Finding the right blend
7 mins read
Successful composites machining is achieved through a combination of correctly specified tooling, machine and software, ensuring little margin for error. Machinery reports
It's easy to see why composite uptake is on the rise – in comparison to aluminium, CFRP (carbon fibre reinforced plastic) typically has more than 14 times greater tensile strength, 19 times lower thermal expansion, five times greater stiffness and yet weighs only half as much. Such is their growing popularity that 25 per cent of the new Airbus A380 is made up of advanced composite materials, while on the new Boeing 787 Dreamliner, this figure rises to 50 per cent. Even modern military aircraft, such as the F-22, use composites for at least a third of their structures – and this is expected to rise to two-thirds in the future.
Structural aircraft components such as wing box, spars, stringers and skins are made from various carbon fibre and matrix materials, while other demanding structures, such as the central (fuselage) wing box, are made from several types of composite material stacked with metal alloys.
A WHOLE NEW BALL GAME
But, with the success of composites as aerospace materials comes new demands on machining capability. In the aerospace sector, typical hole requirements include surface finish of <4.8 Ra, delamination of less than 1 mm over the diameter and no splintering. Producing holes in CFRP requires unique geometries to achieve tolerance and quality, while combining CFRP with a stacked alloy such as aluminium adds another factor, as the two materials vary in machining characteristics. New from Sandvik Coromant (0121 504 5500), CoroDrill 854 and 856 diamond-coated solid carbide drills have been developed to withstand the harsh demands of the latest CFRP materials and aluminium stacks. The geometrical shape of CoroDrill 854 is designed to improve hole-entrance and hole-exit quality on high fibre content materials. In terms of productivity, typical CoroDrill 854 cutting data for producing ¼" (6.35 mm) holes in CFRP/aluminium stacked composite might be: 45 m/min cutting speed; 70 mm drill depth; 0.03 mm/rev feed rate; and 51 mm/min penetration rate. For best results, Sandvik Coromant recommends dry or MQL cutting conditions.
Correct drill selection is critical to successful composite hole making, in order to achieve optimum quality, process security and cost-per-hole economy. With this in mind, CoroDrill 854, with its 'spurred' geometry, is preferred where splintering or fraying is the problem, while customers will find greater benefit with CoroDrill 856, if delamination is proving to be the difficulty in resin-rich materials – this is primarily thanks to its double-angled geometry, which allows soft entry and exit of the workpiece material.
Image: Sandvik Coromant's CoroDrill 854 and 856, targeted at composites machining
Extensive cutting trials demonstrate the productivity and long tool life of these new CoroDrill products. For instance, using CoroDrill 854 to produce ½" (12.7 mm) holes in CFRP epoxy resin stacked with aluminium (12 mm + 12 mm) generated impressive results. At a cutting speed of 118 m/min and 0.05 mm/rev feed rate, CoroDrill 854 produced a total of 650 holes through the 24 mm thick stack – the equivalent of 15.6 m of drilling. All holes achieved a surface finish within <1.6 Ra (aluminium) and <3.2 Ra (CFRP), while maintaining IT8 tolerance grade standards. There was negligible delamination upon entry into the CFRP, while burr height upon exit from the aluminium was well within the 0.2 mm maximum allowed.
IN A DIFFERENT ORBIT
An interesting alternative to conventional drilling of composites is orbital drilling. This is where a tool is rotated in the machine spindle around its own axis and fed axially into the workpiece in an off-centreline position to progressively generate the desired diameter. By adjusting the offset of the tool to the centreline, precise sizing can be achieved. The process also enables a single tool to be used for a variety of diameters in the same component, and holes can be off-set on different centrelines, be produced with complex shapes or profiled with chamfers and even used to straighten crooked holes, because the process is not influenced by any pre-machining for positioning.
Helping fulfil this task is a new series of solid carbide and PCD tooling from LMT (01676 523440). Both are available with two helical flutes with through-the-tool feed for air or coolant in diameters from 3.4 to 10 mm. Flute lengths can be up to 60 mm.
Image:LMT offers tools to support orbital drilling of composites
New machining solutions are in high demand from a sector that is currently receiving extensive funding to help composites realise their full potential. For instance, at the end of last year, previous Business Secretary Lord Mandelson announced new investment totalling £22 million to advance the development of composite materials, which includes £16 million for a new National Composites Centre in the Bristol area.
In the UK Composite Strategy document, Lord Mandelson estimates that the composites market is currently worth about £1 billion to the British economy, while the UK offshore composite wind turbine blade and aerospace wing market alone could be worth £22 billion by 2020.
In another recent funding announcement, £1.4 million has been awarded to investigate new ways of using composite materials for wing panels in aircraft. The research represents collaboration between engineers from the universities of Bath and Bristol and the aerospace industry. It is funded by the Engineering & Physical Sciences Research Council (EPSRC) and aircraft manufacturers Airbus and GKN.
The research will focus on using carbon fibres that are curved within flat plates to produce damage-tolerant, buckle-free structures. It is hoped this will lead to cost and weight savings of 10-30 per cent on structural components, saving fuel and reducing CO2 emissions.
The rewards on offer for suppliers of manufacturing technology that match up to requirements are being ably demonstrated by Flow International Corporation, the parent company of Flow UK (01455 895300), which has received approximately $11 million of orders for composite machining centre abrasive waterjet and routing machine tool systems, to be completed over the next 12-18 months. The majority of the purchase orders are from Airbus supplier partners working on multiple Airbus programmes.
"As waterjet continues to become integral to the aerospace industry's adoption of composite materials, we are proud to again be selected as the only provider of composite machining technology," says Charley Brown, Flow International's president and CEO.
PROGRESSIVE INVESTMENT
In the UK, progressive companies specialising in composites machining are also pursuing investment programmes. A case in point is Melton Mowbray-based KS Composites, which has recently installed an Italian-built Promac Sharav GVT 3.5, 5-axis moving bridge milling centre supplied by UK agent Lemoine (01252 403613) – the first Promac Sharav to be sold in the UK.
Image: The first Promac Sharav to be sold in the UK went to KS Composites
Chris Mayne, managing director, Lemoine (UK), says: "This new machine will provide KS Composites with high performance 3- and 5-axis contouring capability, coupled with high accuracy, for all machining applications in composites and resin model board, as well as steel and aluminium." But successful machining of composites is not all about machine tools and cutting tools; software plays an integral role. With this in mind, aerospace subcontractor AIM Ltd of Clevedon recently invested in a series of 5-axis CNC machining centres, along with CADCAM software from Open Mind (01865 338026).
The AIM business model sees it conducting R&D projects, repair schemes and the updating of commercial and military aircraft, with particular emphasis on airframe structures, which increasingly utilise materials such as composites.
AIM acquired four, 5-axis machining centres and loaded them with high specification tooling. Despite the investment, there was still a bottleneck with part programming.
"We invited Open Mind to program one of our components," says managing director Rob Kendall. "They programmed the part in just over two hours, even with three of our programmers continually interrupting to ask questions. Using the previous system, our programmers took almost 10 hours to program the same component. The decision was made to acquire two seats of hyperMill. Our production times are now up to 50 per cent faster on our 5-axis machines and, while set-ups and the elimination of second operations is a major contributor, it is hyperMILL that has enabled us to move parts across with confidence."
