High-speed machining: productivity and accuracy

Citation

(1999), "High-speed machining: productivity and accuracy", Aircraft Engineering and Aerospace Technology, Vol. 71 No. 6. https://doi.org/10.1108/aeat.1999.12771fab.003

High-speed machining: productivity and accuracy

Keywords Machining, High speed

Precision engineers, mould-makers, the aerospace industry and the motor industry are today discovering the virtues of high-speed machining. With a high-speed spindle and a suitable structure, these machines make it possible to increase the speeds of rotation of cutting tools by a factor of ten or more. They ensure productivity gains of 30 to 40 per cent on all the operations involving removal of swarf. They also make it possible to machine metal down to very fine parts, with thicknesses of less than one millimetre.

Realmeca has the advantage of being both a manufacturer and a user of machine tools for high-speed machining. "We have been manufacturing high-speed machining centres since 1987", says technical manager Francois Lhuillier. "We use eleven of them ourselves in our workshops and they have accumulated a total of 70,000 hours of experience".

Realmeca's machining centres have high-speed spindles that operate at up to 45,000 revolutions per minute with a power of 17kW. They manufacture parts in volume from 10 to 400mm per side. These compact machines are built from high density composite materials. They are fully streamlined for the safety of workers and the protection of the environment against oil leaks and gas emissions.

"Machining at high-speed is very suitable for light alloys and composites", emphasises Lhuillier. "We use it ourselves to manufacture medical prostheses in titanium or electrodes in graphite for spark erosion". Graphite is difficult to manufacture with conventional tools.

"The energy released by high-speed machining is 95 per cent dissipated in the swarf with little heating of the part", notes Lhullier. "The milling tool literally cuts the metal without subjecting it to internal stresses. It is why the result is better than with conventional machining". Mould makers who manufacture moulds for the glass industry also appreciate high-speed machining. It makes it possible to manufacture in bulk the very hard metals for glassmaking moulds without them having to undergo heat treatment.

The well-known manufacturer Huron, located not far from the European Parliament in Strasbourg, has been developing high-speed machining centres since 1992. "We introduced in 1998 a new range of machining centres that is much appreciated by mould-makers because of its gantry structure", notes Guy Bitterolf, Huron's sales manager. These new machines are equipped with a vertical high-speed spindle operating at 15,000 or 24,000 revolutions per minute with a power of 10 to 20kW. Their structure makes it possible to support a high torque on the milling high-speed spindle. The tool works in a volume with a side of up to 1,000mm. These features are especially suitable for manufacture of injection moulds, and dies for forging and cutting and other tooling.

The aerospace industry, a large user of light alloys, is convinced of the advantages of high-speed machining. "The milling of our large dimension parts can take dozens of hours", explains Daniel Marchand, an R&D engineer at Dassault Aviation. "To shorten this time we examine all possible methods to accelerate the machining". In its factories located close to Paris, the French plane maker has been testing high-speed machining since 1981 on large milling machines fitted with spindles with magnetic bearings. This world innovation was not pursued further since there were not available at that time the numerical control and axes of displacement capable of tracking the tools at more than 40,000 revolutions per minute. Dassault Aviation today is equipped with machines fitted with high-speed spindles. The most recent is a gantry machine with a worktable 11m long and a high-speed spindle capable of operating at 30,000 revolutions per minute with a power of 12 kW and a feed rate of 12kW. This machine carries out milling on structural parts of up to 4.5m. The parts are prepared in time concealed by the operator at one end of the table while the machine works on the other end.

The manufacturer of the machine, Forest-Line¨ designs and manufactures large dimension gantry machining equipment for plane makers throughout the world, as well as more compact machining centres for mechanical engineers and automotive manufacturers. It was the first manufacturer to exhibit high-speed machining equipment at the Chicago biennial exhibition in 1984.

At the world exhibition of 1999, Forest-Line¨ introduced another innovation with a vertical milling machine equipped with linear motors. These motors replace the traditional ball screws. They provide displacements of the high-speed spindle with very high accelerations - up to 0.5G, or 5m/s². Displacements can reach a speed of 60m per minute, or three to five times higher than with ball screws. Linear motors considerably reduce maintenance for the machine because they have very few parts. They also have an impressively silent operation that is appreciated by the workers.

PCI designs and manufactures high-speed machining centres. High-speed machining offers a productivity almost as high as traditional transfer machines but it brings the flexibility of conventional machine tools. "It is only necessary to change the tool, the part rest and the machining program to change from one production to another", asserts Olivier Vidal. "It only takes a few dozen minutes".

PCI's machining centres are designed to operate for hours and hours without supervision, with automatic supply of parts and a built-in tool store. Automotive manufacturers such as Peugeot SA and Renault are in the process of setting up vast machining shops with dozens of machining centres of this sort, lined up as if on parade.

PCI has designed its high-speed machining centres to give them a very stable structure, capable of supporting ultra-high accelerations and rotation speeds. "All the planes of the axes for the guidance of the high-speed spindle pass through the centre of gravity of the machine", explains Roland Blin, manager of the industrial procedures office. "This is an innovation that ensures perfect stability for PCI machines, even with very high accelerations". The transverse X axis was designed with two parallel ball screws, rather than only one, to enable the other Y and Z axes to pass through the centre of gravity of the machine.

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