PRECISION WATERJET CUTTING IN THE COMPOSITES INDUSTRY UTILIZING ROBOTS FOR HIGH QUALITY ACCURATE MACHINING Duane Snider Advanced Robotics Application Business Manager, Flow International Corporation USA Abstract 5 Axis Gantry robots and 6 axis Articulated arm robots have been used with plain waterjets for many applications especially in the automotive industry. This paper is on extending the use of these robots to abrasive waterjets and for a much wider range of applications primarily in the composites market. This paper discusses the cutting process of the ultra high pressure waterjet and its technical advantages over conventional mechanical cutting tools. The integration of the abrasive waterjet process on robotic arms has been successfully developed to address the end effector, supply of high pressure water and abrasives to the cutting head, and operational safety. Off line programming, calibration, and inspection are discussed. Advanced software packages typically used in the aerospace industry have been successfully adapted. A few case studies are presented in this paper addressing composite trimming for wing skins used in aircraft and wind turbines, small airframe composite parts, glass trimming for high efficiency solar panels, and three dimensional machining of relatively small parts used in jet engines. Background The need for enhanced accuracy performance using first article inspection results is discussed. A few case studies are presented in this paper addressing composite trimming for wing skins used in aircraft and wind turbines, small airframe composite parts, and three dimensional machining of relatively small parts used in jet engines. It was found that 6-axis robot arms can easily be implemented when moderate accuracies, from 0.010 inch (0.25mm) to 0.015 inch (0.38mm) are specified. However, accurate calibration and first article inspection procedures were found necessary to obtain accuracies below 0.01 inch (0.25mm). Much tighter accuracies are achieved with stiff gantry systems. Introduction Today, thousands of multi axis robotic trimming cells are deployed worldwide in a variety of high production environments. These include; manufacturing standard and customized components, prototype designs, low & high volume mass production with option changes and service parts for OEM and aftermarket sectors. In other words, these robots are used when flexibility is critically required so continuously changing demands are met. Accurate processing of a work piece using an automated trimming system requires understanding and control of several variables. These variables can stack up and resulting in out-of-tolerance parts. These variables could be related to the manipulator, fixture, material, tool/process, and the environment. The manipulator, for example is a complex assembly of interconnected links, gears, gear trains, servo drives, harmonic drives and even belt drives. These result in positional and path errors which need to be compensated for. Also, recognizing, the idiosyncrasies of the material, such as homogeneity, shrinkage, warping etc will lead to minimize errors by proper compensation. In this paper, we first address robots as waterjet tool manipulators. This will be followed by describing their interface with high pressure. The general attributes of waterjet cutting of composites will then be discussed by presenting example applications and case studies. Page 1
Robots and Calibration In this section we address 5-axis gantry robots and 6-axis robotic arms because they are the most used in multi-axis waterjet cutting. 5 Axis Gantries The 5 axis Gantry systems, Figure 1, are Cartesian manipulators and are designed with relatively high degree of structural stiffness in order to achieve the required accuracy. It is typical in the design of these systems to study their structural, kinematic, and dynamic behaviors using sophisticated software packages. Figure 1: Gantry Design CMC For example, Figure 2 shows the results of a finite element analysis used to determine deflections under different loads. Figure 2: Gantry Structural Analysis using Finite Element Analysis software to determine structural stiffness Page 2
