Corundum is an extremely hard oxide of aluminum. Emery is a very hard, granular variety of corundum, containing a small amount of magnetite or hematite. Ground to a powder, these substances are used for polishing, grinding, or abrading stone, metal, glass, etc. In the crushing and grinding process, which is conducted in machines more or less enclosed, considerable fine dust is given off. After sifting and grading according to fineness, the product is stored in appropriate compartments, from which it is taken as needed. Wheels are made of emery or other abrasive material.

The proper selection of a grinding wheel may be the means of saving much money and time, as each metal requires some special difference in the wheel. Wheels are of different coarseness and grades, and when ordering, the diameter, thickness, size of hole, and grade number must be given. It is not reasonable to expect a wheel which was made for cast iron to grind properly brass or steel. Some wheels are made so that they will stand a constant stream of water running over them, while others will not. When moisture is to be used with the wheel, this fact should be stated in ordering.

The grade letter of a wheel denotes the hardness to which the wheel has been baked in the retorts. The number of the emery denotes the particular grade of that substance which is used in making the wheel. The number of emery and the grade letter of wheels to be used for some of the most important materials are as follows;

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

O-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 multiplying 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.