Pores are necessary for the transport of cooling lubricant to and chips away from the cutting point. They become more critical for high material removal rates and high-speed grinding processes to get enough cooling lubricant into the grinding
gap. Grinding wheels with discontinuous cutting faces have similar effects as highly porous wheels [BORK92, p. 36].
Porosity can be influenced by the volumetric composition of abrasives and bond material via Eq. 6.1. In principle, there are two ways to actively create porosity [YARN69], by either substances burning up during tool manufacturing, or hollow substances breaking up during the abrasive machining process.
The proportion of pores varies by grinding wheel bond type. Resin bonded wheels have nearly no porosity, whereas vitrified tools can have porosity up to 70 % of volume [MARI07, p. 113]. The pores in metallic bonded grinding wheels cannot hold the cooling lubricant as well as pores in vitrified bonded tools. The active generation of pores is explained in the manufacture of vitrified bonded tools (Sect. 3.2.2 “Manufacturing of Vitrified Bonds”) and sintered metallic bonded tools (Sect. 3.3.3 “Manufacturing of Metallic Bonds by Sintering”). Super-porous grinding wheels can have pores exceeding the grit size by several times [BORK92, p. 35].
Porosity of abrasive tools affects their mechanical strength. Yarnitsky [YARN69] states that an increase in porosity by 20 % can reduce the modulus of elasticity to about 60 % of its original value. The tool becomes softer and the effective dynamic hardness is affected. Pores can be seen as a discontinuity in the abrasive tool material bringing about a stress concentration. The latter can lead to cracks and fatigue fracture as consequence. The effect of the discontinuity, however, depends on the bonding system, geometric pore structure, pore size, uniformity and distribution. In brittle (vitrified) bonds, the effect of material discontinuity is worse than in soft bonds like plastic or bronze. Moreover, spherical pores are preferable to elongated or sharp-edged pores. A varying pore concentration in the abrasive layer leads to changing properties such as non-uniform density. Softer zones will wear quicker and can even cause tool destruction. In addition, heat conductivity is lower in zones with higher porosity. [YARN69]
The volumetric percentage of porosity in a grinding tool can be determined by several methods:
• The porosity can be calculated from the weight of the grinding wheel compared to the theoretical weight of the pore-free tool [YARN69].
• The archimedic principle can be applied if it is assumed that all pores are hollow and connected to the surface [DAUD60]. The ascending force relates to the weight of grits and bonding. The amount of remaining air in the grinding tool can be reduced by heating of the water, slowly submerging of the tool, and adding agents that reduce surface tension [DAUD60]. This method is only useful for conventional tools.
• The surface of the pore cross-sections can be measured on photographs of the wheel [YARN69]. The quotient of pore surface area to total cross-sectional area gives the porosity. This method can be applied to superabrasive tools.
Remarkable trends in changing porosity are “ultrahigh porosity vitrified wheels” [MARI07, p. 113] resulting from long needle-shaped grits (see Sect. 4.1.2 “Special Grinding Wheel Types”) and lubricated vitrified wheels (see Sect. 9.2.2 “Developments in Tool Design”).
It is common practice to use a second type of abrasive grits, which might or might not be displayed in the tool specification. In resin bonds, secondary grits decrease bond wear. In vitrified bonds, secondary grits are fillers in the bond or take part in chip formation. Combining primary grits with secondary grits can have a synergistic effect [HAY90]. During manufacturing, secondary grits increase mold packing density [WEBS04]. Mixing different sizes of abrasive grits and bond material has the same effect.
Adding a secondary abrasive can reduce the cost of the grinding wheel, for example in the case of pricier sintered corundum [HAY90]. Secondary grits for sintered corundum can be fused alumina, co-fused alumina zirconia, SiC, BC, garnet, emery, flint, CBN, diamond, or mixtures thereof [HAY90, WEBS04]. Secondary grits for vitrified bonded CBN can be Al2O3 and/or SiC and work mainly as filler and support grits [LINE92, p. 39]. Corundum grits are likely to be etched by the vitrified bond [LINE92, p. 39].