Strategic aircraft component developed by bavius and Mapal

As part of a joint project, Mapal and machine manufacturer bavius have designed and manufactured a demonstration part for the aerospace industry. The aluminium component, measuring approximately 3 x 1m, is based on an actual rear spar enhanced with a variety of complex features. Besides its complexity, the component is also impressive due to the short machining time of 10hr thanks to the productivity of a bavius AeroCell and the special Mapal tools utilised for aluminium machining.

bavius technologie, based in Baienfurt in Southern Germany, is a specialist in the high-speed volume machining of structural components made of aluminium, which are typically required in the aerospace industry for wings and fuselages. Previously known as Handtmann A-Punkt Automation, bavius has been active as an independent family business since 2017, concentrating on two product lines — HBZ and AeroCell horizontal machining centres and PBZ profile machining centres. With around 120 employees, bavius generates more than 80% of its turnover in the aerospace industry. As an OEM or supplier, its customers produce components for commercial and military applications, as well as parts for satellites and rockets, such as the Ariane 6.

Most bavius machines are located in Europe, but they are also present in North American and Asian markets. While the company has offices in the USA, production for all markets worldwide exclusively takes place in Baienfurt in two air-conditioned halls with a production area of approximately 10,000m>sup>.

Pictured left: Custom programming and CAM simulations were the first step of the project

A particular strength of bavius’s five-axis machining centres is that they are vert dynamic, which ensures high productivity during high-speed machining, and to ensure maximum productivity, bavius also manufactures its own milling heads. The flagship of its product range is the bavius AeroCell 160|400 with a spindle power of 140kW and top speeds of 30,000rev/min. The 140kW are already reached at 18,000rev/min — the maximum power is available over a wide range of speeds. Dominik Merz, director global sales said: “When we do something, we make sure it is done perfectly – we are global technology leaders with our AeroCell 160|400.”

The rear spar is selected as a demonstration part because it fits perfectly on the machine with a table height of 1.6m and width of 4m. Furthermore, the rear spar is a common structural component in wings — ribs run between the rear spar and front spar, which are arranged perpendicularly to the spars. These structural components define the geometry of the wings, and the outer skin is riveted onto them. The completed component is made of 7075 aircraft aluminium and measures 2,977 × 748mm. Its flat form measuring 138mm high is typical of aircraft components. From 1-ton of starting weight, only 70.61kg remain after machining — a proportion that is quite common in the aerospace industry.

Among other factors, legal reasons prevented bavius and Mapal from using a real aeroplane component for the machining. This however gave those responsible the freedom to use the design for a variety of applications. As a result, this demonstration piece is more complex than any normal rear spar and includes features that are not necessarily associated with a spar but could be useful for other components. Mr Merz said: “A customer who knows structural components and sees our part will recognise it and notice characteristics that are also found in their own components.”

Two clamping setups
Machining takes place in Baienfurt in two clamping setups. While setup one uses low tension, setup two harnesses vacuum for secure hold. At first glance, the front looks simpler than it actually is — he surface is not flat but curves slightly outward over a radius of 9.5m. This means that the component cannot simply be face milled. Instead, five-axis machining is necessary. For roughing and finishing, Mapal employs the NeoMill-Alu-QBig with a 50mm diameter and the OptiMill-Alu-Wave with a 25mm diameter. The surface finish is performed by a PCD custom milling cutter.

The machining of the back is particularly sophisticated. It is separated in nine different sections, each with its own special features. Like any rear spar, the demonstration part has many pockets. They are however completed here in all sorts of forms: rectangular, triangular, round, open, closed, some with inclined or curved bottoms. The ribs are very thin; the walls are mostly inclined. The pockets are up to 137mm deep.

