158 H. J. Grayson : 



oil alone proved somewhat too viscous, especially with the finer 

 abrasives. 



The screw having been evenly coated with abrasive, the nut is run 

 into a central position upon it, and pressui-e brought to Ijear, 

 evenly and slowly, rotating the nut by hand while this is being 

 done. In order to ensure a positive and uniform pressure the 

 clamping screws are advanced alternately until they are felt to grip 

 gently, and are then slightly relaxed. We thus ensure a condition 

 for both nut and screw in which it will he safe to run for a short 

 time. During all starting operations, close attention is paid to the 

 machine, until we are assured the running is satisfactory. Provid- 

 ing the pressure on both rings is properly proportioned, and the- 

 condition of the screw is known to be of a fairly uniform character, 

 it is quite safe to run at a speed of 120 revolutions per minute, 

 with an abrasive (alundum) of a uniform average diameter of grain 

 of about .0007 inch. This was found to be a suitable grade to 

 follow that of emery used in the hand grinding already described. 

 A mixture of equal parts of alundum and carborundum was, how- 

 ever, later substituted for alundum alone. Having allowed the nut 

 to travel up and dowai the screws a few times, the former was- 

 run off and reversed, and re-engaged by hand over several threads; 

 the latter being a precautionary measure never omitted, otherwise 

 damage might result to either nut or screw or both. The nut having 

 attained a central position once more, some readjustment of pres- 

 sure may be required. This is effected exactly as l^efore, and when 

 found uniform, the work of grinding should proceed for the same 

 time interval as before reversal. After the lapse of this interval, 

 it will probably Ije found that the abrasive and oil tend to work 

 away from the centre to opposite ends of both screw nut, and if 

 attention were not given to correct this tendency, it would even- 

 tually lead to a slight tapering of the screw from its centre towards 

 each end, and particularly to that end which was lowest during 

 grinding. At a later stage, it was found possible to counteract 

 this tendency, partly by varying the distribution of the abrasive, 

 and partly by extending or restricting, as the case required, the 

 travel of the nut upon the screw. It might readily be supposed 

 that the central portion of the screw would be more rapidly ground 

 than the ends, providing the nut does not travel over the full 

 length of the screw after each reversal — to permit which is not 

 usual or wise. As a rule it was found best to reverse the motion 

 of the nut when from two-thirds to three-fourths of the screw 



