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Chapter 2
2.3 Tool geometry
Researchers observed that parameters related to the tool geometry i.e. rake angles and tool edge radius cause size effect during micro-cutting. The presence of tool edge radius not only causes ploughing but also alters effective rake angle during micro-cutting, where tool edge radius is comparable to the uncut chip thickness as shown in Fig. 2.9.
Albrecht [63] was among the earliest investigators to suggest the pushing of the material in front of the edge radius predominantly into the chip and some into the workpiece. Kountanya [26] performed orthogonal cutting experiments with different tool edge radius on materials such as pure zinc, cast iron and Al-2024. Fig. 2.10 shows the effect of tool edge radius on size effect during micro-cutting of zinc. It is seen that as the uncut chip thickness decreases, the specific cutting energy increases nonlinearly and the increase is further higher as tool edge radius increases. Lucca et al. [23] showed that at higher negative rake angles, the specific cutting energy is higher during micro-cutting experiments using a single crystal diamond tool (see Fig. 2.11). The dominance of ploughing under these conditions was used to explain the increase in specific cutting energy. In another study, Lucca et al. [64] suggested that the tool edge condition has a significant effect on the resulting forces at small ratio of the uncut chip thickness to the tool edge radius.
When uncut chip thickness is of the same order as or even smaller than tool edge radius, the effective rake angle may become negative and contribute to the specific cutting energy. Research on ultra-precision machining reveals that there is a transition from cutting to ploughing, as edge radius increases. The rake angle also has a significant influence on the specific cutting energy at small depths of cut. The specific cutting energy is also related to the minimum chip thickness. When the uncut chip thickness approaches or is smaller than the minimum chip thickness, the specific cutting energy increases exponentially with the decreasing of the uncut chip thickness [65]. It is observed that the nonlinear effect is more prominent if cutting is performed below the minimum uncut chip thickness. Taminiau and Dautzenberg [66] also witnessed the increase in specific cutting energy while machining brass with a decreased chip thickness. They found that the specific cutting forces depend on the ratio of the uncut chip thickness to the cutting edge radius, when the uncut chip thickness is smaller than the edge radius.
Material strengthening, sub-surface deformation and process parameters such as tool edge radius, tool rake angle and cutting speed have been extensively studied by various researchers. Limited literature is available on the effect of microcrack formation, gross fracture phenomenon and microstructure during micro-cutting on the size effect. The next three sections focus on the review of literature in these fields.
2.4 Microcrack formation in the shear zone
2.5 Gross fracture phenomenon ahead of
tool-tip
2.6 Workpiece microstructure effect
2.7 Conclusions from the literature review
2.8
Objective and scope of the research
2.9
Approach to the work
