Datum targets are also used to establish the datum reference frame. Datum targets are used to establish orientation on irregular parts. They may also be used for large surfaces where it would be impractical to use an entire surface as the datum because of size, or it may be subject to warping, bowing, or other distortions. Usually datum tar- gets are specified on castings, forgings, and weld- ments, or on any other application where it may be difficult to establish a datum. Datum targets may be points, lines, or areas of a part that pro- vide an orientation-dimension origin. A datum target symbol is used to establish contact with da- tum simulators and to identify the datum planes. DATUM TARGET SYMBOL
The datum target symbol is a circle divided in half by a horizontal line. When the symbol is used to identify a circular target contact area, the top half will contain the diameter. Where it is im- practical to specify the circular target area inside the upper half of the datum target symbol, the area may be specified outside the symbol and at- tached to the upper half with a leader line, as shown in Figure 3-9.
Figure 3-9 Specifying datum target area.
Figure 3-10 Datum target identification number.
The contact area diameter is BASIC. When the symbol is used to identify any other datum target, the upper half remains open (see Figure 3- 10). The lower half of the symbol always contains the datum target identification number. The iden- tification consists of a letter (the datum reference letter) and target number. Sufficient targets are specified to satisfy the three-plane datum concept and required points of contact.
Datum target symbols are attached to the da- tum target with a leader line. A solid leader line indicates the datum target is on the near side of the part; a dashed or hidden line type leader line indicates that the datum target is on the back or far side of the part, as illustrated in Figure 3-11.
1.500±.010 .500±.010 23° 2.500 .278±.005 Z Y .008 .008 AA .005 A [x,y,u,v]A [x,y,u,v] B [z]B [z] .003 .003 A A B Y X
Figure 3-8 Application of a customized datum reference frame.
A2 A2.5 A2 .750 OR B1 B1 B1 B1
Figure 3-11 Using leader lines with datum targets.
Datum targets are dimensioned with BASIC dimensions or toleranced dimensions (see Figure 3-12). When basic dimensions are specified, tool- ing tolerances apply. BASIC dimensions do not have tolerance, but to aid in locating datum tar- gets properly, some tolerance is required. There- fore, tooling tolerance (usually one-tenth of spec- ified feature tolerance) is permitted. The feature tolerance used to calculate the tooling tolerance is the specified tolerance for the feature oriented from the datums. Usually BASIC dimensions are specified because of subsequent machining and inspection processes.
Figure 3-12 Datum target located with BA- SIC dimensions.
DATUM TARGET—POINT
A datum target point is identified with an “X”, as shown in Figure 3-12. This datum target
point is usually located dimensionally with BA- SIC dimensions. The target point regularly iden- tified is a front view of the part. Where there is no front view, the point location is dimensioned on two adjacent views. Datum target points are nor- mally simulated or picked up with the point of a cone or spherical radius pin. Tooling pins are spherical or pointed pins that are used to make contact with the part for holding and inspection; they are located by Target Points given on the print. In cases where tooling area is given, flat ended pins of the same size as indicated in the Target Area Symbol are used.
DATUM TARGET—LINE
The datum target line is identified with a phantom line in the direct view. The phantom line is shown in the front view and “X” in an adjacent view. These symbols are located with BASIC di- mensions, as shown in Figure 3-13.
Datum target lines are simulated with the edge contact of pins.
Figure 3-13 Datum target line represented by a phantom line. C1 C1 C1 1.750 .625 C1 C1 .750 -.000+.002.002 1.250 3.750 B1 B1 B2B2 B1 B1 B2B2
DATUM TARGET—AREA
The datum target area is identified with a phantom line circle, with hatch lines inside the circle. The size is specified in the upper half of the datum target symbol. If it is a circular area, the size is preceded with the diameter symbol. Phan- tom lines and dimensions are specified to define the size and shape of target areas. Dimensioning of diameters is not required. The datum target symbol specifies the target area diameters, as shown in Figure 3-14.
Datum target areas are simulated with a flat nose pin. In the case of diameters, the pin is the diameter specified in the datum target symbol, us- ing gage tolerances for the pin diameter. The tar- get area is indicated with hatch lines.
Figure 3-14 Use of datum target area symbols.
DATUM TARGETS—OUT OF DIRECT VIEW Occasionally it may be necessary to identify a datum target point or line without a direct draw- ing view. Some design requirements may force the datum identification to other drawing views. Figure 3-15 shows point target dimensioning.
When the datum target line is specified out of the direct view, the locating dimensions are only specified in one direction. In Figure 3-16, the pre- ceding part with a datum target line is shown.
Fig. 3-15 Datum target point dimensioning.
Fig. 3-16 Datum target line dimensioning.
DATUM TARGETS—COMPLEX OR IRREGULAR SURFACES
When datums are identified on complex or irregular surfaces, the datum feature symbol should be attached only to identifiable datum fea- tures. When datums are established by datum tar- gets, the datum feature symbol is not required. In this example (see Figure 3-17), like many com- plex parts, no surface can be identified as a datum
.688 3.250 1.000 A2 A2.3 A1 A1.3 A3 A3.3.3 .834 C1 C1 .500 .834 C1 C1 A1.4
feature. The datum feature symbol may be ap- plied to extension or center lines as appropriate.
MOVABLE DATUM TARGET
This symbol may be used to indicate the po- tential for movement of the datum target in order to establish contact with the datum feature simulator.
DATUM TRANSLATION
This modifier is used in the Feature Control Frame (discussed in Chapter 4) to indicate that the BASIC location of the datum feature simula- tor is free to translate, within the tolerance, to ac- complish full contact with the feature. It may be necessary, for clarity, to include direction of movement for the simulator.
INCLINED DATUM FEATURES
When the datum reference frame includes an angular feature, the angle of that feature must be
C1 C1 2 X 2.000 2.000 12.000 12.000 4.000 P D1 D1 D2D2 D3D3 P 2 X .500 E1 E1 E2E2 Figure 3-17 Datums established by datum targets.
.250 .250 45° E F TRUE TRUE CONTACTING CONTACTING PLANE D
Figure 3-18 True contacting plane oriented at the BASIC angle.
specified as BASIC. Then to position the part in relation to the specified datums, a true contacting plane must be oriented at the BASIC angle of the feature (Figure 3-18).