434 
ELEMENTARY CHEMICAL MICROSCOPY 
Table VI. 
CHARACTER OF ABRASIVE WHEEL REQUIRED. 
Grain or grit. 
Grade or Hardness. 
Alloy, aluminum type. 
20 to 36 
20 to 46 
20 to 36 
20 to 36 
30 to 54 
20 to 46 
30 to 54 
60 to loo-f 
14 to 20 
46 to 80 
30 to 54 
100 to 180 
Hard 
Hard 
Hard 
Medium-hard 
Medium-hard 
Medium-hard 
Medium 
Medium to medium- 
hard 
Hard 
Medium 
Medium 
Hard 
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 
