Edge Trimming of CFRP- Surface Roughness Measurement and Prediction

Use of carbon fibre composites has been increasing in the aerospace industry. However, there is still a need for finishing operations by conventional machining in the manufacturing of composite parts. Composites have a very different machinability to metals and can suffer from a number of surface defects during machining. The fibres are also highly abrasive and can cause rapid tool wear which in turn leads to increased likelihood of machining defects. This project has focussed on the machined surface quality developed during machining using new surface inspection techniques and additional surface roughness parameters. It is important to be able to accurately measure the surface roughness in order to ensure the integrity of in service components and quantify surface damage from machining. The aim of this project is to develop new numerical modelling techniques for the edge trimming of carbon fibre reinforced plastic (CFRP), and develop methods for the prediction of surface roughness. Different experimental techniques have been used to analyse post-machining damage, including scanning electron microscopy (SEM), computed tomography scanning (CT) and a focus variation system for measuring surface roughness. CFRP specimens have been edge trimmed using a poly crystalline diamond (PCD) cutting tool, and compared for different machining parameters, tool wear and material fibre orientations. Cutting forces were recorded and the surface quality was inspected using the optical focus variation method. Regression models from experimental data have been combined with finite element (FE) models to create a surface roughness prediction tool which includes the effects of tool wear. Areal surface roughness Sa measurements were taken using the optical system and the advantages of the system have been compared with conventional stylus roughness measurement methods. Experimental data was used to validate 3D and 2D FE milling models using MSC Marc. New FE models were developed using adaptive re-meshing, and user subroutine to control the cutting tool movement and simulation idle time. Progressive levels of tool wear have been implemented in the 2D model by using cutting edge radius measurements from experiment. FE and experimental results show that tool wear and material fibre orientation have a significant effect on the cutting forces and surface roughness. Regression models showed that the surface roughness was most affected by tool wear, feed rate and cutting speed. A reasonable comparison has been found between FE and experiment and the FE models were capable of predicting the effects of tool wear due to cutting edge rounding. 3D models were found to better predict thrust forces than 2D FE model. The optical system was found to be useful technique for measuring surface roughness of machined fibrous composite surfaces and is more reliable than conventional roughness measurements. New strategies for roughness measurement have been recommended.