880 REPORT— 1900. 



I think that the correctness of these views cannot be better demonstrated than 

 by showing what is the general practice and experience with regard to screws 

 used in the gun trade. lu both sporting guns and military rifles the screws are 

 subject to severe vibration as well as sudden strains, and are therefore extremely 

 liable to work loose. To obviate this the gunmaker uses a thread vs^ith a 

 •well rounded top, and care is taken that the whole of the thread fits well, but more 

 especially the top of the thread, where the frictional contact on the greatest 

 circumferential portion of the screw will prevent Liosening. We have thus a thread 

 that differs from any recognised standard. It is shallow, with a large angle of 

 the sides generally about G0°, and is admirably suited for the purpose of resisting 

 vibration. 



I will now refer to a shape of thread which merits consideration, namely, flat- 

 topped threads, which are very suitable for many purposes. It is also a shape of 

 thread to which most accurate gauges can be made ; but while admitting their un- 

 doubted suitability for gauge making, I must remark that gauges for threads with 

 rounded tops can also be made satisfactorily, for all practical purposes, both as 

 regards size and form, and so as to be perfectly reversible. Such gauge making, 

 however, certainly requires a skill and experience which can be attamed in but few 

 tool shops. The flat-topped thread can be most accurately formed with a single 

 tool on the ordinary screw-cutting lathes, or on machines having a leading 

 screw or former. The tool can be easily ground to correct shape, and so as to 

 have the cutting clearance which is necessary for the durability of the tool and 

 for the production of clean and accurate work. 



There are, however, serious objections to the adoption of such a thread for 

 screws used in cycle work, and for screws subject to viljration. It is certain 

 that the flat-topped thread cannot give the frictional resistance to vibration 

 which is the case with the round top ; and in the economical production of 

 such work it would be very difficult to maintain the correct shape of the 

 thread. In this production, screwing dies are chiefly used, and these tools show 

 the first and most rapid wear on the parts forming the sharp edges or corners 

 of the thread. For this reason it will be found a serious matter to keep up 

 the screwing tackle, male and female, in the proper working condition necessary to 

 produce flat-topped threads, especially if they should have a small angle of the 

 sides. I am regarding this matter from the commercial point of view, that is, 

 the production of work in quantities to be profitable and accurate, so far as 

 accuracy is commercially possible. 



In my experience the most favourable shape of thread for production with 

 screwing dies and taps is a shallow thread with a large angle of the sides. 

 This will give the best cutting clearance in the screwing tools. AH the faults and 

 errors in screw threads, and the difficulties in manufacture, can generally be 

 traced to the bad cutting clearance in screwing dies and taps for high threads with 

 small angles of the sides. Thus, through the strain put on the sides of such 

 threads, there is a liability to breakage of the threads on the sci'ewing dies 

 and taps, and it also causes the screw to elongate and produces a fewer number of 

 threads to the inch than standard pitch requires. This pitch error is a most 

 serious fault, as the strain which should be distributed over all the threads is 

 often taken by only one or two of them. 



Owing to the rapid wear of dies and taps with a bad cutting clearance, a 

 faulty shape of thread is produced, especially at the sides of the thread. The 

 angle of the male thread is often different from that of the female thread, and, 

 in such case, the bearing surface at t'le sides of the thread is, of course, con- 

 siderably rf-daced. This fault is especially si-riousin long-sided threads. 



The spreading or elongation of the thread i-* another matter whii'h I may here 

 refer to. [t is found necessars in tapping holes ro drill or reara^^r the hole larger 

 'than the bnttom of the thread on t!ie male screw. In the ])rocess ot tapping, the 

 thread elongates so as to fill the cavities between the threads and the tap, and 

 upon the completion of the operation the hole will be found to be considerably 

 smaller than when the tap was first inserted. This elongation also occurs in 

 the male thread, but to a less degree, and if proper allo!\-ance for it is neglected, 



