192 



BELL SYSTEM TECHNICAL JOURNAL 



difl&cult to do this graphically, in which case it is necessary to indicate by 

 notes the particular interpretation which the designer intends. As an 

 example, if the part of Figs. 5 and 6 functions by being located in position 

 by means of the four holes numbered 1, 2, 3 and 4, the intentions of the 

 designer are readily indicated by the following notes : 



1. Functional datum line I is midway between the centers of holes 1 and 

 2 and the centers of holes 3 and 4. 



2. Functional datum line II is perpendicular to datum line I at a point 

 midway between the centers of holes 2 and 4. 



These notes establish both horizontal and vertical center lines specifically 

 in terms of the center of the one set of dimensions between the holes marked 



io.ooo"±-oi6"- 



0.375"±.005" ■ 



4.500 

 ±.006" 



0.375"±.005"-' 

 0.375"±.005"-' 



0.500" + .005V 



_^ 0.625" R 

 ±.005" 



^ 



^ 



--1'^ 



O 



_3.I25"_ 

 i .005" 



iy 



J^ 2.750"_ 



± -006" 



>-^ 



^5 



o 



.3.125" 

 + .005" 



6.000 

 ±.016" 



4.250 

 ± .005" 



3.000 

 ±.oos" 



Fig. 7 — Flat plate of Fig. 5 functionally dimensioned from outside edges with tolerances 



1, 2, 3 and 4. The term functional datum line is suggested as completely 

 descriptive but other equivalent terms might be used. This information 

 could be indicated on the drawing without the use of notes by the adoption 

 and use of some standard convention or symbol to indicate the particular 

 dimension bisected by the center line. 



If the functioning of this part were determined by location against the 

 outside edges, this could be readily indicated by dimensioning the part as 

 shown by Fig. 7 and using notes establishing the line A-B as one datum 

 line and the perpendicular to it through A as the other. 



In either of these cases the drawing becomes completely definite and sub- 

 ject to only one interpretation. In drawings of this type no change in the 

 method of dimensioning may be required and the problem is solved simply 

 by the addition of suitable notes or symbols indicating the intention of the 

 designer as to functional datum lines. 



