Carbon fiber reinforced plastics (CFRP) inspire designers in lightweight aircraft construction, but make machining experts sweat, as the coating specialist Oerlikon Balzers reports. On its way through a composite, a drill sometimes encounters super-strong carbon fibers and softer epoxy resin, and in the case of layered composite material, sometimes layers of titanium or ductile aluminum with completely different machining properties. Delamination, i.e. fiber tears when the drill enters and exits the material, must be prevented. This is because these ultimately lead to deviating tolerances in terms of diameter, shape and surface quality of the many thousands of holes that are intended to accommodate rivets for the stable assembly of components.
Examine each tool application in detail first
This is an extremely tricky task for the Austrian precision tool manufacturer Scheinecker from Steinhaus near Wels, which will be celebrating its 50th anniversary in 2025 and has 35 years of expertise in the aerospace sector. "We examine every application down to the smallest detail on site at the customer's premises before the design comes into play," says Dino Lirk, Technical Director at Scheinecker.
This was also the case when a renowned aircraft manufacturer came knocking and asked for a drill that could handle machining after a material change better than the previous tool. The material in question was a stack material consisting of several carbon fiber and aluminum layers for components in the wing area of a passenger aircraft. In order to machine such heterogeneous materials cleanly and reliably over a long period of use, the substrate, geometry, micro-cutting edge structure and coating of the tool must fit together perfectly. When designing the geometry, the Austrians pay attention to chip flow, controlled chip breaking and tool stability. The focus is also on surface quality, concentricity and rigidity of the drill.
Coating makes the tool stronger
Because the coating plays a key role in ensuring the best performance, Scheinecker opted for Baldia Composite DC, a diamond coating from surface specialist Oerlikon Balzers. Specially developed for machining composite materials, the nanocrystalline CVD (Chemical Vapor Deposition) coating protects against wear caused by highly abrasive CFRP and thermal stress as well as against adhesion and burr formation caused by aluminum.
The capabilities of this diamond coating are not limited to drilling CFRP or stack materials with titanium or aluminum. Tools coated with Baldia Composite DC also cope very well with aluminum alloys containing more than 12% silicon. According to the manufacturer, precise graphite molds are another area of application.
Micrometer-precise coating to maintain tight tolerances
According to Oerlikon Balzers, the surface protection is produced with micrometer precision and the coating thickness is controlled for precise compliance with tolerance values that lie in the middle of the applicable upper and lower limits. Typical coating thicknesses are between 4 and 15 µm. Baldia Composite DC therefore enables very tight tolerances for tool diameters and bores. A special pre-treatment further supports the maintenance of very sharp cutting edges, high process reliability and consistent bore quality. The layer hardness HIT is between 80 and 100 GPa, measured by nano-indentation according to ISO 14577. In the case of multilayer coatings, the hardness of the individual layers can vary and the hardness can be adapted to the respective application. The manufacturer quotes a maximum application temperature of around 600 °C based on practical experience, depending on the pressure present.
Scheinecker designed the solid carbide tool for use in a semi-automatic drilling feed unit (ADU/Advanced Drilling Unit) - a first, as the components in question were previously only drilled manually. The geometry and drill were measured and produced at great expense using state-of-the-art machines. Tests were carried out at the company's own stations as well as at the customer's.
Over 130% longer tool life in just four weeks
The very first test achieved 600 drill holes with one tool, and this figure has now risen to well over 1,000. "With 1,000 drill holes, we are already 130% above the customer's requirements. Further adjustments to the coating thickness and geometries are expected to increase tool life by up to 30%," summarizes Lirk. It only takes around four weeks from the first inspection of the component to the finished, delivered tool - not a special case, but the average lead time at Scheinecker. More than ten weeks is standard on the market, says Lirk.
The tool life and speed of implementation were not least the result of the collaboration with Oerlikon Balzers. "In projects like this, we ask about all the relevant factors straight away and then use our experience to decide very quickly which coating and thickness is suitable. We receive all the necessary information from Scheinecker at an early stage so that we can coat and deliver test tools quickly," explains Dirk Schmidt, Product & Key Account Manager Diamond at Oerlikon Balzers.
"This is why we work exclusively with Oerlikon Balzers as our coating partner," explains Ulrike Scheinecker-Graul. In a way, the Managing Director of Scheinecker received confirmation of the good teamwork directly from the customer: The customer described the costs per borehole achieved with the drilling tool as the best result he has ever been able to achieve in a project of this kind.