VLC 450 DG from EMAG: Perfecting the coated car brake disc in the grinding process

The Euro 7 standard is coming and with it very strict limits for tire and brake system wear. From 2035, cars will only be allowed to emit around three milligrams of particulate matter per kilometer, which amounts to a radical reduction, as this value is currently up to 40 milligrams depending on the vehicle! But how can this be achieved technologically? "Coating brake discs with hard material" is the answer from automotive designers and developers. EMAG's laser cladding in particular opens up many possibilities: Matrix material and hard material carbide are fed into a laser tool and welded to the preheated surface. In this way, first an adhesive layer and then the overlying wear protection layer are applied - at least in the case of so-called two-layer systems. There are also developers who rely on a one-layer solution, e.g. for the rear axles in compact vehicles.  

But one question is obvious: How can this approach be integrated into a holistic and high-volume production solution - including the subsequent grinding processes that give the wheel its final surface? "This question is our top priority," explains Jannik Röttger, Head of Grinding Technology at EMAG. "Our unique selling point is that, on the one hand, we can control and influence the turning process and the entire laser welding process on EMAG machines if the customer so wishes. On the other hand, we keep an eye on the subsequent grinding process right from the start. After all, the turning process and central coating parameters should be configured in such a way that the surface produced results in the shortest possible grinding cycle times and low grinding wheel wear with the highest component quality. Isolated solutions for just one of the two processes generate problematic performance figures."

Perfecting grinding processes

A look at the VLC 450 DG grinding machine - a special solution for brake discs that has been designed by EMAG experts in every detail for the respective coating system and the desired production process - illustrates what this means in concrete terms for mechanical engineering. In every case, the focus is on double-sided face-grinding as a process that is both productive and safe. It creates cross-ground structures on the component that exhibit very advantageous tribological behavior. At the same time, cross-grinding facilitates direct quality control, as the course of the grain entering and exiting confirms that all axis angles are set correctly. 

There are also other process advantages associated with double-sided face-grinding. The associated symmetrical force application enables maximum process feed rates. The large-surface contact between the grinding wheels and brake discs ensures optimum component support and thus high component quality of the brake discs, which are very unstable in the axial direction. The coolant used is mineral oil-free water (i.e. not an emulsion), from which the small grinding particles can be easily filtered out. Close cooperation takes place here with the manufacturers of cutting fluids and filter systems. "Incidentally, this is where users benefit from the vertical arrangement in the VLC 450 DG, as the grinding sludge contains high-strength abrasive particles. With a horizontal tool arrangement, this can quickly lead to problems on the guideways. We have this challenge under control with our machine," says Jannik Röttger. 

In general, the entire EMAG machine concept is designed for the highest productivity level and durability in order to meet the high demands of batch production. For example, a comprehensive cooling concept ensures good thermal stability, the Mineralit machine base minimizes vibrations and a machine shuttle outside the work area forms an ideal interface to downstream automation. The pick-up spindle, which brings the components from the shuttle into the work area, minimizes chip-to-chip times. The grinding tool of the VLC 450 DG also sets new standards in the machining of brake discs: It is designed as an interchangeable ring and has a low weight of less than five kilograms. As a result, tool change times take less than ten minutes - remarkably short! Overall, the vertical concept with a very easily accessible work area offers a high level of set-up friendliness. All change parts are easily accessible for the operator.

A holistic approach is essential

Last but not least, it is important to take a holistic view of the entire process chain for the hard coating of brake discs, as both processes have a decisive influence on each other. It is important to know that although laser cladding always produces a similar surface roughness (Rz up to 100 micrometers), the strength of this surface varies greatly depending on the coating system. Consequently, single-layer and duo-layer systems are ground in different ways: The first variant allows for high cutting speeds, whereas the second variant requires a motor characteristic with rather lower motor speeds and high torque at the same time. In other words, the monolayers are similar to hardened steel, while the wear layers of the duo-layer systems have a cutting behavior like carbide. EMAG's own grinding spindles provide the right performance parameters for both applications to enable efficient machining.

There are also other parameters, such as the choice of abrasive, which are crucial for the productivity of the grinding process. "A direct exchange of information between the development departments of the machine manufacturer, grinding wheel manufacturer and grit material supplier is essential here. Only if there is a continuous exchange of knowledge can the optimum grinding wheel be jointly developed for each coating specification," says Jannik Röttger.

"Overall, it is crucial that grinding, coating and laser experts work closely together and establish the best possible overall solution on this basis, so that the coating has optimum properties in the braking system and is easy to machine at the same time," emphasizes Jannik Röttger. "We offer this complete package of mechanical engineering and application knowledge to our customers, including in the context of prototyping - and this naturally also applies when EMAG only develops the grinding technology. In any case, with the VLC 450 DG, customers benefit from a machine that has been specially developed for this application and at the same time has all the typical qualities and decades of experience of EMAG mechanical engineering."

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