Hardness of Grinding Wheels (Bond Hardness)

Tip: This text can also be downloaded in PDF format (Swiss catalogue, Chapter XX, German only)

The hardness explained here has nothing to do with the hardness of the abrasive material. It relates to the behaviour of the particular bond towards external influences, not only the various bond contents (grain content, bond composition, structure and porosity, any additional pore-forming agents and production method), but also the nature of the bond playing a role. This hardness is therefore also called the static hardness, because it changes under the conditions of use – that is to say in the dynamic state. The dynamic hardness of the grinding wheel is referred to in this case. Both hardnesses are equally of importance within a grinding process. The static hardness is chosen on the basis of the process parameters. It is shown on the wheel's label, usually as a letter. The dynamic hardness, on the other hand, results from the grinding parameters, of which the cutting speed and its possible variations have the greatest influence.

The static hardness can also be defined as follows: It is the force causing the abrasive grains to break out of the bond composite. There are various methods of measuring this, but no exact standardisation in respect of procedure and allocation. NORTON was the first manufacturer to specify a scale, the so-called NORTON scale, for specifying the designation of wheel hardness, namely from A to Z. The letters from E (extremely soft) to about T (extremely hard) are important. The FEPA hardness scale emerged later from this, and nowadays is also specified under ISO.

Why is the hardness of a grinding wheel important? Depending on the process parameters (adjustable settings), a particular contact area results between the grinding wheel and the workpiece.  A grinding pressure builds up in the contact zone, this being broken down into two components, into the peripheral or tangential force and into the normal force. Both forces are determined by the instantaneous power requirement. So that it has the required degree of self-dressing – depending on its specification – the grinding wheel must now be loaded with the correct pressure in the contact zone. If this pressure is too low, the wheel merely blunts and starts to build up pressure. Dimensional errors, poor surface qualities and usually also abrasive burning (thermal damage to the peripheral zone) are the consequence. If the pressure is above the level which would guarantee self-dressing, the wheel collapses. This also manifests itself in a poor surface and very rapid loss of dimensions and form. It is clear that: The static hardness must be chosen such that the dynamic hardness in the process renders good self-dressing properties of the wheel possible.

Needless to say, dynamic adjustments can be made within certain limits via the variable settings (cutting speed, workpiece speed, infeed). It therefore also greatly depends on what changes the particular grinding machine – and in the end the overall grinding process – allows. Even the lubricating properties of the coolant emulsion used affects the hardness effect of the grinding wheel. The same wheel has a significantly harder effect if it is cooled with a high-additive grinding oil than if it is used with an organic, non-lubricated solution.

It can be clearly seen that the wheel hardness must be chosen very carefully, according to the grinding task and overall conditions.