**5.2 Ductile regime machining of brittle materials**

In conventional macroscale machining process, the brittle materials are subjected to fracture as a result of median and lateral cracks [14] as shown in **Figure 11**. This brittle failure can be prevented using a very low uncut chip thickness. Machining of brittle materials takes place in ductile mode at very low uncut chip thickness and therefore it is known as ductile regime machining. Ductile regime machining of brittle materials leads to crack-free mirror finished surfaces. In case of brittle materials, there exists a critical uncut chip thickness below which ductile regime machining takes place and above which the brittle machining takes place. Blackley and Scattergood [15] proposed a machining model depending on critical depth of cut and subsurface damage depth which indicated that fracture takes place above the critical uncut chip thickness (dc) and is subsequently propagated in the workpiece material. The chip thickness varies from zero at the tool centre to a maximum at the top of the uncut portion as shown in **Figure 11**. If the fracture does not penetrate into the cutting plane, the brittle mode machining will take place. Thus the critical chip thickness is a deciding parameter between the two machining modes.

Smooth, damage free and optical quality surface is generated if the uncut chip thickness along the tool nose is below critical chip thickness. This transition of mechanism from brittle to ductile machining is also known as brittle - ductile transition. According

#### **Figure 11.**

*Schematic of brittle and ductile regime machining in brittle materials [15].*

to another ductile regime machining model proposed by Nakasuji et al. [16], the brittle to ductile transition takes place from in the diamond turning process of brittle materials. For the larger uncut chip thickness, the stress zone is such that it contains a number of defects which nucleate to expand the cracks and cause brittle fracture. With the extremely small uncut chip thickness, the defects are too less in the stress zone to form cracks and therefore, it enables the material removal in ductile mode (see **Figure 12**).

Shimada et al. [17] used a different model to elaborate the brittle-ductile transition and suggested that there are two ways of material removal: one is the ductile machining due to plastic deformation in the slip direction on the characteristic slip plane and the other is the brittle mode machining owing to cleavage fracture on the characteristic cleavage plane. When the resolved shear stress *τ*slip in the slip direction on the slip plane exceeds a certain critical value *τ*c inherent to the workpiece material, plastic deformation occurs in a small stressed field in the cutting region of a specified scale. On the other hand, a cleavage occurs when the resolved tensile stress normal to the cleavage plane *σ*cleav exceeds a certain critical value *σ*c. The mode of material removal depends on which criteria dominates or precedes *τ*slip *> τ*c or *σ*cleave *> σ*c for the stress state under a particular machining condition (see **Figure 13**).

*Diamond as a Precision Cutting Tool DOI: http://dx.doi.org/10.5772/intechopen.108557*

**Figure 13.** *Ductile and brittle regime models of chip removal [17].*
