Abrasive wheels are normally made of aluminum oxide and silicon carbide.
This rock-forming mineral is a chemical compound of aluminum and oxygen. Because of its hardness, it can scratch almost all other materials, that’s why it is mostly used for steel applications. It is universally indicated with “A”.
It is the most common aluminum oxide abrasive. It is highly resistant and strong, ideal for heavy-duty work such as snagging steel castings, grinding of welding seams, iron and steel cutting. Its crystal structure and fracture characteristics make it suitable for high-precision deburring and big stock removal. It is also used for tools sharpening.
It is particularly suited for grinding all kinds of steels. It has a friable structure and it is used for applications where high precision is required, such as for sharpening high-speed steels and cast alloy tools and cutters, as well as for cylindrical, surface and internal grinding of tools, dies and gages.
Universally indicated with “C”. It is better suited for grinding very hard and dense materials like cemented carbides and ceramics, most non-ferrous metals and non-metallic materials such as glass, rubber and stone, as well as cast iron.
WHAT TO CHOOSE
As a rule, large surfaces require the use of coarse grits and abrasives of medium-soft hardness. Vice versa, small surfaces require the use of fine grits and abrasives of higher hardness.
Three things are important:
1) type of abrasive; 2) grit dimensions; 3) disc hardness.
1) TYPE OF ABRASIVE
Characteristics of the material to be grinded. Abrasive to use.
Steel in general, other ferrous materials, hard bronze, Aluminum oxide in its different types.
cast iron (it depends on the type and the application).
Metallic carbides, cast iron (it depends on the type and the Silicon carbide in its different types.
application), brass and soft bronze, copper, aluminum and
its alloys, stainless steel, special alloys, non-metallic materials.
2) GRIT DIMENSIONS
Soft and non-treated materials are easily grinded with coarse grit discs, while hard, fragile and treated materials require the use of a finer grit wheel, because the depth of penetration is limited, so a more adequate cutting action is achieved through the removal of numerous small shavings.
3) DISC HARDNESS
As a rule, hard and treated materials require the use of softer grits. As these materials resist the abrasive grain penetration, they get easily blunted. If the wheel is soft, the grains unravel by fracturing and releasing the underlying new sharp granules to form new cutting points extending the life of the abrasive. Soft and non-treated materials, conversely, can be operated with harder discs.
A wheel that is too soft, that is too aggressive and sharp, does not perform to obtain good finishes.
An excessively hard wheel, however, tends to create burn marks on workpieces and produces vibrated and marbles surfaces.