ON A GAUGE FOR SMALL SCREWS. 291 



The most important points to be borne in mind in its selection are : — 



(1) The threads must be easily cut with the class of screw-cutting 

 tackle ordinarily met with in workshops. 



(2) The strength of the threads on the male and female screws must 

 be so correlated that the liability of either to strip is a minimum. 



(3) The resistance of the core to torsional stress when force is applied 

 in rotating the screw must be a maximum. 



(4) The friction should be as small as possible, in order to reduce 

 wear. 



15. In regard to the first of the above conditions it is to be observed 

 that very many of the screws considered by the Committee are usually 

 made by means of a plate in which are round, tapped holes. Such a hole 

 forms a thread by causing the metal to ' flow ' from a space towards a 

 thread, and its action is obviously of quite a different character from the 

 cutting action of dies or of a chasing tool. In the case of plates with 

 notched holes the cutting and squeezing actions are combined. 



16. As bearing on the second condition, it is evident that, as the 

 strength of the threads depends so essentially on the materials of which 

 the screw and nut are made, and these are very varied, no precise and 

 invariable rule is attainable. If strength were the only point to be con- 

 sidered, a purely triangular form without any rounding would be best, 

 contact being assumed to take place over the entire surface. But in 

 practice it is impossible to secure such perfect contact, and it becomes 

 needful to round off the crests from all the threads ; and this rounding 

 is the more necessary as the screws are smaller and irregularities in the 

 manufacture become relatively more marked. This modification is also 

 necessary in view of condition (1) already considered. 



17. The third point — namely, the resistance of the core to torsional 

 stress — is determined primarily by the depth of thread. If the sectional 

 area of the ring cut away is less than that of the core, the probability ot 

 the latter breaking across may be regarded as approximately equal to 

 that of the threads stripping ; but it is impossible to maintain a constant 

 ratio, as such a condition would require the thread to be so fine in the 

 case of small screws that there would be no sufficient hold in the nut. 

 Thus in the very smallest screws (those below •030-inch in diameter) 



the ratio is less than 1, and it gradually increases 



sectional area of thread 



till a proportion of between 2 and 3 is attained. 



18. Condition (4) is evidently best satisfied by a square thread. 

 Such a form is, however, impracticable in the case of the small screws 

 under consideration, but it is obviously approximated to according as the 

 angle of a triangular thread is made less and the rounding greater. 



19. The angles that have been adopted in practice show, as might be 

 expected, considerable variation. On the one hand an angle of 60° is 

 rarely exceeded, the thread being thus derived from the equilateral 

 triangle, and, on the other hand, 45° may be taken as the lower limit. 



20. The depth of a thread is evidently a function both of its angle 

 and of the amount of rounding at the top and bottom. It may con- 

 veniently be expressed as a fraction of the pitch (taken as unity). In the 

 case of the small screws in general use the mean value of the depth thus 

 expressed is found to be - 5G3, the maximum being 0771, and the minimum 

 0'311. It is evident that any increase in the depth beyond what is 

 essential will materially and needlessly increase the difficulty of manu- 



u2 



