70 H. J, Grayson .- 



exactly similar to those of the preceding grade. It was noticed^ 

 ahnost at once, that this somewhat finer abrasive had a different 

 grinding sound, and somewhat smoother action, appreciable to the 

 touch when the nut levers were held by hand for a short time. A 

 longer time was given to grinding with this particular grade, the 

 '.vork extending over the greater pai't of a month. The improve- 

 ment effected could then be easily appreciated in the smoother move- 

 ment of the nut, apart from that seen under the microscope, which 

 was satisfactory throughout the full length of the screw. It was 

 therefore decided to use one of the finest grades of emery and carry 

 on the grinding with this to a finish. This work occupied another 

 week or more, and proceeded satisfactorily, although the effect 

 upon the screw was less pronounced than might have been expected. 



As it now appeared that little further imj^rovement could be 

 effected by grinding, both nut and screw were carefully cleaned, 

 and the latter slightly polished with the harder residneo of washed 

 rouge, that is the purplish portion, which imparted a slight gloss 

 only to the finished work. 



The screw having been freed from all traces of polishing material, 

 the nut was traversed to and fro upon it, with a trace of oil only, 

 and pressure just sufficient to ensure the closest approximation to 

 contact with the screw which could be obtained without risk. As 

 this procedure gave no indication of inequalities of pitch or dia- 

 meter, and direct microscopical examination and comparison by 

 the methods described was equally satisfactory, it was decided to 

 mount tlie screw in its permanent position upon the bed of the 

 machine, and sul>mit it to the test of an acutal ruling or series 

 of rulings, in order to determine under working conditions its 

 accuracy and other behaviour. Preparatory to this step, it was 

 necessary to eut away a portion of the threads from each end of 

 the screw, to provide space for bearings and the ratchet head. To 

 effect this, the original centres of the screw bar were availed of for 

 the preliminary turning doAvn to obtain a first approximation to 

 the limits of accuracy required. As the original axis of the bar 

 and that of the ground thread were probably no longer coincident 

 or parallel, and as it was of the first importance that the thread axis 

 and ]:)earings should be strictly in alignment to ensure this condi- 

 tion, the grinding nut was first mounted upon the lathe carriage, 

 so as to travel approximately parallel with the ways of the l>ed : 

 the screw was then threaded through the closely fitting nut. The 

 bearing portion of each end of the screw Avas next slowly revolved 



