Clogging or loading of the abrasive layer describes the adhesion of chips to the abrasive grits or interlocking of chips in the pore space. With more wheel clogging, the danger of thermally induced damage of the workpiece surface layer, the workpiece roughness and the grinding wheel wear rise [LAUE79]. Lauer-Schmaltz [LAUE79] pointed out that the effect of grinding wheel loading depends not only on the particle spacial size, but furthermore on the particle positioning within the abrasive surface layer.
Lauer-Schmaltz [LAUE80] and Koenig defined three loading types: Chip nests, welded chips, and grit adhesions:
• Chip nests or clustered chips are long, solitary chips in front of active cutting edges [LAUE79]. This type of loading results from mechanical clustering of
chips in the neighbored pores of active grains and does not necessarily disturb the grinding process.
• Welded chips consist of pressure welded chips and adhere, in contrary, on the active cutting edge and, thus, increase the effective cutting edge radius. As consequence, the single grit chip thickness increases. Lauer-Schmaltz [LAUE79] found areas up to 2 mm2 of the grinding wheels covered.
• Grit adhesions cover groups of grits thinly and do not impair the cutting edge shape, but only the friction properties at the flanks [LAUE79]. This type of loading was examined during the machining of chromium and nickel containing materials as well as titanium and aluminium alloys. There can be chemical reactions in the boundary layer between chips and active grains [LAUE80].
Wheel loading depends on the machined material. Pai et al. [PAI89] compared chips from grinding ductile and brittle materials and found that the chips tend to be longer in ductile machining. These longer chips can cause clogging problems of the grinding tool. In comparison, brittle materials tend to exhibit a smaller chip storage problem than ductile materials and hence allow a greater removal rate for the same available chip storage volume [PAI89].
Nagaraj and Chattopadhyay [NAGA89] concluded that chemical reactions have an effect on loading, for example, iron oxide layers are formed on machined C45 steel and prevent adhesion to the abrasive grits. Titanium alloys in contrast might form strong bonds to the corundum grits and form adhesions [NAGA89]. Pure iron led to a relatively high wear rate because of its high ductility and, consequently, chip layers [LUDE94, p. 89 ff]. Furthermore, steels with abrasive Zementit particles in a soft Ferrite matrix like C135 W induced wear due to loading [LUDE94, p. 92]. On the one hand, the Zementit crystals were pushed into the softer matrix and caused minor abrasion. On the other hand, adhesions of workpiece material on the grits resulted in high tangential forces and grit particle break-out.
Normalized steels with higher carbon content lead to less overall wear, because the increasing Perlit content decreases clogging [LUDE94, p. 93]. Nevertheless, the Zementit lamellas of the Perlit cause wear by abrasion, but to a minor proportion compared to the wear by adhesions.
To grind highly ductile material like pure iron, a grinding tool with open pores and low bond strength is advantageous, so that loaded abrasive cutting edges can break out off the abrasive layer [LUDE94, p. 102]. With higher possible material removal rate, higher grinding wheel wear goes along.
There are approaches to monitor grinding wheel wear and loading via CCD camera images [ARUN07, HEIN12]. Various texture analysis methods such as variance, energy, ASM, diagonal moment, IDM, and Ga parameter can be used to characterize texture changes, which are related to the grinding wheel surface condition. However, it is difficult to assure constant measurement conditions, such as illumination. The analysis for each grinding wheel and workpiece material has to be calibrated [ARUN07].