Greater precision and productivity when milling large parts
Portal milling machine instead of travelling column
The Alstom plant in Netphen has chosen a Bimatec Soraluce system for its latest major investment in milling machines, as it has done almost every time over the past 20 years. How did this brand loyalty come about? And why did Netphen choose a portal milling machine this time instead of the travelling column systems used previously? An interview with managers from both companies.
“The Netphen site is the centre of excellence for bogies within the Alstom Group,” explains Production Manager Annika Lichtenfels. The French company develops and manufactures modern and efficient mobility solutions, specialising in rail systems. Wheelsets and bogies for a wide range of rail vehicles, from high-speed trains to trams and locomotives, are developed, manufactured and maintained at the Netphen site. Customers include railway companies and train manufacturers as well as regional and local transport authorities. Only about one-third of the more than 100 customers are Group companies; the rest come from the open market.
Some 2,500 bogies are manufactured each year and 1,000 overhauls and repairs are carried out.
Machining plays a key role in Netphen The site covers all the essential steps from the procurement of raw parts such as frames, wheels and axles and the necessary ancillary components to the delivery of complete, ready-to-install wheelsets and bogies. The demands on the quality and accuracy of the machining and the punctuality of the completion are very high. Many workpieces have large dimensions of up to 4,000 x 3,000 x 1,000 mm. They are machined on three large-format milling centres, including two Bimatec Soraluce machines.
Horizontal rather than vertical clamping
“Until 2019, Soraluce FR series travelling column milling machines with vertical clamping of the components were traditionally used in Netphen,” says Frank Lörchner, Product Manager Automation at Bimatec Soraluce. But this approach had two disadvantages: The lateral attachment of the clamping device to a clamping bracket inevitably caused deflections, which had a negative effect on the accuracy of the machining. In addition, the force of gravity affects the accuracy that can be achieved, depending on the overhang of the ram. Although the machine designers do their best to compensate for the deflection in the case of long overhangs, the effect cannot be completely avoided. The higher the accuracy requirements, the more disturbing it becomes. In addition, the two unfavourable influences overlap and reinforce each other.
As the demands on the precision of the parts have increased over the years, the management in Netphen wanted to change the system in 2019: instead of a travelling column milling machine, they decided on a PM 4000 2T portal milling machine with a fixed portal. The travels are 5,000 mm (X), 4,500 mm (Y) and 2,000 mm (Z). In order to avoid the previously unavoidable main time loss during clamping, the system has two independently moving tables, each measuring 4,000 x 3,000 mm. This means that clamping can be carried out on one of the tables without interrupting machining on the other. Specially designed fixtures are available for each component, providing two clamping positions. This means that all six sides can be machined with just one change of clamping position. The accuracy requirements are extreme; one of the frames has to meet a tolerance of 1,700 mm distance with an accuracy of just 2 μm. Special measuring equipment was purchased for the inspection.
There are two additional special heads in the tool magazine, which are managed and changed by the tool changer in the same way as tools. The control is a Heidenhain TNC 640.
The large number of tools is due to the fact that all tools for all types of machining, including drilling and tapping, are stored in the magazine. This is the case for all six of the frames that are intended for machining on the machine.
“In order to utilise the machine as fully as possible, it has been equipped with a very high degree of automation,” adds Ramazan Ersin, Site Facility Coordinator at the Netphen plant. The tool magazine, known as the “tool arena”, has 395 tool positions and is located at the side and operated by a robot. There is also an automatic milling head changing system including a pick-up station with five milling heads. Four milling heads are currently supplied and one slot remains open for future integration.
Useful technology packages
“In order to reliably meet our requirements, we ordered a number of additional technology packages for the machine,” explains A. Lichtenfels.
- “Adaptive Control” detects typical roughing problems such as blank cuts or variations in dimensions, material hardness or workpiece stiffness by monitoring spindle power. The programme automatically adjusts the machining parameters according to the actual power consumption. The system can reduce machining times by up to 25%.
- The DAS+ active anti-chatter system reduces the vibrations that often occur when using large milling systems, which can otherwise damage the machine and tool and affect the surface quality of the workpiece. With DAS+, sensors detect the occurrence of these vibrations and control the anti-vibration system to operate at 100% power throughout the working area. This reduces cycle time by up to 45% and increases productivity by up to 300%.
- Dynamic Head Compensation (DHC) improves machining accuracy. The self-calibrating system automatically measures the machine’s kinematics and instantly compensates for different milling head positions. During machining, the control constantly checks the position of the milling head and calculates the compensation values accordingly.
- In addition to DHC, the Accura Heads extension enables automatic checking and correction of the milling head kinematics to increase accuracy. The system compensates for the deviation in head position accuracy for a given head position. The measurement cycle is performed immediately before the milling process and ensures the highest accuracy of the machine.
A complete process solution was needed
“With this system, we didn’t just buy a milling machine, but a complete process solution consisting of machine, CNC programmes and tools,” reveals R. Ersin. Milling time studies were carried out in advance at the Bimatec Soraluce site in Limburg. The supplier had to take responsibility not only for the performance of the system as such, but also for ensuring that the work results would be achieved. This applied both to the quality of the work results and to the productivity in the execution of the agreed workpieces.
The scope of supply therefore included all the expertise required in the Alstom factory to ensure that the system could always be operated with the agreed performance data. The acceptance criterion was the timely completion of 15 production orders on frames, which had to be measured completely “green” during the subsequent quality inspection. The guaranteed process reliability also had to be demonstrated on the moulds. As a result, the creation of the programmes and the transfer of all other know-how were also part of the agreed training. The system has been in 24/7 operation since acceptance in December 2022. The overall equipment effectiveness (OEE) achieved since then is over 90%.
Service around the clock
The constant availability of the new milling machine is very important for the Netphen site,” says A. Lichtenfels. The workforce has shown great commitment to the procurement project and the high level of investment involved, as this is seen as an important contribution to safeguarding know-how. Another important factor in the decision was the confidence in the reliability of the Bimatec Soraluce systems, which have been installed for more than 20 years, and in the high level of service provided by the supplier’s branch in Limburg, which is relatively close by. There is an extensive stock of spare parts for the machines. The modular design of the machines means that spare parts are also available for older models.
There is also a special service for the milling heads. If a milling head requires repair or maintenance, one of the more than 60 replacement heads in stock is available to the customer within 24 hours. In the case of a special head supplied specifically for the system, the complete set of parts for a replacement head is always available in a separate bulk store in the event of service.
Text: Klaus Vollrath b2dcomm.ch