Machining composites takes resolve, ingenuity and, of course, the supporting manufacturing technologies. Steed Webzell rounds up the latest developments in this evolving segment
Composites clearly differ to other engineering materials, such as metals and plastics, when it comes to machining. As is well documented, hardness is not the issue; rather, it is the material's abrasive nature and tendency to delaminate under cutting force.
Waterjet manufacturers will argue that their technology is one way to negate unwanted delamination, as well as side-step other hazards associated with conventional cutting tools, such as heat and fibre pull-out. These advantages are certainly proving popular with the aerospace manufacturing community. According to Paul Castle, business manager at Flow International (01455 895300), among those using Flow's waterjet and routing CMC (Composites Machining Centre) dual technology machines are GKN (A350XWB), Airbus (A350XWB), Bombardier (CSeries) and Boeing (787).
Believe it or not, Flow launched the first iteration of its CMC way back in 1991. Today, of course, it has evolved tremendously and current standard sizes are 6-50 m in the X-axis, 2-7.5 m in Y and 1-2.5 m in Z. According to Flow, other benefits of waterjet over routers include preferred part finish, up to 100x tool life, the elimination of secondary operations, no airborne dust, simple fixtures and higher feed rates.
Two Flow CMC machines are currently in service at the GKN Aerospace Western Approach facility in Bristol, where they are being used to machine A350XWB and A400M wing spars of 27 and 20 m in length respectively. The latter represent the first primary wing structures for a large aircraft ever to be created entirely in CFRP. The CMC machines offer abrasive waterjet cutting, followed by milling of the datum faces on the outer mould line, along with drilling of the fastener hole patterns and routing.
Gewefa (01225 811666) HSK 63A heat-shrink clamp toolholders are the preferred toolholder for the milling operations, offering the high rigidity and grip on the cutter shank necessary in these challenging operations, while Gewefa hydraulic chucks are used for drilling the fastener locations. Importantly, though, because of the hostile (wet) cutting environment, it has been necessary to manufacture the toolholders from stainless steel. Furthermore, with the machines having maximum cutting speeds of up to 30,000 rpm, all the Gewefa toolholders are supplied fully balanced.
Another supplier of dual technology waterjet/milling systems is WardJet UK (01142 218002). The company's gantry- style GCM series combines an Infini Winder 360° 5-axis waterjet head, as well as a 24,000 rpm, 5-axis milling head in a single gantry with over 1.5 m of Z-axis travel.
The cutting envelope is split, with one section having a standard waterjet tank, leaving the remainder of the cutting envelope open to allow very large parts to be mounted on the floor.
One such machine is now installed at the University of Sheffield's Advanced Manufacturing Research Centre (AMRC). At a recent open house event, the WardJet GCM cut and milled a carbon fibre cylinder with an outside diameter of 679 mm and height of 717 mm. The diamond of the WardJet logo was milled into the 5 mm wall at a depth of approximately 2 mm. A special fixture was prepared to hold the cylinder and to absorb the remaining energy of the waterjet stream to protect the opposite side of the cylinder.
"When you are rotating the cutting head, you have high pressure water, abrasive and air, and all the controls and power needed at the head," explains Rich Ward, president and founder of WardJet. "What we've done is come up with a method that lets us bring all that through a system with unlimited rotation and no need to unwind. To do that with water at up to 6,200 bar is quite some achievement."
For the next stage of the machine's evolution, WardJet is looking to integrate metrology."This is so we can perform 3D imaging of the component and then compare the images to the model. From here, we cut and measure the part, then compare the outcome to what we had originally."
Metrology in the form of touch-probe technology is also a packaged feature of the Antares 5-axis CNC machining centre produced by CMS Industries (01159 770055). In fact, one such package has just been supplied to the composites facility of T&R Precision Engineering at Colne, near Burnley (pictured). The Antares will be used initially to machine complex carbon fibre components for an aerospace customer, although T&R soon plans to roll it out on a subcontract basis to all sectors. The package also includes part location software, dust extraction and tooling.
Jason McQuillan of T&R Precision has been pivotal in the project and, together with his colleagues, has seen a threefold reduction in the time taken to produce the component from the initial estimate.
Another company to take recent advantage of technology supplied by CMS is Havant-based Formaplex, which has expanded its pattern manufacturing capacity in the UK by nearly a third, with investment in two new CMS Aries 4826 5-axis machines. The acquisition of the machinery completes a £750,000 investment programme in the company's composites and pattern manufacturing services. The new machines are producing patterns throughout the day and night for the majority of the 2013 Formula One field, which roared into action in Australia in mid-March.
Moving to cutting tools, recent market entrants include the new CoroMill Plura compression end mill for composites from Sandvik Coromant (0121 504 5500), which combines positive and negative helix design to 'compress' the top and bottom of the component edge. This minimises any opportunity for splintering, a common defect when machining CFRP and many other types of engineering composites.
Suitable for edge milling applications on material with a minimum thickness of 6 mm, the CoroMill Plura compression end mill features six effective cutting edges for surface finish (Ra) of well under 4 micron and high material removal rates. Conventional up-milling strategies are recommended, as these give less vibration.
SGS Carbide Tool (01189 795200) has also recently introduced a new line of composite cutting tools. The new Series 120 8-facet drill has been designed to create accurate holes, without splintering or delamination of the composite stack. Also added is the Series 31 carbon composite router, which features fewer and deeper flutes than the original Series 20 CCR to avoid clogging.
Finally, for milling and finish-cutting operations on composites, the new Series 25 compression router has a left- and right-hand helix, and is designed to compress the workpiece, such that it eliminates fraying.
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New robot for new materials
With the newly developed RX170 hsm (high speed machining) robot, Stäubli UK (01952 604984) says it offers a new solution for milling composites.
"Manufacturers who want to work with CFRP need machinery that is fast, precise, easy to operate, flexible in application and able to cover a large work area," says Manfred Hübschmann, managing director at Stäubli Robotics.
The robot does indeed bring new freedom and flexibility into play. Thanks to its range of 1,835 mm, even large workpieces are easy to process and, when used in combination with a linear axis, the area of work can be expanded at will, so even 1 m long components can be machined without difficulty. Its repeatability of ±0.04 mm also exceeds the general requirements that apply in CFRP processing.
First published in Machinery, April 2013
Author: Andrew Allcock