Study of Manufacturing Defects and Tool Geometry Optimisation for Multi-Material Stack Drilling

Abstract

In the aeronautic industries, composite materials are increasingly being used for structural parts. Carbon Fibre Reinforced Plastics (CFRP) are often used in combination with metallic materials, mostly aluminium alloys. This raises new problems in aircraft assembly when it comes to machining the holes for thousands of fasteners. The preferred method for this is a one-shot drilling-reaming-countersinking operation usually using a power tool and with the need to respect tight dimensional and geometric specifications. The solutions proposed so far with existing cutting tools, involving reduced feed rate, are unsatisfactory from an economic point of view. This study first focuses on identifying machining defects and difficulties encountered during drilling of aluminium/CFRP stacks. Then, based on the results of different works on drilling [1,3], an experimental study is proposed to define the critical macro-and micro-geometric parameters of a carbide drill. The criteria relate to the fragmentation of the metallic chips, burr avoidance and zero damage to the CFRP. The first results obtained with the new generation cutters developed show the importance of a constant axial rake angle and of the tool point angle in the chip fragmentation phenomenon but also in preserving the health of the CFRP material. The influence of the constant rake angle on the axial forces generated is also shown. Finally, an optimal combination of the tool geometrical parameters is achieved in order to obtain the expected results.