Modelling and Analysis of Surface Roughness in Micro-End Milling Process

There is a growing need for fast, direct and batch production of miniaturized functional parts from a broad range material including metals, polymers, ceramics and composites. The demand for miniaturized products with high aspect ratios and superior surfaces has been rapidly increasing in different fields such as aerospace, automotive, biomedical, optical, defence, micro-fluidics, robotics and electronics. In response to this demand, mechanical micromachining with advanced numerical controls, such as micro-end milling, is rapidly gaining momentum in industrial productions because of its viability to directly produce complex three dimensional (3D) geometries from a wide range of materials. However, with the advancement of the micromachining technology, machining performances associated with surface quality and modern machine-tool structures have not been fully addressed and are currently limiting performance in some applications.Surface quality of machined parts is important in engineering since it plays an important role in their wear resistance and strength. The quality of machined surfaces can be used as an index to evaluate the performance of the machining process. Therefore a salient feature in the modelling of the micro-end milling process is capturing the tool-workpiece interface because machined surfaces reflect the paths traced out by the individual cutting edge of the milling tool. The cutting tools for the micro-end milling process have very low stiffness due to their unique shape and deflect under the action of cutting forces during machining which in turn influences the surface texture formation and dimensional error of the machined products. There is a pressing need for computer-based simulation models that can be used for the purpose of generating surface profiles and analysing the machining perforce during the planning stage.Thus the focus of this research was to develop a comprehensive model which can be used to describe the generation of machined surface profiles and to estimate the surface roughness during the micro-end milling process. The model was developed considering the trajectory and cutting geometry of micro-end mills. The model included the effect of tool deflection caused by the cutting force during machining. Therefore a cutting force model and a tool deflection model were developed to estimate the tool deflection during the cutting process. This tool deflection was then included into the surface roughness model. In order to understand the tool deflection effect, two surface roughness models were developed: the first is a model without considering the tool deflection, and the second is a surface roughness model considering the tool deflection effect. Algorithms for simulating three dimensional (3D) surfaces were developed. The models have been simulated in the Matlab environment.The models have also been applied and validated for the micro-end milling process. Due to difficulties in measuring tool deflections, a finite element (FE) method was used to predict and show that a simple tool deflection model was adequate for predicting the tool deflection. The true shape for entire overhang length of the micro-end mills was modelled and simulated with the ANSYS software. When compared with the ANSYS results, the developed deflection model was shown to estimate the tool deflection with a maximum error of 3.45% for the tool diameters used in this research. Experimental studies were carried out to provide data to validate the cutting force and surface roughness models. Slots were machined on Al 6061-T6 aluminium alloy samples for different cutting parameters. For all cutting tests, 8 mm long slots were cut using a desktop size micro-end milling setup and the cutting force data were recorded using a force dynamometer. The floor surfaces of the slots were scanned with an atomic force microscope (AFM) and a BRUKER 3D optical microscope to obtain 2D and 3D surface profiles and surface roughness parameters. Both 2D and 3D surface profiles were used for detailed comparisons. The accuracy of both the cutting force and surface roughness models was verified through observing good agreements between the simulation and experimental results. The surface roughness model was found to be a suitable tool for generating 2D and 3D surface profiles and for estimating average roughness values of the floor surfaces. The model was able to capture the effect of tool deflection on the surface texture formation in the micro-end milling process. The model can also be applied for studying the effects of cutting parameters on the surface roughness. An additional advantage of the surface roughness model is that there are no arbitrary constants in the model that need to be experimentally determined