These units are installed on a machine tool foundation and usually housed in a controlled environment. Helical rack and precision gear boxes and closed loop control systems contribute to smooth motion and high accuracy. Before using these machines, the linear axes are compensated using laser interferometers. Linear encoder scales are also used to increase the accuracy on large scale systems. Figure 3 shows a picture of a typical 50-m long gantry system used for trimming wing skins for the Boeing 787. Figure 3: Large Gantry for Wing Trimming 6- Axis Robotic Arms 6-axis Robots are mass produced machines, Figure 4, and although highly repeatable, they are not accurate until they are mastered, i.e., compensated. Figure 4: 6 Axis Robots applied to mass production lines and Job Shops Page 3
Robots are typically manually taught by eye from a teach pendant, Figure 5, using a pointto-point format. Figure 5: Typical hand help teach pendant hardware for Point-to Point teaching and programming control versus Offline Programming using 3 rd Party CAD Design software and Post processors. The programmer may choose to use a custom pointer as the simulated Tool Center Point (TCP) or a peripheral laser pointing device, aimed at the target zone or they may simply guess and cut a part, measure the deviations and correct it with another pass and so on. During this procedure the operator will program the path, consisting of lines and circles and also define speeds, corner positioning etc and assign any I-O (inputs and outputs) to control or monitor external functions. These external devices can be the nozzle, vacuum system, clamps etc. The robot can also be oriented in a variety of positions to suit the work environment. Moving forward from this manual procedure to an offline programming environment will enable elimination of the teach pendant. This will require a custom post processor and process specific offline programming software relative to the particular brand of robot. In order to do this, the robot, the end effector TCP and the work holding fixture must be defined in the real world. As with the gantry systems, robot rigidity is also a very important factor to ensure high accuracy calibration. Waterjet Cutting A Green Technology Waterjets (without abrasives) have been used for cutting and trimming a wide range of automotive interior materials (2). Among these are: Floor carpets and roof liners. Dash sound insulation material. Plastic/Fabric composites (i.e. rear shelf and transmission tunnel components). Sheet molded composites (SMC). Glass fiber components (body panels etc.) Carbon fiber and /or Aramid fiber composite. Several system styles such as open and enclosed systems have been used where off line programming was critical as more than one arm may be working on one workpiece. Figure 6 shows an example. Page 4
Figure 6: Typical Robotic Cutting of Automotive Carpet with Dual Arm Articulate robots To increase the capability of waterjets, abrasives were added to form an abrasive-waterjet (AWJs). Figure 7 shows a schematic of an AWJ nozzle. Figure 7: Abrasive Water Jet (AWJ) Cutting head In this nozzle, water at 94,000 psi (650 MPa), is expelled through an orifice to form a highvelocity, coherent waterjet. Typical waterjet diameters are 0.004 inch (0.1 mm) to 0.010 inch (0.25 mm). Garnet abrasives are then added to mix with the waterjet and form and abrasive Water Jet (AWJ). The abrasive jet stream size is typically 0.040 inch (1.0 mm). Any material can be cut with AWJ including metals, ceramics, glass, and composites. Page 5
AWJs are now being used in high production applications across the globe. They compliment other technologies such as milling, laser, EDM, plasma and routers. Waterjets can be considered as a Green technology since there are no noxious gases or liquids are used in waterjet cutting, and waterjets do not create hazardous materials or vapors. No heat effected zones or mechanical stresses are left on a waterjet cut surface. In the following section we focus on composites using generals and specific case studies. Composite cutting characteristics Composite materials are more difficult to machine than metals, because they are anisotropic, non-homogenous and their reinforcing fibers are abrasive. In comparison to mechanical methods such as Polycrystalline Diamond (PCD) router bits there are several advantages of the Abrasive WaterJet (AWJ) process. The mechanical router method tends to peel away at the material versus the fracturing method induced by the high pressure abrasive stream, Figure 8. Figure 8: Router peeling (top) and AWJ fracturing the composite material (bottom) The immediate result of the mechanical method is a localized heat affected zone which melts the resin inducing a recast state and evidential micro cracks as illustrated in Figure 9. Figure 9: Resultant Router Re-Cast and Heat Zone Resin Melt Page 6
