8.4.1 Introduction
Not only the abrasive grain, but also the grinding wheel bond is increasingly subject to wear. The reason is abrasion by ground material particles, which have an abrasive effect on the binding material. With increasing wear, the bond is set back. In the case of long-chipping materials, this bond damage may occur at the grain cutting edges through the flowing chip, whereas in the case of short-chipping materials, wear occurs through a lapping process in the chip space.
For efficient grinding, new multilayer grinding wheels usually have a grain protrusion of 20% to 30% of the nominal grain diameter. This grain protrusion is necessary for the cutting process to evacuate the removed material volume and to let the cooling lubricant reach the active area.
8.4.2 Balancing Grain and Bond Wear
Constant process conditions require constant grain protrusion above the bond level. This implies a uniform grain and bond wear. Besides the specification of grain and bond, the application criteria are decisive for the wear balance. A balance occurs in the so-called self-sharpening range, when blunt grains constantly detach from the bond giving way to succeeding sharp grains as new cutting edges in the grinding process. Thus, the grinding wheel is constantly ready to work; see Figure 9.8 [Warnecke et al. 1994, Anon. 2003a].
If the grinding wheel is badly adjusted, the grain and bond wear are unbalanced. If the bond wear is too low compared to the grain wear, the grinding layer becomes blunt with insufficient grain protrusion. The process behavior is characterized by high process forces with thermal and mechanical overstress. This results in thermal damage and chatter marks on the component.
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If bond wear is excessive in a super-sharp grinding wheel, the embedded depth of the grains decreases and the grain-holding forces with it. The result is excessive radial wear of the grinding wheel, which makes the process uneconomic. Hence, the ideal bond is not that with the lowest wear, but the one with the best adjustment to grain wear.
Grinding with continuous in-process sharpening has been developed as a reaction to topographic changes in the grinding wheel during the process leading to nonstationary process behavior. This technology allows for machining processes, which, under conventional process conditions, lead to system overstress. Moreover, a specific control of the parameter level is possible for nearly all grinding tasks [Spur 1989, Tio 1990, Cartsburg 1993, Liebe 1996].