High-speed machining revolutionises production at Boeing Helicopters
High-speed machining revolutionises production at Boeing Helicopters
Keywords Helicopters,Machining, Marwin
The less an aircraft weighs, the more passengers and freight it can carry and the more money its operator makes. A great deal of research and development has, therefore, gone into finding ways of making aircraft components as light as possible, consistent with them having the specified strength. A supplier in the related technology of high speed machining of thin-wall structural members from solid aluminium is Wolverhampton-based Marwin Production Systems, which supplied to Boeing Helicopters, in Philadelphia, earlier this year (1998) a £2.75 million cell for manufacturing components up to 4m long.
Called Automax IV, the twin-spindle, five-axis machining centre is the largest machine of this design ever built by Marwin. Intended for continuous, unmanned production, the machine has three stations for 4,000mm × 2,200mm pallets, one in the machining area, another waiting with a fixtured workpiece and a third in the separate load/unload station. A chip management system has been incorporated to cope with the large volumes of aluminium chips, which are compressed into briquettes within the machine. A 160-tool store and loading system, integrated with automatic tool length compensation and tool life monitoring, serves the machine.
Overall dimensions of Automax IV are 20m long × 8m wide × 9m high, its installed weight is about 80 tonnes and the entire machine is enclosed by guarding to its full height (Plate 1).
The trend in the aircraft industry is away from riveted and fabricated sheet aluminium towards manufacturing parts wherever possible from the solid. Mechanical joining is unpredictable under stress, so a safety factor has to be built in, increasing component weight. Conversely, the performance of a component machined from a solid billet of aluminium may be assessed precisely using finite element analysis, resulting in a substantially lighter part. Moreover by avoiding the inaccuracies of riveting, subsequent assembly at the aircraft build stage is faster.
Intricate and accurate machining of components from solid is inevitably expensive, so great emphasis is placed on reducing manufacturing costs through high-speed machining. The problem is that wall thickness for an aircraft part is generally about 2mm, while non-structural components may be as thin as 0.75mm. This means that to avoid distorting the part during metal removal, a large number of very light cuts must be taken successively, rather than a few deep cuts.
Thus machining feed rate and acceleration/deceleration have to be high to maintain productivity. On the Automax IV, they are 20m/min. and 0.3g respectively. Cutter rotation also needs to be fast to maximise metal removal. The 24,000rpm spindle of Automax IV features variable bearing preload capable of delivering 50kW of power over a wide speed range.
These are maximum figures,so how has the machine performed in practice? With a 25mm diameter solid carbide cutter, programmed spindle speed is 22,000rpm while the full 24,000rpm is utilised for cutters of half that diameter. High-helix, two-bladed cutters are used for which optimum chip load is 0.25mm per blade, allowing feed rates of 10m/min. Boeing Helicopters is looking to increase this figure to 15m/min.,especially on the larger components, which is well within the scope of the machine's 20m/min. top feed speed.
Tools are 45 taper and held in a temperature chuck for good concentricity and rigidity.
Plate 1 Line illustration of the new Automax IV twin-spindle, high-speed machining centre from Marwin Production Systems, as used at Boeing Helicopters, in Philadelphia, USA
They are generally manufactured in the UK under licence from Marqart of Germany by Hydra Tools,Marwin's R&D partner in high-speed machining. However, Boeing has specified similar cutters from Tooling Innovations in the USA.
Currently, 10m/min. is programmed into the GE Fanuc 15MB control for around 90 per cent of the total of each milling cycle. Acceleration of 0.3g ramps the axis feeds to this speed on straight sections within 25mm from a standing start. Using high-precision contour control with advanced look-ahead facilities to anticipate the position and radius of impending curves, feed rates slow to a minimum of 2m/min. around the tightest turns.
Cycles based on these parameters achieve accuracies well within Boeing Helicopters' general requirement of 0.25mm on true position. First-off parts from the Automax IV were checked on a co-ordinate measuring machine and showed all profiled points to be within 0.18mm. The machining itself caused only a small amount of this inaccuracy, component deformation accounting for the majority. Moreover, surface finish was better than specified and showed a significant improvement over previous machining techniques, reducing the amount of subsequent finishing.
