184 BELL SYSTEM TECHNICAL JOURNAL 



ances, as for example, drill rod instead of cold drawn steel through economy 

 in the manufacture of associated parts. Similarly manufacture of cantilever 

 springs from sheet stock produced to closer tolerances may reduce the cost 

 of subsequent adjustments. Therefore, when individual part tolerances are 

 involved consideration must always be given to the size tolerances of raw 

 materials. 



The same situation exists in the case of tolerances on dimensions pro- 

 duced by a manufacturer's own tools. While close overall limits will re- 

 quire greater overall accuracy of the tools provided and greater frequency 

 of set-ups the most economical distribution of tolerances will be that based 

 upon the normal tolerances that can be expected from various manufactur- 

 ing operations. Certain degrees of accuracy are inherent in certain types 

 of machines and tools and allowing variations not in proportion to these 

 values serves little if any purpose. Also there are types of combination 

 tools and automatic machines, familiar in mass production practice with 

 which wide tolerances are not an economy because accuracy is required for 

 locating or nesting the part for subsequent operations. Since the dis- 

 tribution of tolerances involves such complex factors of manufacturing 

 method and cost as these, it is desirable for the designing engineer to de- 

 termine and to indicate unmistakably the effect of tolerances upon func- 

 tioning and, where interchangeability of individual parts in service is not 

 involved, to allow manufacturing considerations to determine the distribu- 

 tion of tolerances in an assembly. 



It is apparent that considerable study of the requirements for functioning 

 of the design, of available materials and the limitations of manufacturing 

 process are required to establish the most economic balance between per- 

 formance of the apparatus and the required tolerances. Consideration 

 should be given to these tolerance factors in cooperation with manufac- 

 turing engineers in an early stage of a design problem so that they may 

 influence the trend of design. This step may avoid the necessity for slow 

 and costly manufacturing developments and delays in starting production. 

 However, completely rigid adherence to the status quo of tolerances is not 

 necessary in long range planning of major design projects. In such cases 

 the trend of progress in materials and manufacture should be determined 

 and anticipated. For example, some cantilever spring design requiring 

 narrow control has been based on sheet material produced to tolerances 

 not commercially available at the time but made so by the time it was 

 needed for production. The extent of progress in this direction is shown 

 by Fig. 2. 



Similar progress in manufacturing technique can also be expected. For 

 example, the development of broaching from a comparatively crude opera- 

 tion to the precision method it is today is recent and outstanding. 



