Grinding is a method of removing excess material in order to shape or smooth manufactured articles. It is usually done with a grinding wheel, which contains hard sharp particles that act as millions of cutting tools when the wheel rotates at high speed. Fine grinding can produce a finished surface, but additional finishing processes are required in many cases.
Grinding wheels are made of abrasives, or tiny cutting particles, held together by materials called bonding substances. For example, for thousands of years, grindstones have been made out of a naturally occurring mixture of tiny quartz crystals embedded in sandstone.
In modern wheels different abrasives are used. Silicon carbide crystals are used in wheels to grind most non-metals, such as glass and marble, and softer metals, such as brass and copper. Aluminum oxide crystals are less brittle than those of silicon carbide and are used on stronger metals, such as steel and wrought iron. Diamonds are sometimes used to grind extremely hard substances, such as quartz.
Grinding wheels are made by mixing an abrasive with a bonding substance. This mixture is then molded into the desired shape and made to harden. The most commonly used bonding substance is clay. Clay-bonded
Grinding a tapered shape on a grinding machine wheels are fired in an oven in a process, called vitrifying, which makes the clay hard and glasslike. These wheels are suitable for most grinding purposes.
Other bonding substances are sodium silicate, shellac, rubber, and synthetic resin. Sodium silicate wheels are suitable for low-speed grinding. Shellac-bonded wheels are suitable for producing a fine surface. Rubber and resin-bonded wheels are used for high-speed grinding and the rapid removal of material.
Grinding wheels are classified according to the type of abrasive, the type of bonding substance, the size and density of the abrasive particles, and the hardness of the bond. They are manufactured in various sizes and with different cross sections, such as straight, tapered, and recessed.
Grinding wheels are easiest to maintain when they are properly selected and used. Most grinding wheels are self-sharpening if the bond is of the correct type and strength. The cutting edges of the abrasive particles will break out of the wheel as they become dull, exposing fresh cutting edges underneath. Because a hard material will dull the edges more quickly than a soft material, the bonding for grinding wheels to be used on a hard material should be soft so that the particles can break away as soon as they are dull. When grinding is done on a soft material, the bonding should be hard so that the particles will not break off too soon.
The grinding wheel must remain circular and centered on its axis, or it will grind unevenly. In one operation, called trueing, a diamond cutting tool is used to trim enough material off the wheel to restore it to its required shape. Another process, called dressing, removes dulled edges and caked dust from the wheel's cutting face. Dressing must usually be followed by trueing.
While they are being used, grinding wheels are usually continuously sprayed with oil or a water emulsion. The spraying cools the wheel and the object that is being ground. In addition it washes away dust and chips that are formed during the grinding.
Aside from hand grinding machines, which are simply rotating grinding wheels against which simple work can be held by hand, three main kinds of grinding machines are used in industry: surface, cylindrical, and internal grinders.
Surface grinders have wheels mounted on a horizontal or vertical axis, and they are used to grind flat surfaces. A wheel on a horizontal axis grinds with its edge. A wheel on a vertical axis grinds with its circular face. The table that holds the work rotates or reciprocates in order to expose a large work surface to the grinding wheel.
Cylindrical grinders are used to grind the outer surface of shafts, axles, pistons, and other cylindrical objects. They are also used to grind some tapered parts. The workpiece is rotated and slowly moved past the grinding wheel.
Internal grinders are used to grind the insides of tapered or cylindrical holes, such as the cylinders of an engine. The grinding wheel rotates around its own axis and the axis of the hole. The wheel also moves up and down the depth of the hole.
Special grinders are usually variations of one or more of the above types. Grinders have been developed for such tasks as producing threads on screws and cutting teeth on gears. Many grinding operations have recently become partly or completely automatic.