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Composite machining with abrasive waterjet (AWJ) for aerostructures
The use of carbon-‐fibre composites as a structural material for commercial airframes is increasing significantly. This material represents 50% by weight of the new Boeing 787 and 52% on the Airbus 350XWB frames. AWJ cuts offer an acceptable surface finish and high integrity. CuLng rates are faster than with convenMonal rouMng. Significant advances have been made in the hardware, soPware, and process reliability, flexibility, and producMvity, making the AWJ process a mainstream tool for airframe manufacturers and their subcontractors.
The use of advanced materials such as composites has been expanding rapidly over the past three decades, especially in the aerospace industry, coincident with the introduc:on of AWJ technology to the marketplace in the 1980s. This ar:cle briefly discusses AWJ hardware, soEware, and common machining processes such as trimming and drilling of carbon fibre composites. Composite Carbon-‐fibre-‐based composites for aerostructures were ini:ally used in military airframes. Now, carbon-‐fibre composites are used extensively in commercial aircraE to produce parts such as wing components (covers, spars, leading edges, flaps), fuselage components (panels, stringers, frames, clips, doors), tail components (VTP & HTP covers, rudder, and flaps), keel beams, centre wing boxes, or belly fairings. AWJ-‐machined components are used in the following examples: -‐ Boeing 787: wing box, wing skins, spars, stringers, fixed leading edge, -‐ Boeing 777 horizontal and ver:cal stabilizer, -‐ Airbus 350XWB: wing spars; wing skins, fuselage, -‐ Airbus 320: Empenage skins, ribs -‐ Bell Helicopter V-‐22 Osprey: wing skins, spars, ribs, fuselage, s:ff rotors; -‐ Raytheon Premier I: fuselage; Most revolu:onary in the use of composites on commercial liners is the Boeing 787, which will contain 50% composite structure by weight and 90% by volume, and the Airbus A350XWB with similar composite usage. In comparison, the 777, which entered service just over ten years ago, contains only 10% composite structure by weight. AWJ technology offers several advantages over conven:onal machining methods. Among these advantages are: -‐ no distor:on due to limited jet forces and the nature of micromachining ac:on, -‐ no heat-‐affected zones, -‐ higher cuang speeds than routers, -‐ reduced fixturing and tooling, -‐ no delamina:on, -‐ no subsequent processing, -‐ no splintering or fraying edges, -‐ possible process automa:on and mul:-‐opera:ons, -‐ no dust.
AWJ technology advances for composite machining There have been many advances in AWJ composite machining for airframes. These can be grouped into two basic categories: machinery (hardware) process, and soEware advances. Some of these advances are described below. Hardware Cu#ng heads The use of vacuum assist in AWJ cuang heads (see Figure 1) has been cri:cal for successful shape-‐cuang of composites. An external vacuum source is used to draw abrasives into the cuang head before star:ng the waterjet. This insures instantaneous ac:on of the AWJ upon firing the waterjet. It has been shown that no delimita:on occurs when piercing composites using this approach.
www.JECcomposites.com
Mohamed Hashish, SR. Vice President, Technology at Flow Interna:onal Corpora:on
Fig. 3: Composite stringer
Flexible fixturing Flexible fixturing is a key component in AWJ machining cells due to the wide variety of shapes and contours of airframe parts. Flow has developed versa:le fixtures that consist of several linear actuators with vacuum cups, as well as hard loca:on surfaces and points for accurately loca:ng the part and rigidly holding it during cuang. SoPware Most airframe parts are designed using CATIA solid modeling soEware. In order to develop a CNC program to trim and drill parts, a post-‐processor program is needed to translate the drawing so that the machine controller can execute it. This postprocessor also includes informa:on about the process, such as cuang speed, tool diameter, etc. This is a most cri:cal element of the AWJ machining system, as most parts are complex 3-‐D shapes. Waterjet processes The most common waterjet processes used for airframe are trimming, shape-‐cuang, and drilling. Trimming is typically performed on the edges, while shape-‐cuang is performed on the interior surfaces to produce openings such as access holes or windows. Trimming and shape-‐cu#ng A wide range of parameters has been found acceptable for cuang a wide range of thicknesses (2 to 100 mm) of graphite epoxy. The cut surfaces of befer than 10 micrometers (as typically specified by Boeing) are achieved at rela:vely high produc:vity levels. For example, a cuang speed of 500 mm/min can be used to trim a 10-‐mm thick carbon fibre using a 0.15 mm AWJ at 3800 bar. Trimming I-‐beam (stringer) composites requires special tooling such as catchers and cuang heads. Figure 3 shows a cross sec:on of a stringer with the top flange trimmed at 90 degrees with an AWJ while the bofom flange is cut at 45° chamfer. These stringers, especially the ones used in the wings, are rela:vely long, over 40 m, and thus they require special machinery to handle them during cuang.
The end trimming of composite stringers (I-‐beams) is another applica:on that requires a careful 5-‐axis manipula:on strategy as not to cause the exit jet damage the opposite sides of the stringer. Figure 3 shows a trimmed sec:on from the end of a stringer. A rate of 25 mm/min can be used to cut through a 76-‐mm thick sec:on producing an acceptable 10.16 micrometer surface finish. A most challenging cuang applica:on is the cuang of composite honeycomb structures as shown in Figure 4. Cuang this class of materials introduces addi:onal challenges, as the cuang path is not con:nuous. A cut at the bofom of a honeycomb structure appears as a series of punched holes. Cuang at a lead angle (a few degrees) has been found to be effec:ve for minimizing this effect, as shown in Figure 5.
Fig. 4: Examples of honeycomb structures
Fig. 2: Hybrid waterjet system
Fig. 1: AWJ cu#ng head with vacuum assist.
Hybrid waterjet system In order to trim and rout composites, both waterjets and solid tool routers have been incorporated on special hybrid systems, as shown in Figure 2. In these systems, two 5-‐axis masts are used: one for the AWJ and another for the router.
Fig. 5: BoKom surface of 25-‐mm-‐thick honeycomb cut