The percentages of grit volume, VG, bond volume, VB, and pore volume, VP, add up to 100 % (Eq. 6.1). The mass, m, of the abrasive layer is composed of the grit mass and bonding mass, defined by their respective densities, pG and pB (Eq. 6.2).
Vg + Vb + Vp = 100 %
m = mG + mB = PgVg + PbVb
The volumetric composition of grits, bond and pores can be displayed in ternary diagrams (Fig. 6.1). The phase diagram in Fig. 6.1 displays the lines with iso-properties, such as iso-grit volume, iso-bond volume, iso-porosity, and iso-grit-bond ratio [DECN70, MALK08, p. 31 ff]. The iso-grit lines are often considered as “iso-structure lines” [DECN70]. Moreover, the iso-grit lines commonly define the packing number in conventional wheel designations or in the concentration number for superabrasive wheels [MALK08, p. 32].
Maximum packing density is obtained by shaking and pressing the grinding wheel mixture before hardening or sintering; a tool with lower packing density still needs to have enough grit contact, so that the tool does not loose its shape during hardening or sintering [DECN70].
Lines with iso-grit-bond ratio in Fig. 6.1 all pass through one tip of the ternary phase diagram, Vp, where the abrasive layer theoretically has 100 % porosity [DECN70]. The maximum grit-bond ratio with the minimum bond equivalent is defined by strength requirements of the abrasive tool body; minimum grit-bond ratio is imposed by practical manufacturing experience [DECN70].
Several researchers have displayed the most common ranges of grinding tool compositions in ternary diagrams (Fig. 6.2) [KLOC09, p. 45, MENA00, MARI07, p. 111, MALK08, p. 31]. The boundaries for these ranges can be overcome by adding artificial pore builders or using hot pressing methods [KLOC09, p. 45]. In
Fig. 6.1 Phase diagram of abrasive layers with iso lines [MALK08, p. 31 ff, DECN70]
general, superabrasive grinding wheels have a lower volumetric percentage of abrasive grits [METZ86, p. 52].
The simplified diagram in Fig. 6.2 right suggests that the hardness coincides with the iso-porosity lines for the shown grinding wheel brand [MENA00]. For most grinding wheels, however, hardness grade does not usually coincide with the iso-porosity lines and wheel manufacturers have more complex hardness lines in the ternary diagram [MALK08, p. 33, DECN70]. Grit concentrations for CBN tools tend to be higher than for diamond wheels (up to 50 % by volume), especially for internal and many cylindrical grinding applications [MARI04, p. 419]. Therefore, the structure number of CBN wheels is limited to a smaller range [MARI04, p. 419].
Higher bonding proportion with a constant grit proportion leads to thicker bonding bridges holding the abrasive grits tighter (Fig. 6.3 (a) and (b)). This results in increasing wheel strength, Young’s modulus, hardness, and density [KLOM86].
The amounts of grit and bond content can be calculated either by keeping the bonding specification constant and varying the grit concentration or by varying both bonding specification and grit concentration. A common approach is to calculate the grit volume, Vg, from Eq. 6.3 corresponding to the structure number, Ns [BORK92, p. 35, MALK08, p. 16].
Vg = (62 — 2NS)% (6.3)
Ns structure number in the ranges of 0-4 (close structure), 5-8 (medium), 9-14 (wide structure)
Achieving balanced composition is a complex task (Fig. 6.3). Increased bond volume at constant grit volume leads to stronger bond bridges and raises the overall bending resistance and elasticity [BOTS05, p. 92]. A high bond volume might lead to a high proportion of grit splintering resulting from the strong retention force [BOTS05, p. 93]. A bigger grit volume implies more active cutting edges and decreases the load for the single grit resulting in less wear but increased friction.
Homogeneous distribution of abrasive grits, bonding material, and pores in the abrasive layer is crucial for a constant process performance. A non-uniform distribution of abrasive material leads to an uneven material removal process and respectively to a change in the chip thickness. This results in varying loads on the grits, affecting generated workpiece surface as well as wear behavior of the grinding wheel [KLOC05c].
Bot-Schulz published micro-computer-tomography (CT) pictures of a vitrified grinding tool sample for the first time [BOTS05, p. 108 f.]. Challenges lie within the resolution of the computer tomography microscope and the small differences between the densities of the vitrified bonding, the sol-gel corundum grits, and the white corundum grits as secondary grit material. Micro-CT pictures enable the realistic modeling of the bonding bridges between grits [BOTS05, p. 108].