Dry Machining of Multilayer Composite Materials in Aircraft Construction
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In the final assembly stage, tool manufacturers need to control not only a range of materials and deliver maximum precision but must also master the cooling methods and tolerance specifications as well as the machinery used. Mapal is now launching a tooling concept for the reliable dry machining of material combinations such as CFRP-aluminium or various aluminium alloys that meet the high demands in this area.
The Challenge of Final Assembly
High-strength yet lightweight materials are of paramount importance in aviation. New combinations of materials allow weight to be further reduced, strength and corrosion resistance to be increased and assembly to be simplified thanks to an integrated design. While structural components made from aluminium, titanium or high-strength steels are processed on machining centres or gantry machines, components in the final assembly stage are usually processed using hand-guided machinery, drill feed units and robots.
The demands placed on tool manufacturers and tools for the final stage of assembly are therefore significantly different from those in component production. While the parts processed in the component production stage might cost somewhere in the range of € 1000 to € 50,000, the components in final assembly are significantly more expensive, costing anywhere between € 50,000 and € 2,000,000 depending on how far advanced assembly is. Errors in processing must either be corrected through time-consuming and costly manual rework, or the components must be replaced completely. This is the reason why the supplier selection process is very painstaking.
A further challenge for tool manufacturer is the sheer variety of materials, especially when several materials with differing characteristics are to be machined at the same time. In order to qualify as a tool manufacturer, it must be possible to machine all materials reliably and economically. It takes between one and five years to qualify as a tool manufacturer. Tools and processes also require additional qualification. Existing processes are only ever modified in exceptional circumstances. For this reason, care must be taken to ensure that all machining steps are performed with the same consistent quality. For example, drilling during final assembly must guarantee low variation in bore diameter; a CpK value greater 1.7 must be assured.
Bore Holes for Rivet Joints
Aircraft manufacturers use rivet joints to connect the outer skin with the structural elements underneath. This requires a large number of bore holes to be drilled. The rivet heads are countersunk into the aircraft’s outer skin in order to achieve the lowest possible flow resistance (a low cW value). This requires an additional counterbore to be produced at the bore entrance. Whereas in the past, this often involved a process with up to four individual processing steps (drilling, boring, reaming, counterboring), single-step machining, where drilling and counterboring are implemented in a single operation, are today state of the art. Only this allows automated processing by robots. In the past, this type of processing was performed using minimum quantity lubrication (MQL). After machining, the components had to be dismantled, cleaned and reassembled. What is more, with this process the coolant entered the interior of the aircraft, where other assembly steps were being performed in parallel. This led to a call for tools that can be used to dry machine various multilayer composite materials.
Call for Dry Machining
No coolant at all is used in dry machining. Coolants are primarily used to dissipate heat and reduce friction between tool and workpiece through lubrication while they also support the removal of chips. Since no coolant is now used, these tasks must be compensated for by the tool itself. The main challenge when switching the drilling process to dry machining is thus the concept of heat dissipation, or the avoidance of heat generation, as well as the removal of chips. If the heat cannot be dissipated, the temperature will rise too far and the material will be damaged. For example, too much heat input with fibre-reinforced carbon results in the resin burning, making the material brittle. Aluminium, on the other hand, exhibits increased burr formation.
The machining concept has a major influence on tool geometry.
The primary factors influencing the formation of heat during drilling are, first, distortion and friction in the shear zone and, second, friction on the guiding chamfer on the bore wall. Furthermore, chips between the tool and the workpiece cause additional friction. The goal is to keep friction as low as possible.
An adapted tool geometry allows the contact surfaces between the tool and the workpiece to be significantly reduced, thus minimising the resultant friction. Adapted chip flutes and coatings that reduce friction between the tool and chips improve chip removal considerably. Furthermore, the chips produced can be removed from the bore using suction or compressed air. All these measures result in significantly less heat.
Moreover, dry machining has a positive effect on the tool. For example, tool designs that do not use compressed air do not need coolant exit bores. The lack of coolant means that there is no thermal shock load, which results in improved tool life. As no attention needs to be paid to coolant channels in the design of the tools, they can be designed to be more robust.
Unlike a multi-step drilling process, a combination tool must perform all the worksteps (drilling, boring, reaming, counterboring) and complete the bore for the rivet joint. This ensures not only the position of the bore but also the alignment between the cylindrical part of the bore and the counterbore. The possibility of an angle error or misalignment — as sometimes occurs in multi-step operations — is therefore eliminated.
