Neiu Engine for Riding. 77 



diameter and made of the very hard greenish variety of carbor- 

 undum, were obtained. Each wheel was mounted upon its own 

 steel spindle, from which it was not again removed, and with the 

 aid of suitable diamond tools its periphery was trued and cut to the 

 shape necessary to reproduce by grinding the form of teeth required. 

 The wheel spindles were adapted to run in a lathe cutter frame witL 

 hardened centres, with overhead drive and provision for speeds 

 up to 6,000 revs, per minute as free as possible from vibration. 

 A series of wheels were thus fitted varying in grade of grain, but all 

 relatively fine and made up with a vitreous bond. The coarsest of 

 these wheels were used as originally made for the first roughing cuts 

 in which extreme accuracy was not called for. But the carefully 

 formed edges of the finer grade wheels, used for finishing the teeth,, 

 in which process both great accuracy and high finish were required,, 

 were charged with diamond powder obtained by crushing and grad- 

 ing after the method described for preparing other fine abrasives. 

 This grading was of course done on a proportionately small scale 

 and with the aid of petrol in place of water. Several grinding 

 discs were so charged, each with its own grade of diamond in the- 

 following way : — 



The spindles, carrying each its own wheel finally trued and turned 

 to the correct shape, were supported on centres in a small frame 

 convenient to a tiny blow-pipe flame arranged to impinge upon 

 the edge of the disc and parallel with its face. The disc or wheel,, 

 carefully freed from grease or dirt of any sort, is then slowly 

 revolved and its edge moistened with a strong solution of soda car- 

 bonate, using a fine camel-hair brush and working with the aid of a 

 magnifier, which is necessary as the dimensions of the trued edge 

 of the disc are very small. The application of the alkaline solution 

 ensures the even distribution and adhesion of a film or layer of a 

 thin cream (made up with water) of glass enamel composed of 

 extremely fine ground moderately hard glass, containing about one 

 part in three of diamond powder of the particular grade suited to 

 the work to be done. The liquid enamel thus prepared, fills up the 

 pits and crannies in the edge of the disc, which are of course very 

 small in the case of a fine grade wheel. As soon as the layer of 

 enamel is dry, that is in a few minutes, it is ready for firing. To 

 effect this the wheel and spindle are first slowly and uniformly 

 heated with a bunsen flame, after which the blow-pipe jet is 

 brought into position to play quietly upon the extreme edge of the 

 disc, which is quickly brought to a white heat, fusing the painted 