First on the Automax IV was a prototype under-floor beam for a new nine-seater, tilt-rotor aircraft. It was used during the development stages of the machine to explore the feasibility and limitations of machining large, thin-wall components. Size of the beam is 1,500mm long by 100mm at its widest point by 38mm deep (Plate 2).
Plate 2 Automax IV high-speed cutting trials on the under-floor beam for the D600 tilt rotor aircraft showing (top) the result using a conventional control and (bottom) how much thinner the walls are using high precision contour control
Prestretched, annealed aluminium is used to minimise component distortion during machining. Despite the high-technology, computer-controlled machining there is still a degree of "black art" involved in determining where, and by how much the part, will distort as it is machined. With this knowledge, steps can be taken to bring the finished machined part back into shape by selecting machining cycles which incur equal and opposite bending moments.
The billet is clamped on the machine pallet by vacuum, using a bespoke fixture and grooves with rubber seals. Five bolts provide extra holding power, especially where the base will eventually become narrow. Some 6mm is first skimmed off the entire top surface of the billet, which then forms the back face of the component. The effect of this face milling is to cause the billet to bow so that it is about 3mm proud at its centre. However, subsequent removal of large amounts of metal to form the thin-wall structure, results in the fully machined component being perfectly flat.
Next to be machined was an actual production part a leading edge rib for a Boeing 767 wing. Dimensions are 865mm × 305mm × 25mm deep and required thicknesses of base and wall are 1.78mm and 2.03mm respectively. It is clamped in a similar fashion to the floor beam using vacuum and mechanical fixturing. Left- and right-handed ribs are required, so rather than re-program using the mirror image capabilities of the control, Boeing Helicopters adopts the unusual policy of using left- and right-hand milling cutters and reversing the actions of the Automax IV.
Two parts are produced simultaneously within a cycle time of 80 minutes, which includes component and tool probing to determine the location of the parts on the pallet and to verify the length and diameter of the cutting tools. Automax IV automatically compensates for variations in tool length and diameter. First, a 25mm diameter cutter is inserted, removing 5mm of material at each pass, evenly, across the part surface to reduce distortion.
There are five internal webs which stand 12.5mm from the base. They are left at 6.35mm thick by the time the 25mm cutter has finished. A 12.5mm diameter cutter is then used to reduce the web and outside wall thickness to 2.03mm down to the level of the base. Again, only 5mm is removed at each pass with a high feed rate of 10m/min.,cutting being successively balanced either side of the walls. The cycle is completed by drilling two 6.35mm diameter datum holes and subsequently reaming them.
Another component has, so far, been produced, also for the tilt-rotor aircraft but this time a fuselage wing mounting. It has three locations for bolting the wing to the fuselage, a floor of varying thickness, one curved outside wall which is vertical and another which is angled at 25°. In addition, the walls vary in thickness from 2.5mm at the top to 3.3mm at the bottom. The angled wall profile was successfully generated in three CNC axes using a ball end mill. Left- and right-handed components were required, so rather than producing a second program for the opposite hand component using the mirror image capabilities of the control, Boeing Helicopters adopts the policy of using left-hand milling cutters and reversing the actions of the Automax IV.
The traditional way of producing such components in Boeing Helicopters' Philadelphia plant has been to machine them on a three-spindle, horizontal-bed, 5-axis gantry mill, with the spindles running at 3,000rpm. Despite the Automax IV having two spindles, its high-speed machining allows it to produce in one day what previously took one week to complete on the gantry mill.
The company now intends to produce small batches of dozens of different components on the Marwin machine,which has been designed for quick changeover. One of the plant's core products is the Chinook CH46 helicopter, for which a number of components are currently being redesigned to take advantage of the capabilities of Automax IV.
Additionally, the availability of spare capacity on such a productive machine is allowing Boeing Helicopters to import subcontract work from other Boeing factories.
For further details contact Marwin Production Systems Ltd. Tel: +44 (0) 1902 366633; Fax: +44 (0)1902 366573.
