The fascinating power of
|Press shop||Stamping shop|
|Employees||ca. 1,200||ca. 325|
|Fertigungsfläche||40,600 qm||11,600 qm|
|Range of components||More than 800 serial parts; Range of components extends from small add-on pieces up to side panels with a length of four metres||Approximately 4,100 serial parts; Range of components encompasses add-on pieces, structural parts and parts produced by means of the Internal High-pressure Forming method|
|Consumption of material||1,200 t / day||325 t / day|
|Production volume||more than 300,000 / day||up to 1,250,000 Teile parts / day|
|Number of presses||79 - Press forces between 300 up to 9,500 t||25 - Press force from 100 to 5,000t|
|Tool weight||up to 50 t||up to 40 t|
|Retooling process - dependent on the facility||up to 40 retooling processes/day, Retooling times: 7 up to 50 minutes||up to 130 retooling processes/day, Retooling times: 20 up to 120 minutes|
The body assembly shop – Bonding, welding, bolting done to perfection.
Whilst in the pressing plant it is quite hard to imagine that the stacked sheet steel will take the shape of an automobile, it is much easier to get an idea of the end product in the body assembly shop. At BMW the construction of the body-in-white incorporates the gradual joining of several hundred individual steel and aluminium parts of many different sizes and thicknesses. In the body assembly shop the foundations are laid for the car’s final characteristics such as its crash behaviour. The main objective in the construction of the body -in-white is in simple terms the production of a high-accuracy body ready to be passed on to the experts in the paint shop.
An intelligent mixture of materials – large-scale use of high-strength steel and aluminium where it is feasible. Lightness, stiffness, paintability and corrosion resistance are just some of the ever-increasing demands modern bodies have to fulfil nowadays. This is why the use of a specific, intelligent mixture of materials has long been a tradition at BMW, most obviously and consistently realized in the production of the new 7 Series model’s body-in-white.
Light metals such as aluminium are used for this trendsetting automobile particularly where they entail true advantages for the customer, so for example in the production of side panels and the hood. The basic material used is steel of a higher strength. The intelligent mixed construction leads to a desirable reduction of weight resulting in a lower fuel consumption as well as an optimum distribution of weight on the axles, which has a positive effect on the dynamic performance as everybody knows.
82 percent of the BMW 7 Series model’s body-in-white consists of highstrength steel which offers advantages in terms of crash behaviour and weight. The rest is made up of aluminium. About 480 steel sheet and aluminium parts with a thickness ranging from 1 mm to 2.25 mm are welded, bonded and bolted in the body assembly shop. Almost 5,800 welding spots, the application of adhesive on more than 150 running metres, weld seams of an overall length of more than four metres, bolted joints on lids and fenders as well as further conventional inert gas weld seams make the new BMW 7 Series model’s body-in-white an integrated whole.
In order to limit the great variety of variants before assembly in order to comply with the customer-oriented sales and production process KOVP, only four different bodies-in-white are produced for the new 7 Series model: a left-hand drive version, a right-hand drive version, one variant with and one variant without sunroof. Bolts and fasteners are positioned in such a way as to permit the construction of all variants at a later date. With the introduction of the stretched version in 2002 the number of variants will increase to eight. For comparison: At the moment there are more than 100 body variants for the 5 Series models.
Body structure – driving pleasure and safety.
The outstanding body stiffness of the new 7 Series model decisively contributes to driving pleasure. Thus the initial flexural resonance is at 26 Hz, and the initial torsional resonance at 29 Hz. This means in plain language that the body is extremely insensitive to bumpy road surfaces or vibrations caused by the drivetrain.
This advancement was made possible by the use of state-of-the-art CAD systems for the computation and design of the structure and, amongst other things, the use of highly advanced body adhesives for additional reinforcement of the body joints. Compared with an unbonded body, flexural and torsional stiffness is up by up to 15 percent.
In addition to that, adhesive bonding results in an increase in the absorption of energy by approximately 15 percent, which is of great benefit to passive safety.
