Grinding wheel hardness

The term “grinding wheel hardness” does not refer to the hardness of abrasive grit, but to the ability of the bond to resist breaking off of abrasive grit.

The hardess is determined by the following factors:

1)Abrasive grit size

2)Bond ratio

3)Bond type

4)Porosity

5)Structure

Hardness is rated from A-Z with 'A' being the weakest bond and 'Z' being the strongest.   A weak bond is preferred for grinding harder materials while a stronger bond is desired for softer materials.   A typical weak bond for steel would be in the 'F, G or H' range.   A medium hardness would be in the 'I, J or K' range. And stronger bonds would be in the 'L, M, or O' range.   Hardness is dependent on the grain size, porosity, and a number of other factors.

Hardness grades are typically linear: If you increase the hardness by one letter grade (An H to and I for instance) it could give you double the wheel life.   Many people mistakenly believe that such a move (from an H to an I) would only be marginal -- Don't be misled here: A move of just one or two hardness grades could have a dramatic effect on your process!

It is important to note that it is almost impossible to match one grinding wheel manufacturer's wheel hardness to another manufacturer:   Differences in factory processes, measuring instruments and the lack of a standardized hardness system do not allow for direct cross-overs.   One company's 'G' hardness would be a 'F' with another and even a 'H' with another.   We get calls all the time on this:   We simply cannot guarantee one wheel to be the same as another.   Even when our manufacturers switch their production to a different factory in another state or country we will see some variance.   This can sometimes be considerable.