Some parts, such as castings, may have rough surfaces and if such surfaces are placed on simulated datums it may wobble. In this type of situation, the part designer may use datum target symbols to specify how exactly the part will be supported during inspection. Typical places where datum targets are used are: casting, forging, warped or bowed surfaces which may rock when in contact with simulated datum. Instead of specifying the whole surface as a datum feature, the designer may specify point contact, line contact or area contact with a theoretical flat plane. A point datum target is shown as 'X", a line datum target is shown as a phantom line, and an area datum target is shown as an area or a diameter on the part drawing.

Different methods of showing the area targets are shown in Figure 6. The line datum target may also be shown as an 'X' in an edge view. The positions of the datum targets are given in terms of basic dimensions. A rectangular box around the dimension identifies the basic dimensions and they do not have any tolerance associated with them. These dimensions are to be exactly followed at a gage maker's precision level. From each datum target, a leader line attaches to a circle, which is the datum target symbol (Figure 6). It identifies the datum, target number, and the area of contact information where applicable. The line connecting the datum target and the datum target symbol can be solid or hidden type, meaning the datum targets are either on the visible surface or on the hidden surface. Figures 7, 8 and 9 illustrate examples and applications of point, line and area datum targets and the respective gages that will be needed to establish the datums. Remember that the datum target symbols on a drawing have nothing to do with the part geometry. They only specify how the part will be supported during inspection.

Figure 6. Datum target symbols. (Top)

Figure 7. Application of point datum target (Top)

Figure 8. Application of line datum target (Top)

Figure 9. Application of area datum targets (Top)