GRINDING WHEELS 435 



cone pulleys or. better yet, to employ a shunt-wound electric 

 motor and rheostat and thus obtain a variation in speeds. 



One of the greatest troubles we encounter when dealing with 

 abrasive wheels or papers or powders is the non-uniformity of 

 grain size. A few large grains present, often a single one in a 

 small area of the grinding surface, will so deeply scratch the 

 specimen as to render its proper preparation almost impossible. 

 If a wheel is found upon trial to have any such projecting par- 

 ticle the wheel should be abandoned at once, and never be 

 employed save for the crudest sort of grinding. It is this dif- 

 ficulty which leads many workers to discard abrasive wheels for 

 all save the roughest dressing of a specimen and use only laps 

 fed with very carefully ground, sifted and floated abrasive 

 powders. 



Laps may be either horizontally or vertically driven. The 

 beginner will find that satisfactory surfaces are obtained easier 

 upon the horizontal lap, but it is open to the objection that it 

 does not readily clear itself and any dust or dirt falling upon 

 it or any large particle of abrasive will be apt to deeply groove 

 the specimen. The vertical lap on the other hand is difficult to 

 keep charged with pasty abrasive or thin suspensions of abrasive 

 and polishing powders. 



In the case of soft alloys, facing to a smooth surface is most 

 easily accomplished by means of files, rough dressing with a 

 10 or i2-inch bastard cut file and passing to an 8 or lo-inch 

 single cut. With moderately soft .materials such as brass, lay- 

 ing a single cut mill file flat upon the work bench and pushing 

 the specimen down the file against the cutting edges will be 

 found to yield good smooth surfaces with less practice and skill 

 than by holding the specimen in a vise and pushing the file. 

 The specimen should be pushed lengthwise of the file with gentle 

 pressure until it reaches the tang end, then lifted off; the file 

 turned edgewise and struck a sharp blow upon the bench to 

 remove filings, again laid flat and the specimen again laid upon 

 the file and gently pushed toward the tang end, and the process 

 repeated until a small plane surface is obtained. Specimens 

 should never be rubbed back and forth upon an abrasive surface, 



