440 , REPORT — 1900. 



The adoption of a flat-topped thread with a clearance would, we 

 believe, completely obviate the difficulty of producing satisfactory gauges, 

 the question to which the attention of this Committee was originally 

 directed. The construction of these is referred to later in the report. 

 Other elements of the screw have received the attention of the Committee 

 as follows. 



Mr. George M. Bond, of the Pratt and Whitney Company, has 

 expressed to the Committee a strong opinion that the angle of 60° 

 employed in the Sellers thread is most suitable for screws because of the 

 ease with which it is formed. Tools can be ground without ditficulty, 

 and with great accuracy, to any desired angle, and Mr. Bond's reason 

 appears to the Committee insufficient of itself to justify a change in 

 practice. Considering, however, the extent to which screws of tlie 

 Sellers form are employed in foreign engineering work, the Committee 

 desired to obtain some evidence of the exact value of the particular angle 

 of 60°, since, if this angle wei-e found to possess a great advantage over 

 the angle of 47^°, the adoption of the Sellers thread would have the 

 additional recommendation of bringing the small screw pi'actice into line 

 with an already extensive engineering pi'actice, while giving effect to the 

 conclusions already reached by the Committee of the desirability of clear- 

 ance and a flat-topped thread. Some experiments on lines suggested by 

 Mr. Crompton have been carried out by Messrs. J. Marshall Gorhain 

 and AV. A. Price, and their results are printed as an appendix to this 

 report. They concluded that an angle of 47V' is better for ?^-crevvs than 

 an angle of 60°, on the ground that it offers much less frictional resistance 

 to screwing and unscrewing on a given tensional load, and the general 

 tendency of this observation is corroborated by tiie practice of using a 

 thread for the leading screws of lathes, the screws of carpenters' clamps, 

 and of screw jacks, in which the working surface is perpendicular to the 

 axis of the screw. Another consideration leads us to think it undesirable 

 to adopt an angle of 60°. The advantage of bringing small screw-practice 

 into line with that of foreign engineers will only be fully gained if their 

 rule for the size of the flat top of the thread is also adopted. This rule 

 gives a maximum possible clearance of -108 pitch when the thread is cut 

 to a perfectly sharp V -"it the bottom. This clearance would be sufficient 

 V<ut tools with perfectly .sharp points are maintained with difliculty, and 

 it would not generally obtain. A tool of 47^°, ground to give a clearance 

 of -1 of the pitch, has a flat at the point one-seventh of the pitch wide. 

 For small screws Professor Thury's angle of 47V° had the same sanction 

 of practice among clockmakers as a larger angle had among engineers 

 when it was adopted by Dr. Sellers ; and though it is often difficult to 

 assign exact reasons for the particular practice of practical men, yet it 

 caimot be disi-egarded unless the reasons for its use are quite clear, and 

 arc sliown to be insufficient. We see no sufficient reason to change the 

 preseiit angle of 47.\°, especially as a change of angle would make existing 

 stocks and tools altogether useless in conjunction with the existing form. 



The existing series of pitches and diameters, with their designating 

 numbers, is generally approved, and the Committee have received no sug- 

 gestion that it is otherwise than satisfactory. 



Thus far it has been assumed that, given the necessary tools, all forms 

 of thread can be produced with the same ease. This, however, does not 

 apply to the small screws used in watches, which are produced by turning 

 the blanks into a hard die without cutting edges. In such a process 



