198 'BELL SYSTEM TECHNICAL JOURNAL 



Required Standardization 



It is not suggested that the drawings shown and the notes referred to 

 represent a final practice on datum planes. A standard practice in desig- 

 nation of planes and standard terminology and understanding on gauge 

 points and gauge dimensions is required. It will probably be desirable to 

 adopt some symbol or designation for use on drawings to distinguish gauge 

 dimensions which are invariable from ordinary unlimited dimensions to 

 which manufacturing engineers for their own purposes usually add shop 

 tolerances. One thing is certain and that is that datum planes, dimensions 

 and tolerances when established should be primarily in terms of the re- 

 quired functioning of the apparatus. When that is done no one using the 

 drawing in any capacity will have any doubts as to the designer's intention 

 and this results in a great reduction in the discussions and analysis which 

 might otherwise be necessary. 



Summary 



In summary it may be said that the whole approach to these problems in 

 dimensions and tolerances should be on the basis of functioning. However, 

 good engineering of dimensions and tolerances requires knowledge of what 

 can reasonably be produced and the sources of reasonable tolerance values 

 are: 



1. Raw material limits including some knowledge of future trends and 

 developments. 



2. The normal accuracy of manufacture, also including anticipation of 

 future improvement. 



3. Discussion of trend of design with manufacturing engineers. 

 Solution of tolerance problems in the final design may involve all of the 



following steps: 



1. Study of the effect of combinations of tolerances on functioning, 

 allowing for statistical effects in accumulations of tolerances. 



2. Discussion of this analysis with the production planning engineer 

 because the analysis of tolerance combinations is important in the 

 design of long life tools. 



3. Indication of the results of such an analysis by the method of dimen- 

 sioning drawings. 



4. Indication on drawings of functional datum positions, lines or planes 

 established on geometrically correct principles to permanently and 

 unmistakably record the intentions of the designer regarding com- 

 binations of variations wherever this is necessary. 



