194 BELL SYSTEM TECHNICAL JOURNAL 



The clear expression of the designer's intentions by datum plane dimen- 

 sioning will be appreciated by all concerned with the drawing or the re- 

 sulting part. Inspection of the part is expedited no less than production. 

 The inspector can usually by means of gauge blocks or simple fixtures set 

 the part up on a surface plate as indicated by the drawings datum planes 

 and positions. He can then establish the conformance of the part with the 

 drawing by simple measurements to the indicated horizontal and vertical 

 planes. When production quantities justify special gauges the required 

 design of the gauge is established clearly by the datum planes. 



Invariable or Gauge Dimensions 



The drawing of Fig. 8 just described illustrates the use of gauge dimen- 

 sions. The dimensions .265" and .718" and the indicated half-inch di- 

 ameter for the pole face are all gauge dimensions without tolerances and 

 some statement must be made or understanding reached that they are 

 considered invariable and tolerances not permitted. They represent, it 

 might be said, theoretical dimensions, on the drawing, or in practice they 

 represent tools or gauging apparatus made to the highest standards of 

 accuracy. These invariable dimensions are necessary in order to establish 

 a starting point for the dimensioning of the part. It may appear at first 

 that stating that a dimension has no manufacturing tolerance or variation 

 is a hardship upon the manufacturer but this is not really so because the 

 dimensions are not ones which are actually manufactured in the part. They 

 represent usually dimensions built into tools or gauging equipment which 

 are made to a precision greatly superior to that represented by part tol- 

 erances. 



Invariable dimensions, or better, gauging dimensions or whatever it is 

 proposed to call them are really not a new invention and it is possible to 

 cite easily recognized examples. For instance, the dimension 2.473" on 

 Fig. 8 is an invariable gauging dimension not associated with the setting 

 up of datum planes but typical of long standing use of invariable dimensions. 

 We all can recall also the use of the term "theoretically correct position" 

 and it is present practice in the case of vacuum tube bases and similar ap- 

 paratus to designate the location of the contact studs in terms of a gauge 

 having holes located on "true centers." Last but not least a minimum or 

 maximum limit in its application is itself an invariable dimension. 



In effect, datum lines or planes established when necessary by use of 

 invariable or gauging dimensions remove the uncertainty as to the de- 

 signer's intentions and prevent misunderstandings between design, pro- 

 duction and inspecting engineers. Admittedly they do not completely 

 solve all problems of dimensions as probably nothing will. They do, how- 

 ever, transfer whatever problems remain from the field of tolerances on 