There will also be resultant internal voids due to fiber pull out, Figure 10. Figure 10: Resultant Router Fiber Pull Out Real time user experience for wear rates generally result that a 0.400 (10mm) router bit will last on average 40 minutes to 1 hour, whereas the average AWJ nozzle with a 0.040 (1mm) stream diameter will last over 80 hours continuous cutting. The main process advantages of the AWJ is that it is non-thermal, (no heat or mechanical stresses), no fiber pull out, no delamination and the cutting speed is typically twice as fast. The fixturing required can be designed very light weight as the process induces less than 4.4 lbs (2 kg) of vertical force while cutting. The AWJ is not prone to the material damage as caused by the mechanical method when encountering ply orientation at +/- 45 degrees and 0 degrees, although at 90 degrees it is acceptable, Figures 11-15. Figure 11: Resultant good surface finish, no delamination with router Figure 12: Resultant poor surface finish, appearance of exposed fibers with router Page 7
Figure 13: Resultant undesirable surface finish, delaminating with router Figure 14: Resultant undesirable surface finish as fibers tend to buckle and delaminate with router Figure 15: Scanning Electron Comparison Data Image Page 8
Aircraft Applications Because of the above advantages of AWJ over mechanical tools, they have been used extensively in many aircraft composite cutting and trimming applications since the late 1980s. Typical aircraft composite parts that have been cut with AWJs are: Wing: Covers, Leading Edges, Flaps Fuselage: Panels, Frames, Clips, Doors Tail: VTP & HTP Covers, Rudder, and Flaps Keel Beam, Centre Wing Box, Belly Fairing Example aircraft and corresponding AWJ-machined components are: Boeing 787 - center wing box, wing skins, fixed leading edge, vertical stabilizer, Boeing 777 horizontal stabilizer, Airbus 350XWB wing spars (High Performance Composites July, 2006) Bell Helicopter V-22 Osprey - wing skins, stiff rotors (Modern Machine Shop - March, 2007) Raytheon Premier I fuselage. For AWJ trimming of composites, a 6-axis machine is used to manipulate the jet and also to position the catcher under the exiting jet. Figure 16 shows a model and a picture of an end effector. Figure 16: Focal Point End Effector Page 9
Observe that the catcher cup arm needs to swing (6 th axis) as not to collide with the structure being trimmed. To trim in tight spaces such as stringer flanges, Figure 17 used on many structural components, a special cutting head, Figure 18, was developed so cutting can be accomplished from the inside of the stringer. Figure 17: Composite Stringer Flange for wing structures Figure 18: 50-mm High Side-Fire Head for 45 Degree Beveling This nozzle design was important as not to allow the jet from impinging on the opposite flange and cause it to be damaged. The low profile nozzle design also allowed for cutting close to stringer roots in other applications. For example, see Figure 19 where access holes were cut near the root of a stringer and preventing any close wall damage. Figure 19: Access Hole Drilling at the Root of a Wing Box Stringer Page 10
Shape cutting is also another application which requires piercing of a starting hole. Some systems use a mechanical drill to pre-drill the starting holes; but most commonly now, AWJ is used to drill these holes as part of the shape cutting sequence. The example trimming and shape cutting shown in Figure 20 is quite complex due to the 3-D nature of the trimming in different planes which required a 6-axis robot. The design of a holding fixture in this example is of critical importance. Figure 20: AWJ Trimming and Cutting of Complex 3-D Composite Wing Part The Raytheon Premier 1 was the first all-composite fuselage aircraft built. An AWJ mounted on a 5-axis column type machine was used to shape cut the opening in the fuselage as shown in Figure 20. For these types of applications, special jet catchers are used to catch and contain the spent jet. Figure 20: AWJ Trimming and Cutting of Complex Fuselage with a Column Gantry Page 11
