Automotive technology for aeroplanes

Die F. Zimmermann GmbH aus Neuhausen auf den Fildern ist heute vor allem für ihre großen Portalfräsmaschinen bekannt. Über 550 Anlagen hat der exportorientierte Maschinenhersteller mittlerweile in 37 Länder der Welt geliefert. Oft stehen sie in den R&D-Abteilungen großer Unternehmen, um dort die gesamte Produktentwicklungskette vom ersten Designentwurf bis zu verschiedenen Stufen von Anschauungsmodellen zu begleiten. „Alles was einmal eine Form erhält, ist zuvor als Urmodell auf einer unserer Maschinen gewesen“, erläutert Steffen Nüssle, Verkaufsleiter Export und Leiter der Anwendungstechnik bei Zimmermann. 

Übliche Materialien für den Modellbau sind Holz, Styropor und Ureol. Der Einstieg von Zimmermann in die Metallbearbeitung vor mehr als 20 Jahren erfolgte mit Aluminium. Das war zugleich der Türöffner für die Flugzeugindustrie. Nach und nach wagte man sich an die Bearbeitung von Verbundwerkstoffen, Stahl und hochwarmfesten Werkstoffen wie Titan, Waspaloy, Inconel und Invar. Bei diesen Materialien liegt der Fokus nicht mehr auf Modellen und Mustern. Die Aerospace-Industrie nutzt die Portalfräsmaschinen hier als Produktionsmaschinen für die Teile­fertigung.
 

Bei dem Bauteil handelt es sich um eine rund 1,50 m lange Aufhängung. Eine besondere Herausforderung bei der Bearbeitung stellt eine Reihe von zwölf Bohrungen in Laschen dar, die auf einer Länge von 990 mm nacheinander am Werkstück sitzen. Dabei verlangt der Kunde hohe Präzision. Bei einem Bohrungsdurchmesser von 17 mm ist Toleranz H7 vorgeschrieben. Auf der ganzen Länge des Bauteils muss die Konzentrizität der Bohrungen unter 0,05 mm liegen. Die gleiche Genauigkeit wird für die Rechtwinkligkeit der Bohrungslaschen zur Bauteiloberfläche verlangt.

Zimmermann setzte bewusst auf MAPAL als Werkzeugpartner. „Unsere Anwendungstechnik macht bei so einem Turnkey-Projekt keine Kompromisse, da kommen nur sehr gute Systempartner in Frage“, sagt Steffen Nüssle und berichtet von besten Erfahrungen, die er selbst mit dem Werkzeughersteller gemacht habe. 

In zurückliegenden Turnkey-Projekten von Zimmermann ging es um Flügel-Holme für den Airbus A350 sowie um Senkungen und tiefe Bohrungen an Windkraftflügeln aus Glasfaser. Als er bei einem anderen Maschinenhersteller tätig war, hatte Nüssle ebenfalls mit MAPAL zu tun. Dabei ging es um die Bearbeitung der Lagersitze für Kurbel- und Nockenwellen großer Dieselmotoren mittels Line Boring. Mit solchen Reihenbohrstangen hat MAPAL im Automobilbereich sehr viel Know-how aufgebaut. Nun war dieses Know-how auch beim Aerospace-Bauteil gefragt, dessen Laschen sehr an die Stege in der Kurbelwellenlagergasse eines Verbrennungsmotors erinnern. 
 

The project partners brooded over the material for the part. The customer announced forged blanks made of the titanium alloy TA15m. “All of us have experience with aerospace titanium, so we knew what we were getting into, but even our full-fledged aerospace experts had never heard of this special alloy”, Steffen Nüssle admits. Jens Ilg, who works in the Aerospace & Composites segment at MAPAL, agrees: “The material was exotic for us, too.” They discovered that TA15m is a workpiece material that is manufactured powder-metallurgically. The sintered material is redensified in a special process, whereby the workpiece acquires the characteristics of a forged part.

