Grinding wheels are composed of thousands of small abrasive grains held together by a bonding material. Each abrasive grain is a cutting edge. As the grain passes over the work piece it cuts a small chip, leaving a smooth, accurate surface. As each abrasive grain becomes dull, it breaks away from the bonding material.

 

Types of abrasives

 

The manufactured abrasives most commonly used in grinding wheels are Diamond, cubic Boron Nitride, Silicon Carbide and Aluminum Oxide.

 

Diamond: Two types of diamond are used in the production of grinding wheels: natural and manufactured. Natural diamond is a crystalline form of carbon, and very expensive. In the form of bonded wheels, natural diamonds are used for grinding very hard materials such as cemented carbides, marble, granite and stone.

Recent developments in the production of manufactured diamonds have brought their cost down and led to expanded use. Manufactured diamonds are now used for grinding tough and very hard steels, cemented carbide and aluminum oxide cutting tools.

 

Cubic boron nitride (CBN): Cubic boron nitride is an extremely hard, sharp and cool cutting abrasive. It is one of the newest manufactured abrasives and 2.5 times harder than aluminum oxide. It can withstand temperatures up to 2,500°F. CBN is produced by high-temperature, high-pressure processes similar to those used to produce manufactured diamond and is nearly as hard as diamond

 

Silicon carbide: Silicon carbide wheels are harder and more brittle than aluminum oxide wheels. There are two principal types of silicon carbide wheels: black and green. Black wheels are used for grinding cast irons, non-ferrous metals like copper, brass, aluminum, and magnesium, and nonmetallics such as ceramics and gemstones. Green silicon carbide wheels are more friable than the black wheels and used for tool and cutter grinding of cemented carbide.

 

 

Aluminum oxide: General-purpose aluminum oxide wheels, usually gray and 95 percent pure are the most popular abrasives used. They are used for grinding most steels and other ferrous alloys. They are used for grinding most steels and other ferrous alloys. White aluminum oxide wheels are nearly pure and are very friable (able to break away from the material easily.) They are used for grinding high-strength, heat-sensitive steels.

 

Types of bonds

 

Abrasive grains are held together in a grinding wheel by a bonding material. The bonding material does not cut during grinding operation. Its main function is to hold the grains together with varying degrees of strength. Standard grinding wheel bonds are vitrified, resinoid, silicate, shellac, rubber and metal.

 

Vitrified bond: Vitrified wheels are strong and rigid. They retain high strength at elevated temperatures and are practically unaffected by water, oils or acids. One disadvantage is that they exhibit poor shock resistance. Therefore, their application is limited where impact and large temperature differentials occur.

 

Resin bond: Resin bonded grinding wheels are second in popularity to vitrified wheels. Phenolic resin in powdered or liquid form is mixed with the abrasive grains in a form and cured at about 360F. Resin wheels are used for grinding speeds up to 16,500 SFPM. Their main use is in rough grinding and cut-off operations.

 

Metal bond: Metal bonds are used primarily as binding agents for diamond abrasives. They are also used in electrolytic grinding where the bond must be electrically conductive.

 

Abrasive grain size

 

The size of an abrasive grain is important because it influences stock removal rate, chip clearance in the wheel and surface finish obtained.

Abrasive grain size is determined by the size of the screen opening through which the abrasive grits pass. The number of the nominal size indicates the number of the openings per inch in the screen. For example, a 60-grit-sized grain will pass through a screen with 55 openings per inch, but it will not pass through a screen size of 65. A low grain size number indicates large grit, and a high number indicates a small grain.

 

Grain sizes are broadly defined as coarse (6 to 24), medium (30 to 60), fine (70 to 180), and very fine (220 to 1,000.) Very fine grits are used for polishing and lapping operations, fine grains for fine-finish and small-diameter grinding operations. Medium grain sizes are used in high stock removal operations where some control of surface finish is required. Coarse grain sizes are used for billet conditioning and snagging operations in steel mills and foundries, where stock removal rates are important and there is little concern about surface finish.

 

Grinding wheel grade

 

The grade of a grinding wheel is a measure of the strength of the bonding material holding the individual grains in the wheel. It is used to indicate the relative hardness of a grinding wheel. Grade or hardness refers to the amount of bonding material used in the wheel, not to the hardness of the abrasive.

 

The range used to indicate grade is A to Z, with A representing maximum softness and Z maximum hardness. The selection of the proper grade of wheel is very important. Wheels that are too soft tend to release grains too rapidly and wheel wear is great. Wheels that are too hard do not release the abrasive grains fast enough and the dull grains remain bonded to the wheel causing a condition known as "glazing."

 

 

Grinding wheel selection

 

Before attempting to select a grinding wheel for a particular operation, the operator should consider the following six factors for maximum productivity and safe results:

 

Material to be ground: If the material to be ground is carbon steel or alloy steel, aluminum oxide wheels are usually selected. Extremely hard steels and exotic alloys should be ground with cubic boron nitride (CBN) or diamond. Nonferrous metals, most cast irons, nonmetallics, and cemented carbides require a silicon carbide wheel. A general rule on grain size is to use a fine grain wheel for hard materials, and a coarse grain wheel for soft and ductile materials.

 

Nature of the grinding operation: Finish required, accuracy and amount of metal to be removed must be considered when selecting a wheel. Fine and accurate finishes are best obtained with small grain size and grinding wheels with resinoid, rubber or shellac bonds.

 

Area of contact: The area of contact between the wheel and work piece is also important. Close-grain spacing, hard wheels and small-grain sizes are used when the area of contact is small.

 

Condition of the machine: Vibration influences the finish obtained on the part as well as wheel performance.

 

Grinding wheel speed: Wheel speed affects the bond and grade selected for a given wheel. Wheel speeds are measured in surface feet per minute (SFPM). Vitrified bonds are commonly used to 6,500 SFPM or in selected operations up to 12,000 SFPM. Resinoid-bonded wheels may be used for speeds up to 16,500 SFPM.

 

Grinding pressure: Grinding pressure is the rate of in-feed used during a grinding operation; it affects the grade of wheel. A general rule to follow is that as grinding pressures increase harder wheels must be used.