Drilled for accuracy

When British Aerospace Defence called for bids to supply machine tools for the production phase of the Eurofighter in 1993, its request was met with bemusement in the industry. Besides calling for a machine capable of achieving unheard-of accuracy, BAe’s specification expressed its requirements in volumetric terms; most machine tool makers were accustomed to thinking […]

When British Aerospace Defence called for bids to supply machine tools for the production phase of the Eurofighter in 1993, its request was met with bemusement in the industry.

Besides calling for a machine capable of achieving unheard-of accuracy, BAe’s specification expressed its requirements in volumetric terms; most machine tool makers were accustomed to thinking of errors, and quoting accuracy, in linear terms.

As one expert puts it: ‘Most machine tool companies quote directional repeatability. They stay away from quoting volumetric figures because it might reflect badly on their machines.’

‘At the time, there was little understanding of volumetric accuracy,’ adds Malcolm Blount, a specialist engineer at BAe Salmesbury.

Now, following a unique collaborative development process involving BAe, machine-tool maker DS Technology and Huddersfield University, the required accuracy has been achieved. The solution lay in the development of a volumetric error correction system (VCS), which has so far been fitted to five machines delivered to BAe. DS Technology plans to offer the system on other machines.

BAe had clear reasons for demanding this level of accuracy, says Blount.

The first was interchangeability. Traditionally, aircraft panels have been individually adjusted to fit by hand; so if one part is damaged, replacing it is not a simple matter. For the Eurofighter, the extensive use of composite panels potentially added further complications, since composite parts cannot be adjusted to fit, as metal ones can.

So the Eurofighter specification called for spare panels that could be picked off the shelf and fitted. This implied improved levels of accuracy. ‘The better the accuracy, the more automated the production process can be and the easier it is to achieve interchangeability,’ says Blount.

The second objective was to maintain accuracy throughout the expected 15-year life of what would be large machine tools on large foundations; over time, errors could be introduced through the foundations settling, wear and tear, and through minor mechanical damage caused by accidents involving the cutting head. ‘We needed to compensate for these without lengthy mechanical intervention,’ says Blount.

The £17m-plus contract, for nine machines, was won by DS Technology in 1993. From the start, says Blount, ‘it was evident that a lot of joint development would be needed. Both companies recognised this’.

An agreement was signed setting up a joint development team and a risk-sharing programme.

The first machine DS Technology worked on was a five-axis one for milling the edge contours of panels and drilling holes in them for fixings. Ian Percival, the company’s UK managing director, describes this as a very sophisticated horizontal-spindle machining centre.

Volumetric error is expressed as the size of a sphere within the machine’s working envelope. The specification called for a tolerance of 72 m anywhere within the machine’s envelope of 5m x 3m x 2.5m.

Conceptual design studies led to ‘a totally new configuration’ giving top and bottom support to the Y-axis column; usually it is supported at the bottom only, allowing it to flex when the spindle is at the top. Low- shrinkage concrete was used for the support, which proved superior to cast iron for damping vibrations.

Initially, DS Technology’s engineers believed they could achieve enough accuracy through integrity of the machine and its measurement system, and its careful calibration. When the first machine was tested in 1996 this proved not to be the case.

‘Calculations led us to believe we could get close to the required tolerance through designing mechanically to the best possible accuracy. We calibrated the machine very carefully but even with temperature control we didn’t get the volumetric accuracy we required,’ says Percival.

At this point the team learned of the work of Huddersfield University’s precision engineering centre on error correction. ‘My team specialises in separating out errors and devising algorithms to correct them,’ says Dr Derek Ford, director of the centre.

The DS Technology machine, in its original form, achieved an accuracy of around 200 m. ‘That is a very good figure,’ says Ford. ‘For a typical machine that size the figure would be double that. A CNC machining centre with an envelope of 1m3 would typically give a figure of 80 m.’

Following the initial approach from DS Technology, Huddersfield put the machine through its three-axis correction programme to determine which errors were the most significant. DS then attempted to reduce them by modifying the design.

‘We were close but couldn’t guarantee the accuracy over the 15-year life,’ says Percival.

Huddersfield was consulted again. ‘We said: these are repeatable errors ,’ says Ford. ‘ We can put one of our correction packages on to bring it within specification.’ Huddersfield was brought into the team and the addition of its three-axis correction software was a success.

Though this brought the machine within the target range, there was a feeling that it could be improved even further. ‘We then signed an agreement to develop a package for five axes,’ says Blount. ‘This was a true blue-sky development by all three partners a real innovation that wouldn’t have been possible with any of the team members missing.’

The nucleus of the development team was five strong, with two members from Huddersfield, two from DS Technology and one from BAe. ‘Attacking the problem purely theoretically, to give an exact correction, proved too complex,’ says Blount. ‘We decided to aim for 90%. By correcting for just the 10 biggest errors, we simplified the problem dramatically and achieved the 90%.’

This brought the error factor down to an astonishing 27 m. The system is being added to DS’s other Eurofighter machines supplied so far: two horizontal-spindle machines used for structure components and two vertical-spindle machines for the recently opened High Tech Tooling Centre (The Engineer, 22 May).

DS Technology has the rights to offer the technology to other customers who need ultra-high accuracy machines like a vertical-spindle rotating table grinding machine for a maker of large diameter bearings but not to competitors of BAe.

‘We will add the system to future machines,’ says Percival. ‘The cost is low compared to the overall cost of a large machine. And the beauty of the system is its flexibility: it can be added to a variety of machines.’