furnaces from coke and sand. These abrasives include carborundum,
crystolon, carbolite, corbolon, carbowalt, and corex.
Aluminous Abrasives.—These are prepared in electric arc furnaces
from bauxite, a clay that contains a high percentage of aluminium oxide.
It is a soft light-yellow earth, and is the purest form of aluminium oxide
found. Only in the electric furnace can the nearly pure alundum be separated
from the foreign matters present in the earth. The abrasives obtained in
this way are: alundum, alowalt, aloxite,
borocarbone, carbo-alumina, corowalt,
oxaluma, and rex.
Applications. — Although several
of these abrasives are employed for
similar purposes, yet some are more
suited to certain duties than others.
Broadly, the wheels used for materials
of low tensile strength, such as cast
iron, brass, and aluminium, are not em-
ployed for the steels which have high
tensile strength. In general a carbide
of silicon abrasive is used for the first,
and an aluminium oxide abrasive for
Grain or Grit. — The number
that designates the grain signifies the
number of meshes to the linear inch in
the grating forming the bottom of a
sieve, through which the grains will pass.
The numbers in common use range from
about 20 to 60. Usually all the grains
in a wheel are of the same size, but
" combination " wheels are used, with
the object of enabling them to cut fast
and finish smoothly, and so avoid a
finish grinding with a second wheel.
Grade or Bond.—The efficiency
of a grinding wheel for a definite duty
depends on what kind of material is
employed to cement the grains together. Wheels are " hard" when the
grains are not easily dislodged from their matrix, "soft" when they are
readily torn out. But the size of the grains has a modifying influence,
since a wheel with the same bond is harder if the grains are fine than if
they are coarse. Generally a harder wheel will be used on soft steel than
on the same steel if hardened. The harder the material is, the softer the
wheel should be. The reason is that a hard material will blunt the grains
more quickly than a soft one, and therefore they should be torn out more
rapidly to allow fresh grains to come into action. An exception occurs in
Fig, 30.—Edge Grinding Wheel with Bevelled