280 APPLIED SCIENCE 



Materials Emery No. Grade Letter 



Large Iron or Steel Castings 16-20 P-Q 



Small Castings 20-36 P-A 



Hard or Chilled Castings 16-20 R-J 



Wrought Iron Forging 16-30 0-P 



Lathe and Planer Tool 34-46 N-O 



Brass Castings 20-30 P-R 



The makers of emery wheels usually paste tags on each 

 wheel stating the grade, speed, and order letter, but in some 

 cases the machinist may have to find the speed for special 

 wheels. If the wheel is run at a higher speed than that 

 stated on the tag,, the centrifugal force may, as before stated, 

 break the wheel. 



Some emery wheel houses advise a speed of 5500 ft. per minute. 

 From this we can calculate the speed for any diameter by multiply- 

 ing the diameter in inches by 3.1416, reducing to feet, and dividing 

 this figure into 5500. A 10-in. wheel would give us 10 X 3.1416 = 

 31.416 in., or 2.6 ft. 5500 divided by 2.6 = 2108 revolutions per 

 minute. 



319. The Discovery and Use of Carborundum. The 



discovery of carborundum nearly thirty years ago, brought 

 into use a new and exceptionally efficient abrasive. In 1891, 

 experiments with electric furnaces showed that when clay 

 and crushed coke were heated through a piece of carbon in 

 an electric furnace, the heat fused the two ingredients, and 

 that when the carbon was withdrawn, minute crystals adhered 

 to it. These tiny crystals were found to be amazingly sharp 

 and hard. Subsequent tests proved that the material had 

 great value as an abrasive, and it is now in general use. . 



The principal materials entering into the manufacture of 

 carborundum are coke, which supplies the element of carbon, 



