Manufacturing gear wheels in 34 seconds

Minimising production times to ensure competitiveness is one of the most important challenges in the automotive industry, a factor recognised by Henry Ford over a century ago. After decades of optimisation, it is difficult to reduce machining times further while maintaining the same level of quality. Nevertheless, Volkswagen (VW), near the German town of Kassel, has managed to achieve this in its gearbox production facility using Kapp Niles gear grinding machines available in the UK from the Engineering Technology Group (ETG).

The Volkswagen plant in Baunatal is one of the group’s larger German locations with a workforce of about 17,000. Production here focuses mainly on car gearboxes in 10 different series. Half of the manufacturing lines feature gear production centres from Kapp Niles.

Kapp Niles machines also find use in the production of the DL382 dual-clutch gearbox for Audi. A total of 16 gearings are required to shift the seven gears within this type of gearbox – 10 ground and six honed. The production unit runs 24 hours a day, 5 to 6 days per week, depending on demand. VW strives to achieve an EPEI (every part every interval) value of 1 day in the production unit, which means that all components can be produced on each day for the aforementioned gearbox. This type of streamlined production requires seamless processes and a high degree of flexibility.

Technical clerk Christian Hahn is in charge of the production process of the DL382 dual-clutch gearbox: “We have five gearing centres from Kapp Niles in the wheel production unit and two more in the shaft production unit. To achieve an EPEI value of 1 day, we change over the machine in the wheel production unit twice every 24 hours. This way, we can produce 10 different wheels per day.”

The challenge with flexible production is the short cycle times. With an output of 880 gearboxes per day, one machine in wheel production must produce 1760 parts per day. Including all set-up times and failures, this yields a line cycle time of 34 seconds. An average line cycle time is 39 to 40 seconds.

Bernd Kümpel, application technician at Kapp Niles, analyses these figures: “Saving 5 to 6 seconds per cycle does not sound like a lot at first, but together it can be a 15% reduction. If I consider that at least 40% of segments cannot be influenced, I have to reduce the actual process time by 30 to 40%. Seen in this way, 34 seconds is a real challenge.”

A total of seven Kapp Niles machines are deployed which, with their low space requirement, are suited to the highly automated production lines at Volkswagen. The machines include three KX 100 Dynamic, two KX 260 Twin in wheel production and two KX 160 Twin in shaft production.

Hahn and Kümpel agreed from the very beginning that the desired cycle time could only be achieved with a combination of several measures. To minimise the daily set-up effort, Hahn makes sure that the wheels which are to be produced on one machine have bore holes of the same size. Thus, he has to change over the machine, but not the clamping tools. The remaining set-up time is minimised by the intelligent set-up concept of the KX 100 Dynamic. For one machine, he needs just 20 to 25 minutes.

“The semi-automatic set-up makes the KX 100 Dynamic extremely user-friendly,” says Kümpel, describing the process. “All you need is an Allen key for the entire set-up operation. With it, you operate the hydro-expansion clamping chuck of the dresser roll. Everything else is connected without any screws via HSK interfaces.”

An additional visual aid is available in the form of a menu guide and an easy-to-understand cycle on the machine controller. By completing the step-by-step process and the acknowledgement screen, the operator ensures that no work steps are executed incorrectly or forgotten. It thus becomes possible to prevent high-cost failures. The tools are dressed using full profile rolls, allowing all threads of the cylindrical worm to be approached and moulded simultaneously. Thus, with a five-pass full profile roll, the dressing time can be reduced by more than half without compromising on quality.

The integrated measurement system is another important time-saver. Hahn explains the advantage: “After each changeover, quality measurement has to be made outside the machine. We continue to require this, but I can already check the basic, quality-related parameters with the integrated measurement sensor in the machine itself. It saves a lot of time since we can start production before the results of the external measurement are available.”

The integrated measurement system of the Kapp Niles machines thus accelerates the restart process considerably. Furthermore, the external measurement merely checks more teeth and generates the measurement report to monitor the gear.

The search for optimisation potential also includes the actual grinding process. Cubitron II machine tools by 3M show a highly promising approach with geometrically-specific, triangular-shaped cutter heads, compared with conventionally dressable grinding wheels.

“With these, you can step it up a notch, to say it plainly,” states Hahn. “That is, remove more material in one thread, and remove it faster.”

For this purpose, Kapp Niles provided relevant preparatory work with a large number of grinding tests in-house to use the benefits of this machine tool on the DL382 components.

Says Kümpel: “With CII you can remove a considerable amount of material without any thermal damage to the component. This way, we reduce time consumption by a solid 30% compared with other grinders, depending on the component.”

Production is characterised by a belt chaining (or linkage) that goes through the entire production hall. Among the employees, it has gained the nickname ‘the highway’. The available space is limited, hence the compact KX 100 Dynamic machines are the preferred choice. This machine type has two separate rotatable mounted columns, each with vertically movable pick-up axes featuring one workpiece spindle. While a workpiece is being machined, the other pick-up axis places the machined workpiece and loads a non-machined part on to the workpiece spindle. The workpiece is aligned outside the work area. This allows the workpiece spindle, already accelerated to machining speed, to be swivelled in the work area, keeping non-production times to a minimum.

A transfer unit does the loading and unloading from the conveyor belt. Kümpel says: “We usually move with the belt directly below the machine. However, this was not possible here. With the transfer unit, we compensate for height and distance from the belt to the machine. An integrated automation solution would have been significantly more expensive, at about 25% of the price of the machine. A simple transfer unit costs less than 10% of the machine price.”

The time for conversions and commissioning is, in most cases, very limited. But the highly ambitious goals have been achieved.
“Throughout the process, I‘ve been very satisfied with the on-site support and the local service,” says Hahn. “We were convinced by the machine concept and managed to overcome any obstacles together. The cycle time was a critical aspect. But, we did it.”

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