6.7.1 Introduction
When CBN was introduced into the market in 1969, its cost naturally lent itself to being processed by wheel makers that knew how to handle expensive abrasive — namely, diamond wheel makers — using the dense hot-pressed vitrified systems described above. Unfortunately, these had none of the properties, such as chip clearance and dressability, required for high-production grinding of steels where CBN would prove to be most suited.
Furthermore, vitrified bonds used by conventional wheel makers were so reactive that they literally dissolved all the CBN into the bond by converting it into boric oxide. Grit suppliers tried
to counter this by producing CBN grains with thin titanium coatings on them. Unsurprisingly, it took 10 years and numerous false starts before porous vitrified bonds with the capability of being dressed automatically were finally presented to the market. While some manufacturers still pursued hot-pressed bonds with high fugitive or other filler content [Li 1995], the majority developed controlled reactivity cold-pressed bonds using methods common to processing of conventional vitrified wheels. Just as with conventional abrasives, it was possible to modify the bond formulations to obtain just sufficient reactivity and diffusion to create strong wetting and bonding.
6.7.2 Requirements for Vitrified CBN Bonds
The demands of vitrified bonds for CBN differ again from those for either conventional or diamond bonds. Typical wheel supplier specifications, in compliance with standard coding practice, are shown in Table 6.5. The wheel specification format is dictated by the standard practices of the diamond wheel industry. As such, the hardness is expressed as a grade letter but wheel structure is often not given, or described in only the vaguest of terms. As with vitrified diamond, concentration plays a key role in controlling the number of cutting points on the wheel face. Concentrations for CBN wheels, however, tend to be higher than in diamond wheels at up to 200 conc (50% by volume) especially for internal and many cylindrical grinding applications. This limits the structural number to a relatively narrow range.
6.7.3 CBN Wheel Structures
Typical porous vitrified CBN wheel structures are shown in Figure 6.12. In many ways, the sequence of photographs represents the development of CBN bonds over the last 30 years. The initial wheels were very dense structures with porosity levels of the order of 20%. With the high cost of CBN, performance was focused on achieving maximum possible wheel life. With the development of cylindrical grinding applications for burn-sensitive hardened steels in the 1980s, the porosity levels rose to 30%. More recently, with the rapid expansion of CBN into aerospace and creep-feed applications, porosity levels have risen to the order of 40%. Further development in this area appears key to several wheel makers, for example, Noichl [n. d].