PhD Oral Exam - Mohsen Habibi, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Face-hobbing is the dominant and the most productive machining process for manufacturing bevel and hypoid gears. Bevel gears are one of the most important power transmission components, in automobile to aerospace industries, where the power is transmitted between two non-parallel axes. In current industries, the face-hobbing process confronts two major challenges, the tool wear and trial and error experiments to select machining parameters. In the present work, these two problems are targeted.

The tool wear in face-hobbing happens at the tool corners of the cutting blades due to the multi-flank chip formation and large gradient of working rake and relief angles along the cutting edge at the corners. In addition, the cutting fluid absence contributes in the tool wear phenomena. In the present work, a cutting tool design method is proposed in order to improve the tool wear characteristics especially at the tool corners. The rake and relief surfaces of the conventional cutting blades are re-designed in such a way that normal rake and relief angles during the face-hobbing process are kept constant and consequently the gradients of these two angles are minimized, theoretically to zero. Using mathematical tool wear characterization relationship and also FEM simulation, the improvements in tool wear are approved.

In addition, in the present thesis, semi-analytical methods are proposed to optimize the face-hobbing process in order to select appropriate machining settings. The optimization problem is constructed in such a way that the machining time is minimized subject to the tool rake wear or cutting force related constraints. In order to predict the tool rake wear (crater wear depth), methods are proposed to calculate un-deformed chip geometry, cutting forces, normal stresses, interface cutting temperature and chip sliding velocity.

The un-deformed chip geometry is obtained using two proposed methods numerically and semi-analytically. In the numerical method, the workpiece in-process model is obtained and then the un-deformed chip geometry is approximated using the in-process model. In the semi-analytical method, an un-deformed chip boundary theory is constructed in such a way that the boundary curves of the un-deformed chip are formulated by closed form equations.

The obtained un-deformed chip geometry is discretized along the cutting edge of the blades. Each infinitesimal element is considered as a small oblique cut. The differential cutting forces are predicted for each individual element using oblique cutting transformation theory. The total cutting forces are derived by integrating the differential cutting forces along the cutting edge.

The proposed methods are applied on case studies of non-generated face-hobbing of gears to show the capability of the methods.