40207_27 - COMPOSITE MACHINING Kent E . Kokkonen a nd Nitin...

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COMPOSITE MACHINING 27 Kent E. Kokkonen and Nitin Potdar 27.1 INTRODUCTION The processes used to manufacture composite structures generally require that trimming and other machining operations be performed prior to assembly. Machining processes are required to produce accurate surfaces and holes to allow precision fitting of components into an assembly. Due to shrinkage during the curing stage of the composite structure it is not practicable to place holes in the part during the molding stage, therefore milling, cutting, drilling etc. are considered a post cure opera- tion. Due to the toughness and abrasive nature of modern composites, there is a need for harder and longer lasting cutting tools. A large data- base of machining information for various high speed steel and carbide cutting tool materials exists for machining metal, wood and some thermoplastics. However, much of this data cannot be applied to machining mod- ern composites. Modern composites like graphite-epoxy, aramid-epoxy and carbon- carbon each have their own machining charac- teristics. Composites are not homogeneous or isotropic, therefore the machining characteris- tics are dependent on the tool path in relation to the direction of the reinforcing fibers. Metals or metal alloys have nearly homoge- neous properties throughout the workpiece, but each material in a composite retains its individual properties. Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 27.2 CONVENTIONAL MILLING When milling graphite-epoxy with polycrys- talline diamond (PCD) the chips are formed as small particles of powder dust and fumes. The surface roughness is a function of fiber orien- tation, cutting direction and the angle between cutting direction and fiber direction. The sur- face may sometimes exhibit many small holes due to fiber pull out. When taking heavy milling cuts there is a greater tendency to break comers as the tool exits the material so it is advisable to first machine a step on the edge perpendicular to the final pass. A four fluted end mill will reduce cutting pressure on the laminate and keep it cooler. Climb milling helps prevent the fibers from separating from the matrix bond material. Advantages of machining composites are: 0 improved surface finish unless part surface was directly in contact with the mold sur- face; 0 machined surfaces provide accurate mating surfaces for parts to be assembled; 0 eliminates the majority of the problems associated with part shrinkage and insert movement during the fabrication processes. Tool life factors are: 0 PCD end milling cutters will perform sixty to one hundred times longer than carbide; 0 cutting speed does not have a great effect on the flank wear of PCD cutting tools.
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40207_27 - COMPOSITE MACHINING Kent E . Kokkonen a nd Nitin...

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