New machining processes for aeronautical parts
New machining processes for aeronautical parts
Keywords: Machining, Aerospace industry, Tools
In the 1980s, the development of new generation tools, such as ceramic,polycrystalline diamond and ultra-hard coverings, enabled manufacturers to develop “HSM” machines (high-speed machining) at the same time as conventional machine tools, for working with aluminium, cast iron, steel and alloys of copper and titanium. Synonymous with productivity and cost reductions,this technology was of interest to the aeronautics and automobile industry. However, HSM uses modern means in extreme conditions with spindle rotation and advance speeds multiplied tenfold.
Franc¸ois Lhuillier, Technical Director at Realmeca, one of the first French companies to adopt the technique said, “We have to use a machine designed for HSM, give it a special spindle with a specific speed of rotation(35-45,000 rpm) and power (from 12 to 20 kW), equip the spindle with a special tool holder, carefully select the tools, respect a very specific machining method and master CADAM.” Only then does HSM offer significant improvements in performance, with gains in productivity producing cost reductions of 40 per cent and times divided by a factor of five. The multiplication of machining axes also makes it possible to generate more complex forms, which would be uneconomical with conventional means, such as thin sheets,widely used in aeronautics. It also enables the number of operations to be increased without dismantling as parts no longer require rectification or polishing. This reduces the investment costs of the associated tools, not to mention settings, a real problem on small runs, which only require a single intervention. Forest Liné, one of the European leaders in machine tools,developed all its components uniformly when it launched the Linéar Minumac in 2000. Abandoning ball screw spindles for linear motors in all axes,including rotating, this machine combines speed, velocity (feed rate of 60 rpm,acceleration of 20 m/s2 at the tool point) and trajectory accuracy,allowing the machining of thin walls.
“Direct axes drive systems have many advantages, such as no mechanical clearances, better rigidity, increased feed speeds, high acceleration, no wear and tear, low noise levels and reduced maintenance”, said Sales Engineer Yves Neboit Guilhot. For tooling of aeronautical structure parts, which are very large and tooled from a single block (80 per cent of the initial block ends as swarf), the speed of execution is an essential parameter. The company has two manufacturing plants in France, around one hundred agents in 40 countries,subsidiaries in Germany, the USA and China, and makes 50 per cent of its sales in aeronautics, and has a turnover of e45 million of which 80 per cent is exported.
In terms of dimensional capacity, the machines of Huron Technologie are particularly suited to tooled parts and prototyping. The first experience of this machine tool manufacturer, based near Strasbourg, in HSM dates from 1994. Since then, the company, with sales subsidiaries in Germany, Canada and Portugal, has developed the concept with machines available in three, four and five axes. The latest in the GX range, the GX 45 is a revolution for the manufacturer, using linear motors for the first time; the dimensions of the other machines did not need this system, which, due to ball screw spindles,cannot support the cutting efforts. With this new machine however, the manufacturer is aiming at greater capacities: 4,500 mm in X. In addition, Huron is currently selling the K2X, a transition to ultra-high-speed machining (UHSM)with a spindle speed of 18,000 or 42,000 rpm and maximum acceleration of 6 m/s2.
Dassault asked Realmeca to manufacture aeronautical parts and the Lorraine-based company made use of the need to make ultra-light and complex parts to design its own machines and develop expertise in HSM five-axis centres. The Realmeca range works to hundredths of millimetres and is designed for high-precision milling or complex parts. Currently, the company produces 140 machines per year and has installed over 3,000 machines. The machines are made of composite material which, as well as providing structural rigidity, ensures low thermal drift and better absorption of vibrations. This is an option which helps to improve surface states and the lifespan of the cutting tools, while allowing higher speeds. Till date, this company used high-dynamic motorisation(brushless with digital control) and is also tackling machining of even harder materials such as carbides, ceramics or glass substrates.
The latest generation, the RV 8SP high- precision vertical centre, uses digital techniques to improve the dynamic control of the machines, one of the weak points of HSM. Although HSM is more widely used, technical problems remain unresolved, and depend, in particular, on the development of CADAM systems,numerical control and the reliability of the spindle. Ultra- high-speed machines are also being introduced, assisted by the appearance of spindles with magnetic bearings. The next generation of machine should have feed rates of 50 m/min,acceleration of over 20 m/s2, spindle speeds of over 50,000 rpm and spindle powers of 50 kW.
The vulnerability of the electric spindle remains a weak point of HSM.“Controlling vibrations means increasing the rotation speed while preserving the spindle and improving surface states”, said AndréGreffioz, a specialist in structure calculations, at the head of a small company, ELPS, Etude Logiciels Procédés Spéciaux. Experts in behaviour analysis of high- speed machine tools, they have developed a range of software which, by simulating vibration phenomena on computers, enables operation to be optimised. They have just designed a new machine concept, the Skymill, manufactured by Sud-Ouest système. Designed to machine structure parts, Skymill is a three-axis machine equipped with linear motors and operated by parallel kinematics. Machining is done on the ceiling by a horizontal part support table turned downwards to evacuate swarf by gravity.
The Vosges sub-contractor CMW recently launched the Hexapode machining system after 8 years of development and 1.5 million Euros invested. Quick and easy to install, Hexapode allows HSM (in five simultaneous axes) on a traditional machine with no modifications. Equipped with an electric spindle(40-70 kW) and compatible with all CADAM systems, it is fixed on the existing machine support and is used like an independent milling machine. Special attention was paid to the simplification of use owing to powerful calculation software integrated in the control cabinet. The operator runs it like a simple milling machine. Driven by six motors, with six precision settings, it machines by successive links. With 200 kg of weight in movement it offers speed and accuracy operating /silently, without guideways, and its parallel structure ensures rigidity. Hexapode is available in three versions depending on the rotation speed of the spindle.
Details available from: F TPB. Tel: +44 (0) 207 235 5330; Fax: +44 (0) 207 235 2773.
