Modelling and simulation of shear bands and tool-tip vibration in ultra-precision diamond turning

Pao Yue-kong Library Electronic Theses Database

Modelling and simulation of shear bands and tool-tip vibration in ultra-precision diamond turning


Author: Wang, Hao Victor
Title: Modelling and simulation of shear bands and tool-tip vibration in ultra-precision diamond turning
Degree: Ph.D.
Year: 2011
Subject: Diamond turning.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Industrial and Systems Engineering
Pages: xviii, 193 leaves : ill. (some col.) ; 30 cm.
Language: English
Abstract: Single point diamond turning (SPDT) is an enabling technology in the category of ultra-precision machining with stringent achievable tolerances for the fabrication of precision components for optical, medical and telecommunication applications, etc., which require extremely high geometrical accuracies, in sub-micrometric form accuracy and nanometric surface finish. The development of single point diamond turning is attributed to the advancement of controls, feedback systems, servo drives, and general machine design and construction, etc. In SPDT, a monocrystal diamond cutting tool is used with a nanometric edge radius, form reproducibility and wear resistance. Different from conventional machining processes, the depth of cut in SPDT is in the order of a few micrometres or less. In this regard, the well-established classic theory of metal cutting for conventional and precision machining should be reviewed critically if it is applied in the study of the microcutting process in ultra-precision machining. The theoretical and experimental study of this thesis has been divided into three parts. In the first part, serrated chip morphology with elastic strain induced shear band is studied in relation to its application in the generalised model for shear angle prediction. The second part is dedicated to a study of the physics of high frequency tool-tip vibration with its characteristic twin peak and influences on surface finish, with a proposed representative surface measurement method. With a dynamic model of the tool-work system, the third part reveals the connection between chip morphology and tool-tip vibration. In the first part, the theory of elastic strain induced shear bands is first established to explain the importance of elastic strain concentration in the initiation and formation of adiabatic shear bands with serrated chip morphology and identify the key factor leading to the onset of the highly localised material softening effects. The experimental results and numerical analysis verify the presence of a work softening effect. From a dynamic point of view, the shear band does not propagate from the tool tip to the free edge of the chips. Instead, the free edge creates an elastic strain concentration which makes the shear band initiate at the free edge and then propagate towards the tool tip. The effect of adiabatic shear bands on cutting force and relative tool-work displacement is further studied, which is also the key factor affecting surface generation. Without the widely adopted assumption of rigid-perfectly plasticity, the developed theory reveals the importance of elasticity in chip formation. A numerical analysis is conducted using a finite element method (FEM) to simulate the stress states and the procedure of initiation, propagation and formation of shear bands. The proposed theory of elastic strain induced shear bands finds its immediate application in the answer to the problem why the widely adopted Merchant's model fails in the presence of regularly spaced shear bands (RSSBs) in the microcutting process. A generalised theoretical model is further developed in which Merchant{174}s model and its modified models can be regarded as its special cases under different machining conditions.
In the second part, the theory of high frequency tool-tip vibration (HFTTV) has been built up based on the experimental findings in relation to the characteristic twin peaks (CTPs) identified in the direction of cutting force, which is regarded as one of the fundamental physical phenomena of the microcutting process in SPDT. Based on the experimental data, a physical model is proposed to explain the generation of CTPs and their relationship with the mechanical properties of work materials. The HFTTV is the root cause of the relative tool-chip displacement which consequently leads to the influence on the profile and roughness of the machined surface. Furthermore, a representative measurement method is further proposed to effectively characterise the patterned surface with regard to the mechanism of the HFTTV. The proposed method takes into consideration the effect of sample location and sample area ratio in the measurement. Since the ISO standard provides merely a minimal, not an optimal requirement for surface measurement, a concept encompassing calculation of sample-area-ratio sensitive roughness is defined to objectively study the discrepancies in surface measurement. In the third part, to examine the problem in its totality, a dynamic model is eventually developed to study the effect of HFTTV on the formation of shear bands in the range of high strain rate. The HFTTV and the formation of adiabatic shear bands (ASBs) are intrinsically well-related physical phenomena which can be regarded as a system (ASB-HFTTV). However, based on the rigid-perfectly plastic assumption, the conventional approach inclines to explain the system using a static model with dependency on cutting velocity. The proposed model in this thesis is applied to model the spacing and thickness of shear bands and reflect the dynamic features of the ASB-HFTTV system. The originality and significance of this thesis can be identified by (i) its theoretical framework established for the analysis of the microcutting process in multiple aspects, taking into consideration chip morphology, machining dynamics and surface characterisation; (ii) instead of a computational method and pure theoretical models, the starting point of this thesis is experimental observation, for which the physical explanations are provided; (iii) apart from the theory for conventional machining, the proposed theory in this thesis originates from and is applicable to the ultra-precision machining process; (iv) the research includes not only the machine tools but also the factors of material properties.

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