Experience is everything

The milling and drilling of modern composite products may still be seen as a ‘black art’ by many, but with the aid of machinery developed specifically for the task, suppliers and OEMs have been honing their skills and experience to produce repeatable, reliable products. One such user is Lola, which has been producing highly complex components for its motorsport programmes as well as the safety-critical defence and aerospace industries for some time. Simon Lott reports.

While in the early days of the industry, it was common to see the milling of carbon and glass fibre products taking place on machines designed for metallic components, the wide differences in the nature of the material types and the improved quality requirements of newer products meant that specialist equipment soon entered into the market. CMS Industries has been supplying the market since 1969 and as one might expect, has refined its products to a high standard.

By their very nature, machines designed specifically for the milling of composites need to be more versatile than their metallic counterparts, accommodating not only a wider range of materials, but wider variation in their inherent properties. Lola, for example, has invested in two CMS Ares mills – a 36-26 PX5 and a 48-18 - to aid not only its core motorsport offering, but a whole range of other applications. For example, parts currently produced onsite include large naval radar structures, vertical wind turbine blades and fuselage sections for the Watchkeeper and Mantis UAV programmes, all hand laid in its three aerospace standard clean rooms and cured in one of five autoclaves or two curing ovens.

As many different components covering many different application requirements pass through the facility, the mills employed at Lola are regularly employed for the processing of carbon fibre parts with all kinds of fibre thicknesses, tow thicknesses, resins and layer orientations and which have undergone a variety of curing processes, leading to considerable differences in strength and stiffness between what are often low-batch components. Similarly, the mills are utilised regularly for the trimming of one-off epoxy blocks for patterns and moulds where there is little room for error and can also be used in the milling of aluminium, although Lola rarely does so.

No compromise

Due to the complex and specific nature of many of the components produced at by the manufacturer, most of the patterns produced and milled on the CMS machines are one-offs, so the need to achieve ‘right first time’ quality is of primary importance and extensive work goes into model preparation. There may be as much as £3,000-£4,000 carbon fibre in one raw blank, so, aside from whatever tooling requirements there may be, machines need to behave predictably.

With this in mind there are three other key considerations that define composite mill design: speed (both in the axis guideways and in the spindle), fixturing and dust. CMS UK general manager Toni Nagiel describes the difference between metal and composite milling as ‘the difference between a lorry and a sportscar’. For example, due to the relative stiffness and reduced weight of glass and plastic composites over their metal counterparts, spindle speeds tend to be much higher, with CMS machines typically operating within a range of 10,000-24,000rpm. As a consequence, to maintain the precision required of customers and with force of the materials being much lower, the machines themselves are built with much lighter, stiffer components. This ‘lightness’ then allows the guideways to accelerate much faster, minimising non-cutting time.

Inevitably this speed, and the fact that the majority of components are ‘dry cut’ mean that heat generation is an equally important consideration in the composite arena, to some extent limiting the rate at which material can be cut. Temperatures above 180°C for example can affect some resins and similarly, ‘hot spots’ at the point of cut can substantially affect tool life. Heat propagation can also vary depending on the density of layers, with more tightly packed materials being more forgiving when it comes to finishing. To negate these problems, CMS machines are provided with a refrigerated air blower system, with liquid cooling also available for the spindles to ensure continuous duty cycles are possible.

Material dynamics

Fixturing, on the whole, tends to be a simpler affair, but again depends a great deal on the lay-up and thickness of materials. Thin parts for example, such as aircraft interior footrests produced at Lola, are highly prone to vibration, compromising tolerance and cutter life. As with any component, a whole variety of clamping methods are available, but given the typical lightness of composite products, the machine is also provided with a vacuum system. In contrast, one of the most complex parts that Lola produces is the monocoque for its Le Mans Prototype (LMP) coupé challenger, of which eight competed at this year’s 24 hour race (as well as two open-cockpit Lolas). Second only to F1 in complexity, complete LMP cars can spend anything between 300-400 hours in wind tunnel development, sell for £900,000 - £1 million each and which make for very costly scrap. However, with the material and the overall structures being inherently stiff, only basic fixtures are required.

Given the flexibility of the 5-axis machines provided by CMS, most parts produced by Lola can be completed in a single set-up, however if this cannot be achieved there is an additional fixturing consideration when milling large, complex carbon fibre parts such as the LMP monocoque. A great deal of excess material is trimmed from the initial part, including the windscreen,

door apertures and a variety of of holes, suspension points and inserts, but most challenging is the fuel filler area, which must be strictly parallel on both sides of the car. The problem lies in the fact that the carbon fibre structure is prone to a certain amount of movement after moulding, so when it comes to the second set-up, the component does not exactly match the CAD model. As a result, each of the flanges around this area need to be carefully probed and written into the program to compensate for any twisting that might have occurred. Some suspension parts are also machined in relation to a centre line to ensure that they are not affected by twist.

The final major concern to be addressed is the dust produced in composite milling operations, which manifests itself most obviously in the machine construction. While a major problem, this is not a new one and with years of experience going back to CMS’ experience in woodworking, its composite machines include slideways that use a double seal system, and that are also situated above the workpiece to minimise the amount of contact. Similarly the spindle requires a particular sealing system to ensure that bearings are fully protected. Several vacuum systems can also be provided – either a high volume option extracting from the whole machine or a high velocity option on the spindle.

As with most things in life, many years of experience in the sector has allowed CMS to move beyond many of the problems associated with composite milling, although new types and grades always require careful consideration as far as feeds and speeds are concerned. However, with confidence in their machinery, companies such as Lola are continuing to develop more efficient and repeatable milling processes, ensuring their products remain competitive in price, as well as on the racetrack.

www.lola-group.com
www.cmsukltd.co.uk