"We are responsible for the maintenance of a total of 22 hydropower plants, each with different performance parameters and characteristics," explains engineer Stefan Pfeifer from the mechanical engineering department at Illwerke vkw in Vandans (Austria). The power plant fleet has a pumping capacity of 1.4 GW and a turbine capacity of 2.4 GW. This enables renewable energy from hydropower to be supplied to the grid and surplus electrical energy from the grid to be stored by pumping water up to higher reservoirs. Each of these power plants has different characteristics and performance data. The pumps, turbines and generators used, some of which can be up to 100 years old, are mostly one-offs for which there is no stock of spare parts anywhere. In order to ensure the operational readiness of the power plants, the Mechanical Maintenance CNC in Vandans must therefore carry out all conceivable tasks: from overhauls and reworking for contract welding to the complete new production of components from a wide range of raw parts (sheet metal, forged blocks, cast parts). This involves component sizes ranging from fist-sized clamping nuts to workpieces with dimensions of several meters and weights of up to around 25 tons. A colorful mix of materials such as steels, stainless steels, cast iron, brass and bronze are processed.

Flexible traveling column system replaces fixed-bed milling machine

"With the new FBF-M 6000 moving column milling center from MTE, we have replaced an outdated fixed-bed milling machine," reveals Christoph Schuchter, Maintenance Mechanics CNC at Technical Maintenance in Vandans. With the old machine, it was no longer possible to carry out all the work required, both in terms of the dimensions and the required accuracy. The tasks include, for example, the machining of sealing surfaces inside valves or the boring of bearing seats, in alignment from both sides. The required accuracy must be guaranteed even when the milling head is fully extended.

The characteristics and features of the newly procured system meet the specified requirements. The column has a travel of 6,000 mm in the X-axis, the vertical axis has a stroke of 3,000 mm and the cross slide (Z-axis) has a stroke of 1,500 mm. The clamping area is divided into three sections: The centrally arranged rotary table with a load capacity of 40 t and a clamping area of 3,000 mm × 2,500 mm can be moved outwards by (W) 1,500 mm. This means that even very large or long workpieces can be clamped on the two platen fields arranged to the right and left of it. The automatic head changing station for three milling heads mounted on one of the plate fields can be removed in such cases. The operator is accommodated in a horizontally moving operator cabin, which can also be moved vertically if necessary. This allows the work process to be viewed and monitored directly, even with large components.

"The size and strength of the components to be machined require a robust basic machine design. A high torque even at low speeds is required for spindle operations," adds Pfeifer. From this point of view, the rigid, heavily ribbed design of the MTE system made of vibration-damping cast iron was convincing, as were the INA linear guides in all axes. In the case of the cross slide, flat guides with a scraped Turcite coating provide additional damping. This concept is particularly important when machining large components with a widely extended milling head. Thanks to highly dynamic digital drives, the axes reach speeds of up to 25,000 mm/min in rapid traverse.

The main spindle motor has an output of 40 kW and drives an infinitely adjustable diagonal milling head with "SK 50 Big Plus" spindle system via an oil-cooled 3-stage gearbox in the milling slide. Its simultaneous taper and face contact ensures a very stable connection between the machine and tool. The milling cutter thus achieves up to 4,000 ^rpm and a maximum torque of up to 1,940 Nm. Thanks to its oil cooling, the milling head can be operated with 100 percent ED. It can be automatically exchanged in a change station with three places, for example for a milling drill spindle for roughing.

For particularly delicate work, for example inside narrow housings or in the area of the interfering edges of shaft flanges, the head can be equipped with a slim, offset milling head with a length of 500 mm and a small spindle arranged at a 90° angle. Despite the long overhang, the tool is very rigid, as the machining forces are transferred to the milling head via a stable housing and a Hirth coupling as an interface.

Careful selection process

"We proceeded very carefully when selecting the supplier for this special project and involved several providers with the help of a corresponding tender," recalls Schuchter. For most of these, we not only visited the suppliers themselves, but also reference users in order to get a realistic impression. The focus was less on suitability for heavy-duty machining and more on the high-performance overall concept of the system. The variety of requirements demanded a high degree of flexibility in terms of possible applications. However, as space was limited, another important criterion came into play: the ability to fit into the existing premises. The available space was to be used as effectively as possible, as a new building was out of the question. The decision was ultimately made in favour of MTE, because this supplier was able to implement the maximum possible travel distances within the existing installation space and at the same time meet all other requirements.

"In order to fit the envisaged milling system into the hall, we had to put together a larger 3D puzzle together with MTE," says Pfeifer. This required both companies to work closely together as partners, because not only the conditions on the floor but also the available space for the foundations had to be taken into account. The foundations of the pillars in the hall proved to be a particular handicap, as they were designed to be very massive due to the 25 t crane and still protrude into the hall below floor level. To plan the necessary modifications to the system, MTE received a 3D representation of the hall, including the position of the foundations. These modifications proved to be very extensive and required changes to numerous units such as the chip transport system, for which multiple deflections had to be provided under the floor. In addition, both the machine platform, which carries the hydraulic unit and the control cabinet, as well as the operator platform, had to be modified. These modifications were then incorporated into the foundation planning.

With the help of these planning documents, the schedule for the entire project was drawn up. This included the dismantling and removal of the old milling machine, the demolition of the old foundation, the casting of the new 700-tonne vibration-damped foundation and finally the installation and commissioning of the new system. Despite the tight schedule, the system was commissioned on time in May 2019. "The new milling machining center is now in full operation and has proven its worth," says Schuchter.

Klaus Vollrath, freelance journalist / ag