Safety – basic concept for a high standard of passenger safety:
•an extremely stiff passenger cell 82 percent of which is made of high-tensile steel,
•bonded body joints result in a 15 percent increase in the absorption of energy,
•increased load-carrying ability of the stress-bearing structures in case of frontal and and rear collisions as well as roll-over accidents,
•optimum use of the deformation zones,
•optimum protection against side impacts with the risk of intrusion reduced by 20 percent,
•compatible design of the front end structure.
New: Large-scale use of spot-welding adhesive bonding technology.
In 1999 BMW specialists began to use a new joining technology, the socalled spot-welding adhesive bonding technology, which is meanwhile well-advanced and used in series production. Before spot welding is done, an adhesive is applied to join the steel plates, which increases stability, acts as a seala nt and has a noise-dampening effect. With the start of production of the new 7 Series models in 2001, this technology was realized and used in industrial-scale manufacture for the first time.
Due to adhesion plus spot-welding technology large-surface flanged joints are created. This helps to increase the body stiffness resulting in an improved crash behaviour. At the same time this technology makes it possible to reduce the steel plates’ thickness and to do without reinforcing materials, the use of which has been necessary up to now. Adhesive bonding is a joining technology which has not long been used in the automobile industry’s construction of bodies-in-white but it shows an enormous potential. Through the use of special adhesives it is possible to join materials of the same type and of different types (synthetic materials and steel) to form a bonded and non-positive joint. The adhesives’ double action is taken advantage of in this process. The parts to be joined are bonded by adhesion (adhesion of the glue to the component) and cohesion (internal strength of the glue) at a low temperature.
Whereas in the past adhesive was applied to a flange length of about eight metres, adhesive on the new 7 Series model is applied to a flange length of 150 metres. The application of the adhesive is performed most accurately by robots on a large-scale basis.
New: Flexible inline metrology.
When the production of the new BMW 7 Series model started, the world’s first inline measuring units were installed at the Dingolfing body shop, which consistently monitor the serial production processes and carry out a 100 percent check of the dimensions by means of temperature compensated measuring robots.
The new measuring units are directly integrated into the production line (that is why they are called "inline") and are able to take threedimensional measurements. They help to ensure the accurate operation of the welding robots and are in a position to intervene where it is necessary. The sensor system has been fully integrated into the robots’ control system. The sensors can be programmed directly via the robots’ "teach panel". There is a cyclic alignment mechanism operating between the inline facility and the measuring rooms.
technology is most flexible, as it is robotized and can be reprogrammed within
a few hours. If the body’s measuring points have to be changed, for
example, the reprogramming only takes about 2 hours, whereas conventional stationary installations require much more time to be
reprogrammed. In the production process of the BMW 7 Series model’s body the four flexible inline measuring units check at 100 percent the front
end, the rear end and the underbody at 62 measuring points each as well
as the body carcass at 105 measuring points. A central data base for
all future inline measuring units is in the planning stage in order to
ensure a more efficient documentation in addition to process monitoring and
New: 180° flanging (edging of the shell) of the body-in-white’s rear lid.
The all-new striking design structure of the new BMW 7 Series model’s rear end called for adjustments in the production process. Whilst on the previous 7 Series model edging and the 90° flanging of the rear lid’s shell were done in the press shop, this is no longer possible on the new BMW 7 Series model. The inner panel of the "supported boot lid" can no longer be precisely fitted into the already edged shell. The solution is that the press shop now delivers the shell without flanging. This step has been integrated into the body assembly process. An additional tool device allows 180° flanging resulting in an harmonic joint of the rear lid and the side panel in the interest of good looks and a high degree of functionality. Specialists working in the BMW laboratories carry out endurance tests to check the rear lid’s functionality. During such a test the boot lid is opened and closed 12, 000 times at predefined speeds and different temperatures and must pass this test undamaged. The range oftemperatures extends from minus 30° up to plus 80° Celsius.
