Mechanism Studies And Process Optimization Of Meso Scale Milling Process

Recent years, the production of miniaturized components with complex and small features is gaining increasing importance due to the trend of miniaturization which is determining the development of products for various industries, such as biomedical instruments, electronic products, and defense industry and so on. Most of these components fall into the scales from 0.1 to 10 mm and the feature size from 0.01 to 1mm known as meso scale. Considered as one of the most effective techniques, meso scale milling operation can be used to fabricate these components with complex 3D micro features over a wide range of material types.However, meso scale milling process is not simply downsized from the conventional operation but has its own characteristics, such as size effect and minimum chip thickness. Further advances in both efficiency and quality are limited by our incomplete understanding of the basic mechanism of meso scale milling process. To satisfy the increasing need of miniaturized manufacturing, mechanism studies are becoming more and more important.This thesis presents mechanism studies and process optimization focusing on its characteristics, size effect, minimum chip thickness, tool wear effect and surface generation. The main contents and innovations consist of four parts:(1) Development of Meso Scale Milling Machine ToolIn order to overcome the shortcomings of current technologies for meso scale manufacturing, a meso scale milling machine tool system (mMT) was developed and some tests were performed to evaluate its performane. First, the mMT is developed based on the analysis of the requirements and characteristics of meso milling process. Then, some tests were carried out to analyze the performance and the feasibility of the mMT. The tests consist of the precision positioning of the stage, machining precision and some machining cases. In addition, the dynamic performance of the machine tool is also investigated. Through these tests and measurements, the meso scale milling process is proved to be feasible and practical.(2) Material Modeling and Analysis of Meso Scale MachiningIn this section, a finite element (FE) model is developed focusing on the characteristics of meso scale machining process. Firstly, by using strain gradient plasticity, a modified Johnson-Cook constitutive equation is formulated to model the size effect of material behaviors at micron level as well as the main features in macro machining process. Then, based on the material model, a finite element model is developed considering the size effect, cutter edge radius and fracture behaviour. Finally, the size effect and chip formation are investigated by FE simulations applying the developed model. Some research findings can be drawn: (i). The size effect of the material behaviors at micron level may be the cause of the size effect of meso scale machining and can be well explained by the proposed material model using strain gradient plasticity. (ii). Plaughing phenomenon occurs when the uncut chip thickness is close the cutting edge radius, which in turn causes the minimum chip thickness of meso scale machining; (iii). Rake angle is found to have a significant influence on the chip formation in meso scale machining process.(3) Modeling and Mechanism Study of Meso Scale Milling ProcessThis part presents mechanism studies of meso scale milling process focusing on its characteristics, size effect, micro cutter edge radius and minimum chip thickness. Firstly, based on the FE simulations of meso scale machining process, an analytical milling force model is developed using the cutting principles and slip-line theory. Then, a series of experiments for OFHC Copper meso scale milling using 0.1mm-diameter micro tool were performed on the mMT, and good agreements were achieved between the simulated and the experimental results. Finally, the chip formation and the size effect of meso scale milling process are investigated using the proposed model. Some research findings can be drawn: (i). For meso scale milling, the size effect of the material behaviors is found to be the main cause of the size effect of meso scale machining when the uncut chip thickness is larger than the minimum chip thickness. (ii). Specific shear energy increases greatly when the uncut chip thickness is smaller than the minimum chip thickness due to the plaughing phenomenon and the accumulation of the actual chip thickness.(4) Process Optimization of Meso Scale Milling ProcessThis part presents the process optimization study of meso scale milling process. Firstly, an efficiency based process optimization model is proposed from the meso scale milling process tests. Then, experimental analysis and parametric studies on the tool wear were performed based on systematically designed meso scale milling experiments. A trajectory-based surface roughness model and a milling force model with tool wear effect are developed and validated by meso scale milling experiments of OFHC Copper using 0.1 mm- diameter micro endmills with the mMT. Finally, the process optimization was carried out to improve the production efficiency with the constraints of the surface roughness and the milling forces. The proposed model and its solution provide an approach to improve the production efficiency for meso scale milling process.Focusing on the characteristics of meso scale milling process, this thesis performed the mechanism studies and process optimization. The underlying mechanisms of the size effect and minimum chip thickness are well studied by material behavior modeling and chip formation analysis. These works can provide theoretical basis and approaches for process design, quality control of the meso scale milling process.