New Engine J or Ruling. 59 



a comparatively soft metal as mild steel in its annealed state, as 

 minute particles become imbedded in its surface. Commercially 

 obtainable in the form of very finely divided powders, microscopical 

 examination of even the finest reveals the presence of numerous 

 acutely angular, and frequently needle-like particles, very difficult 

 to separate to a uniform shape and size of grain. Thus, though 

 rapid in action as a cutting and grinding agent, Carborundum is 

 liable to score and scratch finished surfaces to an extent dispropor- 

 tionate to the average grading of the powder used. Moreover, the 

 grades not infrequently contain a considerable amount of graphite, 

 ■which is apt to soil and obscure the surfaces being operated on. 



Alundum, when obtained in the form of an abrasive powder, is 

 usually white and clean. Though not so hard as carborundum, 

 it is tougher and somewhat less variable as to the shape of its. 

 particles. It may readily be separated into the finest grades,, 

 which are clean to work with and effective in action. 



Emery, as an abrasive, is rounder and more uniform of grain 

 than either carborundum or alundum, compared with which it is 

 not so hard as either, but possesses the quality of toughness to a 

 greater degree. Owing to its fairly high specific gravity it can 

 readily be separated into a very effective series of finely divided 

 powders suited for use upon mild steel, on which the finer grades 

 leave a uniformly smooth or even polished surface. 



As, practically, the process for the separation of any of these 

 abrasives is the same, we may describe that used for separating 

 emery as typical, the details being as follow : — 



(i.). — The lyreliminary se'pdrafion of Emery or other ahrasives. — 

 In connection with other Avork, considerable quantities of separated 

 abrasives were required; the experience gained in their separation 

 was availed of in the selection and preparation of the materials 

 required for grinding the screws; the process being, in respect to 

 the separation of emery, as follows : — 



As commercial flour emery contains a relatively large amount 

 of coarse material, that is of grains over .001 incli in diameter, 

 down to small grains unsuited for abrasive work of the kind here- 

 described, the work of separation is best accomplished by first 

 subjecting the crude powder to a preliminary treatment. The 

 quantity dealt with, it must be understood, was propoi'tioned to 

 the work for which this particular separation was required. A 

 third of this amount would, of course, be ample for the supply of 

 material for grinding several screws. 



