Important mechanical characteristics are tool hardness, density, and elasticity [KUEN98]. The mechanical properties of grinding tools result from their inhomogeneous structure [QUIR80, p. 6].
Grinding tool hardness is defined as resistance of abrasives to be pulled out, so hardness is a property of the whole tool not single components [DECN70]. Tool hardness is proportional to the pull-out force [TYRO03]. Tool hardness results from the relative bond volume, the breakage strength of the bond bridges, and the retention strength of the grits within the bond [TYRO03].
A grade letter represents the hardness of an abrasive tool in the tool specification (Table 4.2). Hardness is defined in terms of bond resistance against grit pull-out due to grinding forces. Ideally, the abrasive grits are pulled out as soon as they reach a certain degree of dullness, but not sooner.
5.3.1.1 Hardness Testing with Penetration Methods
Opitz and Rumbach [OPIT42] give an overview on a multitude of hardness test methods. An easy, but subjective method is to break out grits with a hand chisel. The user will relate the breakout resistance to the wheel hardness based on his or her experience [OPIT42]. This method can be improved by measuring the applied force with a spring. During the so called “Winterling” method, a rotating blade under load gives a penetration depth that relates to wheel hardness [RAMM74].
The first sand blasting test worked with blasting of sand during a defined time (e. g. 2 min) to the grinding wheel. The weight loss of the wheel indicates the wheel hardness, but the test cannot be performed more than one time per tool [KLOC05a]. The sand blasting test method invented by Mackensen is applied on a Zeiss apparatus and known as Zeiss-Mackensen method. Here a defined volume of sand (e. g. 20 cm3) is blasted by compressed air onto the grinding wheel [OPIT42]. The depth of the generated crater relates to wheel hardness and grit breakage behavior [OPIT42]. Disadvantages of this method are potential damage to the abrasive layer, deviating results for soft and coarse-grained grinding wheels, and the necessity to assume a homogeneous body [KLOC05a].