Abrasive Grade or ‘Hardness’
The abrasive grade is a measure of bond strength commonly known as ‘hardness’. For conventional wheels, grade ‘A’ is soft and ‘Z’ is hard.

The hardness of a grinding wheel is proportional to the bond strength with which a single grain is tied to the bond matrix and depends upon the:

Abrasive Structure
The structure governs the spacing of the abrasive, and those with the tightest structure (least porosity, highest density) have a structure number of 1 or 2.

Less dense wheels numbered up to 15 have much higher porosity. Even higher numbered wheels can be achieved using the induced porosity technique during manufacture (see extra porous wheels).