On the light track

4 min read

Laser interferometer technology ensures accurate measurement of aircraft and car components can be carried out rapidly and with high precision. Mark Venables reports.

When considering measuring applications, small online probes on metal cutting machines come to mind. But what happens when the item to be measured is a car or a plane?

Because measurement inaccuracies in aerospace and car construction can have enormous negative consequences, quality assurance and measurement technology are paramount.

To ensure quality at its Bremen facility, Airbus uses Leica Geosystems Laser Trackers. The company's engineering and manufacturing functions are centrally managed, but rely on fully integrated cross-functional and trans-national team working practices, which bring together the capabilities and technical knowledge of 16 different locations in Germany, France, Spain and the UK. Each location produces complete aircraft sections, which are subsequently transported for final assembly in Toulouse or Hamburg.

In addition to individual components for the German parts of the aircraft, the Bremen facility produces all the necessary landing flaps for Airbus in the structural assembly process.

Erwin Hoffmann, as the leader of the nine-member measurement technology department, is responsible for examination of large-capacity components, dismantling and removal of production tools, monitoring of examination systems and calibration of all examination systems on-site.

'Thanks to the in-built laser interferometer, measurements can be carried out rapidly and with high precision,' he said. 'For both single point and surface measurements, the Laser Tracker can record data from a single set-up position on objects with a measurement volume of up to 80m diameter, with an accuracy level of plus or minus 10 microns/m, at a measurement rate of up to 3,000 points per second.'

Areas where Laser Tracker can be employed particularly effectively include high-precision tool manufacturing and geometrical examination in the automotive and aerospace sectors, as well as in other industries. Periodic inspections, repetitive tests and additional tasks can be carried out automatically with the help of the integrated absolute distance meter.

The system was used recently to examine the new countersinking facilities which are dismantled and examined using the Tracker, and collimation of tracks and landing flaps during the preparation of wing assemblies for the long-range aircraft (the A330/A340) is performed using the mobile measurement system.

The Trackers are also deployed during the process of putting into operation and approving the entire production system and machinery, as well as during structural assembly at the final examination of all large-capacity components produced at Bremen, and during individual component production.

Back in the UK BAE Systems' Samlesbury facility has a cell that machines carbon fibre panels for the aft fuselage and tail assembly of the F-35 Joint Strike Fighter. This was installed as part of a multi-million pound investment for designing, engineering and building major assemblies for the supersonic multi-role stealth aircraft. The cell comprises a Dorries Scharmann Technologie (DST) five-axis, high-speed machining centre to mill and drill the moulded panels and a co-ordinate measuring machine (CMM) supplied by LK to inspect them.

The innovative precision machining techniques ensure that the F-35 meets its stealth requirements. Very high accuracy is also required so that panel replacements fit perfectly to avoid re-work and aircraft downtime in service, and the CMM measuring equipment underpins these vital interchangeability requirements.

The machining and inspection cell is housed in a 45 x 18 x 9m high room, the temperature of which is controlled to 22.8ºC, plus or minus 1ºC. LK's G90CR rail-type CMM has a quoted volumetric accuracy over one metre of 7.5 microns (an accuracy normally quoted for smaller machines).

A measuring envelope of 5,000 x 3,500 x 2,500mm was specified for the machine, so that the largest 3 x 2.5m component for the F-35 can be inspected while clamped on a vacuum fixture, which in turn is mounted on a machine pallet.

This is exactly how parts are fixtured for the machining centre, eliminating any unnecessary variation in the process. Machining operations involve milling the periphery of the carbon fibre panels, machining the landings where the panels fit to the aluminium/titanium structure, and drilling fixing holes, the number varying from a couple of dozen up to several hundred depending on panel 0size.

Around 150 different composite components will be machined at Samlesbury for the three F-35 variants — conventional, short take-off and vertical landing, and the carrier version — each aircraft containing some 75 carbon fibre elements including both structure and panels.

As well as aircraft, cars are being built and tested to ever-more rigorous criteria, and buyers of the new Mini being produced at BMW Cowley will benefit from the stringent quality standards demanded by the German owner.

Moving production of the new Mini from Longbridge to Oxford presented quality engineers with some interesting challenges. One has been the application of quality standards already in place at other BMW plants, including the QZ quality programme against which all BMW vehicles are measured.

More than £1m was invested in measuring technology over a very short period in 1999 at the Oxford plant. This was originally meant for use in the development and manufacture of the Rover 75, generally agreed to be the highest quality car ever built there.

Dominating this investment are two Wenzel CNC-driven twin-arm RAD CMMs, both equipped with Renishaw PHS1 servo positioning heads.

These two floor-level horizontal arm CMMs are large enough to accept complete vehicles and were the first in the UK to be fitted with the PHS1 heads. Developed with the needs of the automotive industry very much in mind, the PHS1 is a two-axis motorised head with a continuous 360° servodrive and 0.2 arc/second angular encoder resolution.

It has been designed for horizontal CMMs and the head will accept a range of probes, which can use long extensions with a maximum reach of up to 750mm. This enables it to be positioned at any angle and to carry out checks in restricted spaces.

Together they are used to check vehicle build from single supplier parts through to body-in-white and body-less doors, on a range of fixtures that check for fit and alignment. One of the two machines, with a measuring volume of 6 x 3 x 2.1 m, is used in the metrology department for sub-assembly and component checking as well as for developing quality control programmes. The other, with a measuring volume of 5.5 x 3 x 2.1m used for measuring complete body-in-white assemblies and body-less doors, is placed at the end of the production line.