58 H. J. Grayson: 



pressure applied may be in the direction of uniformity of action^ 

 Thus any advantage derived from increased surface action; result- 

 ing from the use of a segmented nut, may be somewhat discounted 

 by unequal pressure. 



The foregoing and one or two minor considerations led up to th& 

 decision to employ a continuous steel nut of sufficient thickness and 

 rigidity to resist any variations in pressure which were likely to be 

 used upon it. This nut was cut, bored and threaded up to a length 

 of about 10 inches, which was nearly equal to the length proposed 

 for the finished screw when in use. 



This long nut was threaded on the lathe used for cutting the- 

 thread of the screw, with a threading tool and such other conditions 

 and precautions as would result in the closest correspondence 

 between the tAvo threads. 



The outer wall of the finished nut was slotted through on one side 

 and partly so on its inner opposite side. The two halves of the- 

 nut could thus be drawn or rather forced together on the applica- 

 tion of moderate external pressure at suitable points. To effect 

 this pressure the nut was encircled with three strong metal rings 

 fitted with set screws bearing directly on its opposite sides. As the- 

 rings, under pressure, Avere slightly elastic or yielding the pressure 

 exerted by them was free from any rigidity likely to lead to seizure 

 between the respective surfaces of nut and screw. Fig. 2, Plate XI. 

 illustrates several of these features. 



II. — Process of Grinding the Screiv. 



As may readily be supposed, success in grinding an accurate 

 screw is so intimately related to the abrasive used, that some- 

 account of the properties and preparation of those^commonly em- 

 ployed is desirable before treating of the method of their application. 



The following three well-known substances, in tlie form of 

 abrasive powders, viz. : Carborundum, Alundum and Emery were- 

 used, at least to some extent, experimentally in grinding the screws. 

 As each of these abrasives possesses qualities and structural peculi- 

 arities of its own, some explanation of the n\ethod of separating or 

 grading, with i-espect to size and uniformity of grain, is essential 

 to an appreciation of their efficiency. 



With respect to their nature and qualities : — 



Carhornndum, whilst by far the hardest of the three substances, 

 is also the most brittle and the least suited for application to such 



