Engineering Problems in Dimensions and Tolerances 



By W. W. WERRING 



Dimensional Units 



The basic unit in most considerations of dimensions in the United States 

 is the inch. The value of the inch is so important that many companies 

 including the Bell System maintain in their measurement laboratories a 

 standard yard bar calibrated against the standard at the National Bureau 

 of Standards. In spite of this it is an interesting and curious fact that 

 though all have been much concerned over the legal value of the dollar 

 there has been little interest even among engineers in the exact legal value 

 of the inch. Actually there is no single answer to so simple a question as 

 "WTiat is an inch?" In fact, we have changed from a British inch and our 

 own legal meter, to our inch and the International meter and now through 

 action of the American Standards Association we are actually using an inch 

 based on conversion from the International meter which is neither our 

 own legal inch or the British legal inch — and the British are using it too. 

 Table I shows this history of the legal inch in the United States. 



It will be seen that under the present status there exists a difference of 

 two parts in a million between the legal inch and the inch used in the di- 

 mensional work of industry. This difference is more theoretical than real 

 in small dimensions and industrial use. The bill before Congress, sponsored 

 by the Bureau of Standards is intended to eliminate this as well as any 

 possible ambiguity in the U. S. inch. 



Decimal Dimensioning 



In subdividing the inch the modern trend in industry is toward the use of 

 decimals instead of the older common fractions although fractions continue 

 to be used, especially for dimensions of certain materials such as iron pipe, 

 lumber, phenol fiber. In fact even a special decimal system based on using 

 only the tenths and fiftieths of an inch is being considerably discussed by 

 general industry. This system would use a scale on which the smallest 

 division is 5^" or .020'' instead of ^" - .0156". It is in use by the Ford 

 Motor Company and the values shown in Table II are some of those used 

 in place of common fractions. The decimal equivalents of these common 

 fractions are also shown rounded to 3 decimal places in accordance with 

 American Standard Rules for Rounding off Numerical Values Z25. 1-1940. 



In the Ford system one and two-digit decimals carry the general toler- 



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