On the Cutting of Metals at the Small Scale: Size Effect, Friction and Plastic Instabilities

Abstract

The “size effect” phenomenon in metal cutting – the substantial increase of the specific cutting energy with the decreasing chip size – is systematically studied. An instrumented ultramicrotome is applied to achieve two-dimensional orthogonal cutting with small depths of cut in the range of 30 nm to 3 µm. Diamond and freshly cleaved glass knives with theoretically sharp cutting edges are utilized to carry out cutting in the absence of edge radius effects.

Based on the measurements of cutting forces and specific energies with multiple metallic systems (copper, zinc, polycrystalline and single crystal aluminum), new evidence is presented in sup-port of tool-chip friction as the primary source of the size effect. It is shown that size effect arises as a result of the non-proportional decrease of the friction component with the underlying length scale. Further direct measurements show that the non-linear dependence of the tool-chip contact length with respect to the depth of cut contributes to this phenomenon. The size dependence of tool-chip friction and contact size is interpreted and quantitatively modeled using a plastic sliding contact mechanics model in which the increased role of the intermolecular adhesion at small tool-chip contacts is shown to be the primary factor underlying the non-linear scaling of the contact length and the size effect. Cutting experiments are also carried out with tools covered with solid and liquid contaminants. These results show that contaminant films effectively diminish adhesion and lead to a drastic reduction in the size effect, suggesting practical benefits for industrial manufacturing processes. Experiments to directly measure the adhesive pull-off force at the tool-chip contact are conducted using the same ultramicrotome platform, and a framework is proposed to calculate the work of adhesion at this contact from the pull-off force. Together, these studies suggest an opportunity to use tool-chip contact in cutting to study friction and adhesion phenomenon at small scales over a broad range of contact conditions.

As a sub-focus, the thesis also studies the problem of surface instability and fold formation in chip formation in cutting of soft metals. A unified framework based on plastic buckling is presented to describe this phenomenon and capture the scaling of instability wavelength (fold spacing) on the depth of cut.