Modelling and experimental investigation of tool wear in ultra-precision raster milling

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Modelling and experimental investigation of tool wear in ultra-precision raster milling


Author: Zhang, Guoqing
Title: Modelling and experimental investigation of tool wear in ultra-precision raster milling
Degree: Ph.D.
Year: 2014
Subject: Milling machinery.
Mechanical wear.
Machine-tools -- Monitoring.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Industrial and Systems Engineering
Pages: xxi, 223 leaves : illustrations ; 30 cm
Language: English
InnoPac Record:
Abstract: Ultra-precision raster milling (UPRM) is a typical intermittent cutting process used to fabricate non-rotational symmetric surface structures with nanometric surface roughness, which has become an essential method for producing precise optical products. However, since wear of the cutting tool in the UPRM process significantly affects the machined surface roughness, it is necessary to determine tool wear characteristics, tool wear effects on the surface roughness, and tool wear on-machine evaluation methods for UPRM. This thesis provides an account of theoretical and experimental research into tool wear and machined surface roughness evaluation methods in UPRM, and is divided into four parts. In the first part, the research focuses on the exploration of tool wear characteristics and their effects on machined surface quality in UPRM. Each tool wear characteristic and its effect on the machined surface quality are discussed separately according to the observed tool wear characteristic: tool fracture, material welding, flank wear land formation, and sub-wear-land formation. The second part of the thesis provides an account of tool wear evaluation methods based on cutting forces. An analytic cutting force model and a dynamic model are established to simulate the cutting force pulse and the free vibration of dynamometer induced by the cutting force pulse. The relationship between cutting force and cutting parameters such as the feed rate and depth of cut, is presented and the power spectrum features of cutting forces at different tool wear stages are explored. In the third part, tool fracture wear and its effects on the machined surface are evaluated on-machine by using cutting chips. During the UPRM process, tool fractures are directly imprinted both on the cutting chip surface and the machined surface as groups of {176}ridges. Through inspection of the location and cross-sectional shape of these {176}ridges{174} on a cutting chip surface, a virtual cutting edge of the diamond tool under fracture wear, and surface topography considering the effects of tool fracture wear, are developed. A mathematical model is developed to simulate the virtual cutting edge and surface topography with two geometric elements, semi-circle and isosceles triangle, used to approximate the cross-sectional shape of ridges. The mathematical model was also utilized to compute the surface roughness taking into consideration the effects of tool fracture wear. In the fourth and final part, tool flank wear and its effect on machined surface quality are on-machine evaluated by using cutting chips. The occurrence of tool flank wear truncates the cutting chips at both the tool entry and tool exit sides of the cutting chips. The width of truncation positions of the cutting chip can be measured and used to calculate the width of flank wear land with the help of a mathematical model. The identified width of flank wear land is also used to calculate the surface roughness with the help of a mathematical model. It is found that with the progress of the tool flank wear, the truncation position in the feed direction moves from two sides to the central position of the cutting chips, meanwhile, the surface roughness decreases at first and then increases significantly. The originality and significance of this research lies in the provision of a novel method to on-machine evaluate tool wear and its effects on machined surface quality in UPRM. The study contributes to the body of knowledge by: (i) identifying tool wear characteristics and their effects on machined surface roughness in UPRM; (ii) analyzing the cutting force compositions in UPRM and indicating two power spectrum peaks that can be used to evaluate tool wear; and (iii) providing a new method for on-machine evaluation of tool wear characteristics (including both tool fracture wear and tool flank wear) and their effect on machined surface roughness by using cutting chips.

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