1.Define abrasives? How they are classified
Ans: Abrasives are hard substances used for grinding, cutting sharpening, polishing and buffing operations. They are characterised by high hardness, high melting point and chemical inertness.
Natural Abrasives – Diamond, Quartz, garnet, corundum, emery.
Artificial Abrasives – Silicon carbide, Boron carbide, Alundum.
2.What is mohs scale? Name the hardest substance known.
Diamond is the hardest substance known.
3. What are soft abrasives?
Ans: Abrasives having their hardness 1 to 4 in Mohs scale are known as soft abrasives. eg. Gypsum, Talc, Calcite.
4. What is hardness of an abrasive? What are its units?
Ans: It is the ability of an abrasive to grind away another surface. It is measured on Mohs scale or Vicker’s scale.
5. What is abrasive power?
Ans: It is the strength of an abrasive to grind away another surface. It
depends upon hardness, toughness and refractoriness.
6. What is diamond? Mention its types.
Ans: It is a pure crystalline carbon. It is the hardest known substance. Its harness is 10 on Mohs scale. It is chemically inert and not affected by acids or alkalis. The off-colour diamond is called borts and black colour diamond is called carbonado. The have no jewellery value.
7. How is carborundum prepared?
Ans: It is prepared by heating a mixture of sand SiO2 and coke (carbon) with small amount of saw dust in an electric furnace at about 2200°C.
8. Mention the important properties of carborundum (SiC).
(v) It has high resistance to abrasion and spalling.
9. What are the use of Silicon carbide?
Ans: a) For making grinding machines, grinding stones, crushers since SiC as hard a diamond.
b) For giving edge to cutting tools.(razors, Knives, Swords, etc)
c) For making carbon rods in resistance heaters.
d) As a refractory material, both in the form of bricks and as loose material.
e) For making abrasive papers and cloths.
10.How is alundum prepared?
Ans: It is prepared by heating a mixture of calcined bauxite, coke and iron in an electric furnace at about 4000 C. this one is actually the artificial corundum.
11. Mention the uses of alundum.
- Alundum is used in grinding of hard steels and other materials of high tensile strength.
- It is used in the manufacture of abrasive wheels.
12. What is norbide? Mention its properties.
Ans: Norbide is manufactured by heating boric oxide with coke at 2700°C.
Properties:
- It hardness is 9 on mohs scale.
- It is light weight and black coloured compound.
- It is highly chemical resistant and resistant to erosion.
- It resists oxidation much better than diamond.
13. How grinding wheels are manufactured? Mention their uses.
Ans: It is manufactured by crushing the abrasive material in jaw crusher and graded according to the grain size. These abrasive grains are mixed with suitable binder and moulded into desired shape and cured.
Uses: They are used in workshops to sharpen tools and to polish the fabricated parts.
14. Mention some important applications of abrasives.
Ans: (i) To clean the surface prior to coating abrasive powders are used e.g., Quartz, garnet
(ii) To prepare smooth wood, metal and plastic surfaces abrasive paper is used eg: Alumina, Silicon carbide.
(iii) To remove the scales from iron surfaces, grinding wheels are used.
15.How are abrasives used?
Ans: Abrasives are used in three forms
- As grinding wheels.
- As abrasive paper or cloth.
- As loose powder.
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