Dry Machining of Multilayer Composites in Aircraft Construction

Materials that are both high-strength and light are of vital importance in the aerospace sector. Thanks to new material combinations, the weight can be further reduced, strength and corrosion resistance increased and assembly can be simplified by means of an integrated design. While structural parts made of aluminium, titanium or high-strength steels are machined on machining centres or portal machines, parts in final assembly are mostly machined by hand-held machines, drill feed units or robots.

The requirements for tool manufacturers and tools for final assembly therefore differ significantly from those for part manufacturing. While machined parts in part manufacturing have a value of around 1,000 to 50,000 Euros, parts in final assembly, depending on the assembly progress, are significantly more cost-intensive with a value of around 50,000 to 2 million Euros. Faulty machining must either be manually reworked, which is time-consuming and expensive, or the parts have to be completely replaced. For this reason, the suppliers for final assembly must be carefully chosen.

A challenge for tool manufacturers is the variety of materials, particularly if several materials with different properties are to be machined at the same time. To ensure a qualification as a tool manufacturer, it must be possible to machine all materials reliably and economically. The qualification for a tool manufacturer takes between one and five years. Tools and processes also require an additional qualification. Existing processes are only adapted in exceptional cases. For it must be ensured that all machining is carried out with a consistent quality. For example, a low scatter of the bore diameters, a CpK value larger than 1.7, must be ensured for drill machining in final assembly.

Aircraft manufacturers use rivet connections for connecting the outer skin to the structural parts underneath. For this purpose, innumerable bores are drilled. To achieve the lowest resistance to the airstream as possible (low cW value), the rivet heads are countersunk in the outer skin. For this, an additional countersinking must be added to the bore entrance. In the past, a process with up to four individual machining steps was often required (drilling from the solid, boring, reaming, countersinking).

Today machining in just one step, where bore and countersink are realised in one process, is state of the art. Only in this way was automatic machining using robots possible. Previously this type of machining was realised with minimum quantity lubrication (MQL). After machining, the parts had to be disassembled, cleaned and remounted. In addition the cooling medium got inside the aircraft where further assembly steps were taking place at the same time. The demand for tools for dry machining different composite workpiece materials
was the consequence.

With dry machining, the use of a cooling lubricant is completely dispensed with. Cooling lubricants are primarily used for heat dissipation and the reduction of friction between tool and workpiece by lubrication and support the removal of chips. By dispensing with a cooling lubricant, these tasks must be compensated by the tool. The main challenge for changing the drilling process to dry machining is therefore the concept of heat dissipation or more specifically the prevention of heat development as well as the removal of chips. If the heat cannot be dissipated in good time, the temperature becomes too high and the material will get damaged. For example, an excessive heat introduction for fibre-reinforced carbon will cause the resin used to burn. The material becomes brittle as a result. In contrast, a high level of burr formation is observed for aluminium.

In contrast to a multi-stage drilling process, the combination tool must undertake all work steps (drilling, boring, reaming and countersinking) during machining and bore the rivet connection in one step. In this way, both the position of the bore and the alignment between the cylindrical part of the bore and the countersinking are ensured. Angular errors or offset that can occur with multi-stage operations are therefore excluded.

Together with other quality characteristics of the machining result such as diameter, transition radius and countersinking angle, the burr at the bore outlet plays an important role. If a burr has formed at the bore outlet for multi-stage bore machining that was manually executed, it can be removed without great effort with the aid of a countersink. If the process is automated in only one step, manual deburring is not possible. For this reason, the relevant tool must be able to drill virtually burr-free. Aircraft manufacturers usually specify a maximum burr height of 0.1 mm. In addition to the burrs at the bore outlet, the interlaminar burrs between the layers can arise. If they form, the multilayer composite must be dismantled at the end of the drilling operation to remove the interlaminar burrs. Dismantling is time consuming and cost-intensive and so these burrs must also be avoided.
 

The machine concept significantly affects the tool geometry. CNC applications on machining centres or portal machines are characterised by a high rigidity and stable machine control. As a result, the tool is very well guided in the bore. Applications with drill feed units, robots or hand-held machines are less stable and require tools with additional stabilising features for high accuracies.

Another special feature of drill feed units are the so-called “nose pieces”, also called guide bushes. The chips are removed above the tool through the long and narrow guide bushes up to a suction channel that is located at the end of the guide bush. To be able to remove the chips, long chip spaces are required that must be correctly dimensioned and adapted.

The bores at the outer skin (fuselage and wing) are drilled with portal machines or robots. The inaccessible drill machining steps, mainly in final assembly, are then drilled with drill feed units or with hand-held drills.

Along with the process and machine concept, the materials used have a significant influence on the tool design. Each material places individual requirements on the tool and process parameters. The selection of the individual material combinations in aircraft construction depends on the loads that act on the part during flight operation. There is also generally always a focus on reducing weight.

The outer skin and ribs of the latest generation of aircraft consist primarily of a composite made of CFRP and aluminium. In addition,  combinations made of different aluminium alloys or CFRP titanium are often used in the aerospace sector. The dimensional accuracy of the bores in this composite material is crucial. The bore must feature exactly the same diameter in both materials of the respective combination. In principle drilling always takes place from the outside to the inside. For example, the bore entrance and the countersinking is in the outer skin that consists of CFRP when machining CFRP/ aluminium, and the bore outlet is in the structure underneath that is designed in aluminium. During the individual machining of CFRP and aluminium materials, the geometries of the tools as well as the cutting data are fundamentally different.

