ON THE SMALL SCKEVV GAI!GE. 437 



tory system of screw threads may cause some apprehension among users 

 of them, and the Committee, many members of which are intimately 

 acquainted with the trouble and inconvenience incidental to a change of 

 the kind, recognise that very substantial reasons are required to justify it. 

 They think it desirable that the considerations which have led them to 

 make this proposal should be fully stated. 



Consideration of the exact cause of the difficulty found in the con- 

 struction of gauges for the British Association thread showed immediately 

 that it was due to the rounded tojj and bottom of the thi-ead. There is 

 no difficulty in making any given angle between the straight portions or 

 sides of the generating tool or chaser, but to arrange that these straight 

 lines shall, at definite points, turn smoothly into circular arcs of a given 

 radius is a matter of some difficulty. The difficulty has been met with a 

 good deal of success. The original threads, cut by Mr. Lehmann, and 

 those produced recently by the Pratt and Whitney Company, are admirable 

 specimens of workmanship, especially when the small size of the pieces 

 is considered ; and to the careful work done by Mr. Lehmann in the years 

 following 1 882, when originating the threads, the success they have 

 achieved is largely due. The production, however, of chasers, even if it 

 can be repeated indefinitely, does not end the difficulty. The hardening 

 of the screws produced by these tools introduces some inaccuracy. They 

 are no longer perfectly straight, perfectly cylindrical, or of perfectly 

 accurate pitch, and the only way to correct them is by grinding. The 

 inaccuracies produced by hardening are not of sufficient importance to 

 affect the use of taps, and in the case of die-plates the errors produced in 

 the diametei's are corrected by opening or closing the die ; but for gauges 

 corresponding to modern ideas of mechanical accuracy the errors pro- 

 duced by hardening are considerable, and much greater than those found 

 in screws whose forms can be finally obtained by grinding. With the 

 British Association thread this process does not seem to be practicable 

 except perhaps in single specimens, and in this lies the inherent defect of 

 the thread. 



A way out of the difficulty ia offered by the adoption of a flat-topped 

 thread, but before this can be discussed it is necessary to consider what 

 are the peculiar advantages of the rounded thread, which have brought it 

 into general use, and led to its adoption by the original Committee. The 

 British Association thread was taken with a slight modification directly 

 from Professor Thury's SavIss system, which had been constructed by 

 finding a formula to represent the average existing practice among Swiss 

 clockmakers. Sir Joseph Whitworth formed his sj'stem of screws in a 

 similar way by averaging the English engineering practice of his time. It 

 appears that the object in view in both these cases was to regularise exist- 

 ing practice, not to effect a reform ; and that an alteration in the form of 

 thread in common use was not contemplated. The same was done in 

 America for the United States thread, so far as the pitches and diameters 

 were concerned, but the form of the thread was determined by Dr. Sellers 

 on general considerations. The origin of the round thread in the British 

 Association system was in the common practice of the Swiss workshops 

 when the rule was constructed. Now, whatever may be the prescribed shape 

 of the thread, it is certain that small screws, produced on screw machines, 

 will have rounded tops, and if a new rule for American threads were con- 

 structed from the shapes of ordinary small screws found in the United 

 States, the form obtained would have a rounded top, notwithstanding that 

 they are all supposed to represent the flat-topped Sellers thread. Since 



