The abrasive truth about composites

Image: Composite cutting with Jobs technology Practically all aircraft manufacturers are turning to composites to replace certain metal components and assemblies. In fact, many manufacturers of high value products are increasingly employing composites to take advantage of their strength, stiffness, durability, corrosion resistance and light weight. Some experts say that, in 10 years, there may be even more composite going into wind turbines than into all aircraft. Furthermore, metal matrix composites are being used for high performance automotive parts, such as brake rotors. And, because composites can be transparent to X-rays, they are likely to find new medical applications as well. The shift towards composite materials entails a similar shift in the way composite parts are made. To better understand what this means for machine shops, some reflection on the mechanical structure of composites is helpful. By definition, composites are not homogenous in the way metal is. A 'composite' is a combination of two or more materials engineered to achieve better properties than either of the component materials could achieve on their own. In a composite, one material is the matrix and at least one other is the reinforcement. Carbon fibre reinforced plastic (CFRP), the predominant composite material in aerospace parts, comprises a plastic matrix with carbon fibre reinforcement. Machine shops attempting to process such materials face a combination of challenges. The matrix could melt from excessive heat, while the carbon fibres do not cut easily, because they fracture instead of shearing smoothly. Meanwhile, the layers that constitute CFRP structures can easily splinter or delaminate during machining. So what special demands are placed on the machine tool when processing composites? Well, the contoured shapes of composite components usually demand a 5-axis machine tool. However, the amount of power and torque required for cutting metal is typically not needed for composites, at least not for CFRP. In fact, CFRP can be machined efficiently on lighter-duty CNC routers that generally never see a metal part. A case in point is witnessed at FACC AG, an Austrian-based specialist in the design and manufacture of composite components for commercial aircraft. With a turnover of around €270 million and over 1,600 employees, recent investment at FACC has seen the installation of a Jobs Jomach 146 gantry-style machining centre, equipped with a direct motor, torque-type milling head and NC suction hood for composite dust extraction, and two LinX linear motor milling centres from the same manufacturer (Macro Engineering, 01920 487711, represents Jobs). The most recent LinX machine features Jobs' Compoflex universal tooling system, designed specifically for the clamping of composite parts. Workholding for machining composites is critical, because clean cutting without fraying, delaminating or otherwise separating the layers requires the part to be secured firmly in the fight against vibration. Compoflex, which can be used with any Jobs machine, offers multi-functional flexibility to reduce set-up times significantly, and FACC is using the system to machine a family of 22 composite nacelle parts (panels to cover aero engines). Compoflex comprises 170 jacks that are independently controlled by software, each with an articulated head equipped with a suction cup at the extremity. Each jack is able to detect the precise position of the workpiece. Motorsport is another sector that is increasing its uptake of composite materials and the high concentration of racing teams based here in the UK has led Brackley-based Crosby Composites to acquire a 5-axis machining centre from CMS Group (0115 977 0055). The first set of 17 components produced on the CMS Ares 3626 was supplied to one of the company's many Formula One team customers shortly after the machine was commissioned late last year. All the parts fitted on to the racing car with no interference, avoiding rework and fitting-up. According to Crosby, it was the first occasion in the team's history that this had happened with any set of composite parts from any supplier. "It is important that we equip ourselves with the very best technology available," says managing director Paul Crosby. "The new machine has extended our pattern-making and component trimming capabilities, which are critical in today's composite machining marketplace." The CMS machine is provided with protection against the ingress of carbon dust and fibres to the electronics, slideways and spindle, as well as an extraction hood that turns in line with rotary axis movement. Image: Flow's combined water jet and milling unit hybrid machine. It offers a significant advantage in reducing set-up times MACHINING ALTERNATIVES Although advances in machine tool and cutting tool technology are developing fast, it is no secret that machining composites has its challenges. When seeking alternatives to traditional machining, initial thoughts probably turn to non-abrasive methods such as laser, EDM, ion beam/electron beam cutting and microwave cutting. All have certain advantages and yet all have their drawbacks, too. For instance, because carbon fibre is difficult to melt, it is easy to introduce a heat-affected zone next to the cut when using laser. EDM struggles with certain materials exhibiting poor conductivity, such as fibreglass and ceramic matrix composites, while ion beam, electron beam and microwave cutting methods are typically only applied to thin sheet material, and cannot be applied to contoured surfaces that require machining to tight tolerances. When it comes to composites, arguably the most successful alternative to conventional cutting tool-applied machining processes is water jet. According to water jet specialist Flow International (UK – 01455 895300), abrasive waterjet (AWJ) technology offers several advantages that include: zero distortion, due to limited jet forces and its nature of micro-machining action; no heat-affected zones; higher cutting speeds than routers; no delamination, splintering or fraying; and no dust. Recent technology advances include the use of vacuum assist in AWJ cutting heads, which has been critical in the successful shape-cutting of composites. An external vacuum source is used to draw abrasives into the cutting head before starting the waterjet. "This ensures instantaneous action of the AWJ upon firing the waterjet and impacting the material, and it has been shown that delimitation does not occur when piercing composites using this approach," says Dr Mohamed Hashish, senior vice president – technology at Flow International. Boeing has used waterjet for years for its 777 tail parts. Aircraft manufacturer Raytheon is another Flow customer and uses water jet to cut composite parts for its Premier 1 jet. The most common waterjet processes used for airframe are trimming, shape-cutting and drilling. Trimming is typically performed on the edges, while shape-cutting is deployed on interior surfaces to produce openings such as access holes or windows. In order to trim and rout composites, waterjets and solid tool routers have been incorporated together on special hybrid systems. In these systems, two 5-axis masts are used: one for the AWJ and another for the router. "The AWJ is used to trim the part using an end 'effector', while the router is used to drill and countersink the required holes or trim some critical areas not easy to address with the AWJ," says Dr Hashish. "This system provides significant advantage in minimising set-up time." Airbus turn to waterjet Image: Airbus has awarded Flow International a multi-million dollar contract to manufacture and install multiple Composite Machining Centre (CMC) abrasive waterjet and routing machine tool systems. The machines will be built and installed over the next six to 12 months for use in manufacturing the new Airbus A350 XWB (Xtra Wide Body) aircraft. Airbus will use Flow's CMC waterjet systems throughout all of its plants across Europe to cut and trim the wings, fuselage, wing spars, keel beam and other parts on the A350 XWB, which is Airbus' response to market demand for a medium capacity, long range, wide body family. First published in Machinery June 2009