PROBLEMS IN DIMENSIONS AND TOLERANCES 



191 



the center line of this drawing could actually be shown in several different 

 places as, for example: 



1. It may be a line through the centers of the two large holes. 



2. It may be a line anywhere from 2.992" to 3.008" from the outside 

 edges. 



3. It may be 2.247" to 2.253" from the small holes in the center of the 

 plate. 



4. It may be 2.615" to 2.635" from the holes numbered 2 and 4. 



In brief, the center line which appears so definitely located on the drawing 

 may actually be rather an indefinite location on the part when the various 



■ 10.000" + .016" 



6.000 



^■°'S" 4.500" 



+ .006" 



3.000" 

 + .008" 



T 



2.250" 



±.003" 



■5.000"±.008'^ *j^ 2.750"- 



-2.750 "±.006* 



<> 



^ 



t 



<> 



<> 



4 



l-^K 



0.625"R 

 ±.002" 



<D 



<^ 



■4.500"±.005'- 



■9.000 +.OiO- 



1.250" 

 ■t-.ooz" 



5.250 

 + 020' 



2.625 



+ .010" 



Fig. 6 — Flat plate of Fig. 5 with the addition of tolerances 



tolerances are considered. While the differences in the possible interpreta- 

 tions are in the order of thousandths of an inch nevertheless this order of 

 magnitude is critical in this part or the indicated tolerances would not have 

 been used. The interpretation of the center line which should be adopted 

 will depend entirely upon the manner in which the part is intended to func- 

 tion and therefore should be indicated by the designing engineer. Obvi- 

 ously, not all designs or all dimensioning will present this difficulty but all 

 should be studied from this viewpoint to determine whether or not they do. 



Functional Datum Positions 



When the type of uncertainty illustrated exists, it is necessary to 

 indicate clearly the effect of tolerances on functioning by establishing 

 the functional positions to which dimensions should refer. It may be 