The exit burr also plays a major role in addition to other quality characteristics of the machining result such as diameter, transition radius and countersink angle. If burr forms around the bore exit during multi-step bore drilling that is performed manually, it can be removed without great effort with the help of a countersink. Manual burr removal is not possible when the process is automated and performed in a single step. For this reason, the corresponding tool must be able to drill virtually burr-free. Aircraft manufacturers usually specify a maximum burr height of 0.1 mm in this case. In addition to burr at the bore exit, there is also interlaminar burr between the layers. If this forms, the composite layer must be dismantled at the end of the drilling operation in order to remove the interlaminar burr. As dismantling is time- consuming and costly, this type of burr must also be avoided.
Consequences of the Machining Concept
Bores in the outer skin (fuselage and wings) are drilled using gantry machines or robots. Inaccessible bores, primarily in final assembly, are then processed using drill feed units or manual drills.
Machining Multilayer Composite Materials
In addition to the process and machinery concepts, the materials used have a major influence on tool design. Each separate material places specific demands on the tool and process parameters. The choice of individual material combinations in aircraft construction depends on the loads that are applied to the component during flight operation. In addition, there is generally a focus on weight reduction.
However, when CFRP and titanium are combined, tools are required whose cutting edge is rugged enough to resist ductile titanium while having the appropriate sharpness to cut CFRP. Whether one drilling process step is sufficient to complete the bore, or whether the bore subsequently needs to be reamed, depends in the case of this material combination on the required bore tolerance.
Tools for drilling multilayer composite materials made from different aluminium alloys, for example 7050 and 2024, do not require any wear-inhibiting coating, since the types of aluminium used in aircraft construction contain little or no silicon and can therefore drill virtually wear-free. This greatly differentiates this multilayer composite from composites that contain CFRP.
In order to create precise bores and avoid damaging the bore wall in the composite layers above, it is essential to remove the chips optimally from the bore. With long-chipping materials like aluminium or titanium, the chips need to be broken in order to prevent any flow chips. Pecking has proved to be an effective way of reducing chip size on drill feed units and thereby simplifying their removal. In this process, the chipping cross-section is varied in order to generate chip break. If drilling is performed on a CNC machine, chipping breakage can be effected by means of programmed feed stops.
Each material places specific demands on the tool and the process parameters.
Tools for material combinations with a proportion of CFRP, for example, are generally given a diamond coating. This counteracts abrasion by the CFRP and enables a long tool life. It is not possible to regrind these tools, as the diamond coating is extremely hard.
When designing the tool geometry, attention must be given to quality requirements, to the material and to the process deployed in order to ensure reliable machining. Since the majority of the bore holes in an aircraft are made with a counterbore because of the rivets, the bore exit must be evaluated more critically in order to exclude costly rework. Delamination and fibre projection must be prevented in the CFRP material, while burr formation must be avoided in aluminium. An important aspect when machining all individual as well as all composite materials is the issue of chip removal. Chips that cannot be removed by suction remain in the flute and damage the bore wall in the layer above. The bore wall is scratched or abraded, with the result that the diameter is no longer within the specified tolerance; this is why chip flutes must be designed and implemented accordingly. Failing to ensure perfect chip removal will result in the bore quality for dry drilling being significantly outside the required tolerances. However, the biggest challenge when developing a dry drill is adapting the tool geometry to the unstable machining system of the drill feed units in combination with the cutting parameters and chuck (concentric collet).
Drilling and Counterboring Tool for Aluminium-Aluminium Combinations
Aircraft manufacturers frequently use multilayer composite materials made from different aluminium alloys, e.g. 7050 and 2024, for the fuselage. In this case, the wrought alloy aluminium 2024 is used for the outer skin. The material can generally be easily machined. Al7...-alloys are frequently used for skin reinforcement or for the truss structures behind it. Al7075, for example, is very easy to machine.
Drilling and Counterboring Tool for Machining CFRP-Aluminium Combinations
No coolant at all is used in dry machining.
Different material combinations, tight tolerance specifications and reduced machine guidance, present tool manufacturers with major challenges. In addition, dry machining is gaining increasing importance in aircraft manufacturing as a result of automated production using robots. Mapal has collaborated closely with leading aircraft manufacturers to develop drilling and counterboring tools to reliably dry machine multilayer composite materials made from CFRP—aluminium and aluminium-aluminium. The systematic design of the tool geometry that takes account of material combination, the machinery concept and drilling process achieves significantly greater process capability and longer tool life in everyday practice. It also prevents bore holes outside the tolerance limits as well as faults at the bore entrance and exit.