434 



ELEMENTARY CHEMICAL MICROSCOPY 



Table VI. 

 CHARACTER OF ABRASIVE WHEEL REQUIRED. 



Alloy, aluminum type 



Alloy, brass type 



Alloy, bronze type 



Alloy, nickel type 



Iron, cast 



Iron, chilled 



Steel, soft 



Steel, hard 



Soft porous material 



Soft friable material 



Moderately hard compact material . 

 Very hard brittle material 



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 avail- 

 able. 



The operating speed of a grinding wheel is usually expressed 

 as " surface 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 



