**5.1 Ductile machining**

**Figure 10** shows the chip formation and various regions along the chip profile in ultraprecision diamond turning. In the chip profile, ratio of uncut chip thickness (a) to toll edge radius (r) varies from zero to some maximum value. The cutting mechanisms vary with different regions of a/r ratios. At a/r ratio sufficiently higher than 1 (region III), maximum amount of material removal takes place along the rake face of the tool and thus facilitates pure cutting. As a/r ratio approaches to 1 (region II), the tool edge radius and uncut chip thickness become comparable which makes the effective rake angle of the tool negative. The negative rake angle leads to extrusion like squeezing action. As a/r ratio becomes less than 1 (region I), the tool is unable to remove the material as a chip and only sliding and elastic deformation takes place.

As the uncut chip thickness is decreased thrust force increases in comparison to the cutting force and mechanisms like plowing, rubbing and burnishing become

**Figure 10.** *Cutting mechanism along the chip profile for the workpiece machined with round nose diamond tool.*

dominant. This is because as the chip thickness is reduced, the tool edge radius becomes significant in comparison to chip thickness and the effective rake angle thus becomes negative. Higher negative rake angle restricts the chip to flow along rake face and therefore the chip flows sideways of the tool. It can be described as tool plows through the workpiece. Further reduction on the uncut chip thickness causes the tool to rub or burnish the work surface. The mechanism of material removal in diamond turning, thus, primarily depends upon the uncut chip thickness and tool edge radius. Since, the uncut chip thickness along the chip profile varies, the mechanisms tend to vary along it. The tool edge sharpness also degrades during the course of machining, which also affects the cutting mechanisms.
