Mapal and Bavius show how precise tools and modern machines can significantly reduce machining time for complex aluminium parts in the aerospace sector.

The pockets with various forms were the biggest challenge of the demonstration part. Some special features were included, such as lugs with bores or a T-stiffener. The pockets milled into aluminium are up to 125 mm deep and have very thin and sometimes inclined walls. Right angles are quite rare on the part.

Demonstration part for the aerospace industry: As part of a joint project, Mapal and machine manufacturer Bavius designed and manufactured an aluminium aircraft component, measuring approximately 3 x 1 m. The part is based on a real rear spar from aircraft manufacturing enhanced with a variety of complex features. Besides its complexity, the component is also impressive due to the relatively short machining time of ten hours thanks to the productivity of the Bavius Aerocell and the special Mapal tools used for aluminium machining.

For the involved partners’ customer presentations and as an eye-catcher for trade fairs, five of these rear spars are produced in Baienfurt. The machining of a part takes a total of almost precisely 10 hours. “A component like this can take between 20 and 30 hours on other machines,” Dominik Merz, director global sales, estimates.

Proudly presenting the finished demonstration part in front of the AeroCell 160 | 400 (from left): Jens Ilg (Business Development, Aerospace & Composites Mapal), Alexander Follenweider (Component Manager Aerospace & Composites Mapal), Stefan Diem (Application Engineer Bavius) and Dominik Merz (Director Global Sales Bavius). The clamping setup can be seen in the foreground.

The rear spar is selected as a demonstration part because it fits perfectly on the machine with a table height of 1.6 m and width of 4 m. 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. The outer skin is riveted onto them. The completed component is made of 7075 aircraft aluminium and measures 2977 × 748 mm. Its flat form measuring 138 mm high is typical of aircraft components. From one ton of starting weight, only 70.61 kg 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 much more complex than any normal rear spar. It includes features that are not necessarily associated with a spar but could be useful for other components. “A customer who knows structural components and sees our part will recognize it and notice characteristics that are also found in their own components,” Merz says, explaining this approach.

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.

5-axis machining in two clamping setups

Structural parts are usually machined vertically on gantry machines with big tools. Cutter heads with diameters of 125 mm are normal. Feeds and speeds remain low. One disadvantage of this way of doing things is that chips are left behind, which can cause scratches. Heat is also transferred to the component. Horizontal machining precludes this.

Machining of the rear spars by the Mapal-Bavius collaboration takes place in Baienfurt in two clamping setups. While setup 1 uses low tension, setup 2 harnesses vacuum for a secure hold. At first glance, the front looks simpler than it actually is. The surface is not flat but curves slightly outward over a radius of 9.5 m. 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 50 mm diameter and the Optimill-Alu-Wave with a 25 mm diameter. The surface finish is performed by a PCD custom milling cutter.

Thin ribs, inclined walls, deep pockets

The machining of the back is particularly sophisticated. It is separated into 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, and some with inclined or curved bottoms. The ribs are very thin; the walls are mostly inclined. The pockets are up to 137 mm deep.

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 experts use different diameters and lengths of the modified shoulder milling cutter.

Mapal also sets great store by efficiency during programming, as component manager Alexander Follenweider explains: “We work with a zigzag strategy in the parallel and counter feed to save on travel time. We thus constantly switch strategies during machining.” Despite the high machining speeds, the aluminium may not be damaged, as it changes properties when overheated.

Chip volume of more than 14 l/min

At top speeds, the Optimill-Alu-Wave achieves a feed of 12 m/min at a cutting depth of 48 mm at 29,000 rpm. The bigger Neomill-Alu-QBig achieves a feed of 25 m/min at 10 mm cutting depth. Alone in the first 55 minutes of machining of the second clamping, 425 kg of aluminium are thus machined. At its peak, this results in a chip volume of more than 14 l/min. “The results we were able to achieve here are excellent – and we were also able to create very good surfaces in the process,” says Stefan Diem, application engineer at Bavius.

No standards – various challenges

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. Bore and reaming operations are also called for in certain areas. “Nothing is normal on our component,” Merz says, 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. (as, tp)

An HPR multi-bladed reamers with the diameter 40 H7 machines the 47-mm-deep fitting bores. It is employed at a cutting speed of 120 m/min and a feed of 0.2 mm per turn.

Source：INTERNATIONAL ALUMINIUM JOURNAL