NUTOOL BT150 - Grinding Wheel Details; Types of Abrasive Grain; Types of Bond; Wheel Hardness (Grade)

20
GB
TABLE 1.
The four main categories of abrasive grains
Coarse
Medium
Fine
Very Fine
12
30
70
150
14
36
80
180
16
46
90
220
20
60
100
240
24
120
TYPES OF BOND.
There are five main kinds of bonding agents namely, ceramic
(vitrified), phenolic resin, rubber, shellac and metal. The
abrasive can be bonded with its particles close together or with
spaces between each grain particle. Bond material creates a
link between each grain. The spaces between the grains will
determine how dense the wheel is, large spaces will give an
open wheel and smaller spaces will give a close or dense
wheel.
The structure is measured on a scale of 0-14 with 0 being very
close and 14 being very open with wide spacing between the
grains.
WHEEL HARDNESS, OR GRADE.
Grade or hardness is the strength of bonding between the
abrasive grains. Wheel hardness is graded alphabetically, in
increasing order of hardness; letters A through D represent very
soft wheels, and letters V through Z represent very hard ones.
Letter K in the illustration below represents a soft to medium
wheel.
9A - 46 - K5 - V 22
9A
46
K
5
V
22
Kind of Abrasive
Grain Size
Grade
Density
Bond Type
Manufacturers
Symbol
The number shown in the above illustration is an example of
a grinding wheel description. 9A means that the abrasive is
aluminum oxide 46 means that the grain size is medium, K
means that the wheel is soft, 5 means that the wheel is dense,
V stands for vitrified bonding and 22 is the manufacturer’s
symbol.
Wheels supplied with bench grinders are generally “N” on the
hardness scale, which is considered hard. Do not try to sharpen
carbide tools, knives, scissors or high-speed tools (router bits
wood turning tools) with this hard wheel. A standard wheel will
not cut
carbide. If you try, you will only create heat and glaze over the
wheel. Hard wheels will burn router bits and other high-speed
tools. Overheating the tools will destroy their hardness and
render them useless. Soft wheels give themselves up while
grinding.
When grinding, only a small amount of material is removed,
and the wheel releases grains a little at a time, presenting new
cutting edges. This releasing activity helps to keep the tool from
heating up and gives a finer grinding result.
Table 2 shows wheel hardness and uses.
TABLE 2.
Wheel Hardness (Grade)
Uses for designated hardness
Description of hardness
ABCD
Not used very often
Very soft
E I F J
GK H L
Use these soft wheels to sharpen high-
speed-steel tools, such as router bits,
drills, and tool bits.
Soft to medium
MQNROSPT
Use these wheels for rough grinding on
soft materials
Medium to hard
U
This hard grade is used for dressing sticks
to dress grinding wheels.
Hard
VY W Z X
Not used very often
Very Hard