JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
In the present paper, we present the issues around the milling and drilling of carbon-fibre-reinforced composite panels, with a reminder of the material’s specific properties, the technical equipment required, the potential conditions, and distinctive features about the way machined surfaces degrade. We will also discuss the use of PCD tools, with details of the tool geometry and mechanical action, before concluding on the industrial results obtained and potential developments for the near future.
(Published on June-July 2005 – JEC Magazine #18)
BENOÎT FURET, BENOÎT JOLIVEL, DIDIER LE BORGNE, IRCCYN – UMR CNRS 6597 - ECOLE CENTRALE/ECOLE DES MINES/UNIVERSITY INSTITUTE OF TECHNOLOGY IN NANTES AIRBUS – RESEARCH AND TECHNOLOGY DEPARTMENT – NANTES PLANT.
On civil aircraft, composite materials now can account for as much as 20% of the structural weight. This proportion is expected to keep on increasing, as is happening with the A380. According to an AIRBUS study, the proportion of composite materials in aircraft structures should reach 35 to 50% in military aircraft programmes in 2006.
All of the basic composite parts are obtained using different manufacturing processes, such as hand layup or automatic lay-up followed by autoclave cure, RTM, or resin infusion. They also require milling to obtain functional surfaces of sufficiently high quality, and drilling to obtain the bore diameters needed for mechanical assembly.
Machining carbon-composite parts
In milling, carbon-fibre-reinforced composite parts need to be contoured to eliminate excess material from the blanks, and finished by surface grinding. Cutters with polycrystalline diamond (PCD) brazed tips are mostly used.
Five-axis CNC machining centres are required for variable-angle trimming all around the edges of parts. These machines often need to be quite large to machine parts that are becoming longer and longer.
To drill parts for assembly that often are made of different materials (carbon composites, aluminium alloy, etc.), helical interpolation on special machines with PCD cutters can be used. But in most cases, automated axial drilling with pneumatic hand drills placed manually on the positioning die and a gun drill with PCD brazed tips is used.
Because the fibres used in composite materials tend to be abrasive, in most cases only PCD tools are strong enough to be used industrially with the above-mentioned machining techniques.
Degrading modes of machined surfaces
Due to the heterogeneous composition and anisotropy of composite materials, machining procedures can damage the material in ways that directly affect the mechanical properties. This is not the case for metals.
The mechanical action during cutting can cause a loss of adhesion between the fibre layers. If this occurs deep in the part thickness it is called delamination, and if it occurs at the surface, it is called peeling or flaking. Sometimes uncut fibres in the top or bottom ply can be observed in machining. This defect is due to the type of weave and to operating conditions.
Lowering the incidence of these defects or eliminating them entirely during machining procedures requires identifying and being able to control the forces involved in PCD cutting of carbon composites.
Analysing the cutting forces in milling
The experiments were carried out at the Nantes University Institute of Technology on an Hermle C800U high-speed machining centre equipped with a Kistler dynamometer.
The signal waveforms obtained during the machining of carbon composites with a PCD cutter with brazed tips is similar to those obtained when milling metals.
The summary of results in figure 10 shows the changes in Fx and Fy as a function of feed per tooth. This indicates a certain linearity in the change in intensity of cutting force as a function of the chip cross-section. With respect to the quality of the milled surfaces, and particularly of damage defects like the ones discussed earlier in this paper, it appears advisable to have control over the direction of the cutting force. It is important to choose the work mode that is in phase with or in opposite phase (on the part side), as a function of fibre orientation.
Analysing the cutting forces in axial drilling
The experiments were carried out in Nantes on an Anayak machining centre supplied by Europe Technologies group subsidiary Pelletier/Mecarec, and equipped with a Kistler dynamometer.
The drilling/force measuring tests were carried out for a range of operating parameters. We observed the following: - axial force and torque are feed-value-dependent; - cutting speed has little effect on the torque and axial force.
The axial force and torque are a function of the material, especially for aluminium alloys, for which the magnitudes are two to three times larger than for carbon composites.
To optimise axial drilling for multi-material assemblies such as carbon/aluminium, it appears worthwhile to dissociate or uncouple speed and feed functions to allow them to vary in one way or another independently as a function of the material being drilled. It also appears worthwhile to be able to monitor the physical variables that participate in cutting during the drilling process, for example, spindle power or the axial force value, to allow automatic monitoring of the process for broken tools or significant wear that could cause bore diameter defects.
The milling or drilling of thick structural parts made of carbonfibre- reinforced composite materials is in its early stages of development. The use of PCD cutters gives good results for contouring or surfacing operations, although it is necessary to have good control of the operating conditions. Measuring cutting force shows up a certain linearity to the proportionality of cutting-force intensity as a function of chip cross-section. It also appears absolutely necessary to have good control over the radial and axial cutting angles in order to be able to reduce damage such as delamination or peeling to machined surfaces, as it has a very adverse effect on the mechanical properties of the parts produced. For drilling composite parts or multi-material stacks, tests showed that it is absolutely necessary to have good control over the characteristic angles of the cutting tool; that the operating conditions have significant effects on the mechanical forces involved; and that it is necessary to be able to control the cutting conditions. Joint research on materials, cutting tools, spindles, vibrating behaviour, machines, and CAD/CAM needs to be continued. A new research project, the Aluminium and Composites HSM Project, started in December 2004.