m '!? 



21)2 



iiKi'oiiT— 1884. 





) 



I 

 III 



facture wlion ;i screw-plate is used; at the same liino tlie (Je|»tli must not 

 1)0 tor "uctli rcdiKH'tl on account of the greater teiideney of the thread 

 to stnp. It is further important that the additional toision involved in 

 catting a <leep tiiiend, which materially increases the risk of tearing the 

 metal acrosa, should not be lost sight of. 



21. The Committee, after comjiaring together a large inimber of 

 different forms of thread, .some of which are in actual use, wliile others 

 have only been suggested, were much tempted to recommend the Wiiil- 

 worth thread for adoption by the British Association, becauso it is so 

 well known in this country, and experience has provetl indisputably that it 

 is excellent when employed for engineers' bolts, &c'. But, as appears from 

 sections 10 and 18, in the case of small screws the tendency should rjither 

 be to increase the rounding on nccount of the diflienlties of manufacture, 

 and the depth of the Whitworth thread is G-t of the pitch, which is 

 considerably in excess of 0"r>03, the average adopted in practice. The 

 Whitworth thread is, moreover, characterised by a greater angle than is 

 usual in small screws. 



22. The advisability of modifying the form of thread of small .screws 

 as compared with those of greater diameter is fully recogniseil by tlic; 

 Swiss Committee, their thread for the former having an angle of 47.V\ 

 while that for the latter is r»;>°, nearly tlie same as that of the Whitworth 

 thread. In the case of small screws made in the FiUhre Hnlsur the crest 

 of each thread is rounded off with a radius equal to ,Uh the pitch, and the 

 hollow with 1th the pitch. The actual depth is OGO the pitch, somewhat 

 less than in the Whitworth thread. 



23. While approving the general form of thread here dosci-ibed, this 

 Committee could not; but feel that the difference in the roundings (,Uh at 

 the top and !th at the bottom) was unnecessary. Looking, moreover, to 

 the fact that very matiy of the screws of the sizes now under consideration 

 are for electrical and telegraphic instruments, and, therefore, may be of 

 hras.s, and that, with threads of such dimensions, it is impossible lor tlie 

 eye to ascertain whether a given screw satisfies the required conditions in 

 regard to such .small ditVerencea between the crest and hollow of the thread, 

 the Committee feels that an equal rounding (I'l ths of the pitch) at the top 

 and bottom would be preferable. This would maintain the angle of thread 

 and the depth the same —namely, 474° and jJths of the pitch respectively. 



24. Having now discussed the three main points that require to bo 

 considered in awy system of screws — namely, the pitches, diameters, and 

 form of thread, it seems desirable to enumerate briefly the recommenda- 

 tions at which the Committee has arrived. These are : — 



(1) That the series of diameters for screws from ^ ,Vjyth inch to \i\\ 

 inch be that given in millimetres in Column V., the nearest thonsandtlis 

 of an inch being given in Colnmii II.; these diameters being the series 

 calculated by making V, in the formul.i D =i G Pj, have in succession 

 the following values : — 



I (or OO") mm. ; 00' mm. ; Q-^S^ mm. ; O-O' mm. ; 00" mm. 



Only two significant figures are taken to represent the diameters. 



(2) That the pitches of the.se serew.s be the above gradually decreas- 

 ing series, each pitch being j''„ths of its ])rcdece.ssor, but that only two 

 significant figures be used in their expression. The series thus obtained 

 is given in Column VI. 



(3) That in view of the desirability of securing a system of small 



