Manufacturers of large structures often need to remove weight from the product, which has pushed the greater use of composite materials. Leading the way are aircraft structures, wind turbine blades, Formula One monocoque cabins and, increasingly, structures on cars. The Jaguar F-Type has carbon-fibre wind mirror covers, bonnet louvres and a variant has a carbon-fibre roof.
The growth of carbon-fibre applications has driven new machining technologies for composites. Machining techniques, cutting tool designs and workholding solutions have been refined, but the biggest area is cutting tool design, with a growing range of tooling companies introducing new designs to overcome the problems of machining composites, which behave very differently to metals. Layers, or plies, of fibre and resin bonded together react differently when placed under the force of a cutting tool.
“The biggest difference is in the chip formation mechanism, which you have no control of for composites, as the material shatters,” said Dr Kevin Kerrigan, composites machining technology lead at the Advanced Manufacturing Research Centre (AMRC) with Boeing near Rotherham. Dust is the predominant feature when composite machining, as the cured epoxy resin layers disintegrate and throw up micro-sized particles.
The challenge for tool designers is to create a geometry that first deals with the chip formation mechanism and then the damage you encounter with fibre (FRP) or glass-reinforced plastic (GRP).
Damage mechanisms can vary with the bonding of two different materials, the epoxy plastic and the fibre itself. Under mechanical stress, these layers can separate or delaminate.
“Delamination is one of the biggest challenges for tool suppliers, in that there are so many damage types and every application has a different requirement,” said Kerrigan. “The tool designer has to design a product for the customer’s own needs.”
The solution is more cutting edges and diamond-coated carbide tools. Diamond coatings reduce burring on the material surface and extend tool life – diamonds have a low wear rate. The coating type depends on the application. In a high-volume application such as aerospace parts, many companies use diamond-coated chemical vapour deposition (CVD) tools. Carbide is chosen as it is cheaper than diamond to manufacture.
The fastest-growing type of coating is polycrystalline diamond (PCD), made by sintering micro-size single diamond crystals at high temperature. PCD is cheaper to manufacture than monocrystalline diamond.
”Delamination is one of the biggest challenges for tool suppliers, in that there are so many damage types and every application has a different requirement.
Dr Kevin Kerrigan, Advanced Manufacturing Research Centre (AMRC) with Boeing
Manufacturers experiment with the thickness of the PCD layer on the tool substrate to derive different effects. The AMRC is researching ways to bond crystalline carbon to carbide and to how much of the contact surface. “Some tool manufacturers are looking to minimise that contact to reduce the cost of the tool, or more to maximise the life of tool – it’s a fine balance,” said Kerrigan.
The main composite machining types are drilling and edge trimming. Each present different challenges. The variables for both are how the material is stacked and bonded, and the direction of tool approach.
With drilling composites, the approach to the surface will have huge effect on the damage it creates. “With a stack of laminate laid up horizontally, your tool design and the parameters set in your process need to be such that when the tool breaks through, you don’t delaminate or generate fibre or epoxy pull-out,” said Kerrigan.
A common problem is that when a tool reaches the bottom layer, it bursts through and separates the bottom ply from the rest, or pulls out large amounts of fibre from that bottom layer.
Engineers can use backing plates and peel ply layers to get around this, but some users will place the whole structure inside the machine, so there is no access to the back of the composite.
Consequently, some research, notably by Tobias Pfeifroth in Germany, has been carried out on the effect of the point angle and cutting speeds on hole quality – delamination, fraying and burr formation. Tool companies are devising clever solutions for this.
Exactaform, an independent diamond tooling company, has developed one-shot tooling. “A single tool is used for a single pass at high speeds, removing the need for a roughing and a finishing pass, typically required using conventional tooling,” said technical sales engineer Jamie White. Tool costs can be high, but for large volumes the cost is mitigated and even reduced by improved productivity, less tooling and fewer tool changes.
“One-shot routing using PCD tooling is the future for successfully mitigating the difficulties faced with composite-based projects and improving productivity to enable us to keep up with demand,” said White.
Edge trimming is often done in the same plane as the laminate sheets, posing problems. The specific needs of edge trimming have led to design innovations. The general rule is that more cutting surfaces compensate for the composite material splintering. KOMET has developed an nanocrystalline diamond (NCD) and HSC NCD multi-tooth composite milling cutters for this application.
A ‘burr-style’ tool design from OSG is the DIA-HBC4. This has two separate helices, a deep helix like a normal tool, then a second helix going in the opposite direction in the shank of the tool at a much shallower depth. “Those two helices replicate the chip formation process, providing a lot of small teeth making more individual cuts to compensate for the disintegration of the material, which is driven by the strain rate of the material,” said Kerrigan.
Carbon fibre has a higher strain rate than metals, so the mechanism failure depends on the fibre orientation to the contact of the tool. Depending on the angle, the effect could be bending of the fibre, and eventually it will snap.
While more cutting edges help – Exactaform has a 12-flute router and companies are working on 16 edges in a 10mm router – the challenge is to fit all those edges into the geometry, where currently the best combination is 12 edges, according to Kerrigan.
Innovations in tooling also include the diamond-to-substrate bonding process. Seco Tools’ solid-carbide cutters and drills use a six per cent cobalt substrate and the DURA coating.
Due to the high-grain boundaries-to-volume ratio, the coating can be applied in different thicknesses per application, increasing tool life. According to Seco, “getting the diamond to bond onto the substrate is always a problem due, in part, to the relatively sharp edges of the cutter – but more because of the presence of cobalt, used as the binder material.”
So before applying the DURA coating, the cobalt on the surface is reduced using a deep- cleaning process. This roughens the surface, allowing for the deeper seeding of the diamond – resulting in superior bonding to the substrate.
The next step change for composites is how technology can improve tool life. Companies such as Exactaform offer a process to recoat a PCD-coated tool. Another approach is to regrind a PCD tool to rebuild the tool geometry. The potential cost savings for customers who could have regrindable PCD burr geometry is profound.
Big companies such as Boeing, Airbus and BAE Systems can incur tooling costs that are upwards of 50 per cent of their manufacturing process. “Any tool cost reduction is massive for the overall process; that’s where PCD dominates over CVD.”
As well as better tool design, high-volume composite machining has low feed rates that keep throughput down. Ultrasonic machining produces 10-micron amplitude motions of a tool, at more than 20,000Hz or up/down movements a second, while the tool is doing its normal speed and feed. In a trimming operation, as the tool is rotating it is moving in the vertical direction by 10 microns. DMG Mori claims that ultrasonics can improve feed rates by 20 times. Modelling of this at the AMRC shows that the forces exerted on the workpiece are massively reduced.
The race is on to perfect recoated or regrindable diamond-coated tools with more edges, using ultrasonic machining in some cases, to supply the parts in to delivery schedules that some big OEMs insist on.
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