Good vibrations!

  Lightweight, boasting high tensile strength, yet not easy to machine – these are the features which characterise fibre composite materials. Machining such compounds formed from multiple materials (stacks), which are mainly used in aircraft construction, is particularly demanding: a typical application is drilling rivet holes. One of Walter's main focuses in this field is vibration drilling using tools custom-designed for this purpose. There is currently a great demand for composite materials due to their lightweight properties and incredible tensile strength. Use of these materials is becoming increasingly widespread in vehicles, sports equipment, medical products and other products. However, the aircraft industry is where the highest number of applications, the widest variety of materials and the largest components are found. Many fuselage and wing parts are manufactured from carbon fibre-reinforced plastics (CFRP). A typical task is drilling rivet holes – riveting continues to be an important connection method for aircraft due to the reliability it provides. However, cutting through the delicate carbon fibres using a drill is not simple, as there is a great tendency for these fibres to tear in most cases. The fibres are also highly abrasive, which means that conventional cutting edges blunt rapidly. Drilling becomes even more challenging when composite materials, which are referred to as stacks, made of CFRP and metal layers, are involved. CFRP/Ti, CFRP/Al and CFRP/Ti/Al combinations are common. Application examples include door frames, formers and reinforcement profiles. During drilling, the tool comes into contact with materials that have a wide range of different properties. Complying with the strict diameter tolerances, usually around 40µm, for all material layers is no easy feat. This is particularly difficult when the ratio of the depth of the drilled hole to the required diameter is large. <Getting to know the drill> Vibration drilling has proven to be an effective method for some time now. This involves the standard feed movement being complemented by an oscillating action (sinusoidal oscillation) in the feed direction of the drilling tool. The component manufacturer has fitted a vibration unit to the spindle of their machining centre for this purpose. This unit usually contains a wave washer and a bearing. One part remains fixed while the other rotates. The values for the vibration frequency and the amplitude depend on the design. If the large fuselages or wing parts no longer fit on a machining system or are difficult to access, the aircraft fitters work with mobile, semi-automatic drilling machines: these are now available with vibration units too. Stefan Benkóczy, aerospace component manager at Walter, explains: “Vibration drilling is a particularly good option if the first layer to be drilled into is CFRP, followed by the metal layer.” The aim of this method is to use the tool's longitudinal oscillations to produce short metal chips without increasing the process time, as is the case for pecking (chip removal feed strokes) for example. The oscillation means that the tool is not cutting constantly, causing a break in the chip length. Long chips rub against the wall of the hole drilled into the CFRP when being transported back through the drill's helical flutes and may wear down this wall – beyond the tolerance limit of the drilled hole in the worst-case scenario. On the other hand, chips that are broken off when they are short cause far less damage. There is also another aspect to be taken into consideration. “The deformation energy is mainly transferred to the chips resulting in an increase of their temperature,” states Benkóczy. “Short chips are removed in an optimum manner; therefore the process temperature is halved for vibration drilling in comparison to conventional machining using MQL. This protects the composite material matrix in particular and minimises diffusion wear on the tool. This results in an increase in component quality due to vibration drilling. Tool life is also increased while the process time remains the same.” <Innovations with oscillations> To achieve the best possible results, all process parameters must be carefully matched to the application and the available equipment. There is a relatively large number of influencing factors: cutting data, vibration frequency, amplitude, tool cooling, geometry, etc. With conventional mechanical vibration units, the drill's oscillation frequency still also depends on its rotational speed. 500 to 3,000rpm is a feasible range for a drilling diameter of around 5mm. Wave washers which produce 1.5 or 2.5 longitudinal oscillations per revolution result in a typical frequency range of approximately 12 to 125Hz. A large amplitude increases tool wear and can even lead to tool breakage. “In order to avoid placing unnecessary stress on the tool, the amplitude must be selected so that the chips break off straight but cleanly,” explains Benkóczy. “However, in the case of unstable clamping arrangements, the amplitude must be significantly increased to ensure that the chips still break off.” Special machining methods require special tool solutions. Drills with PCD cutting edges or a diamond coating, such as those normally used for machining composites, are only an option for vibration drilling under very specific conditions. The extremely hard diamond cutting materials are generally too brittle for this application. The solution provided by Walter for vibration drilling is the Walter Titex VFT1A. This drill's key features include four lands to ensure that the drill remains on course, particularly when drilling through different layers of material. An aluminium-chromium-nitride coating protects the drill from becoming worn quickly. Its internal coolant channels are designed to provide the minimum quantity of lubrication. “Vibration drilling in composite materials increases process reliability,” concludes Benkóczy. “Short chips are evacuated in an optimum manner and there is reduced variation in the diameters. In conjunction with an automated system, vibration drilling represents a very economical alternative to conventional drilling. There is still potential for further improvement, which is why Walter is working on other projects.” www.walter-tools.com