Research On High Speed Milling Of High-performanced Graphite

In recent years, high-performanced graphite materials have been used more and more in EDM applications of die & mould to manufacture products in the fields of automobile, home appliances, communication, electronic industry, etc. In comparison with copper electrode, graphite electrode has higher strength, less electrode wear and less thermal expansion, so graphite can be used to manufacture complicated electrodes with many special structures, such as thin walls and micro holes. Graphite becomes the prevailing electrode material over copper in EDM, especially for manufacturing complicated mould cavities with narrow and deep slots and micro holes. High speed milling with micro endmills is one of the major process methods to manufacture thin-walled graphite electrode with high efficiency and good precision. However, because of lack of further researches on high speed milling mechanisms of graphite, tool wear mechanism and optimization of high speed milling technology, there are many problems in its applications. In this paper, systematic researches were conducted on chip formation mechanisms of high seed milling of graphite, tool wear mechanisms, surface finish, cutting forces and optimization of cutting parameters and programming strategies for high speed milling of thin-walled graphite electrode, and the research results were validated rationally and practicably through a instance of high speed milling of thin-walled graphite electrode.On chip formation of high speed milling mechanisms of graphite, online photography method and microscope analysis method of materials, the basic characteristics of graphite chip formation were studied through orthogonal cutting experiments and high speed milling experiments. Combined with the geometric relationship between the maximum depth of cut of single flute of micro endmill a_pmax and feed per tooth and radial depth of cut, the relationships between high speed milling conditions and graphite chip micrograph, chip size distribution, micrograph of machined surfaces, ratio of surface damage and surface finish were established. The relationships between chip formation process and characteristics of cutting forces and tool wear. High speed milling mechanisms of graphite was proposed. The major results show that: in high speed milling of graphite, with increase of a_pmax, graphite chips changed from semicontinuous chips to crushed particle chips and fracture block chips; Chip formation process in high speed milling is influenced by feed per tooth and radial depth of cut through a_pmax, and ratio of surface damage can be reduced by decrease of feed per tooth and radial depth of cut. Bigger fracture block chips can be facilitated by use of tool with positive rake angle; clearance angle and helical angle have little effect on graphite chip formation.On cutting forces in high speed milling of graphite, combined with the influence of cutting conditions on graphite chip formation, ratio of surface damage and frictional coefficient between tool flank and workpiece surface, the effect of cutting parameters, geometric angle of endmill and graphite materials properties on cutting forces in high speed milling of graphite were studied. The variation of the characteristics of time-domain waveform and frequency components with tool wear progression were analyzed. By use of orthogonal array experiments based on Taguchi method, the major factors which had the greatest influence on the cutting forces were studied to get the optimized combination of cutting parameters aiming to decrease cutting forces to the minimum.On friction and wear characteristics of graphite/ carbide pair, in order to simulate the friction and wear characteristics at the friction interface when graphite chip and workpiece material are sliding against carbide tool surface, standard disk-pin friction experimental machine was used to conduct sliding friction and wear experiments, and modified disk-pin friction and wear experimental equipment was designed to do abrasive wear tests. The sliding friction and wear behaviors and abrasive wear behaviors of graphite/ carbide pair were studied, in order to provide basic frictional theories for research on tool wear mechanisms. (1) On the sliding friction and wear behavior, the effect of normal load and sliding speed on the surface characteristics of friction pair, coefficient of friction and frictional temperature. The results show that graphite transferfilms were formed on the surface of carbide pin during sliding; the worn scar of carbide showed the characteristics of " polished" abrasive wear; and the rise of normal load and sliding speed can facilitate the formation of graphite transferfilm and decrease coefficient of friction and frictional temperature. (2) On the abrasive wear behavior, the effect of WC grain size, Co content, normal load, sliding speed and coating on the micrographs of worn surface, specific wear rate and coefficient of friction were investigated. The results show that the worn surfaces of carbide have characteristics of" polished" abrasive wear and "micro-cutting" abrasive wear; the specific wear rate and coefficient of friction of carbide decreased significantly with decrease of WC grain size and Co content, increased with increase of normal load, and changed little with variation of sliding speed. AlTiN coating has inconspicuous anti-wear effect on carbide. (3) With the introduce of graphite chips bwteen the frictional surface, the effect of graphite chip on the sliding friction and wear behavior of frictional pair was studied. The results show that graphite chips can reduce the coefficient of friction and frictional temperature, and make the coefficient of friction decrease with the decrease of normal load and rise of sliding speed.On the tool wear and failure behavior of micro endmill in high speed milling of graphite, the frictional conditions were analyzed, and the patterns and mechanisms of tool wear and failure were revealed. The results show that coating premature declamination is the major premature failure mechanism, and " polished" abrasive wear is the major wear mechanism in the steady wear stage of micro coated carbide endmill. The effect of WC grain size and Co content on the abrasion resistance and impact resistance were investigated. The results show that the abrasion resistance increased with the decrease of WC grain and Co content, but when the Co content was too little, the impact resistance of micro carbide endmill decreased again obviously. Superfine-grained carbide 0.2μmWC-8%Co is the best carbide substrate material for high speed milling of graphite. By use of orthogonal array experiments based on Taguchi method, the major factors which had the greatest influence on tool wear were studied to get the optimized combination of cutting parameters aiming to decrease tool wear to the minimum.On the high speed milling technology of typical thin-walled graphite electrode, the previous results in this paper were applied synthetically. According to the technology characteristics of high speed milling of typical thin-walled graphite electrode, the programming strategies, process technologies, cutting parameters and cutting tool suitable for high speed milling of typical thin-walled graphite electrode were constituted and optimized for roughing, semi-finishing and finishing. High speed milling process and CNC programs were established to perform a instance of high speed milling of typical thin-walled graphite electrode. A thin-walled graphite electrode with thickness of 0.3mm and thickness/ height ratio of 1: 53.3 was achieved by high speed milling technologies.