With lightweighting in vehicle construction being a hot topic, AMS visited longtime aluminium users Jaguar at the company’s Castle Bromwich facility to see how they produce luxury cars from this material
Jaguar’s new all-aluminium sports car, the F-Type, has been making the headlines recently, but it’s only the most recent vehicle in the range to benefit from the company’s long experience in using this lightweight material in mass production. In 2003 the XJ (X350) was built with an all-aluminium body; a major technical advance for Jaguar.
The latest incarnation of the XJ (X351) began production at Castle Bromwich in 2009 and continues the company’s development of aluminium construction. This lightweight alloy offers significant weight savings (up to 40% lighter than steel in BIW) but issues with the joining methods have limited its use in mass production. Current welding technologies (laser and Delta spot) are making the process quicker for higher volume production but Jaguar has opted for a riveted and bonded construction method. This process is slower than welding but suits the medium volume of XJ production. Another advantage for Jaguar is that it provides a very rigid platform, making the XJ very light and agile for a large luxury car. It’s also a method the company has perfected over the years.
There are two press lines providing both aluminium and steel parts for the XJ. The large multistage Schuler press produces parts ranging from door skins and wings to complete side frames. The loading of the stamped blanks is automated and there are two loading areas, one on each side of the press to enable a continuous feed of blanks; the robot switching sides when the supply needs replenishing. At the other end of the process the off-loading of the pressed parts is done manually to allow for a visual inspection and avoid any damage to the parts. The press alternates between steel an aluminium parts as demand requires but a minimum run is around 600 pieces. The dies, arranged in order and stacked in sets ready for changeover, are colour coded for either steel or aluminium and have a system of simple but effective visual indicators to alert the press operative as to the status of the die (if it is ready for use, in need of cleaning or maintenance or not to be used). This is a necessary precaution as, given the size and weight of the dies, there isn’t enough space to store them in the die shop.
The dies for aluminium parts require more frequent checks and cleaning as these are more prone to the softer material ‘picking up’ on the die surface. Switching dies between components or materials takes about two hours (depending on the number press operations required for that part). This is facilitated by an overhead crane which employs a simple method for aligning the hoist to the die. A series of ‘bullseye’ targets marked on the floor between the die sets, and located by a camera on the crane, indicates the stopping position for the crane with great accuracy.
Heath and safety is one of the key reasons for employing this system. The second smaller press (also Schuler) also produces both steel and aluminium parts but these are smaller structural components. Throughout the plant storage bins are identified as for either steel or aluminium parts to avoid any contamination of the components. Jaguar is currently planning on expanding the press shop to allow the stamping of blanks in-house; currently this process is outsourced.
Moving from the press shop to BIW there is a display XJ body revealing all the elements of its construction. The main feature here is the use of rivets instead of spot welds. A selfpiercing, self-sealing type of rivet is used, coated to avoid any corrosion issues. The rivets vary in length depending on the application and are fed to the rivet guns via rolls of plastic strips. The rolls of rivets are mounted on hubs of different diameters depending on rivet length; this helps avoid the wrong length rivet being placed on the wrong station. Each body will require over 3,000 rivets and 150 metres of bonding adhesive in its construction. The adhesive is cured as the body is heated in the paintshop. Suspension towers and door window frames are made from cast aluminium and smaller brackets are made from steel (again given a special coating to avoid corrosion). Perhaps the most striking visual aspect of this method of construction is just how neat and precise it looks even before painting. It’s easy to imagine the rivets distorting the alloy panels, especially where they holding together multiple layers, or the bonding adhesive oozing from every joint, but modern vehicles are built very precisely and Jaguar’s expertise and experience with this method of construction is clear to see.
As well as producing a very light, rigid body, an additional benefit is a better working environment for the staff than if spot welding were used, says the company. The XJ production line is 50% automated with approximately 110 robots in use from the press shop to finished vehicle.
Interestingly, parts of the body construction are manual operations. While the lion’s share of riveting and bonding is automated, there are a number of stages where riveting and bonding require manual operation. This indicates the volume of production which allows this almost ‘handmade’ approach. It should be noted that this isn’t a reluctance to embrace new technologies but rather opting for the process that works best for the application. The lower volume of completed XJ BIWs per day contrasts with the production volumes of steel, welded mass produced vehicles.
It also highlights one of the reasons for the limited number of all aluminium models on the market; the production process is slower than producing spot welded, steel vehicles. An ABB robot cell performs three hemming and clinching operation for boot lids, bonnets and front wings, but these parts are then fitted to the body manually to ensure correct gaps are achieved. Given the prestige segment Jaguar’s cars occupy, quality control is a high priority. Perceptron vision systems are used throughout the bodyshop tp provide accurate quality checks. Again the doors are hung on the body and adjusted manually. Special weights are placed in the doors to simulate the weight of glass, trim and electrics to be fitted at final assembly, ensuring a perfect door gap and fit at that stage.
Diversification of specification
Before leaving the bodyshop for painting the bodies pass through a highlight inspection bay where operatives wipe a highlighting fluid over the body to reveal any damage to the panels that might need reworking prior to painting.
After painting, the bodies are sent to a marriage line where the powertrain and suspension are fitted. The wiring looms are stored in a preheated unit on the lineside which makes fitting the looms easier. Although the volumes are not high the levels of customisation are, with a number of engine, trim and equipment options available – and with a high number of vehicles for export, the diversification of specification increases.