The customer did not provide any blanks to design the machine with. Thus, the project partners had no other choice but to take a titanium bar and mill the form of the blank with its measurements from the solid block. In this way, a blank was imitated, which represented the starting point of manufacturing for the customer. This preparatory work alone took 30 hours of machining time.
 

At the same time as the machine was being constructed by Zimmermann in Neuhausen, MAPAL began with tool tests in Aalen. The tool manufacturer could rely on a new generation of titanium tools for this purpose. In order to achieve high removal rates, the NeoMill-Titan shell end face milling cutter was chosen. MAPAL developed the new topography of the indexable insert in such a way that the chips are optimally formed and removed. An equally new cutting material concept minimises wear and tear and the formation of built-up edges.

Even experienced application engineers were surprised how smoothly and almost silently the tool works. The tool life is also impressive. The 60 minutes that Bernd Scheurenbrand expected were exceeded considerably. “Some of the tools were at work for almost seven hours – that’s a sensational tool life”, the experienced application engineer from Zimmermann praises. This result was made possible, among other reasons, due to the modern protective coating with a good adhesion to the tool, an effective temperature barrier and a very smooth surface that reduced heat on the cutting area.
 

Excellent cooling is essential when machining titanium to remove the heat that develops. MAPAL MillChucks with integrated coolant supply were used to bring the coolant pressure to the milling cutter. Zimmermann chose the company Blaser Swisslube as a further project partner to provide the cooling lubricant as their coolants are ideally suitable for high pressures.

For the bores on the sides of the lug, MAPAL developed a custom tool with angular drill head based on the MEGA-Speed-Drill-Titan. The drill bores, then deburs back in the countersink step and finally produces a chamfer in reverse.
 

The 685 mm line boring bar with a diameter of 17 mm, which does the fine machining in the end, has two adjustable inserts and is led between the lugs in guide bushes. The first insert bores the main bore, thus eliminating any offset that might have occurred during pre-machining. The second insert then completes the final diameter in H7 quality. The long boring bar reminds Jens Ilg of a lightsabre: “The swinging motion that positions the tool on the part brings Star Wars to mind.” The gantry machine, a six-axis FZ42 with a VH60 milling head, offers more than enough room for the machine’s spectacular lunges. The working area is 8.50 x 3.90 x 1.50 m large and is separated in an area with a fixed machine table and an area with a round table for further processing.

Transferring the results from MAPAL’s test centre to the machine in Neuhausen was a critical stage of the project, Steffen Nüssle remembers: “After the successful test runs in Aalen, we knew where we were headed. Our gantry machine is a lot bigger than the machining centre in Aalen, has bigger moving masses and lever lengths. Thus, it isn’t quite as rigid.”

The misgivings were quickly allayed: The desired accuracy was achieved with high process reliability in Neuhausen, as well. This was particularly true for the middle of the workpiece, where the bores meet each other from both sides and a small offset could occur. The measured results prove that the machining was symmetrical. Just 8 µm difference was measure here. The line boring process ensures that the precision that is once reached can be reproduced for years to come, as the machine only provides the feed and spindle speed, while the actual geometrical guidance is provided by the fixture.
 

Zimmermann paid close attention to vibration dampening while designing their fixture. The many buckles and clamps however also resulted in many tool restrictions. Here, the machine manufacturer harnessed its expertise in model making to first create a blank out of plastic. “If the first attempt failed, we would have neither damaged the expensive titanium or the tool, let alone the machine,” says Nüssle. But it worked from the get go. Only the cutting parameters had to be adjusted for the first titanium part.

Steffen Nüssle was very impressed by the good cooperation between Zimmermann and MAPAL: “I’ve already been involved in many turnkey developments but have never experienced a project that went as smoothly as this. It was a lot of fun.” The geographical proximity between Neuhausen and Aalen was also a big advantage: They only had to drive an hour to be able to discuss the next steps together right at the machine.