Crush ratio can have a profound effect on the dressing action. Ishikawa and Kumar [1991] reported a study on dressing of vitrified bonded wheels containing coarse grade 80# GE 1 abrasive. They distinguished between three forms of grit fracture: “micro,” “medium,” and “macro” as illustrated in the micrographs in Figure 7.21. It was determined that at a modest qd = +0.2, there was a definite shift from predominantly a microfracture regime at ad = 1 pm to a macrofracture regime at ad = 3 pm; changes to ad as small as 0.5 pm had a significant impact on grind power and finish. Microfracture led to a high surface abrasive concentration and, therefore, a higher wheel life, but also relatively high grinding forces; macrofracture with its lower surface concentration of sharper abrasives led to lower wheel life but lower grinding energy. As the crush ratio was increased from +0.2 to +0.8, the level of macrofracture increased dramatically to dominate the process accompanied by increased bond loss. This result is important because coarse grade GE 1 abrasive was the workhorse of cylindrical grinding and it, therefore, defined the required dressing infeed accuracy and crush ratio requirements for the earliest grinders designed specifically for vitrified CBN.
These results are specific to a particular grade and size of CBN that is relatively easy to fracture. It is, therefore, to be expected that a tougher grade of CBN, or blockier shape or finer grit size would require either a higher crush ratio and/or deeper depth of cut to achieve the same degree of grit fracture. Evidence for this is suggested in the work by Takagi and Liu [1996] who found that when dressing much tougher 80# GE 500 abrasive at 5 pm depth of cut, microfracture still dominated at qd = 0.5; only at qd = 0.9 was this replaced by macrofracture.
With the introduction of several new, generally tougher, but sharper, grades of CBN within the last few years, further investigation of dressing characteristics as a function of grit size, morphology, toughness, wheel speed, and vitrified bond strength is badly needed.