Machinability study of carbon/peek composites

Composites are generally fabricated to near nett-shape, however, there is often a need to conduct some additional machining operations such as trimming of the edges. High tool wear and the need for good surface finish are some of the major concerns in machining. This study deals with the machining of Carbon/PEEK, a high performance thermoplastic matrix composite, which is being used extensively in aerospace industries because it is light and posses high specific properties compared to conventional metallic materials. The technique of the full factorial design of experiment is used to investigate the relative effects of the various machining parameters on the amount of tool wear and the surface quality of the workpiece. Experimental results show that the cutting speed, depth of cut and feed-rate are the main factors that contribute to tool wear of the inserts. The surface finish is found to be independent of the machining parameters and much better than those obtained in the conventional machining of metals provided a critical cutting speed is exceeded. This critical cutting speed is found to be in the range of 70m/min to 75m/min. The cutting point temperature at this speed is found to have exceeded the glass transition temperature of the material, indicating that cutting is done in the rubbery regime. This leads us to conclude that the smooth surface finish of the workpiece is caused by some form of polymer softening action rather than determined by the tool geometry of the cutter.