318 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



into six parts, and within the lines formed hj the sides of the thread and 

 the top and bottom dividing' lines, inscribing a circle which determines the 

 form of top and bottom of thread. Judging from the practice in this coun- 

 try, the English form of thread has not met with the same favor that has 

 been accorded to their pitches. Its advantages over the sharp thread arc 

 increased strength to the screw from the absence of acute corners, and the 

 greater security from accidejital injury which the rounded top possesses. 

 Its objectionable features are, first, that the angle of 55 deg. is a difficult 

 one to verify; it is probable, no gauges to this angle, made independently 

 of each other and without special tools, would correspond with sufficient 

 accuracy. Secondly'-, the curve at the top and bottom of the thread of the 

 screw will not fit tlie corresponding curve in the nut, and the wearing sur- 

 face on the thread will be thus reduced to the straight sides merely. It is 

 not to be inferred from this that these curves cannot be made to fit, but 

 only that the diflSculties in producing contact are so much increased by the 

 peculiar form, that in practice it will not be accomplished. Thirdly, the 

 increased cost and complication of cutting tools required to form this kind 

 of thread in a lathe, it being requisite that this tool shall have at least 

 three cutting sides, in order to form the round top between the two of them. 

 The English practice for small work, is to rough out in a slide-lathe with a 

 single point tool having sides of the proper angle, and finish in a hafd 

 lathe with a comb-chaser, which has been dressed to the proper form upon 

 a hob kept for that purpose, requiring three kinds of cutters and two lathes 

 to perform what with our practice requires but one cutter and one lathe. 

 On large work, the screw is finished in the slide lathe, with a chasing tool 

 dressed to the proper form upon a hob; and as these hobs are necessarily 

 the standards of form until worn out, it is fair to suppose the shape must 

 be undergoing a continual change. The necessity of guarding the edge of 

 the thread from accidental injury, becomes more and more apparent as the 

 size of the bolt is increased, and we have recognized this by finishing- such 

 bolts with a small flat upon the top of the thread; but no plan has been 

 proposed for general adoption upon all screws, nor have any proportions 

 been suggested where a flat is desired, or wheru from tlie size of the bolt it 

 would seem to be necessary. As it is very desirable that some uniform 

 rule should be observed in the formation of all threads, and as the sharp 

 top is objectionable upon large screws, this form must be abandoned if we 

 would accomplish our object. It being conceded that the flat angular sides 

 are necessary, we have only to choose between the rounded and flat top; 

 and having examined the former, it only remains to notice whether the fla.t 

 will be found free from the objections urged againsr the round. As the 

 sides of the thread are the only parts required to be fitted, and as these are 

 of the same shape as the sharp thresid, the one will be as easily made as 

 the other. The width of the flat top will be determined by the depth to 

 •which the thread is cut, so that the same tool can be used in both cases. 

 The flat on the top of the thread being required to protect it from injury, it 

 is evident a simdar shape at the bottom would give increased strength to 

 the bolt as well as improve its appearance. To give this form requires 

 only that the point of the cutting tool shall be taken off, and then it is evi- 

 dent this thread can be cut in a lathe with the same tool and in the same 



