In vitrified bonded wheels, the natural packing of the abrasive particles leaves certain porosity [DAVS04]. Additional pore builders produce higher porosity when they sublimate or burn off during the sintering process. Pore builders are typically either hollow particles or fugitive materials [MARI07]. Hollow particles such as hollow ball corundum, glass beads, or mullite maintain a stronger wheel structure [MARI07]. Conventional fugitive pore builders include nut shell powder, sugar, starches, polymeric materials, plast granulat, naphthalene, ceralith (from rye groats), wax balls, etc. [DAVS04, HUZI12]. Fugitative pore formers allow flexible pore shapes and sizes [MARI07].
Naphthalene is a fugitive pore builder, which boils at 218 °C [GEST12]. Because naphthalene is regarded as carcinogenic, tool manufacturers attempt to substitute it with natural components, supercritical CO2, liquid CO2, or other materials [GEST12, DAVS04].
Other substances that build pores during the tool production are often organic such as wood shavings, salt, etc. Variations in pore distribution can lead to non-uniform tool shrinkage in the manufacturing process, especially for tools produced at high temperatures. This is a result of two factors. On the one hand, the effective thermal expansion coefficient of a composite body depends on the relative contents of the various components. On the other hand, denser zones with fewer pores undergo a more thorough sintering action with stronger contraction. To minimize discontinuities in the abrasive layer, the mixture of pore builder and the other ingredients should be well blended. [YARN69]
One problem of pore builders can be carbon residues [DAVS04]. Possible defects in the grinding tool arise from swelling, slumping, off-gassing, or collapsing of pores during sintering shrinkage [DAVS04, HUZI12].
3.2.2.5 Finishing
Connecting body and abrasive layer for superabrasives is explained in Sect. 5.1 “Body Concepts”. Conventional grinding wheels are machined to achieve the final dimensions and tolerances. Superabrasive tools are near-net-shape and need less effort if any.