The distinction made between “hard” and “soft” grinding wheels refers to the bond structure, whose job is to retain the abrasive particles responsible for cutting. Regardless of the label, softer bonds are known to produce a more aggressive cut than harder bonds. Keeping this in mind, let us explore how the bond structure impacts the grinding process.
The main difference between softer bonds (like resin) and harder bonds (like metal) is the abrasive renewal rate. This refers to the amount of time it takes for the bond to wear down, releasing the dulled abrasive particle on the surface and revealing new, sharp grit for cutting. Softer bonds wear more quickly, thus revealing new grit more often. This allows for more aggressive cutting, whereas harder bonds will retain the same surface particles for longer, resulting in an inability to remove large amounts of material at a quick rate.
Ranking from softest to hardest, resin is known as the softest bond, followed by polyimide. However, abrasive concentration can have a hand in the hardness of the wheel. For example, a C50 abrasive concentration wheel would be softer than a C100 with the same bond. The run speed can also affect the relative hardness of the wheel. A higher RPM can make a soft bond act like a harder bond, and vice versa. Finally, coolant (or lack of coolant) plays a key role in the performance of a grinding wheel. Harder bonds work best in water-based coolant applications, while softer bonds are best for oil- based coolants.
Softer bonds work best for dry grinding and oil-based coolant applications and can be used in centerless grinding and CNC tool manufacturing. Harder bonds like metal and hybrid are often used in ceramic and glass working, for grinding single edged tools and fluting processes.
In conclusion, the wheel needs to be formulated based upon the grinding application, material being ground, machine type and power and the coolant type. Speaking to an application engineer will ensure the correct wheel bond and concentration is used for consistent grinding results.