PURE ALUMINIUM

XK body structure

So what's the big deal about aluminium? Beneath every XJ's multiple layers of mirror-finish paintwork is a body structure elaborately constructed of the famously lightweight stuff. Cars that are lighter can accelerate faster, handle with more agility, stop more eagerly, use less fuel and produce fewer emissions.
But also important is the way all the aluminium components come together, to create a body structure that's not only extremely light, but also exceptionally strong, setting the XJ apart. Which is why we're at Jaguar's revolutionary Castle Bromwich body assembly plant, in the heart of the English Midlands, to find out how the XJ's body is created.

  • Jaguar XJ aluminium body structure

    "We didn't decide to use aluminium for the XJ's body structure simply because it was new or something different. It's because aluminium delivers real and significant benefits for the people who drive the XJ,” says Bob Holtham, Body-In-White superintendent at Castle Bromwich.
    "Ultimately, an owner may not care whether the body is made of aluminium or steel," says Bob. “But what the Jaguar owner does care very much about is the way the car performs, the way it rides and handles. And that's where aluminium delivers."
    Bob says the one unique feature of the whole XJ body assembly operation is its use of rivet-bonding technology. It's a similar process to the way an aircraft's fuselage and wings are built. Zinc-coated, self-piercing, boron steel rivets are used in combination with aerospace industry-sourced epoxy adhesive to 'weld' the aluminium pressings, castings and extrusions together, and to create the super-stiff structure.
    In the construction of the XJ's body, a team of 88 robots applies almost 3,200 rivets and more than 100 metres of structural adhesive. The result: a body that is immensely stiff - 60 per cent stiffer compared to a steel equivalent. Something that helps the XJ's suspension work better to deliver benchmark ride comfort and responsive handling.
    It also results in a body shell dramatically lighter than before - around 200kg less than the average rival in its class. Less weight means less mass to move, so improved fuel economy and lower emissions.
    It also helps make the XJ safer. Bob explains that being lighter than an equivalent steel body, the XJ's structure carries less kinetic energy into a collision, reducing the amount of energy that has to be absorbed by the vehicle body.
    Walk into the XJ body assembly plant and you might be surprised to discover that the assembly process doesn't start with twirling robots, or gooey adhesives. It starts in a massive, 9,600 square-metre, state-of-the-art stamping plant adjacent to the assembly operations.
    It's here where 13 giant presses stamp out no fewer than 125 different aluminium parts. These massive, earth-shaking hydraulic presses deliver up to an incredible 2,000 tonnes of pressure, transforming flat sheets of shiny aluminium into curvy body sections. Everything from door skins to roof sections, to spare tyre wells and the boot lids and bonnets that go into making the XJ's entire structure.

  • Jaguar XJ aluminium - Robots

    Different thickness aluminium sheets are used for different areas of the car - the gauge, for example, is 0.9mm for the outer body panels and 3mm where the loads are greater and extra strength is needed.
    Lift up some of the body components produced at the plant and you can feel the weight-reduction in your own hands. An XJ's front door, for example, weighs just 10.5kg - that's 45 per cent less than an equivalent steel door. That beautifully sculpted bonnet?
    Despite its size, it tips the scales at just under 11kg - 54 per cent less than the previous model.
    And aluminium isn't just used for stamping. It's used for various extrusions and castings that carry the heavy loads in the body structure. Bob points to the XJ's doors as a good example of how all three forms of aluminium come together to provide strength with low weight. Here, the hinge pillar is a casting, the outer skin panels are sheet stampings, while high-strength extrusions are used for the impact beam inside the door and the doorframe.
    In total, the number of parts that make up the XJ's so-called ‘body-in white' - that's the bare-bones body shell of the car that you can see on the previous spread - add up to 334, which comprises 284 stampings, 35 extrusions and 15 castings.
    Is every part of the XJ's body made from aluminium? Bob shakes his head and points to the cross-car beam that runs behind the dashboard. "It's magnesium. Stronger than aluminium, and just as light."
    The assembly of an XJ's body structure doesn't start on ground level with the floor of the car, but with a section of aluminium pressing that makes up the car's dashboard.
    "The car comes to life here - a computer prints out a bar code that shows whether the model being built is either left- or right-hand drive, long- or short-wheelbase, with sunroof, or without,” explains Bob. “That bar code also determines everything from the power train to be fitted, to the body colour, to the type of leather and wood used inside. And that sticker will travel with the car all the way through the assembly process to the paint shop."

  • From here, it's a frenzy of activity as robots twirl and whirl, and featureless aluminium pressings quickly come together to create the gorgeous curves that form the complete bodyshell.
    And the major difference between the Castle Bromwich plant and most other body assembly plants in the motor industry? Sparks. Or lack of them. Here there are no explosions of fiery molten metal as body panels are welded together. Just the near-silent ‘tush-ah' as rivets are injected into aluminium.
    "It has transformed the way cars are built. Castle Bromwich used to be noisy and challenging, with sparks from the welders flying everywhere. Not any more," says Bob.
    To see the robots in action is a piece of pure mechanical ballet. We watch as a robot's arm suckers-up a panel from a stack, lays it in a jig, then switches the sucker for an adhesive gun and lays a line of cherry-red bonding around the perimeter.
    Then a second arm swings across with another body pressing, lays it on top, and with that muted hiss, injects a string of self-piercing rivets to merge the two together. Ken Rudge, who runs the frame and body side part of the assembly line, explains that the bonding adhesive has to flow easily so that it can be nozzle-applied by the robot. Yet it has to be strong enough, even in its uncured form, to hold a pressing in place – even if vertical – until it can be riveted.
    "The adhesive doesn't cure and create its incredible bonding strength until the bodyshell has been painted and is bake-hardened in an oven at 540 degrees Fahrenheit," he says.

  • As the completed bodyshell comes to the end of the line, there's one vital area it still has to go through. Here at the so-called Perceptron Station, no fewer than 67 laser-guided cameras measure hundreds of points on the body surface, checking that each section is correctly aligned and that the panel gaps meet specification.
    "Most car makers aim for plus-or-minus 3mm. But our aim is plus-or-minus 0.5mm," says Lloyd Pugh, group leader on the body framing line. “It's what helps give the XJ its look and feel of quality and craftsmanship."
    Before the XJ's body starts the next part of its journey, heading for the paint plant, it has to go through one final check. In a tunnel of stark, bright-white lights, the aluminium shell is checked and re-checked for imperfections by a team of white-gloved inspectors.
    "To us in the factory, these body shells are nothing less than works of art," says Bob. "The panels look so good in the raw aluminium, it almost seems a pity to paint them."

  • THE HUMAN TOUCH

    The robots on the XJ's production line are very good at moving chassis parts quickly and efficiently. They are also very good at precisely applying the bonding and rivets in the right place every time, day after day. The laser-guided cameras are very good at making sure that each part and each panel is exactly aligned, but there's still a need for the human touch. Before each XJ makes it to the paint shop, it is checked, caressed, sanded and dusted by a team at the end of the production line to ensure that every panel is absolutely perfect.

    [Issue 2, 2008]

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