PREPARING OPAQUE OBJECTS 387 



indicate at once, to the worker, whether he is employing the 

 proper grade, grain and speed. It is strongly urged upon the 

 beginner to carry out experiments in this manner and spend 

 considerable time, if possible, in ascertaining just what different 

 wheels will do under like speeds. 



Table VI may serve as a rough guide to the selection of the 

 wheel which will prove satisfactory with the materials indicated. 



If the grinding room equipment is limited to two or three 

 wheels it is evident that the widest range of applicability will be 

 found in the following selection: 30 hard, 40 medium-hard, and 

 60 or 80 medium, providing a sufficiently high speed is available. 



The operating speed of a grinding wheel is usually expressed 

 as " surf ace velocity" in feet per minute in order that wheels of 

 different diameters may properly be compared. 



Surface velocity = Diameter wheel in feet X 3.1416 X R.P.M. of arbor. 



Most small wheels used for grinding are designed to run with 

 a surface velocity of from 2000 to 4000 feet per minute. This 

 requires that the. grinding head shall rotate at the rate of approx- 

 imately 1800 to 3000 revolutions per minute for a five or six inch 

 wheel, if the data given in Table VI are followed. For slower 

 speeds it will be necessary to select finer grains and harder 

 grades. In order to permit some latitude in the selection, it is 

 best to have the grinding head and driving motor provided with 

 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 em- 

 ployed save for the crudest sort of grinding. It is this difficulty 

 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. 



