Grinding wheels are self-sharpening over time. It seems contradictory as dressing is required after certain period of time to re-sharpen the wheel.
In order to clarify this contradiction, it is essential to understand the chemistry of grinding wheels, grinding forces and the external factors affecting the grinding wheel surface. Grinding wheels contain abrasive grains that are held together by a bonding agent. Each individual abrasive grain acts as a cutting tool to remove the workpiece material.
As the grinding wheel applies forces to remove material from the work piece, the work piece applies resistance forces to the grinding wheel. Overtime, these forces lead to fracturing of abrasive grain from the bond in the case of vitrified and metal bond wheels.
In the case of organic and resin bond wheels, new abrasive grains are released under heat. The resultant is a release of dull grains and exposure to the new sharp grain beneath it.
Now the question arises, why does it seem like the grinding wheel dulls over time instead of self-sharpening. It is because the face of the grinding wheel is covered in swarf over time and the porous structure of the wheel is loaded up, which hinders the grinding action.
This swarf forms a layer between the abrasive particles and the work piece. To regenerate the sharp surface of the grinding wheel and eliminate the loading, the swarf needs to be removed.
This is the reason why dressing is such an important aspect of grinding. It eliminates all the impurities from the grinding wheel face and allows the abrasives to cut the work piece efficiently.