We often get questions on how should select the correct grinding wheels. Here is a general guideline that one should follow.

Selecting the correct grinding wheel

A grinding wheel consists of three basic elements: abrasive (grains), bond and pores. The type and combination of these three elements result in very different grinding wheels.

These three categories are explained in more detail in the table below.

Abrasive

The grains should be as hard and tough as possible, to ensure maximum stock removal from the workpiece. There are four categories of abrasives:

Category	Material	Comments	Sub-categories
Corundum	Hard and soft steel, unalloyed and alloyed steels up to 63 HRc*. *Hardness Rockwell C	Most frequently used abrasive. Easily dressable.	Normal corundum, semi- noble corundum, noble corundum, single-crystal corundum, sintered corundum.
Silicon carbide	Hardened and alloyed steels up to 65 HRc, gray cast iron.	Higher diamond wear during dressing.	None
Cubic boron nitride (CBN)	High-alloy, hardened steels, tool steels.	Expensive abrasive, difficult to dress.	None
Diamond	Hard metal, glass, ceramic.	Very expensive abrasive, difficult to dress.	None

The size of the grain can vary in any of these four categories. The following generally applies: The smaller and finer the grain, the finer the surface of the workpiece will be (and vice versa). In addition, a rougher-grained grinding wheel is normally used for pre-grinding and a finer grained grinding wheel is used for finish-grinding.

Bond

The bond is necessary to hold the grains together in the wheel. It determines the hardness, strength and cut- ting ability of the grinding wheel. The following four bonds are used most frequently:

Bond	Characteristics	Suitable abrasive
Vitrified bond	Suitable for all four abra- sives, CBN and diamond only dressable under rotation.	Corundum, silicon carbi- de, CBN, diamond.
Resin bond	Conditionally dressable, must be dressed under rotation.	CBN, diamond.
Galvanic bond	Good bond for CBN and diamond, non-dressable and therefore limited working life of the grin- ding wheel.	CBN, diamond.
Metallic bond	Difficult to dress.	CBN, diamond.

Grade

The grade is the strength of bonding of a grinding wheel, and is frequently referred to as its hardness. The higher the letter designation, the stronger the bond.

Wheel Structures

Wheel structures (the spacing between grain particles) range from open to dense and vary with different grinding operations, depending upon the area of contact and type of material being ground, rate of stock removal and finish required. Carborundum offers standard structure wheels that will perform well on a wide range of materials.

Pore

The pores are determined by the relationship between grains and bond. They are responsible for the removal of chips and for transport of the cooling lubricant to the workpiece.

Pores	Characteristics
Large pore volume	Porous, cool grinding, less heat develops during grin- ding. Consequently better stock removal rate.
Small pore volume	Good edge stability (e.g. during thread grinding).

Conclusion

Soft grinding wheels should be used for hard materials and vice versa. The hardness of a grinding wheel is primarily determined by the bond. When hard materials are ground with soft grinding wheels the grains break out, so that new easy-cutting grains are used again. This is known as a «self-sharpening mechanism».

 

Source: Studer Kerber Schleifring, Art of Grinding.

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