Datum Referencing Machinery's Handbook, 31st Edition
632
Target area (where applicable)
12
P1
Datum reference letter
18
18
12
12
or
P1
P1
Target number
18
18
Target C2 is on the hidden or far side of the part.
12
C2
18
18
Fig. 11. Datum Target Symbols Datum Target Points: A datum target point is indicated by the symbol “X,” which is dimensionally located on a direct view of the surface. Where there is no direct view, the point location is dimensioned on multiple views. Datum Target Lines: A datum target line is dimensionally located on an edge view of the surface using a phantom line on the direct view. Where there is no direct view, the location is dimensioned on multiple views. Where the length of the datum target line must be con trolled, its length and location are dimensioned. Datum Target Areas: Where it is determined that an area or areas of flat contact are necessary to ensure establishment of the datum, and where spherical or pointed pins would be inadequate, a target area of the desired shape is specified. Examples include the need to span holes, finishing irregularities, or rough surface conditions. The datum target area may be indicated with the “X” symbol as with a datum point, but the area of contact is specified in the upper half of the datum target symbol. Datum target areas may additionally be specified by defining controlling dimensions and drawing the contact area on the feature with section lines inside a phantom outline of the desired shape. Positional Tolerance.— A positional tolerance defines a zone within which the center, axis, or center plane of a feature of size is permitted to vary from true (theoretically exact) position. Basic dimensions establish the true position from specified datum features and between interrelated features. A positional tolerance is indicated by the position symbol, a tolerance, and appropriate datum references placed in a feature control frame. Modifiers: In certain geometric tolerances, modifiers in the form of additional symbols may be used to further refine the level of control. The use of the MMC and LMC modifiers
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