Pictured right: The Mapal NeoMill-Alu-QBig with 50mm diameter quickly machines the front. The slightly convex form of the part necessitates five-axis machining

After pre-machining with the NeoMill-Alu-QBig, the pockets are cleared out by an OptiMill-Alu-Wave of various lengths. The semi-finishing is performed by a shoulder milling cutter modified specifically for aerospace applications. Thanks to its special geometry, the tool is particularly suitable for machining residual material in the corners as well as subsequent finish milling of the floors and walls. The special core rise ensures optimal stability during the machining process. To machine all the areas efficiently, Mapal engineers use different diameters and lengths of the modified shoulder milling cutter.

Mapal also sets great store by efficiency during programming. Component manager Alexander Follenweider explained: “We work with a zig-zag strategy in the parallel and counter feed to save on travel time. We thus constantly switch strategies during machining.”

Excellent results
Despite the high machining speeds, the aluminium cannot be damaged as it changes properties when overheated. At top speeds, the OptiMill-Alu-Wave achieves a feed of 12m/min at a cutting depth of 48mm at 29,000rev/min. The bigger NeoMill-Alu-QBig achieves a feed of 25m/min at 10mm cutting depth. Alone in the first 55min of machining of the second clamping, 425kg of aluminium are thus machined. At its peak, this results in a chip volume of more than 14 l/min. Stefan Diem, application engineer at bavius, said: “The results we were able to achieve here are excellent – and we were also able to create very good surfaces in the process.”

The various pockets are not the only challenges on the component — the bores on the four lugs can only be reached via an angled head — undercuts are required elsewhere. A ‘T-stiffener’, which is common for structural components and provides rigidity, is also found on the demonstration part and is machined with a special PCD tool.

Pictured left: View into the working area of the bavius AeroCell 160|400. Here we see pockets being milled and other features machined in the second clamping setup

Bore and reaming operations are also called for in certain areas. Mr Merz said: “Nothing is normal on our component,” referring to the fact that you will be hard pressed to find a right angle anywhere on the part. Such oblique constructions are however the norm in the aerospace industry.

For the involved partners’ customer presentations and as an eyecatcher for manufacturing trade fairs, five of these rear spars are produced in Baienfurt. Much to the satisfaction of all those involved, the machining of a part takes a total of 10hr. Mr Merz said: “A component like this can take between 20 and 30hr on other machines.”

Structural parts are usually machined vertically on gantry machines with big tools, and cutter heads with diameters of 125mm are normal, while feeds and speeds remain low. One disadvantage of this method is that chips are left behind which can cause scratches — heat is also transferred to the component, but horizontal machining overcomes this. Together with powerful tools that enable high cutting data, the dynamic AeroCell 160|400 opens entirely new possibilities thanks to its extreme acceleration.

Pictured right: The rear spar was machined using about 20 different tools from Mapal. Both special and standard tools with high-performance cutting material from Mapal’s aluminium portfolio were used here

Mapal and bavius have been profiting from their close cooperation for many years. While Mapal was developing the NeoMill-Alu-QBig and OptiMill-Alu-Wave, it was able to test and further optimise prototypes of the new tools in Baienfurt. As a tool manufacturer, Mapal doesn’t have equally powerful machines in Aalen. On the other hand, bavius depends on innovative tools.

Mr Diem explained: “For our premium machines, we need top tools by suppliers like Mapal offering high-volume milling cutters that take our machines to their limits. That doesn’t work with universal tools – we don’t gain any insights with them.”

The partners first demonstrated what the AeroCell 160 can do two years ago. During test operations with overloading, the solid-carbide milling cutter OptiMill-Alu-Wave achieved chip volumes of up to 20 l/min while full slot milling normal aluminium. The top results achieved here are every bit as impressive: 12 l/min for a component made of the AL 7075 with its higher tensile strength. Mr Diem continued: “I really appreciate the collaboration with Mapal because we speak openly and treat each other fairly, and because the tooling supplier always provides a solution.”

Jens Ilg, business development (aerospace and composites) at Mapal, concluded: “I also get an optimal feel of our own products when I see them being used in real and demanding conditions. I can then offer them to customers with corresponding recommendations.”