Neiv Engine ior Ridiitg. 57 



this should involve a slightly heavier cut than that just named. 

 Also it is material that the over-all threaded length of the bar 

 should exceed by several inches the portion it is pro^Dosed to use in 

 the finished screw. 



The thread " pitch " and " angle " should be appropriate to the 

 work for which the screw is designed. The pitch value of the screw 

 here described is 20 threads to the inch, a value convenient for sub- 

 division, and the thread angle 50°, permitting of a somewhat deeper 

 thread than the usual Whitwortli standard; both "crown" and 

 " root " of the thread, previous to grinding, remained as left 

 from the threading tool. The greatest care was taken so that the 

 intersections of the thread walls with any plane through the axis of 

 the screw should be straight lines — a precaution applying equally 

 to the counterpart threads within the nut. Should the work for 

 which a screw is being cut justify tlie expenditure of the time 

 involved, it will pay to cut several screws at the outset, selecting two 

 of the best for final grinding. 



(i.). — The. grinding nut . — For grinding his screw, Rowland, accord- 

 ing to the Encyclopaedia article, appears to have made use of a brass 

 nut constructed externally so as to taper from the centre towards 

 -each end, and split longitudinally into four equal segments which 

 could be fitted to the screw and held in position by means of 

 sliding sleeves or collars adapted to the tapered ends of the nut. 

 The opposing sleeves were connected with bolts and nuts, and could 

 thus be drawn together as the work of grinding proceeded. 



Soine objection may be taken to the use of this form of nut for 

 grinding a precision screw for the following reasons : — (1) The 

 iise of a brass nut upon a steel screw has the disadvantage due to 

 the wide difference in the coefficients of expansion of the two metals, 

 as some heating will take place in the process of grinding. To 

 avoid heating, the Avork ol grinding must either proceed very slowly 

 or else be conducted under water or oil, which entails very serious 

 disadvantages. Hence any gain from the use of brass, due to its 

 superior action with abrasives upon a harder metal, is lost. (2) 

 The division of a grinding nut into four segments appears to be 

 faulty, in that it is possible for one or more sections to move 

 .slightly in relation to the others providing, as is almost sure to be 

 the case, there are periodic or other irregularities in the screw. 

 While the method of tightening up the segments is likely to produce 

 uneven pressure, so that one or more sections of the nut may do more 

 than their share of grinding, even though the general trend of the 



