Tc 9-524 fundamentals of machine tools

GRADES OF HARDNESS
The grade of a grinding wheel designates the hardness of the
bonded material. Listed below are examples of those grades:
A soft wheel is one on which the cutting particles break
away rapidly while a hard wheel is one on which the bond
successfully opposes this breaking away of the abrasive
grain.
Most wheels are graded according to hardness by a letter
system. Most manufacturers of grinding abrasive wheels
use a letter code ranging from A (very soft) to Z (very
hard). Vitrified and silicate bonds usually range from very
soft to very hard, shellac and resinoid bonds usually range
from very soft to hard, and rubber bonds are limited to the
medium to hard range.
5-6

---

TC 9-524
The grade of hardness should be selected as carefully as
Figure 5-8 illustrates sections of three grinding abrasive
the grain size. A grinding abrasive wheel that is too soft
wheels with different spacing of grains. If the grain and bond
will wear away too rapidly, the abrasive grain will be
materials in each of these are alike in size and hardness, the
discarded from the wheel before its useful life is
wheel with the wider spacing will be softer than the wheel
realized. On the other hand, if the wheel is too hard for
with the closer grain spacing. Thus, the actual hardness of
the job, the abrasive particles will become dull because
the grinding wheel is equally dependent on grade of hardness
the bond will not release the abrasive grain, and the
and spacing of the grains or structure.
wheel’s efficiency will be impaired.
GRINDING WHEEL ABRASIVE
ABRASIVE WHEEL STRUCTURE
MARKINGS
Bond strength of a grinding wheel is not wholly dependent
Every grinding wheel is marked by the manufacturer with a
upon the grade of hardness but depends equally on the
stencil or a small tag. The manufacturers have worked out a
structure of the wheel, that is, the spacing of the grain or its
standard system of markings, shown in Figure 5-9.
density. The structure or spacing is measured in number of
grains per cubic inch of wheel volume.
For an example use a wheel marked A36-L5-V23. The A
refers to the abrasive which is aluminum oxide. The 36
represents the grain size. The L shows the grade or degree of
hardness, which is medium. The 5 refers to the structure of the
wheel and the V refers to the bond type.