to the concerned feature surface outline, extension line, dimension line, or feature control frame as follows:
(a) placed on the outline of a feature surface, or on an extension line of the feature outline, clearly separated from the dimension line, when the datum feature is the surface itself. See Fig. 3-3.
(b) placed on an extension of the dimension line of a feature of size when the datum is the axis or center plane. If there is insufficient space for the two arrows, one of them may be replaced by the datum feature triangle. See Figs. 3-4(a) through (c).
(c) placed on the outline of a cylindrical feature surface or an extension line of the feature outline, separated from the size dimension, when the datum is the axis. For CAD systems, the triangle may be tangent to the feature. See Figs. 3-4(d) and (f).
(d) placed on a dimension leader line to the feature size dimension where no geometrical tolerance and feature control frame are used. See Figs. 3-4(e) and 5-2.
(e) placed on the planes established by datum targets on complex or irregular datum features (see para. 4.6.7), or to reidentify previously established datum axes or planes on repeated or multisheet drawing requirements.
(f) placed above or below and attached to the feature control frame when the feature (or group of features) controlled is the datum axis or datum center plane. See Figs. 3-5 and 3-23.
3.3.3 Datum Target Symbol. The symbolic means of indicating a datum target is a circle divided horizontally into halves. See Fig. 3-6. The lower half contains a letter identifying the associated datum, followed by the target number assigned sequentially starting with 1 for each datum. See Fig. 4-30. A radial line attached to the symbol is directed to a target point, target line, or target area, as applicable. See para. 4.6.1. Where the datum target is an area, the area size is entered in the upper half of the symbol; otherwise, the upper half is left blank. If there is not sufficient space within the compartment, the
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