PROBLEMS IN DIMENSIONS AND TOLERANCES 181 



Many in industry have probably grown used to the standard gauges in 

 particular fields but though gauge numbers were undoubtedly initiated as 

 a simplified identification the variety of gauges and the variety of names 

 for the same gauge now merely increases confusion. Sheet metals are 

 handled in terms of a number of gauges such as B&S gauge, U. S. standard 

 gauge and BWG gauge; and sheet soft rubber is even designated in decimals 

 of ^ such as ^^" . It has become good practice to specify sizes by decimal 

 dimension values and not by gauge numbers and holes by actual decimal 

 size rather than by drill numbers. The actual sizes used, however, are 

 determined in many cases by the values corresponding to old gauge num- 

 bers long used commercially, though in large running items mills will and 

 do manufacture to any specified decimal size. For some time it has been 

 the practice of material manufacturers and other large industries thus to 

 discontinue the use of gauge numbers though still using the decimal values 

 of gauge sizes. 



There is now under way an effort, organized under committee B32 of the 

 American Standards Association, to eliminate the old wire and sheet metal 

 gauge systems entirely and set up a rational series of American standard 

 thicknesses for all metal sheets and preferred diameters for wire, and insure 

 availability in these sizes. The basic conception of a rational series of 

 sizes is that a uniform degree of choice should be presented between suc- 

 cessive sizes. Therefore each size should differ from the next by a fixed 

 percentage. The series should therefore be geometric. A variety of geo- 

 metric series could be used but in order to permit extending the series in- 

 definitely by shifting the decimal point, the particular series based on the 

 root of 10 has been established internationally as the Preferred Numbers 

 Series for standard sizes. The 5 series is one having 5 numbers between 1 

 and 10 (or between 10 and 100) and is produced by using as the multiplier 

 the fifth root of 10; the 10 series is produced by multiplying by the 10th root 

 of 10; the 20 series by multiplying by the 20th root of 10 etc. The complete 

 Preferred Numbers Series is explained and listed in various forms in Ameri- 

 can Standard Z17. 1-1936. 



The subcommittee working on the sheet metal sizes has recently issued a 

 proposed American Standard of preferred thicknesses for all uncoated flat 

 metals thinner than .250". These thicknesses are all decimals based on the 

 20 series of preferred numbers rounded in the standard manner to 3 decimal 

 places. The Preferred Numbers and the proposed thicknesses are shown by 

 Table III. It happens that this series closely approximates the Brown and 

 Sharp gauge used in the nonferrous metals which simplifies that portion of 

 the changeover. If this proposed American Standard is generally approved, 

 as now appears most promising, we will be able to choose thicknesses of any 

 metal interchangeably without the restrictions of ancient gauge sizes es- 



