From The Geometrical Tolerancing Desk Reference: Creating and Interpreting ISO Standard Technical Drawings

Overview

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.

Example 1: Interpretation of a datum target
Example 2a: Type of datum target
Example 2b: Type of datum target
Example 2c: Type of datum target

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
Example 3: As shown on the drawing

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 ).

Example 4: Datum targets (as shown on the drawing)
Notes
  • Datum targets Al, A2 & A3 establish datum A

  • Datum targets Bl & B2 establish datum B

  • Datum target CI establishes datum C

Copyright Paul Green 2005 under license agreement with Books24x7

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