Demand for carbon fibre reinforced composites continues to rise, with an estimated 10% increase in 2019. This amounts to global sales of nearly $18 billion, according to Carbon Composites, a network of companies and research organisations working within carbon fibre composites.
Carbon composite materials are highly specialised and for the most part restricted to high tech applications. A notable example is Formula 1, where their lightweight and high strength make them ideal for use in the car chassis. At the same time, modern aircraft such as the Boeing Dreamliner uses more than 50% composite materials by weight.
One surprising potential use of composites however is in machine tools. Machine builders can use these materials for similar reasons to car designers: to create lighter, stiffer parts, and improve damping. The end result will be machines that run faster, with higher accuracy and reliability. This has a positive effect on productivity and part quality.
Despite the obvious advantages, there are a few hurdles to overcome when specifying composites for machine tools. One is their greater cost, which often puts them beyond the reach of more mainstream applications. Another is that awareness in composites is some way behind that of traditional materials such as steel and aluminium. There has also been a reluctance to switch from established methods of manufacture to a new way of machine design and construction.
Machine builders need to be aware of the specific benefits offered by composite materials if they are to consider taking advantage of them.
First, they are making larger machines, with moving elements that may be many metres long. At the same time, they need high-speed motion systems that can accelerate and decelerate rapidly. To add a further challenge, a high degree of stability, accuracy and repeatability is required.
In addition to these challenges, designers must also select suitable structural elements. These need to be low in mass, but offer enough strength and stiffness to support both static and dynamic loads with minimal deflection.
Composite parts can be ideal in these applications because of their combination of high strength and low weight. For instance, a carbon fibre machine element can weigh around a quarter of a steel one, for the same strength. These materials can also be engineered to deliver 20 times the vibration damping properties of steel or for zero thermal expansion in one dimension.
Real-world applications rarely focus on a single physical attribute, but carbon composite machine parts can be precisely engineered to offer a compromise between characteristics.
If the natural frequency of a part, or its walls, is too close to the operating frequencies of the machine, the resonance can impair performance. This can lead to instability or inaccuracy in use. The low mass and high stiffness of carbon composite parts already provides good damping compared with steel or aluminium alternatives. This can be enhanced by adjusting a part’s dimensions and wall thickness, to adjust its natural frequency. Alternatively, damping materials such as rubber and cork fillers, or internal foam reinforcements, can be incorporated into the structure.
In terms of thermal stability, composites are less prone to thermal expansion than metals, which helps to reduce potential distortion. This is particularly important over large spans.
All parts must integrate effectively into the overall structure of a machine. Appropriate fixing points can be built into the structure of a composite part during manufacture. These include extra layers of machinable material, to allow for the drilling of holes, metal inserts or pads to support tracks and brackets.
Cylindrical parts can be made using a technique called filament winding. Here, the fibre takes the form of a continuous ‘string’, which is impregnated with resin, wound at an angle around a cylinder, then cured. The resultant structure is very strong and light.
CompoTech has perfected its own version of filament winding, called axial fibre placement, over the past 20 years. The technique improves resistance to bending loads by winding the fibres along the length of the cylinder, rather than around its circumference. This aligns all the fibres in the axial direction, which increases strength: stiffness is 10-15% higher, while bending strength is around 50% higher.
The tool also imparts improved surface roughness. This means, in certain circumstances, it can perform the job that normally requires two steel tool sets, for rough and final machining. This increases milling productivity, decreases machining time and reduces machining cost.
The tool is made by depositing carbon and graphite fibre onto a steel part using a process called robot assisted filament laying (RAFL). The steel body acts as a mandrel, while also connecting the tool to the toolholder and the toolholder to the spindle. It also provides a way to attach the tool to the milling teeth.
Composites can also be used to make motor spindles. These are usually hybrid structures, with steel used in the bearing surfaces and threads.
Professor Atsushi Matsubara of Kyoto University in Japan has carried out a detailed study of the behaviour of spindles with CFRP shafts. His research shows that the shaft of the hybrid spindle is 70% lighter than a steel version of the same design. At the same time, the spindle shows 75% less displacement when heated to 70°C. The damping behaviour of the composite spindle was 16 times better than the steel version.
The research also revealed that the hybrid spindle reached its maximum rotation speed 17% faster than a steel spindle due to its lower inertia. Most significantly for machining operations, the stability limit curves showed that twice the limit chip thickness could be safely achieved at many spindle speeds. Cutting depth could be increased from 5.5 to 10mm with the hybrid spindle.
The main benefit of carbon composite machine parts however usually comes from improvements in speed, throughput and quality.