In contrast for CFRP-titanium combinations, tools with a cutting edge that is sufficiently stable are required to withstand the ductile titanium and simultaneously have the appropriate sharpness to cut the CFRP. Whether merely one boring process suffices to produce a bore or whether the bore must be subsequently reamed depends on the required bore tolerance for this material combination.

Tools for drilling composite materials made of different aluminium alloys, for example 7050 and 2024, do not need a wear-inhibiting coating. This is because the grades of aluminium used in aircraft construction contain no or very little silicon and can therefore be drilled virtually without wear. This multilayer composite is significantly different to composites that contain CFRP when machining.

Tools that are used for material combinations that contain CFRP are generally provided with a diamond layer. This counteracts the abrasion of the CFRP and enables long tool lives. Regrinding these tools is not possible as the diamond layer used has a very high hardness.

• Qualitätsanforderung 
• Material 
• Bearbeitungsprozess

Da die Großzahl der Bohrungen im Flugzeug aufgrund der Nieten eine Senkung benötigen, ist der Bohrungsaustritt kritischer zu bewerten, um kostenintensive Nacharbeiten auszuschließen.  

Im Werkstoff CFK:

• Delaminationen
• Faserüberstände

Im Werkstoff Aluminium:

• Gratbildung

Wichtig ist bei der Bearbeitung aller Einzelmaterialien sowie aller Schichtverbundwerkstoffe zudem die Spanabfuhr. Ist diese nicht gewährleistet, liegt die Bohrungsqualität beim Trockenbohren schnell deutlich außerhalb der geforderten Toleranzen. Die größte Herausforderung bei der Entwicklung eines Trockenbohrers stellt aber die Anpassung der Werkzeuggeometrie auf das labile Bearbeitungssystem der Bohrvorschubeinheiten in Kombination mit Schnittparametern und Spannsystemen (ConcentricCollet) dar.

Für die Trockenbearbeitung der Schichtverbundwerkstoffe aus unterschiedlichen oder gleichen Aluminiumlegierungen, hat MAPAL einen Bohrer mit Senkstufe entwickelt. Durch spezielle Geometriemerkmale hält das Werkzeug die Gratbildung gering. Zudem erreicht der Bohrer eine verbesserte Zentrierung. Die Beschichtung des Werkzeugs verhindert die Bildung einer Aufbauschneide an der Schneidkante. Speziell ausgeformte Spannuten stellen die optimale Spanabfuhr sicher. Gekühlt wird mit Luft, was sowohl die Überhitzung an der Werkzeugschneide als auch die des Aluminiums und damit die Gratbildung verhindert. Zudem werden mit der Pressluft die Späne ausgeblasen. 

• Einsatzort: Längsnaht im hinteren Hauptfeld
• Durchmesser: 4,748 mm
• Senkstufe: 100°
• Drehzahl: 2.959 min-1
• Vorschub: 0,154 mm
• Standzeit: 1600 Bohrungen
• Toleranz: 4,73-4,805 mm

Um Schichtverbundwerkstoffe aus CFK und Aluminium prozesssicher zu bearbeiten, hat MAPAL ebenfalls einen Bohrer mit Senkstufe zur Trockenbearbeitung entwickelt. Die spezielle Geometrie des Werkzeugs sorgt dafür, dass die entstehende Bearbeitungswärme nicht an das Bauteil abgegeben wird. Zudem werden weder das Bauteil noch die Arbeitsumgebung durch Kühlmittel verschmutzt. Der zweischneidige Bohrer aus Vollhartmetall vereint die Eigenschaften eines Bohrers zur Bearbeitung von Aluminium mit denen eines Bohrers zur CFK-Bearbeitung. Durch die speziell ausgeführten Spanräume ist die prozesssichere Abfuhr der Späne sichergestellt. Da CFK ein extrem abrasiver Werkstoff ist, ist der Bohrer diamantbeschichtet. Damit wird gegenüber einem unbeschichteten Bohrer die achtfache Standzeit erreicht.

Das Bohr-Senk-Werkzeug zur Trockenbearbeitung von CFK-Alu-Kombinationen ist erfolgreich bei Kunden im Einsatz. Es wird mit einer Drehzahl von 5.000 min-1 und einem Vorschub von 0,1 mm gearbeitet. Das Werkzeug überzeugt in der Praxis nicht nur durch die erreichten Ergebnisse hinsichtlich Prozesssicherheit, Standzeit und Bearbeitungsergebnis, sondern auch durch den ruhigen Bohrprozess.

Unterschiedliche Materialkombinationen, die engen Toleranzvorgaben sowie die geringe Maschinenführung stellen die Werkzeughersteller vor große Herausforderung. Im Hinblick auf die automatisierte Fertigung durch Roboter gewinnt zudem die Trockenbearbeitung in der Luftfahrt immer mehr an Bedeutung. In enger Zusammenarbeit mit führenden Flugzeugherstellern hat MAPAL diese Herausforderungen gemeistert und innovative Bohr-Senk-Werkzeuge zur prozesssicheren Trockenbearbeitung von Schichtverbundwerkstoffen aus CFK-Aluminium und Aluminium-Aluminium entwickelt. Die gezielte Auslegung der Werkzeuggeometrien im Hinblick auf Werkstoffkombination, Maschinenkonzept und Bohrprozess ermöglicht in der Praxis eine signifikante Erhöhung der Prozessfähigkeit sowie der Standzeit der Werkzeuge. Bohrungen außerhalb der Toleranz sowie Defekte an Bohrungsein- und -austritt gehören damit der Vergangenheit an.