Simulation tool ROBCAD.
ROBCAD, a graphic three-dimensional software simulation tool is used for the most realistic planning of optimum processes, workstations and assembly operations prior to the start of series production. This system generates virtual solutions as to production which help to plan, check, simulate and program offline the production and engineering process. This tool is used to achieve a quicker and much smoother transition from design to actual production.
Aluminium: A highly sensitive material.
The 7 Series is the first model series on which BMW uses aluminium shell parts (front lid and fenders) on a large-scale basis. For many years the BMW specialists have built up their know-how necessary for dealing with this lightweight material. The BMW Z8, for example, has an allaluminium body, which is nearly fully hand-finished in the BMW Group’s aluminium competence centre in Dingolfing. Aluminium was also used by BMW several times in the past for different components, so for example for the aluminium engine block of the legendary BMW 507 in the fifties, or in 1973 for the BMW 3.0 CSL, whose front and rear lids and doors were already made of aluminium.
As far as the new BMW 7 Series model is concerned, automation and mechanization instead of manufacture are given top priority when dealing with aluminium. Aluminium is a very sensitive material to deal with. The tolerances are very tight when processing this material.
The body shop in Dingolfing – a short survey.
|Number of employees||approximately 2,800|
|Area||about 166,000 square metres|
|Main points of the production programme||5 Series and 7 Series body-in-white; Z8 body-in-white|
|Characteristics of the new BMW 7|
|Series body-in-white||around 95%|
|Application of adhesives to||a 150 metre length of flange|
|Welding seams||7.40 metres|
|Weight||arround 430 kg|
Within the BMW Group’s production network Dingolfing is also the centre of production for BMW chassis components and systems. Work is done by about 2,400 employees on an area of approximately 80,000 square metres. At the Dingolfing plant the BMW engine and chassis division mainly produces front and rear axles, front wheel and rear wheel drives for all BMW model series as well as wheelsets for motorbikes. The new BMW 7 Series model’s all-aluminium chassis is the result of a consistent extension and optimization of the employees’ know-how gained in lightweight construction and processing aluminium when constructing the 5 Series models. Back in 1995 BMW was the first automobile manufacturer to build a welded aluminium chassis on a large-scale basis. Not only does the aluminium chassis help to reduce weight, it also enhances traction, increases the car’s dynamic performance and lowers fuel consumption. When producing the allaluminium chassis of the new BMW 7 Series model advanced and all-new production technologies are used.
Example: High-performance welding process – tandem welding.
This technology provides for the use of two wire electrodes instead of one wire electrode when setting the weld seam at the front axle crossmember. Thanks to this technology more material can be melted on and the process speed for weld seams is two to three times higher than before.
Example: High-speed processing of aluminium axles.
The so-called High Speed Cutting (HSC) technology is used in this process. A high-speed milling technology for the mechanical processing of up to 140 surfaces, edges and bores. The processing time for the front and rear axle is reduced by approximately one third.
Example: Geometric simulation for the virtual determination of processes.
On the new 7 Series model all welding and handling tasks the construction of the suspension involves have for the first time been predetermined virtually. In the planning stage prior to the start of series production it was thus possible for the engineers to determine by means of computer simulation the optimum position of the installations and robots even before the hardware design. Thus the engineers had the opportunity to check the installations for accessibility and ergonomic layout well in advance.
Example: Internal high-pressure forming.
BMW is the world’s first automobile manufacturer to use the internal high-pressure forming technology in the construction of rear axles. The use of this technology incorporates the bending of tubes, which are later inserted into a forming tool. After that, hydraulic fluid is pressed into both ends of the tube in a high-pressure process, so that the tubes adopt the contours of the tool. Thanks to this innovative production method components can be formed that are so favourable in their geometry that they make optimum use of the area available in the underbody in the interest of the axles’ stiffness and stability and have considerable advantages in terms of weight over shell-type constructions.
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Source: BMW Group. March 2002.
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