Abrasives
Abrasive is a hard material. It can be used to cut or
wear away other materials. Small abrasive particles are used in
grinding wheels. They are called abrasive grains. Abrasives may be
classified into two types.
Natural abrasives
These are produced by uncontrolled forces of nature. These are found from mines. The following are the natural abrasives.
a. Sandstone or solid quartz
b. Emery
c. Corundum
d. Diamond.
Natural
abrasives lack uniformity of properties and reliability has been
largely replaced by manufactured or artificial abrasives.
Artificial Abrasives
These
are manufactured under controlled conditions in closed electric furnace
in order to avoid the introduction of impurities and to achieve
necessary temperature for the chemical reaction to take place. These
abrasives have better cutting properties and higher efficiency than
natural abrasives. The various manufactured abrasives are
a. Aluminium oxide
b. Silicon carbide
c. Artificial Diamond
d. Boron carbide
e. Cubic boron nitride.
Aluminium Oxide
It
is the crystalline form of aluminium oxide. This abrasive carries very
hard and tough grains having sharp cutting edges. It is manufactured by
fusing mineral Bauxide in an electric arc furnace mixed with coke and
iron scrap. Here, iron scrap acts as a flux. After fusing, it is
crushed, washed and treated with alkalis. Again, it is washed and
finally, ground.
Aluminium oxide is tough and less
brittle. It is used for grinding materials of high tensile strength
like high speed steel, malleable iron, wrought iron etc.
Silicon Carbide
It
is made from Silicon dioxide, coke, sawdust and salt. The ingredients
are thoroughly mixed and heated in an electric furnace about 2000 C for
34 Hours. The mass under the action of intense heat fuses and the
following chemical reaction takes place.
The
silicon carbide mass is crushed, washed and treated with alkalis. It is
again washed and finely ground into small particles. Silicon carbide is
hard and brittle. It is used for grinding materials of low tensile
strength like grey cast iron, brass, copper, aluminium etc. The common
trade name for this abrasive is ‘Silicon carbide’ ‘Carborundum’
‘Crystolon’ and ‘Electron’ etc. It is represented by the letter C.
In general, the physical properties of aluminium oxide are compared with silicon carbide as follows:
1. Silicon carbide is harder than aluminium oxide.
2. Aluminium oxide can withstand greater stresses than silicon carbide.
3. Aluminium oxide is tougher than silicon carbide.
Artificial Diamond
Artificial
diamond is a form of pure carbon which is mainly used for truing and
dressing other grinding wheels for sharpening carbide tools, and for
processing glass, ceramics and stone.
Boron carbide
It
is harder than silicon carbide but not as hard as diamond. It is
produced from coke and boric acid at tremendously high temperatures in
an electric furnace. Boron carbide is mainly used for grinding and
lapping very hard metals, hard alloys, glass and jewels.
Cubic boron nitride
It
is a never synthetic abrasive that is harder than either aluminium
oxide or silicon oxide. It is a combination of boron and nitrogen. Boron
Cutters, grinding tool-steel, punch-press dies, grinding some
hardenable stainless steel and for internal grinding of all ferrous
metals.
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