From The Geometrical Tolerancing Desk Reference: Creating and Interpreting ISO Standard Technical Drawings
A datum target is either a point, line or area on a workpiece. The datum target is to be the place of contact (supports of the workpiece) for the manufacturing and inspection equipment.
When a part has an irregular contour (casting, forging, sheet metal parts etc ) the use of a datum target could be required.
Three-Plane Datum System
Normally for orientation tolerances the use of one datum or two are sufficient. Sometimes when using positional relationships a three-plane datum system is required. In a three-plane datum system all the planes are perpendicular to each other.
Figure 7.1: The three-plane datum system
Figure 7.2: The three-plane datum system from example 3 above
Another example of a three-plane datum system is by the use of datum targets. The datum target is either a point, line or area on a workpiece. The datum target is to be the place of contact (supports of the workpiece) for the manufacturing and inspection equipment. The use of a datum target could be required when a part has an irregular contour (casting, forging, sheet metal parts etc ).
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