ON TflE SMALL SCREW GAUGE. 443 



used for makiiag sci-ews will not be suitable for making taps. In small 

 workshops this may sometimes cause mistakes, but in shops having a 

 se23arate tool-room this extra specialisation should present no difficulty. 



Objections have been raised to the above proposal on three grounds. 



It has been represented to us that in finely fitted work the screws 

 should fit their holes perfectly and all over, and that the existence of a 

 clearance gap all round the edge of the thread is inconsistent with a high 

 standard of workmanship. This objection is evidently to some extent a 

 matter of opinion, and it is always possible to use taps of the same form 

 as the screws, so that the screws will fit the taps all over as in a non- 

 clearance system. 



It has been objected that the introduction of the proposed system will 

 seriously interfere with existing stocks of screws and the repairs of existing 

 instruments. In fig. 2 it is' shown that the new thread differs from the 

 old one by the addition of the small corners, (j B h, h B k, to the screw, 

 and d A' e, e A' f, to the nut. In making screws and nuts with dies and 

 taps, these corners will always be rounded off' to some extent, though the 

 re-entrant angles at A B' will be as sharp as the tool which makes them. 

 In some screws and nuts prepared experimentally to test this point, the 

 outer edges of the thread were fairly rounded, and they were perfectly 

 interchangeable with the B.A. screws of an existing manufacturer's stock. 

 The Committee believe that screws made to the proposed new thread will, 

 owing to the inevitable rounding, be interchangeable with existing stocks 

 in a great majority of cases, and that only in cases where great care has 

 been taken to work closely to the old standard will any difference be 

 noticed. 



It has Ijeen objected that the proposed thread is unsuitable for such 

 work as bicycles and small arms which are subject to violent concussion 

 and vibration, whereby the screws are liable to be shaken loose and to 

 drop out ; and the case of alternating current arc lamps has been Jiien- 

 tioned to the Committee as one in which the same thing is liable to occur. 

 Mr. O. P. Clements of the Birmingham >Small Arms Company contri- 

 butes a paper to the Mechanical Section on the practice of his firm in the 

 manufacture of screws for bicycle parts, for which it is found necessary 

 to use rounded threads fitting very closely all over. It is clear that no 

 one form of thread can be suitable for all purposes, and we have direct 

 evidence that the form of thread we propose does not fail in instrument 

 work in the way Mr. Clements anticipates that it would do in bicycle work. 



We beg to report that the system of screw threads recommended by 

 the British Association for the use of instrument makers, and knoAvn as 

 the British Association screw threads, should be modified in the following 

 way for all screws from No. to No. 11 inclusive. 



For screws. — That the designating numbers, pitches, outside diameters, 

 and the common angle of 47 1° remain unchanged ; but that the top and 

 bottom of the thread shall be cylindrical, showing flats in section, and 

 that the depth of the thread shall be increased by one-tenth of the pitch, 

 the diameter of the solid core being, in consequence, diminished by one- 

 fifth of the pitch. 



For nuts. — That the designating numbers, the pitches, the diameters 

 of the clear holes, and the common angle of 47i° remain unchanged ; but 

 that the top and bottom of the thread shall be cylindrical, showing flats 

 in section, and that the depth of the thread shall be increased by one- 

 tenth of the pitch. 



