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BELL SYSTEM TECHNICAL JOURNAL 



one part. However, there are obvious generalities which can be established 

 and Fig. 9 shows some of them. 



An example of functional datum plane analysis and dimensioning in three 

 dimensions of a complicated part is shown by Fig. 10. This is the die cast 

 frame for a special selector switch. It is the base upon which many in- 

 terrelated parts and subassemblies are mounted. The proper functioning 

 of the completely assembled switch depends in large measure on proper 

 manufacture of this casting. In effect, the switch is designed around a 

 vertical shaft passing through points P and Q and planes 1 and 2 are, 

 therefore, established through the axis of this shaft. The production 

 planning engineers intend to design the die and withdraw die plugs from 

 such directions that the mounting surfaces will be smooth, flat and without 



UNSATISFACTORY 



LONG EDGE 

 ROUGH SURFACE CURVED SURFACE WARPED SURFACE LARGE SURFACE 



TAPPED HOLES 



DEEP SLOTS 



PROVIDED WITH BOSSES 



SATISFACTORY 

 FIXTURE POSITIONS GIVEN 



FINISHED POSITIONS 



Fig. 9 — Types of datum positions 



any taper and they intend to use these surfaces as guiding points for their 

 jigs and fixtures. It is for this reason that Plane 1 is established parallel 

 to these mounting surfaces and an indicated distance from them. The 

 other planes are established as shown on this drawing and described by the 

 notes. With this arrangement of planes the designer's analysis in terms of 

 Plane 1 is easily worked out and the reference of Plane 1 to the mounting 

 surfaces permits the production or tool engineer to translate the design of 

 the part into the design of his tools without necessity for further analysis 

 and without the possibility of different interpretations. It will be noted 

 that invariable or gauge dimensions are again used. The complete draw- 

 ing of this part is very com.plicated and occupies a drawing practically 4 

 ft. X 6 ft. The perspective sketch shown and the accompanying notes are 

 incorporated in the drawing as a separate view. 



