292 . eepout — 1884. 



facture when a screw-plate is used; at the same time the depth must not 

 be too much reduced on account of the gi-eater tendency of the thread 

 to strip. It is further important that the additional torsion involved in 

 cutting a deep thread, which materially increases the risk of tearing the 

 metal across, should not be lost sight of. 



21. The Committee, after comparing together a large number of 

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

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

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

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

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

 sections 16 and 18, in the case of small screws the tendency should rather 

 be to increase the rounding on account of the difficulties of manufacture, 

 and the depth of the Whitworth thread is 064 of the pitch, which ia 

 considerably iu excess of - 563, the average adopted in practice. The 

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

 usual iu small screws. 



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

 as compared with those of greater diameter is fully recognised by the 

 Swiss Committee, their thread for the former having an angle of 47j°, 

 while that for the latter is 53°, nearly the same as that of the Whitworth 

 thread. In the case of small screws made in the Filiere Suisse the crest 

 of each thread is rounded off with a radius equal to J.th the pitch, and the 

 hollow with ith the pitch. The actual depth is 060 the pitch, somewhat 

 less than in the Whitworth thread. 



23. While approving the general form of thread here described, this 

 Committee could not but feel that the difference in the roundings (£th at 

 the top and 1th at the bottom) was unnecessary. Looking, moreover, to 

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

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

 brass, and that, with threads of such dimensions, it is impossible for the 

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

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

 the Committee feels that an equal rounding (-j3j-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, 47r>° and ?-ths of the pitch respectively. 



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

 considered in any 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 y^yth ^ ncu to 4th 

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

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

 calculated by making P, in the formula D = 6 P-|, have in succession 

 the following values : — 



1 (or 0-9°) mm. ; 0-9' mm. ; 09 2 mm. ; 09 3 mm. ; 0-9" mm. 



Only two significant figures are taken to represent the diameters. 



(2) That the pitches of these screws be the above gradually decreas- 

 ing series, each pitch being f^ths of its predecessor, but that only two 

 Bignificant figm-es 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 



