Machinery Repairman

Figure 5-4.—Standard marking system for grinding wheels (except diamond).
the wheel. Since different grinding jobs require different
abrasives, you should learn this identification system
so you can identify the grinding wheel you need for a
given job. The six stations are (1) type of abrasive,
(2) grain size, (3) bond grade, (4) structure, (5) type of
bond, and (6) the manufacturer’s record symbol.
Figure 5-4 shows the six stations that identify nearly
all abrasives except diamond, which we’ll explain in
later paragraphs. Follow the stations in the figure
from left to right as you read an explanation of each
station in the following paragraphs:
1. Type of abrasive: There are two types of
abrasives: natural and manufactured. Natural
abrasives, such as emery, corundum, and diamond,
are used only in honing stones and in special types of
grinding wheels. The common manufactured
abrasives are aluminum oxide and silicon carbide.
They have superior qualities and are more economical
than natural abrasives. Aluminum oxide (designated
by the letter A in station 1) is used to grind steel and
steel alloys and for heavy duty work such as to clean
5-3
up steel castings. Silicon carbide (designated by the
letter C in station 1) is harder but not as tough as
aluminum oxide. It’s used mostly to grind nonferrous
metals and carbide tools. The abrasive in a grinding
wheel makes up about 40 percent of the wheel.
2. Grain size: Grain sizes range from 10 to 500.
The size is determined by the size of mesh of a sieve
through which the grains can pass. Grain size is rated
as follows: coarse: 10, 12, 14, 16, 18, 20, 24;
medium: 30, 36, 46, 54, 60; fine: 70, 80, 90, 100, 120,
150, 180; and very fine: 220, 240, 280, 320, 400, 500,
600. Fine grain wheels are preferred to grind hard
materials-they have more cutting edges and will cut
faster than coarse grain wheels. Coarse grain wheels
are generally preferred to remove metal quickly from
softer materials.
3. Bond grade (hardness): The bond grade runs
from A to Z, (soft to hard). It’s a measure of the
bond’s ability to hold the abrasive grains in the wheel.
A grade of soft or hard does not mean that the bond or