Abrasives

CHAPTER FOUR
ABRASIVES

Notes made while reading Audel‘s Machine Shop Basics visit them at www.audelbooks.com
Grinding is relativity a new art with most of the development having taken place since the 1890’s. The definition of an abrasive is a substance (i.e. sandpaper, emery) used for grinding polishing etc. Before this, the only abrasives were prepared from sandstone. Because of harder metals and alloys, abrasives that are more efficient were needed.

Natural Abrasives

Emery and corundum are hard natural metals, naturally occurring in aluminum oxide, corundum has larger crystals and contains fewer impurities. These materials were used in grinding wheels until the turn of the century, since then more efficient man-made material started being used.

Manufactured Abrasives

As the need, grinding increases so did producing abrasives whose quality is controlled. Progress has made in the development of bonding materials when ceramic clays and pottery kiln used in grinding wheel manufacture.

Composition of Abrasives

At first emery was used in grinding wheels, it was not until silicon carbide, aluminum oxide, and diamonds were developed to replace emery that true progress was made. Silicon carbide is a mixture of clay and powered coke originally used to polish precious gems. Aluminum oxide or Bauxite was fused electrically into a hard material similar to emery and corundum. Its one advantage is it can be produced in uniform grade and higher purity, 93% to 94% Aluminum oxide. Today Aluminum oxide is used 75% of all grinding wheels of one type or another.

Diamonds

The ability to synthesize diamonds in the laboratory was achieved in 1955, prior to this natural diamonds were used in grinding wheels. The need for diamonds was created by tungsten carbide a material so hard ordinary grinding wheel would not work. Today diamonds wheels are used for grinding carbide, ceramics, glass, stone and some tool steels.
Use of Abrasives in Grinding Wheels

Grinding is the process of disintegrating a material and reducing it into small particles of dust by crushing or attrition. Here are some of the newer abrasives used in grinding wheels.
Silicon Carbide Abrasives

Silicon carbide is made of pure silica and carbon in the form of finely ground coke. When heated in an electric furnace silicon carbide crystals are formed, these are mainly used in a large variety of abrasive and refectory products. Here are some brand names

crystolon
carborundum
carbolon
carbonite
Aluminum Oxide Abrasives

The source of aluminum oxide (Al/2O/3) is Bauxite, which is aluminum, combined with water and varying amounts of impurities. Manufacture of aluminum abrasive is done with Arc-type electric furnaces. The finished product is called a “pig” of crystalline aluminum oxide. Here are some brand names of aluminum oxide abrasives:

Alundum
Aloxite
Lionite
Borolon
Exolon

Diamond Abrasive

Both natural and man-made diamonds have fields of application in which they excel. Diamond grinding wheels are made in the following three different bond types.

Resinoid
Metal
Vitritied