MACHINING


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 An engineer


inspects another component


for one of Lola’s LMP challengers


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 make for very costly scrap. However, with the material and the overall structures being inherently rigid, only basic fixtures are required. Given the flexibility of the 5-axis machines provided by CMS,


An LMP monocoque after machining


on the CMS Ares 36-26 PX5


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 means 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


24 | Composites in Manufacturing | Autumn 2010


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 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 new and with years of experience going back to CMS’ experience in woodworking, its composite machines include slideways that use a double seal system, which is 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. 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.


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


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