Section 13. qxd

ABRASIVE PROCESSES
(See also Sec. 6.)
Abrasive processes
consist of a variety of operations in which the tool is
made of an abrasive material, the most common examples of which are
grinding (using wheels, known as 
bonded abrasives
),
honing, and lap-
ping. An abrasive is a small, nonmetallic hard particle having sharp cut-
ting edges and an irregular shape. Abrasive processes, which can be
performed on a wide variety of metallic and nonmetallic materials,
remove material in the form of tiny chips and produce surface finishes
and dimensional accuracies that are generally not obtainable through
other machining or manufacturing processes.
Grinding wheels
have characteristics influenced by (1) type of abra-
sive; (2) grain size; (3) grade; (4) structure; and (5) type of bond. (See
Figs. 13.4.18 and 13.4.19.)
Selection of Abrasive
Although a number of natural abrasives are
available, such as emery, corundum, quartz, garnet, and diamond, the most
commonly used abrasives in grinding wheels are 
aluminum oxide 
and
silicon carbide,
the former being more commonly used than the latter.
Aluminum oxide is softer than silicon carbide, and because of its fri-
ability and low attritious wear it is suitable for most applications.
Silicon carbide is used for grinding aluminum, magnesium, titanium,
copper, tungsten, and rubber. It is also used for grinding very hard and
brittle materials such as carbides, ceramics, and stones. 
Diamond
and
cubic boron nitride
grains are used to grind very hard materials and are
known as 
superabrasives.
Selection of 
grain size
depends on the rate of material removal desired
and the surface finish. Coarse grains are used for fast removal of stock;
fine grain for low removal rates and for fine finish. Coarse grains are
also used for ductile materials and a finer grain for hard and brittle
materials.
The
grade
of a grinding wheel is a measure of the strength of its bond.
The force that acts on the grain in grinding depends on process variables
(such as speeds, depth of cut, etc.) and the strength of the work material.
Thus a greater force on the grain will increase the possibility of dislodg-
ing the grain; if the bond is too strong, the grain will tend to get dull, and
if it is too weak then wheel wear will be high. If glazing occurs, the
wheel is