Ultrasonic-assisted grinding with an oscillating wheel is divided into different process modifications. In the case of ultrasonic-assisted cross-peripheral grinding, the feed movement takes place vertically to the tool spindle axis and thus to the generated oscillation. With this process, grooves, gaps, and radii can be machined in brittle-hard materials. The feed movement in the case of ultrasonic-assisted face grinding is, however, parallel to the tool axis. Bores may be machined with this process.
20.6.1 Ultrasonic-Assisted Cross-Peripheral Grinding
During cross-peripheral grinding, the cutting edge engagement is not interrupted because the peripheral side of the wheel is in contact with the workpiece. Rather, a sinusoidal grain engagement can be observed as a result of the axial oscillation. The engagement at the front face of the wheel takes place according to the movement conditions described for the peripheral longitudinal grinding with radial excitation, leading to local individual engagements.
Figure 20.11 shows the achievable surface topographies and roughnesses after ultrasonic — assisted cross-peripheral grinding of different ceramics. Surface roughness achieved corresponds to values after conventional grinding.
Under certain kinematic conditions, the result of ultrasonic movement vertically to the workpiece surface is a complete interruption of the workpiece-tool contact. On all materials, typical pocket-type surface structures can be observed as an effect of the axial ultrasonic movement. The topography depends on the fracture toughness of the machined material. Bending strengths of components ground conventionally or with ultrasonic assistance are similar, although higher residual compression is imparted to the subsurface of the material through the changed stress in ultrasonic — assisted grinding [Engel and Daus 1999].
Figure 20.12 shows the achievable material-removal rate during ultrasonic-assisted cross-peripheral grinding of different ceramic materials. The feed speed was controlled in the tests by the determination of a maximum process force in the direction of feed. Smaller process forces lead to higher feed speeds and thus to growing material-removal rates. In contrast to ultrasonic-assisted grinding with constant feed speed, this kind of process control prevents inadmissibly high stresses on the tool.
As a result of the ultrasonic superposition of the grinding process, higher material-removal rates can be stated for all investigated materials and used diamond grain sizes of the grinding tools. It can be seen that the diamond grain size is not decisive for the achievable material removal rates during ultrasonic — assisted grinding. There is a correlation between this statement and investigations by Pei and Ferreira [1999], who observed this behavior during the machining of zirconium oxide. It becomes clear that the mechanical properties of the machined material have a significant influence on the machining result.
The surface qualities obtainable are greatly dependent on the characteristics of the machined materials and of the machining process. This can be proved by comparing the surface qualities of ceramics, which were machined with ultrasonic-assisted grinding and plane parallel lapping (Figure 20.13). Excluding the material silicon carbide, the arithmetical mean deviation of samples ground with ultrasonic assistance is slightly above the value established for that of plane parallel lapping.
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FIGURE 20.11 Surface topography and roughness of ceramic materials after ultrasonic-assisted cross-peripheral grinding.
For the materials silicon carbide and zirconium oxide (ZN 40) the best surface values were measured during ultrasonic-assisted grinding. Summarizing, the surface qualities obtainable for ceramic materials machined with ultrasonic-assisted grinding can be established at the level of conventional finishing procedures. Despite higher mechanical loads, surface qualities comparable to those of lapping can be achieved.
Bending strengths of ceramic workpieces were investigated for the machining parameters in Figure 20.10. Ultrasonic-assisted grinding gives bending strengths similar to or higher than those of plan-parallel lapping. Moreover ultrasonic-assisted grinding leads to insignificantly higher deviations of bending strengths. These are caused by the alternating load on the workpiece subsurface [Spur et al. 1999].