**4. Mechanisms of material removal in ultraprecision machining with diamond tool**

Ultraprecision machining is achieved by reducing the scale of machining process which in turn brings the "size effect" into picture. Difference between conventional machining and ultraprecision machining lies in the ratio of uncut chip thickness and edge radius (see **Figure 8**). Unlike conventional machining process, diamond turning process with a highly sharp single crystal diamond tool exhibits the size effect due to its reduced scale of material removal. Cutting tool edge radius (r) becomes significant with respect to uncut chip thickness during diamond turning process (see **Figure 8(b)**). Resisting shear strength of the material increases considerably as the chip thickness is reduced.

At the macroscale in conventional cutting, while the uncut chip thickness is larger than the tool edge radius, the tool is able to remove cluster of grains. The shearing action takes place along the easy slip plane. The easy slip plane is governed by the stress concentration along the grain boundaries, defects and vacancies in the material. When the uncut chip thickness is reduced from few microns to a few nanometers, cutting within a grain takes place. It requires more energy to cut the solid grain with round edge than to remove a number of grains with sharp edge tool (see **Figure 9**). Therefore, the specific energy is higher in the ultraprecision machining process like

$$r\_2 \gg r\_1 \ll t\_2 \ll t\_1$$

**Figure 8.**

*Schematic of (a) conventional and (b) ultraprecision machining [3].*

**Figure 9.** *Size effect in ultraprecision machining.*

diamond turning compared to the conventional cutting. With the reduction in chip thickness up to few nano meters, plastic deformation viz. rubbing and burnishing become more dominant than cutting due to very small effective cutting zone. In diamond turning, the tool edge radius is continuously deteriorated and increased with constant uncut chip thickness, which in turn deteriorates the machining process.
