Conversion to dry processing

The drilling and countersinking tool for dry machining of CFRP-aluminum laminates combines the properties of a drill for machining aluminum with those of a drill for machining CFRP. (Image: Mapal)

Dry machining completely dispenses with cooling lubricants. The main challenge when converting the drilling process to dry machining is heat dissipation or the prevention of heat generation and the removal of chips. If the heat cannot be dissipated in time, the material will be damaged. For example, if too much heat is applied to fiber-reinforced carbon, the resin used burns and the material becomes brittle. With aluminum, on the other hand, a higher burr formation can be observed.

In contrast to a multi-stage drilling process, the combination tool must produce the hole for the riveted joint in a single step. This ensures both the position of the hole and the alignment between the cylindrical part of the hole and the counterbore. Angular errors or misalignment as in multi-stage operations are thus eliminated.

In addition to other quality features of the machining result such as diameter, transition radius and countersink angle, the exit burr plays a major role. If a burr has formed at the exit of the hole during manual multi-stage drilling, this can be removed using a countersink. If the process is carried out automatically in a single step, manual deburring is not possible. The corresponding tool must therefore be able to drill almost burr-free. Aircraft manufacturers usually specify a maximum burr height of 0.1 mm. In addition to the burr at the hole exit, there is the interlaminar burr between the layers. If this forms, the laminate must be dismantled at the end of the drilling operations to remove the interlaminar burr. This disassembly is time-consuming and cost-intensive, so this burr must also be avoided.

A special CFRP with "copper mesh" is often used in aircraft construction. Any delamination or fiber protrusions at the bore entry are avoided. (Image: Mapal)

The machine concept also has a significant influence on the tool geometry. CNC applications on machining centers or portal machines are characterized by high rigidity and stable machine guidance; the tool is thus guided very well in the hole. Applications with drill feed units, robots or hand drills are less stable and require tools with additional stabilization features.

Another special feature of drill feed units are the so-called "nosepieces", also known as guide bushes (Fig. 3). The chips are removed via the tool through the guide bush to an extraction channel located at the end of the guide bush. In order for the chips to be removed, long chip spaces are required, which must be correctly dimensioned and adapted.

The holes on the outer skin (fuselage and wings) are drilled using portal machines or robots. The inaccessible drilling operations - mainly in the final assembly - are then drilled using drill feed units or manual drilling machines.

Customized tools for laminated composites

Each material places individual demands on the tool and the process parameters. The choice of individual material combinations in aircraft construction depends on the loads that act on the component during flight. In general, the focus is always on saving weight.

The outer skin and ribs of the latest generation of aircraft are predominantly made from a composite of CFRP and aluminum. In addition, combinations of different aluminum alloys or CFRP-titanium are often used in the aviation sector. The decisive factor when drilling holes in these layered composites is dimensional accuracy. The hole must have exactly the same diameter in both materials of the respective combination. Holes are always drilled from the outside in. For example, when machining CFRP-aluminum, the hole entry and counterbore are located in the outer skin, which is made of CFRP, and the hole exit is located in the underlying structure, which is made of aluminum. The geometries of the tools and the cutting data are fundamentally different for the individual machining of CFRP and aluminum materials.

The CFRP-titanium pairing requires tools with a cutting edge that is strong enough to withstand the ductile titanium and at the same time sharp enough to cut the CFRP. Whether a drilling process alone is sufficient to produce the hole or whether the hole needs to be reamed afterwards depends on the required hole tolerance for this material combination.

So-called "nosepieces" are used for additional stabilization. These make it more difficult to remove the chips. (Image: Mapal)

Tools for drilling layered composite materials made from different aluminum alloys, such as 7050 and 2024, do not require a wear-resistant coating. This is because the aluminum grades used in aircraft construction contain little to no silicon and can therefore be drilled almost wear-free. This distinguishes this layered composite from composites containing CFRP during machining.

Tools that are used for CFRP material combinations are generally provided with a diamond coating. This counteracts the abrasion of the CFRP and enables a long service life. It is not possible to regrind these tools as the diamond coating used is very hard.

Choosing the right tool

To ensure process reliability, the quality requirements, the material and the process must be taken into account when designing the tool geometry. As the majority of the holes in the aircraft are produced with countersinking due to the rivets, the hole exit must be evaluated more critically in order to rule out cost-intensive reworking. Delamination and fiber protrusions must be prevented in CFRP and burr formation in aluminum. Chip removal is also important when machining all individual materials and all laminated composites.

If chip evacuation is not guaranteed, the bore quality during dry drilling will be well outside the required tolerances. However, the greatest challenge in the development of a dry drill is adapting the tool geometry to the unstable machining system of the drill feed units in combination with cutting parameters and clamping systems (ConcentricCollet).

Processing of aluminum-aluminum combinations

The bore diameters in the top layer when machining aluminum-aluminum laminates with the Mapal tool are reliably within the specified tolerances. (Image: Mapal)

Mapal has developed a drill with a countersink step for dry machining of laminated composites made of different or identical aluminum alloys. Special geometry features keep burr formation to a minimum and improve centering. The coating of the drill prevents the formation of a built-up edge on the cutting edge. Specially shaped flutes ensure chip removal. Air is used for cooling, which prevents both the tool cutting edge and the aluminum from overheating and thus burr formation. The chips are also blown out with compressed air.

At an aircraft manufacturer, the drill is used for drilling the longitudinal seam in the rear main panel, among other things. Here, it works at a speed of 2,959 rpm and a feed rate of 0.154 mm. The drill with a diameter of 4.748 mm and a 100° countersink stage reliably machines 1,600 holes before the holes no longer reach the required tolerance of 4.73-4.805 mm (Fig. 4).

Processing of CFRP-aluminum combinations

The bore diameters in the top layer when machining CFRP-aluminum laminates. (Image: Mapal)

Mapal has also developed a drill with a countersink for dry machining in order to reliably machine CFRP and aluminum layer composites. The special geometry of the tool ensures that the heat generated during machining is not transferred to the component. In addition, neither the component nor the working environment are contaminated by coolant. The double-edged solid carbide drill combines the properties of a drill for machining aluminum with those of a drill for machining CFRP. The specially designed chip spaces ensure reliable chip removal. As CFRP is an extremely abrasive material, the drill is diamond-coated. This achieves eight times the service life of an uncoated drill.

The drilling and countersinking tool for dry machining of CFRP-aluminum combinations is being used successfully by customers. It is operated at a speed of 5,000 rpm and a feed rate of 0.1 mm. In practice, the tool not only impresses with the results achieved in terms of process reliability, tool life and machining results, but also with the quiet drilling process (Fig. 5).

Dry processing on the rise

Different material combinations, tight tolerance specifications and low machine guidance pose a major challenge for tool manufacturers. With regard to automated production using robots, dry machining is also becoming increasingly important in the aviation industry. In cooperation with aircraft manufacturers, Mapal has successfully met these challenges and developed innovative drilling and countersinking tools for the reliable dry machining of CFRP-aluminum and aluminum-aluminum composite materials. The targeted design of the tool geometry with regard to material combination, machine concept and drilling process enables a significant increase in process capability and tool life in practice. Out-of-tolerance bores and defects at the bore entry and exit are now a thing of the past.

Jens Ilg, Center of Competence (CoC) Aerospace & Composites; Thorsten Müller, Research and Development; Sebastian Kuhn, Technical Marketing; Mapal / ag