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Magnesium castings are being used more and more in automotive manufacture, as they are cheaper than aluminium and also lighter, leading to greater fuel economy and better stability when cornering if the component is mounted above the vehicle's centre of gravity. Canadian-based Meridian Technologies Inc, the largest manufacturer of high-pressure magnesium die cast components for the North American automotive industry, opened a manufacturing facility in Nottinghamshire last March (2003) to tap into the growing UK market.
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Alongside the casting machines, rated at up to 3,200 tonnes, one of the first pieces of auxiliary equipment to be installed was a computer-controlled CMM (co-ordinate measuring machine) from LK to underpin all metrology functions from initial prototyping through to, and including, full production. Two of the first cars to benefit from the stringent quality control are the BMW Mini and the Land Rover, whose cast magnesium cross-car beams on which the fascias sit are supplied by Meridian.
When the company was visited last December, the configuration of the LK G-90C machine's 2,500 mm x 1,500 mm x 1,000 mm measuring envelope included a fixture across the back for the Mini cross-car beam and another along the left hand side of the granite table to accommodate the Land Rover beam. The accompanying picture shows a beam for the current Mini being inspected in the former fixture, while the latter was being used for checking a prototype beam for the new Land Rover, so photography of the component was not allowed.
In addition to supporting production of the Mini beam, the CMM was at the time involved in various prototype phases of four other components. The gestation period for a new project can be typically one year. Initially, a prototype die is produced from the CAD model and around 200 castings are run off, inspected, and analysed to identify which holes have to be moved. These are normally the critical features and require tweaking in response to crash test data and build issues.
Next, a production die and trim tool are manufactured and more prototypes produced, including some for machining, and these also need to be checked. Final adjustments to the part are made at this point. The last sample stage is known as PPAP (production part approval process) and requires a minimum of 25-off magnesium castings to be inspected, and up to 120-off to meet the requirements of some automotive OEMs or their first-tier suppliers.
Once in production, the first-off component from every die casting machine is checked at the start of each of the two daily shifts to ensure that a Cpk of 1.33 or better is being achieved. The Mini cross-car beam, for example, requires Meridian to check 45 holes whose positional tolerances are ± 1 mm. The cycle takes 25 minutes and the results are correlated against the CAD model supplied by BMW as a CATIA file and accompanied by hard copy drawings with the tolerances.
LK has supplied a specific CAD-to-CMM interface for CATIA, as used by BMW and Land Rover, to allow direct import of data from this CAD system. A further IGES interface has been provided within the LK CAMIO-Studio software suite. Once resident within the LK system, the CAD data may be used to program the CMM, instead of having to wait for the tool to be made and a sample casting to be run off. Thus as soon as the first components arrive, inspection can start straight away, eliminating a delay of about one week for preparing a new program by putting a prototype casting on the CMM and using the teach-in method.
A further capability within CAMIO-Studio is graphical reporting, which machine operator, Lee Brown, intends to start using in early 2004 to enhance the preparation of reports for both management and production staff internally, and for customers. Production of documents merging text and drawings is straightforward and for the most part automatic, and the style may be varied to suit the recipient's preferences.
Choice of the LK machine was driven by Phil Mangan of aluminium die cast tooling manufacturer, Preconomy, a sister company to Meridian located next door. Preconomy owns the CMM and leases it to Meridian. Mr Mangan has used a smaller G-90C 10.7.6 for over eight years for checking die inserts up to 850 kg as well as cast samples. One of the company's latest projects has been to manufacture the dies to make the cylinder block and head for Triumph's new "Rocket III" 2.3 litre motorcycle, which will be available this Spring (2004).
"We have never had any problems with our LK machine," confirmed Mr Mangan, "but we still researched the market before deciding on the larger model. In the end, it was the superiority of LK's software and the reliability of their service that were the deciding factors."
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