Basic Knowledge of Grinding Wheel

Ⅲ. What is abrasive grain (abrasives)?

Basic Conditions of Abrasives
Type of Abrasive materials
About Grit
Extra Information – History of Abrasive materials-

Basic Conditions of Abrasives

Hardness (Hardness of Abrasive materials)	・Harder than workpiece. ・Fracture (abrasive break apart and creat new cutting edge) against external force during grinding.
Toughness, shatter resistance.	・Resistant to deformation and fracture which received from external force during operation. ・Microfracture
Fire resistance	・Abrasives have a moderate fire resistance which prevents blunting the edge due to grinding heat.
Inertness	・Chemical inertness (no ability to react with workpiece).

Hardness and Toughness are conflicting properties. When hardness is high and *self-sharpening is performed well, whereas low toughness grains increase wasteful consumption and it will effect finishing.
* Abrasives are fallen off when these are worn down and lost sharpness, then these expose new cutting edges constantly.

Type of Abrasive materials

There are many kinds of abrasive materials.
The major types of grain and its application are as follows.

Name	Code	Color tone	Applications
Brown Alumina	A	Brown	Hand-free grinding of carbon steels, Precision grinding of carbon steels
Mono-crystalline fused alumina	HA	Ash gray	Precision grinding of alloy steels, tool steels and hardened steels
Pink Alumina	PA	Pink	Precision grinding of alloy steel, tool steel and hardened steel
White Alumina	WA	White	Precision grinding of alloy steels, tool steels and hardened steels
Zirconia alumina	AZ	Gray	Deseaming, deburring and cutting of steels
Black silicon carbide	C	Black	Grinding, precision grinding of nonferrous metals and nonmetal materials
Green silicon carbide	GC	Green	Grinding of superalloy

About Grit

Size of abrasive particles called “grit”, and it’s selected based on finishing accuracy of grinding surface.
Grit is specified in JIS R6001, it’s denoted by “F + number”.
(Example: when grain size is 36, it will be denoted as “F36”)
The lower numbers mean large spacing of abrasives.

Abrasives play the most important roles in the raw materials as described above.

Extra Information – History of Abrasive materials-

Humans have polished and ground wood, born and stone with various materials since the dawn of history.

Polishing and grinding are very familiar to us.
Various abrasives of sand stone (silica stones), emery and garnet, alumina came into use in the Stone Age, Copper Age and Iron Age respectively but all abrasives are natural stones.

In 1881, American chemist Edward Goodrich Atcheson worked on development of methods to produce artificial diamond. He heated a mixture of clay and coke in arc furnace and found some hard, big crystals. He presumed that it was a compound of carbon and corundum and called it “carborundum”, the result of chemical analysis showed that it was silicon carbide (SiC).
This was the first synthetic abrasive. The technology were implemented by the Carborundum Company and later spread around the world.

In 1897, C. B. Jacobs developed a method to produce artificial corundum by heating a mixture of bauxite (aluminum) and coke with arc furnace. This substance was named “alundum” and was commercialized by the Norton Company. Later alundum was produced worldwide.

In Japan, silicon carbide and artificial corundum was respectively first produced in 1917 and 1918 by Kagoshima tramway Co., Ltd. Although the production was suspended temporarily, many company resumed the manufacture after 1930.

Since silicon carbide grain and aluminum oxide grain were established, with the development of the industry, many countries have improved and produced a variety of artificial abrasives.
There are many kinds of alumina abrasives (WA, MA and PW etc.), AZ is a crystalline structured abrasive that is produced by fusing alumina and zirconia, is a groundbreaking abrasive which has a high toughness combined with desirable microfracture property.
Artificial abrasives have made a great progress due to practical application of cubic boron nitride (CBN) and artificial diamond, commencement of night operation in the precision machining industry.

Nowadays, several companies have developed new abrasive material called ceramic abrasive which has microcrystal structure. It manufactured by chemical syntheses and not by fusion method, and has already been put to practical use in some industries.

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