PROBLEMS IN DIMENSIONS AND TOLERANCES 189 



drawings. This limitation is probably well known to most engineers but 

 it is worthwhile to analyze it because it is important to be always aware 

 of it. 



This limitation is the fact that in drawings the shape of the part and the 

 effect of all nominal dimensions are actually shown graphically whereas, 

 it is possible to indicate tolerances numerically but not graphically. We 

 are therefore apt to visualize the part as it is graphically shown, that is, 

 without tolerances and to think of the numerical tolerances one at a time 

 rather than in combinations as they affect each other and the shape of the 

 part. 



If any dimension, significantly affecting the design of a part, is changed 

 the drawing is immediately corrected so that its meaning will be clear and 

 the functioning of the part can be checked. This obviously facilitates de- 

 sign and manufacture. Yet because they cannot be shown directly by 

 regular drawing methods, we have grown accustomed to not being shown the 

 effect of tolerances or changes in tolerances upon the shape of the part. 

 Nevertheless it is obvious that these effects are critical in the functioning 

 of the part or tolerances would not be set. The fact that these critical 

 features of the design are not actually graphically shown and therefore are 

 not easily seen and understood on the drafting board is a serious detriment 

 in working out a design and in all later analysis of it. The full effect of 

 interrelated variations particularly if in three dimensional space may ap- 

 pear only after tools are in process or the first parts produced and this may 

 be rather late for economy. 



Originally this difficult analysis of the effect of tolerances upon function- 

 ing probably involved only the designer. The manufacturer tried to make 

 the part as nearly as possible to the nominal values shown and variations 

 from them were accidental. Tolerances were looked upon as an indication 

 of the care required and as a means of inspection for acceptance or rejec- 

 tion. With increasingly complex manufacturing tools the permitted toler- 

 ances are utilized more and more in the design of tools to allow the greatest 

 possible wear before defective parts are produced and the tools must be 

 replaced. For mass production parts progressive step type tools are used 

 in which a continuous strip of stock advances by various stages from blank 

 sheet to finished part. Tools of this type are extremely expensive and in order 

 to obtain maximum life full use of allowed variations is made in their de- 

 sign. Design of such tools and the gauges required to maintain quality in 

 mass production therefore also requires analysis of the effect of combina- 

 tions of variables upon the desired part. As the designer has presumably 

 already made this analysis, and incidentally is best qualified to do it, econ- 

 omy and accuracy dictate that his analysis be transmitted to the manu- 

 facturing engineer. The problem is to find means by which he can indicate 



