Bonded abrasives are made by incorporating abrasives grains into a matrix, which can
be resin and filler or vitreous materials. Usually, such abrasives contain porosity which
helps to control the thermal aspects of the process.
Coated abrasives are abrasive grains incorporated or cloth backing.
The formulation and choice of additives are usually a closely guarded secret of the manufacturer.
The structure of the bonded abrasives is described in terms of proportions of
abrasive phase, the volume of porosity and the volume fraction of the bond as described
in Figure below.
Each axis represents 100% of one component with the opposite side corresponding to 0%
and intermediate percentages represented by the fractional distance from the side to the axis.
Actual wheels compositions do not cover the whole composition range represented
by the phase diagram but are restricted to a limit range by technological and practical
factors.
grinding wheels phase diagram
Grains:
Abrasive grains are the tools of the grinding and cutting process.
In the past abrasives grains were derived from natural minerals, such as emery,
quartz sand, and so on, today, the industry is almost entirely dependant upon synthetic
grains, which are far 更多consistent and cost-effective.
Some physical properties of the most important abrasives materials are
Summarized in Table 1.
Table 1: Physical properties of main abrasive grains
|
Property |
Aluminum Oxide |
Silicon Carbide |
Boron Nitride |
Diamond |
|
Crystal Structure |
Hexagonal |
Hexagonal |
Cubic |
Cubic |
|
Density (g/cm3) |
3.98 |
3.22 |
3.48 |
3.52 |
|
Melting Point (C) |
2040 |
~2830 |
~3200 |
~3700 |
|
Knoop Hardness (kg/mm2) |
2100 |
2400 |
4700 |
8000 |
The characteristics of the grains as size, shape, strength, friability are important
and control the wheel's performance as the matrix material.
Table 2: Typical characteristics of abrasive grains.
|
Grain |
Knoop Hardness kg/mm2 |
Friability Index |
|
Aluminum Oxide
Pink/red (3% Cr)
White
Single Crystal
Regular
40% ZrO2
Sintered |
2260
2120
2280
2040
1460
1370 |
65.0
56.6
47.7
35.6
7.9
6.5 |
|
Silicon Carbide
Green
Black |
2840
2680 |
62.5
57.2 |
Fused Aluminum Oxide and its derivatives:
Aluminum Oxide is derived from Bauxite in an industrial high-energy process.
During the melting stage the crystalline structure as well as the chemistry can be
controlled and results in pure crystal Aluminum oxide, white fused Aluminum oxide
(low Iron and Silica content), Semi-friable Aluminum oxide (low titania levels),
Monocrystalline Aluminum oxide (strong cutting edges and a high compressive strength)
and Red and Pink Aluminum Oxide (high Chrome levels).
Applications: For grinding high tensile strength materials, as well as rough grinding,
deburring, snagging, cutting and fettling of low-alloy, ferrous materials (Regular
Aluminum Oxide).
For grinding hardened and high alloy steels up to 62 Rc (White Aluminum Oxide).
For high-alloy steels, grinding operations with high form and dimensional accuracy requirements (Monocrystalline Aluminum oxide).
Aluminum Oxide-Zirconia:
The production of aluminum-oxide Zirconia grains involves similar equipment and
process to that of standard Aluminum Oxide. The presence of Zirconia phase and
its crystal size can improve the toughness of the grain compared to white or brown
Aluminum Oxide.
Applications: Used in steel mills and foundries.
Sol-Gel
Sol-Gel Aluminum Oxide and derivatives:
Aluminum Oxide produced by the sol-gel technique has very fine crystals and therefore
retains sharp cutting edge as the material wear. One of the disadvantages of the Sol-gel Aluminum Oxide is it low thermal stability with respect to fused
Aluminum Oxide.
Applications: Used on both ferrous and non-ferrous materials
Silicon Carbide:
The material is harder than Aluminum Oxide, but it is usually much 更多friable. The manufacturing process makes controlled use of high energy to obtain green or black
silicon carbide. The black silicon carbide has a higher impurity content than the
green grain.
Applications: For grinding low tensile materials like cast iron, non-ferrous and non-metallic.
Resin based:
Phenolic thermosetting resins are still by far the most important organic bonds for grinding
and cutting wheels. The properties of the bond can be varied to give elastic
or thermal properties by incorporating plasticizers or fillers.
Compared to vitrified wheels, resin-bonded wheels are much 更多resistant to shock and loading and heavy-duty use. They are therefore, 更多popular in snagging operations and
for cut-off wheels where uncontrolled lateral stresses can be very high. When extremely high strength is required, for example in high-speed operations (>100 meter per second surface speed), then the resin may be further reinforce with glass fiber. There are limitations,
however, in terms of temperature that can be permitted
at grinding/cutting surface. Also, resins wheels can be attacked by alkaline grinding fluids.
Vitreous based:
Compared to resin-bonded wheels, those with vitrified bonds can withstand much higher temperatures, but they are often significantly less tough. Also, the vitrified wheels are unaffected by water, acids and oils.
The essential raw materials are clay, feldspar and glass frits, similar to those in the ceramics whitewares sector. The vitreous bond phase is made during the firing of
the wheel at high temperature up to 1250 degrees C and higher.
Fillers:
Fillers are added into the mixture for different purposes as described in the table below:
Table 3: Fillers Type and functionality
|
Function |
Description |
Filler Material |
|
Production of pores |
Temporary
Semi-permanent
Permanent |
Nut shell, Wax, Naphthalene
Graphite, coke
Glass balls, Spherical aluminum oxide |
|
Active grinding effects |
Lubricants |
Cryolite, Chlorides, Iron Pyrites, Phosphates |
|
Production aids |
Curing aids
Pressing aids |
Calcium Oxide, Magnesium Oxide
Stearates, Wax |
