COMMON HAND-TOOLS 279 



grain presents a distinct crystalline structure with a sharp 

 cutting edge. From these materials are made a wide variety 

 of grinding wheels and sharpening stones. 



318. Corundum and Emery Wheels. 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 sub- 

 stances 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, con- 

 siderable 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 ; 



