Component repair Process-secure repair of expensive components

As one of the leading MRO providers, MTU Maintenance Hannover masters the continuous growth in orders with consistent modernisation in machining. A DMC 210 FD from DMG Mori has improved the firm’s efficiency by around 60 %.

To make sure that aircraft engines provide reliable service for up to 30 years, regular maintenance, repairs and overhauls are necessary. “On the one hand, the highly stressed components, depending on climatic conditions, wear out, while on the other hand safety regulations specify the maintenance intervals,” explains Andreas Kappe, Industrial Services Manager at MTU Maintenance Hannover. In addition, we hear, economic factors are also critical for airlines and leasing firms: “The bottom line is that punctual maintenance keeps up the value of the engine and therefore of the aircraft.”

An indispensable instrument in the MRO business nowadays is the data recorded by the motor for later evaluations or, if required, sent to a ground station in real time. Sensors measure exhaust and motor temperatures, consumption of fuel and oil, vibrations and also pressure in compressor, combustion chamber and turbines. In total there are 5000 parameters. During a flight, an aircraft engine produces on average a terabyte of data. Andreas Kappe sees it as an important information source: “As a result, maintenance can be adapted to the actual requirements.” For example, wear is worse in desert regions because of sand and dust.

Up to four months pass between delivery and repair

Work on the engines – which are usually delivered from Amsterdam and Frankfurt on lorries with special suspensions – begins with an incoming test which establishes the current performance values. Next come cycles 1 to 3: dismantling, cleaning and inspection of the components, followed by repair and assembly, with a final outgoing test. Two to four months are needed by MTU Maintenance Hannover for the whole process. “We are constantly optimising this process in order to remain competitive,” comments Andreas Kappe. The modernisation of the machining has apparently been essential in this context.

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In 2011, MTU Maintenance Hannover examined its machining processes minutely and came to the conclusion that the flexibility and utilisation of the machine park would not be sufficient for future requirements. “In addition, we worked out qualitatively a large number of possibilities for standardisation,” Marcus Spatz recalls, looking back at the work preparation. This led to the task, shared with his colleague Stefanie Kreftsiek and the CNC programmers Michael Seifert and Rafael Wilgoschesky, of finding a machining solution meeting all demands on productivity and quality. “Since there was no standard machine for our needs, we found in DMG Mori a partner who joined us in working on this complex project.”

The result of this cooperation is a DMC 210 FD with customer-specific special features guaranteeing maximum flexibility for the very varied machining tasks. A great step forward here is the total of five pallets. “Today, setting up parallel to main time eliminates a large portion of the idle periods which limited us in the past,” says Stefanie Kreftsiek, comparing work on the milling/turning centre with the old procedures. The same is true for complete processing: “For some repairs, up to five machines were previously necessary, along with time-consuming re-clamping.” In addition, processing in one clamping has certainly improved quality substantially. The tool magazine with 300 places ensures high flexibility.

The productivity of the DMC 210 FD is so high that MTU Maintenance Hannover also loads the machine with comparatively small components of under 1000 mm diameter. The maximum traversing paths of 2100 mm in the X and Y axes and the workpiece height of 1250 mm were however an important criterion for the specialists at EMO, as Andreas Kappe emphasises: “Here we maintain exclusively medium-sized and large aircraft engines with which we can also fill out this working space.” That is why a special pallet-changer cycle for setting up oversized workpieces of up to 2600 mm has been integrated. Lasers and scanners monitor the pallet changer area and contribute to safety.

Omnipresent at MTU Maintenance Hannover is the awareness of the high quality and enormous cost of the workpieces in basic alloys of titanium, steel and nickel. One of the component groups assembled here can be worth up to two million euros. “There is no room for mistakes here,” states Andreas Kappe. The absolutely essential process security, we are told, is achieved by the greatest possible standardising of the machining. Marcus Spatz points to the in-process measuring: “The only remaining risk is that a worker may start the wrong program, but even this is checked by the machine in time when the component is measured.” The integrated process monitoring furthermore watches spindle power and feed forces as well as coolant pressure and through-flow.

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Additional security is provided by the DMG Mori Virtual Machine, with which MTU Maintenance in Hannover has been working for some time. The virtual machine integrates machine geometry, kinematics and dynamics, as well as all functionalities of the NC controller and PLC. Programs which have been tested successfully in this 1:1 simulation run with absolute process security on the real machine. There is also additional security due to the machine protection control, which protects the high-cost components and the machine from a collision. Michael Seifert sees a further advantage: “Working with the virtual machine reduces alignment and setup times on the milling/turning centre, and simplifies planning of the orders because we know the precise throughput times.”

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In contrast to the manufacture of engine components, cutting rates in repair are very low. Instead, the aim is to get the component back into its original state. To this purpose, a metal plasma is applied to the damaged or worn area, which the DMC 210 FD mills or turns to the original dimensions. The precision of the turned parts in particular is close to two hundredths of a millimetre. This precision is also influenced by the coolant unit, providing up to 80 bar and coolant temperature control. It goes without saying that, in view of these specifications, comprehensive measurement procedures are needed. Before processing, the machine checks, for example, whether the workpiece has been correctly aligned on the motor-driven precision setting-up surface. Further measurements, such as those with the L measurement sensor packet for difficult-access features, are made later in the process.

Intelligent NC programs use such measurement data in real time. Michael Seifert explains: “The machine measures, for example, the diameter of the component and turns only where too much material is present.” This intelligent machining is the result of a steep learning curve which the programmers at MTU Maintenance Hannover went through. “After trai­ning with DMG Mori, we were optimally prepared, but even after that the learning process continues constantly in practice,” Rafael Wilgoschesky reflects regarding the development. DMG Mori technology cycles likewise simplify the programming. This also applies to the range of processes on the DMC 210 FD, which can be further expanded to include a grinding cycle, for example. “The grinding process is also a central topic with the DMC 125 FD Duoblock, which DMG Mori installed here this year,” adds Andreas Kappe.

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Forward-looking investments in machinery are just as decisive for Andreas Kappe as in personnel: “We need good and motivated skilled workers in order to get the maximum out of the modern CNC technology.” The trai­ning of the next generation of wor­kers, advanced trai­ning for existing workers, and attracting experienced CNC experts are therefore constantly recurring topics, we are told. Because of the growth in the sector, MTU Maintenance Hannover looks into the future positively, but is also conscious of the challenges. “New demands on the manufacturers’ side are possible at any moment.” Andreas Kappe speaks from experience. Only recently, DMG Mori installed a new filter unit for the DMC 210 FD: “New regulations specify a filter mesh of 3 µm for grinding and for rinsing clamping equipment.”

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* Sascha Dietze is an editor with the Pressgate GmbH in 50668 Cologne. Further information: DMG Mori Global Marketing GmbH, tel. (0 89) 2 48 83 59 60, stine.meyer@dmgMori.com

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