OX A GAUGE FOR SMALL SCREWS. 289 



to choose from : (1) to have a constant arithmetical difference between 

 successive pitches, in which case either the pitches of small screws would 

 differ b} r too great an amount, or those of the larger screws by too small 

 an amount ; or (2) to divide the entire range into sets, in each of which 

 the differences are constant. The third alternative is to take successive 

 powers of some other simple fraction, for example 0'8, but such a series 

 would not so well correspond with the screws most generally employed. 



8. Accepting this series, it may, however, be urged that it should be 

 based on some aliquot part of an inch rather than on the millimetre. Buc 

 any advantages to be gained by such a modification are inappreciable. For 

 an examination of the numbers at once shows that they are, for the most 

 part, awkward fractions of a millimetre, and the metric system of measure- 

 ment thus enjoys no advantage in this respect over that based on the inch. 

 From the point of view of interchangeability, however, of screws to be 

 manufactured in this country and on the Continent, it is essential that 

 the same basis of measurement of the pitch be everywhere adopted; 

 because, having agreed upon only two significant figures on one basis, 

 terminable decimals are obtained, but such terminable decimals could not 

 be accurately expressed by two significant figures on the other basis of 

 measurement. 



9. Again, it is to be remembered that the use of metric measurement 

 to designate the pitch need not inconvenience English manufacturers who 

 are desirous of cutting the screws in their lathes. For, as has recently 

 been pointed out by Mr. Bosanquet, 1 it is easy to cut a thread whose 

 pitch differs from one millimetre by an amount which may for all ordinary 

 purposes be neglected (ir-^th), with a guide-screw based on the inch by the 

 addition of a wheel of 127 teeth, and thus the series here recommended 

 could, on the rare occasions that it became necessary, be originated on any 

 screw-cntting lathe provided with the requisite wheels. But the Committee 

 do not consider it needful to specially contemplate facility in the origin- 

 ating of the threads, as the screws under consideration are made in a plate 

 or by the aid of dies ; and manufacturers on a large scale would be pro- 

 vided with a special lathe for the purpose. 



10. Whether the inch or millimetre is adopted as a unit of measure- 

 ment the series of pitches for these small screws becomes an ideal rarely 

 attained in practice, for with screws tapped in a plate, or even with dies, 

 the exact pitch aimed at will often not be attained; neither is it safe to 

 assume that two screws, tapped in corresponding holes in different plates, 

 will have precisely the same number of threads per inch. This is 

 especially the case with the smaller screws, as may be proved by accurately 

 measuring the pitches of several tapped in holes that are nominally alike. 



11. The fact here stated affords a reason against extending the practice 

 of designating screws by their number of threads per inch, already some- 

 times resorted to in the case of large screws, to the screws now under 

 discussion. It is found that screws, nominally alike, frequently differ in 

 this respect by as much as five or even ten threads in the inch, nor need 

 this occasion surprise when it is remembered that the screw-plates em- 

 ployed must expand to varying extents in the hardening, that the hole is 

 often not more than three or four threads deep, and that the pressure, 

 applied by hand, must vary considerably. Such a nomenclature would 

 thus involve the use of inconveniently high numbers to express a minute 



1 Phil. Mag. (Firth Scries), vol. xv. pp. 217, 438. 



1884, u 