Other Robotic Composite Applications Hole drilling in composites with either AWJ or perf drill tools (perforation) is an emerging application. The AWJ has been proven to drill high quality holes, free from delamination or adverse effects due to the micromachining nature of the material removal process. The accuracy of TCP can be increased using volumetric compensation as described above. Figure 21 shows a composite nacelle mock-up drilled with AWJ. Figure 21: AWJ Hole Drilling and Piercing Over 30,000 holes were drilled at about 0.9 sec hole-to-hole time. A system with multiple robotic arms can be used to drill a full scale nacelle rapidly. Mounting of 6-axis robotic arms has also been used to achieve maximum flexibility for cutting and drilling 3D parts. Different parts can be located within the envelope of the machine and processed based on the machining needs. A similar approach is used to mount 6-axis robot on a rail for the trimming of relatively long wind turbine blades. The seam between the upper and lower shell is trimmed using a special nozzle that cuts very close to the root of the joint. Figure 22 shows a model of this system. Figure 22: 6 Axis Articulate Robot on a 7 th Axis Linear Rail for Turbine Wind Blade Trimming Page 12
Conclusion 5-axis robots have been in use with AWJ cutting of composites since the late 1980s for a wide range of aircraft components achieving accuracies of +/- 0.002 inch (0.05mm). The use of specific, rigid 6-axis robots is now emerging as their accuracies are now proving to be better than +/- 0.010 inch (0.25). The calibration process allows the robot and the workcell to become more accurate, by precisely defining the Actual or true kinematic model of the robot-cell, and offsetting and compensating for the differences to the CAD or ideal kinematic model. The differences between these two models are defined in a new parameter file working both in the Forward direction for the cutting nozzle and in the Reverse direction to the fixture / workpiece. This method will assure competent offline programming capability. Every robot program created in Simulation within the robot cell can be post processed using the new kinematic parameters. Additionally, some of the parameters can be changed within the offline programming task such as the TCP or the location of the Fixture relative to the Robot base frame, etc. Several waterjet cutting applications have benefited from robots flexibility and accuracy. For example, the large scale 787 wing skin and stringers are trimmed with AWJs. The use of robotic cutting and drilling cells is emerging for such parts as composite nacelles and small scale parts such as clips, fairings, covers etc. as a very cost effective solution. References 1. U.S. Pat. 5,731,256 (Mar. 24, 1998) John Owens, RSL, Newcastle upon Tyne, Great Britain; Off-line Programming Software for Control of Industrial Robots 2. Hashish, M., (2003) Waterjet Applications in the Automotive Industry, Proceedings of the 7 th Pacific Rim international Conference on waterjetting Technology, Jeju, Korea, October 27-29, 2003 3. Snider, D., 1989 Automated TPO Trimming for Quality and Production, Proceedings of the 5 th American Waterjet Symposium WJTA, Toronto, CA. 4. Cook, R., and Wightman, D., (1988) Automotive Applications of Waterjet Cutting, SME paper number MR88-140, WESTEC, March. 5. Hashish, M., (1998) The waterjet as a Tool, Proceedings of the 14th International Water Jet Cutting Technology Conference, BHR group, Brugge, Belgium, October 6. Kumanduri, R., (1997), Machining of fiber reinforced composites, machining Science and Technology, 1(1), pp 113-152 7. Ramulu, M., and Taya, M., (1988), "An Investigation of the Machinability of High Temperature Composites," Proceedings, 12th Conference on Composite Materials and Structures. 8. Hashish, M., (1999) Waterjet Machining of Composites and Ceramics, Chapter 13 in Machining of Ceramics and Composites, edited by Jahanmir S, Ramulu, M., and Koshy, P., Marcel Decker, NY, 9. Hashish, M., (1999) Status and Potential of Waterjet Machining of Composites, Proceedings of the 10 th American Water Jet Conference, Houston, TX, pp. 811-827 10. Snider, D. and Hashish M. Precision Waterjet Cutting in the Composite Industry Utilizing Multi-Axis robots for High Quality Machining. ACMA Composites 2010, Las Vegas, NV, February. 11. SEM Images courtesy of Missouri State University Electron Microscope Center, Dr. G. Munn dated November 2009 Page 13