PROBLEMS IN DIMENSIONS AND TOLERANCES 



193 



It is sufficient to establish datum lines in the case of parts which are 

 practically flat pieces with little depth but when a part has substantial 

 depth it will be noted that center lines or other datum lines on a drawing 

 really represent planes in space. In such parts it becomes necessary to 

 establish datum planes rather than lines and three planes at right angles to 

 each other are required. 



Figure 8 illustrates such a part which might be an armature such as is 

 used in many pieces of electrical contacting apparatus. In the typical 

 operation of such a part its functioning is determined by the relation of its 



POLE FACE AREA SHALL 



NOT BE OUT OF PLANE 



MORE THAN 0.003" 



0-073 ±-002 

 0.173" DR. 



PLANE I 



-0.1285 DR. 



0.109" 



POLE FACE AREA 

 0.500" DIAM. 



Tk" 



'0.718* F 

 PLANE 2 



I- 



1.578" 

 ±.003" 



^^ 



0.062" R 



PLANE 3 



0.062"R 



^ 



Fig. 8 — Functional datum plane dimensioning of magnetic armature type of part 

 Functional datum plane I passes through the common axis of the two .1285 in. diameter 



holes and .265 in. above the pole face gauge position. 



Functional datum plane II is perpendicular to plane I and passes through the common 



axis of the two .1285 in. diameter holes. 



Functional datum plane III is perpendicular to planes I and II and passes midway 



between the finished surfaces which are 1.578 ± .003 in. apart. 



various dimensions to the position of the pole face and the axis support. 

 In order to indicate this on the drawing it is necessary to establish dimen- 

 sioning as shown and add to the drawing the notes shown. 



These notes establish three functional datum planes, the first through 

 the axis at the point of support and a distance .265" from the pole face 

 area; the second at right angles to the first through the axis of support and 

 the third at right angles to both the first and second and halfway between 

 the finished surfaces 1.578" apart. With these planes established the 

 application of all the limits and tolerances shown is based on the operating 

 position and analysis of the design is simplified. The drawing and the 

 intentions of the design engineer cannot be misunderstood. 



