Methods of specifying datum features

A datum is designated by a capital letter enclosed by a datum box. The box is connected to a solid or a blank datum triangle. There is no difference in understanding between solid or blank datum triangles. Fig. 20.18 and Fig. 20.19 show alternative methods of designating a flat surface as Datum A.

Figure 20.20 illustrates alternative positioning of datum boxes. Datum A is designating the main outline of the feature. The shorter stepped portion Datum B is positioned on an extension line, which is clearly separated from the dimension line.

Figure 20.21 shows the datum triangle placed on a leader line pointing to a flat surface.

Figures 20.22, 20.23, 20.24 illustrate the positioning of a datum box on an extension of the dimension line,

Leader Line Engineering Drawings
Fig. 20.23

Fig. 20.24

when the datum is the axis or median plane or a point defined by the dimensioned feature.

Note: If there is insufficient space for two arrowheads, one of them may be replaced by the datum triangle, as shown in Fig. 20.24.

Multiple datums Figure 20.25 illustrates two datum features of equal status used to establish a single datum plane. The reference letters are placed in the third compartment of the tolerance frame, and have a hyphen separating them.

Drawing Position Tolerance

Figure 20.26 shows the drawing instruction necessary when the application of the datum features is required in a particular order of priority.

Fig. 20.25

Fig. 20.18

Fig. 20.19

i

Fig. 20.20

Fig. 20.21

Figure 20.26 shows the drawing instruction necessary when the application of the datum features is required in a particular order of priority.

Primary

Tertiary

Primary

C

A

Fig. 20.26

Figure 20.27 shows an application where a geometrical tolerance is related to two separate datum surfaces indicated in order of priority.

/

0.05

A

Fig. 20.22

Fig. 20.27

If a positional tolerance is required as shown in Fig. 20.28 and no particular datum is specified, then the individual feature to which the geometrical-tolerance frame is connected is chosen as the datum.

7.05

4 Holes 06.95 I EQUISPACED I

Engineering Drawing Tolerance Boxes

Figure 20.29 illustrates a further positional-tolerance application where the geometrical requirement is related to another feature indicated as the datum. In the given example, this implies that the pitch circle and the datum circle must be coaxial, i.e. they have common axes.

Fig. 20.28

Figure 20.29 illustrates a further positional-tolerance application where the geometrical requirement is related to another feature indicated as the datum. In the given example, this implies that the pitch circle and the datum circle must be coaxial, i.e. they have common axes.

6 Holes 09.9 | EQUISPACED |

00.02

6 Holes 09.9 | EQUISPACED |

00.02

Fig. 20.29

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  • prima
    How to specify paint in engineering drawings?
    8 years ago
  • erling
    What are flat features in engineering drawings?
    8 years ago
  • megan
    What is feature engineering drawing?
    8 years ago
  • Olli-Pekka
    What are methods of datum?
    3 years ago
  • Lobelia Gaukrogers
    How to show datum axis for pitch circle in drawing?
    9 months ago
  • raymond
    How to select a datum in engineering?
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