Despite the growing use of composites in the high end manufacturing sectors such as aerospace, automotive and power generation - the research into effective machining of composite material is still at its infancy.
Conventional metalcutting tooling is still widely used in machining of composites and could be detrimental to productivity and product quality. Pioneering R&D through the partnership of Airbus in the UK and precision tool specialists SGS Carbide Tool (UK) is delivering substantial benefits in understanding the most effective machining strategy of aerospace grade carbon fibre reinforced plastics (CFRP) composites for primary aircraft structures. The breakthrough has elevated both parties to the forefront of knowledge in composite machining.
A most recent case study was carried out by SGS with the Manufacturing Engineering Research department of the Airbus Wing Centre of Excellence (CoE) in Bristol. The objective was to evaluate and optimise the machining strategy before production of composite components.
SGS first introduced a purpose-designed CFRP composite cutting tool to Airbus in early 2008. Early test results did not provide any benefits when compared to baseline PCD tooling, but after introducing number of modifications to the existing design and optimisation of machining conditions, significant improvements in productivity, surface quality and tool life were achieved. Further benchmark testing has shown that the redesigned router outperformed PCD tooling in a number of instances when machining CFRP composites.
The Airbus trails have also enabled SGS to develop its carbon composite router through real life test analysis, as field sales manager, Wesley Tonks, explains: “The Airbus collaboration has accelerated the tool's development significantly and we have been developing special derivatives of the tools to test which will be to all our customers' benefit in time. The results have been extremely beneficial both to us and to Airbus in generating a greater understanding of the most effective methods of machining CFRP composites,” he observes.
Airbus' Wei-Ming Sim confirmed that SGS tooling now significantly outperforms PCD equivalents in the controlled tests conducted and, specifically, returns double the tool life, whilst minimising delamination of the top and bottom of the composite laminates. The tool is also far more progressive in performance, giving warning of performance drop off as the tool wears.
Mr Tonks adds: “As further testing progressed, in harmony with further tool development, we have been able to cut deeper with our specially developed long shank tools whilst maintaining consistently high quality results.
“However, the issue that become more critical to effective carbon composite machining in these circumstances is the effective removal of waste material to reduce the tendency of the ultra fine swarf to clog.”
This has triggered additional research with Airbus' machine tool supplier MTorres to investigate and create an effective low pressure zone/vacuum around the tool to remove dust more efficiently.
This research has taken some 24 months to improve understanding of the fundamental of composite machining and the focus of this research was the milling of CFRP composite – commonly known as routing. The review embodied the work carried out by the major players in the aerospace industry, such as BAE Systems, Boeing and GKN Aerospace. At that time, polycrystalline diamond (PCD) tooling was commonly used by the industry.
A number of leading PCD tooling suppliers were contacted and these cutting tools were tested on the baseline CFRP materials, which would be used for the A350 XWB aircraft.
Slotting was chosen as this was the most demanding operation due to the deployment of up and down milling in one revolution cut and the tendency of clogging in the slot. The test criteria included uncut fibres on the top/bottom ply, onset of vibration and residue of dust in the slot.
A typical test condition was as follows: tool diameter, Dc = 12mm; radial depth of cut, ae = 12mm; axial depth of cut, ap = 5mm; cutting speed, vc and feed per tooth, fz = varied.
All tests were performed dry on 5-axis MTorres TorresMill gantry machining centres as shown in Figure 1. The TorresMill has a high speed spindle of up to 35,000rpm and equipped with vacuum to enable dry machining of composite.
Further in-depth testing to characterise the cutting tool quantitatively included cutting condition (vc and fz) optimisation, cutting force measurement, tool wear analysis, workpiece analysis and tool life testing.
The following conclusions have been determined from this research on the carbon composite tool over the last 24 months:
• The choice of cutting speed was critical as it will affect the wear mechanism. Two dominant wear mechanisms observed are abrasive wear and chipping. A high cutting speed leads to premature failure or chipping of the cutting edge.
• High cutting conditions leads to high cutting temperature at the cutting tool and workpiece material contact area. Therefore, the composite dust generated has a greater tendency to bind leading to clogging at the cutting edge and smearing on the fibre matrix. • Coating appears to have a negative effect on the cutting edge of the carbon composite tool's cutting tool. Figure 5 shows an extended tool life for the uncoated cutting tool as compared to coated version and PCD. • The design of the carbon composite tool led to no obvious uncut fibres at the top or bottom ply. As the tool wear increases, the amount of uncut fibres also increases.
SGS Carbide Tool (UK)