Section 13. qxd

ABRASIVE PROCESSES
13-67
In addition to wheel characteristics, grinding wheels come in a very
large variety of shapes and dimensions. They are classified as 
types,
such as type 1: straight wheels, type 4: taper side wheels, type 12: dish
wheels, etc.
The
grinding ratio
is defined as the ratio of the volume of material
removed to the volume of wheel wear. The grinding ratio depends on
parameters such as the type of wheel, workpiece speed, wheel speed,
cross-feed, down-feed, and the grinding fluid used. Values ranging from
a low of 2 to over 200 have been observed in practice. A high grinding
ratio, however, may not necessarily result in the best surface integrity of
the part.
Wheel Speeds
Depending on the type of wheel and the type and
strength of bond, wheel speeds for standard applications range between
4,500 and 16,000 surface ft/min (1,400 and 4,800 m/min). The lowest
speeds are for low-strength, inorganic bonds whereas the highest speeds
are for high-strength organic bonds. The majority of surface grinding
operations are carried out at speeds from 5,500 to 6,500 ft/min (1,750
to 2,000 m/min). The trend is toward high-efficiency grinding where
wheel speeds from 12,000 to 18,000 ft/min (3,600 to 5,500 m/min) are
employed.
It has been found that, by increasing the wheel speed, the rate 
of material removal can be increased, thus making the process more
economical. This, of course, requires special grinding wheels to with-
stand the high stresses. Design changes or improvements involve items
such as a composite wheel with a vitrified bond on the outside and a
resinoid bond toward the center of the wheel; elimination of the central
hole of the wheel by providing small bolt holes; and clamping of wheel
segments instead of using a one-piece wheel. Grinding machines for
such high-speed applications have requirements such as rigidity, high
work and wheel speeds, high power, and special provisions for safety.