Making the new-generation Mini

  • 14-Dec-2013 07:10 EST
MINI 12-13 2a.jpg

Among the high-tech elements of Mini manufacture is measurement of body dimensional compliance.

There has been a population explosion at Mini’s Oxford, England, manufacturing facility: robot count has risen from 600 to nearly 1500, and Plant Director Frank Bachmann is delighted with the resultant increase in production quality.

“There are two particular aspects to enhancing vehicle quality: its design and the manufacturing input," he said. "There remain some aspects of building a Mini that cannot be easily or satisfactorily achieved manually. The consistent repeatability and process ability of a robot is much higher, which is why we have made a huge investment in them.”

Also significant for improved quality is the use of a new flexible, scalable, weight-efficient platform (designated UKL 1) for the MY2015 Mini Hatchback (Hardtop in U.S.) The platform is important “not only for us, but for the BMW Group as a whole, with common parts for both Mini and BMW models to increase flexibility within the whole production system,” said Bachmann. 

And there is expanded powertrain flexibility, with a new range of Mini engines, including three-cylinder gasoline and diesel units that will be built on the same line at Hams Hall.

In total, the BMW Group is investing £750 million in the three U.K. Mini plants as part of a program stretching to 2015. The substantial upgrades to the Oxford facility—where Mini Hatchback, Convertible, Clubman, Coupe, and Roadster are assembled—are complemented by those at its Swindon pressings plant where most of the Mini’s body panels and subassemblies are manufactured, and at its Hams Hall, Birmingham, where engines are built.

Oxford gets the lion’s share to provide a new, purpose-built body shop with the high robot count. “Body part total for the new Hatchback is 435, compared with [depending on version] 340 to 360 previously, so it’s a very big jump,” said Bachmann. Spot weld count has risen from 4000 for the previous Mini to 6000 for the new, larger version.

The body assembly facility accounts for a significant portion of the investment at the factory, its robots arranged around three framing stages rather than the usual two. “This body assembly method allows a greater variety of body styles than was previously feasible,” Bachmann said. At specific stations, eight robots work on a single body simultaneously, operating on three levels. The assembly accuracy of each body is measured using Perceptron cameras that achieve an accuracy of 0.05 mm (0.002 in). They measure the body across 502 points at five stations.

Unusually, the new Mini body shop operates a single common finished process system, explained Bachmann: “While most body shops operate two identical lines in parallel, Mini Oxford functions as a single line, each body undergoing the same process at the hands of the same operators and associated machines. The advantage is more consistent quality and greater efficiency.”

BMW has also invested in 1000 grippers for each of its robots. They facilitate the handling of tools and the wide variety of body panels for the multiple Mini versions. The robots operate using the latest programmable logic controllers; some use vision systems and have an intelligence level allowing them to pick components from a box.

Oxford was the first BMW Group plant to use the company’s Integrated Paint Process (IPP). Environmentally friendly, it eliminates the primer coat application and oven stages, thus saving energy, with a consequent reduction in emissions of more than 10%.

The paint shop now has further automation of the seam-sealing process and additional robotization for the top coat—both developments introduced to improve quality and reduce waste.

The seam-sealing system involves welded joints in the car’s interior, engine bay, roof, and part of the tailgate aperture being automatically protected against water-ingress via a flexible mastic material applied by a set of 12 robots.

“Besides reducing the risk of corrosion, this process also limits the passage of noise into the car. Automation ensures that the application of the mastic is performed to a consistently high quality and with less material waste,” said Bachmann.

Application of top coat to the door, tailgate, and bonnet apertures is now automated.

The new Mini has a double-skinned front bulkhead separating the engine bay from the passenger cabin to reduce the amount of noise transmitted into the cabin. Foam is robotically applied to the component.

Measuring equipment in the Oxford plant’s quality and engineering center includes a geometric optical measurement cell equipped with cameras that record 8 million megapixel images, each pixel representing a datum point. Said Bachmann: “A series of these images can be knitted together with very high precision to produce a 3-D picture of the entire car and every area within it. Color-imaging reveals the dimensional compliance of a Mini’s every element—from an entire panel to a single bolt. It provides an extremely accurate way of measuring a large volume and, among other things, is used for aligning the holes for fixings, ensuring that correct gaps between components are maintained, and that parts follow the correct sculptural profile.”

The overall objective in the quality and engineering center is to ensure that the car and its constituent parts (the Mini Hatchback comprises 3600 individual part numbers) comply with the original computer-aided design drawings—a process that’s fundamental to accurate manufacture.

The center uses a cube—an aluminum master buck that is a full-size replica of the new Mini. Built to a tolerance of 0.2 mm (0.08 in), it is the template for the body and everything attached to it. The cube is used to ensure that the car remains precisely manufactured throughout its production life.

Improvements to manufacturing efficiency include the use of height-adjustable skillets on two production lines; 44 automated guided vehicles to carry preassembled cockpits to the track; and six new robots in the glazing cell, two of which are equipped with cameras to identify different derivatives. The sunroof installation facility is now fully automated. Cameras are used to ensure the accuracy of the process marrying the engine to the car.

The company’s Swindon body pressings subassembly plant is debuting several technologies. These include tactile laser welding applied to the eight pressed parts that make up the Mini Hatchback’s complex hood panel. The technique involves bending the edge of the hood’s outer panel to the point where it touches the mating flange on the inner panel. The two are then joined using a fine weld wire on the laser that creates a continuous and solid joint to provide an effective anti-corrosion solution. A further innovation is the development of a remote laser that can produce long, continuous joints of greater precision; the process is quicker than spot-welding, and a first for the BMW Group.

BMW Group is currently operating 730 programs to reduce the carbon footprint and waste generated by all its manufacturing operations in the U.K.

Particularly significant is a new electro-coating facility. An interesting eco statistic emerges from the more precise control of the flow of air through the paint shop, which brings a 730 t (805 ton) saving in CO2 production thanks to reduced gas usage, and a 1130 t (1246 ton) saving from reduced electricity consumption. A program to better control the level of humidity in the paint shop saves 420 t (463 ton) of CO2 annually.

“Apprentice training is also applied at Oxford, leading to an NVQ [National Vocational Qualifications] Level 3,” Bachmann said. “It enables the company to tailor the education of our apprentices, both academic and practical, to better match the needs of the business in the future.”

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