Bonded abrasives are made by incorporating abrasives grains into a matrix, which can
be resin and fillers 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.  

In order to choose the right wheel material and structure several quest ions must be asked:

• Type of use: cutting, grinding, polishing etc
• Type of article to be ground and materials: Nickel alloy blade, metal cylinder etc.
• Type of abrasive grain needed: Aluminum oxide, boron nitride etc.
• Type of specific physical property: porous, compacted etc.
• Type of bonding: verified, resin etc.
• Type of fabrication process: Cold or hot press etc.

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 grinding wheels phase diagram.

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.

           

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 more 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

Table 2: Typical characteristics of abrasive grains.

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 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 more 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 more resistant to shock and loading and heavy-duty use. They are therefore, more 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

For CGW  products go to: Abrasive products