Fasteners put Mini assembly in the fast lane

When engineers at BMW Group set about designing the Mini, they looked for fastening solutions that provided greater benefits than simple rivet or weld type fixings.

Manufactured at BMW Group plant Oxford, the Mini combines the best features of the original Issigonis-designed street icon with the sophistication of latest generation passenger vehicles.

According to Body Engineering Design Leader, Ian Hill, the primary structure of each model is made up of approximately 300 individual panels, which are welded together by robots.

"This gives the vehicle its structural integrity and is the framework to which mechanical and trim components are subsequently secured.

Today, we increasingly employ automated rather than manual assembly methods throughout the plant.

This places more emphasis on accuracy and the minimisation of process variables - and is one of the main reasons we chose Profil for so many applications on the vehicle", he explains.

Profil UK's Adrian Ellis takes up the story.

"During an intensive two day assessment session with the design team, we not only presented the fundamental benefits of pierce/clinch fastening methods, but got down to discussing detail design and installation issues for a number of application areas.

These ranged from simple brackets on items such as the vehicle's fusebox and foot rests, through to 'safety-critical' areas including the steering column, bumper and subframe mountings".

"As these were in both 'dry' and 'wet' areas of the vehicle, there were associated requirements for water tight sealing in several cases too", continues Senior Body Engineer, Mitch Batten.

Through the company's presentation, Profil demonstrated that it could not only meet the specified performance criteria, but that substantial cost, productivity and quality benefits could also be achieved if fasteners were installed as part of the respective pressing's manufacturing process.

The use of purpose-designed automatic feed units and the inclusion of setting dies within the progression tooling has enabled this approach to be adopted.

As a result, a new finished part - complete with installed fastener - is now produced with every stroke of the press.

"It represents setting rates of more than 100 fasteners per minute - at least an order of magnitude faster than equivalent rivet nut or welded alternatives", adds Hill.

"With production of the Mini running at more than 2000 vehicles a week, this is a major consideration.

Equally, in-press installation ensures positional accuracy of +0.25mm - well beyond the capabilities of traditional welded fasteners".

"The consistency and integrity of pierce fastening methods are also inherently greater than for equivalent welded fixings, which are prone to variations in the base material, surface contamination and operator skill levels", continues Ellis.

A further benefit of the Profil method is that parts simply can't be produced without the fasteners installed.

With manual operations, there is always the risk of individual nuts or studs being omitted, which would cause serious disruption during final assembly.

Equally, welding operations introduce thermal stresses into the sheet metal structure, which may not only act as corrosion centres, but also adversely affect the vibration and fatigue resistance of the finished product.

By contrast, pierce fastening does not degrade or mark the coated materials used on the vehicle's body shell and immediately eliminates any risk of stray weld spatter adhering to threads.

"The latest generation of Profil fasteners are also far superior to traditional rectangular pierce nuts, which can induce high stress levels in finished assemblies", adds Hill.

"With all these advantages, Profil clinch nuts and studs have become the preferred fastening method wherever space is available in the press tools to accommodate the insertion dies", he continues.

"Today, more than 70 of these fasteners are employed in the build of each new Mini".

Ten-off M6 EBF type studs are installed on the fuel tank reinforcing rings, while various M6 and M8 RND types nuts are used on reinforcers in areas such as the bonnet hinge, door mirrors and handbrake.

In addition, M8 RND nuts are employed on the steering column mounting bracket, while a further six-off M8 studs secure the vehicle's front bumper assembly.

Profil fasteners are also specified for the dashboard crossbeam mounting, while the heaviest duty variants are reserved for the vehicle's subframe anchorage points.

These M12 nuts, which are mounted in the floor pan, incorporate large reinforcing flanges in order to provide the required static and dynamic load carrying capabilities.

"The principle of using Profil clinch and pierce nuts is well established and, when installed in-press, this method represents a highly attractive way of providing high strength threads in any sheet metal component", adds Batten.

The technology is particularly suitable for the high volume production runs found in the automotive sector.

However, unlike welded or blind rivet alternatives, pierce fastening technology can be tailored to provide cost-effective installation across the widest spectrum of production requirements.

For one-off and prototype work, pierce fasteners can be positioned and installed manually with as little as GBP 250 investment in tooling.

As customers' production requirements grow, the process can be developed with the introduction of bowl feeders, sorters and semi-automatic insertion equipment; or further enhanced - as in BMW Group's case - by the use of fully automated high speed insertion heads that enable one or more fasteners to be installed with every stroke of the press.

A further option is the use of robot controlled placement heads.

These are also ideal for vehicle bodywork or other large pre-formed components, where fasteners are required to be inserted in different work planes.

"In each of the above production options, final assembly integrity is embodied in the fastener and its placement tooling, rather than any dependence on operator skill levels", notes Ellis.

"With continuing automation of processes throughout the automotive sector, it is clear that the Mini will not be the last vehicle to have its assembly methods driven into the fast lane by Profil pierce and clinch fastening technology".

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