(IN A (i.\i'(;i: von small screws. 



289 



0072 

 0080 

 0-080 

 0-0<.)8 

 Oil 



on 



014 



015 

 017 

 Oil) 

 021 



0-2:; 



0-25 

 0-2H 



o:u 

 0-3.-, 



0-3i) 



0-4:'. 



04 H 



.->:$ 



05'.) 



o-eC) 



0-81 

 0-90 

 100 



wera of O'O for 

 ich dimensions 

 lanlly attain, 

 iticiint figures 

 uracy lil^ely to 

 xcliecl. Rely ill? 

 arrived at i'ov 

 ch in diatnctev. 

 raising 0-'.» to 

 the Committee 



ace merely as a 

 iieasurement, it 



which they ave 

 Mo purpose, find 

 ms Column VL 

 at is practicallv 



is to be observcl 

 iplo alternatives 



uiriscs two screv.N 

 f for microiuctiT' 

 crc this coiuiiicn^^^'^" 



to clinopc from : (1) to have a constant arithmetical dIHercnco between 

 siu'cossive pitches, in vliiuli cas(^ either the pitches of small screws would 

 difl'er by too great an .'iTiiouiit. or those of the larj^er screws by too small 

 an amount ; or (2) to divide tlio entire ranjjc into sits, in each of which 

 the diflVrences are roiistnut. The tliird alternativD is to take successive 

 powers of some other simple fraction, for example U8, but such a series 

 would not so well (•orresp(Mid with the screws most generally employed. 



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

 based on some aliquot p;irt of an inch rather than on the millimetre. IJiic 

 anv advantages to bo gained by such a modification are inappreciable. For 

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

 part, awkward iVactions of a niillimetre, and the metric .system of measure- 

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

 From the ])oint of view oi' interchangeability, however, of screws to bo 

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

 the same basis r.f measurement of the pitch be everywhere adopted; 

 because, having agreed upon only two signilicant figni-es 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. 



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

 to designate the pitch nei'd not inconvenience Knglish manufacturers who 

 arc desirous of cutting the screws in their lathes. For, us has recently 

 been pointed out by Mr. liosanquet,' it is easy to cut a thread whoso 

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

 purposes be neglected (::,'/ ,th), with a guide-screw ba.'-ed on the inch by the 

 atlt'ition of awheel of 127 teeth, and thus the series here recommended 

 could, on the rare occasions that it became neces.sary, be originated on any 

 screw-entting lathe provided with the requi.site wheels. JJut the Committee 

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

 athig of the threads, as the screws under consideration are nuide in a pli.o 

 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 ineli 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 

 assunu; 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 . Tlie fact here stated affords a reason against extending the practice 

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



Jtinies resorted to in the ca.se of large screws, to the sci'ews now under 

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

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

 jtlus occasion surprise when it is remembered that the screw-plates cm- 

 Iployed must expand to varying extents in the hardening, that the hole ia 

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

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

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



1884. 



' /'//,/. .]/,,>/. (Fil'th Scric.-^), vol. XV. pj). 217, t:;S. 



r 



