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Advanced Technologies Supplement: Cutting Composites

 

 

Waterjets do the job for the aerospace industry

 

By Robert B. Aronson
Senior Editor  

 

Composites were used initially in developing some commercial as well as stealth aircraft, but are now realizing wide-scale commercialization in the aircraft industry. Exact numbers are not available, but estimates are that 50–60% of the material used in the new Boeing 787 and Airbus A380 will be composites. Because of the composite’s structure of very strong fibers in a softer matrix, conventional machining techniques do not work as they do on metals, so waterjets are being used extensively to cut and machine these materials. The majority of waterjet systems used in aerospace are abrasive waterjet units. That is, an abrasive, usually garnet or aluminum-oxide grain, is introduced into the high-pressure water stream. Some aerospace applications use water-only systems. Although the process is used for cutting many metals, the main applications are trimming and holemaking in composite materials.

The advantages of composite structures and cutting with waterjets include:

  • Light weight. Even a small weight reduction translates into major cost savings over the life of the aircraft.
  • Strength-on-demand. The type, amount, and direction of the reinforcing fiber can be varied to match performance needs.
  • No delamination. Waterjets eliminate the potential for the delamination of composite materials that can occur with conventional cutting techniques.
  • Near-net shape. The composite part is made to near final shape, so forming and conventional machining operations are minimized.
  • Multifunction part. Aircraft sections formerly made of several components can be made as a single part.
  • No hard tooling. Waterjet cuts use a single tool (cutting head) moved under computer control. No hard tooling is needed for some individual part as in conventional machining.

 

Composite cutting is vastly different from cutting metal because composites are not homogeneous materials. The matrix has vastly different characteristics from the stronger reinforcing, which is much more difficult to cut. When conventional machining techniques are used, this creates the potential for greater tool wear with significant heat generation, which can be damaging to the composite.

Product improvements are evident in most waterjet systems. Manufacturers are working to expand the types of products waterjet cutting can handle, and to take over from more conventional machining operations.

”Outside suppliers, often job-shops, are the most frequent buyers of waterjet systems, because aerospace manufacturers are using subcontractors,” explains John Olsen, vp of operations, OMAX Corp. (Kent, WA). “However, one OEM customer was Rolls Royce cutting Hastelloy parts with an abrasive waterjet. When they used conventional machining, each part took 30 hr, with the waterjet it went to 12 minutes. They now use the waterjet as the starting process for most parts passing through the shop.

“For the future, we are going to ever higher precision. That is the main way we will expand our market. We are now taking over some wire EDM market share. Our process still has 10 times the EDM area error, but we have 10 times the speed of EDM. 

“We eliminate the taper problem with a head that automatically tilts, ensuring square edges. Software automatically generates the proper speed and head-tilt angle to get the finish and edge squareness needed. The adjustments are transparent to the user.

“The type of pump does not affect precision. But it does affect operating cost, and noise.

“There is a trade off between precision and cutting speed. If you want to cut stone you don’t worry about precision, speed is the issue. But for someone making medical or aerospace parts, precision and geometry are the issues,” he concludes.

Product improvements are evident in most waterjet systems. Manufacturers are working to expand the types of products waterjets can handle, and to take over jobs formerly carried out by conventional machining operations. For hardware improvement, researchers are concentrating on nozzle design, while the software improvements are concerned with handling more process variables.

Another area of research is the mixing of the water stream with the abrasive. The goal is to get more of the abrasive to do the cutting work instead of just flowing through the system.

”Waterjet is the preferred method anytime you cut composites,” states Michael Ruppenthal, vice president at Flow International (Kent, WA). “On Boeing’s new 787, the skins for the tail and wing as well as the spars and stringers are all composite. For these components, waterjet is used for both trimming and complex cutting. Similar work is being done on the A350 Airbus.

“Reliability, which is paramount in aircraft design, also favors waterjet because conventional machining may cause hazardous delaminating and fraying. Tests show that a composite part cut with a conventional router can fail 3× faster under fatigue than a part cut with abrasive waterjet due to damage to the edge.

“Waterjet also offers advantages when cutting very thin materials as well as metals. Because there is no heat affected zone, a number of nested parts can be cut from a single workpiece,” Ruppenthal says.

In one of the more recent indications of the waterjet’s acceptance, Flow was given a major contract to manufacture and install a multiaxis Composite Machining Center (CMC), abrasive waterjet and routing machine tool system for A350 Airbus program at the company’s UK facility. The equipment will be used to machine composite wing structures. Within the Airbus group, those in the UK are responsible for design and manufacture of the wings and fuel systems.

According to Mohamed Hashish, Flow’s senior vice president of technology, “Computer control has eliminated many of the problems, such as stream curl in thicker materials, and making tight, clean corners.

“About 95% of the waterjet machining involves edge trimming, holemaking, and creating openings in the surface such as maintenance access ports.

“Usually, we can rough and finish in a single pass at a speed that is 1.5× faster than conventional machining,” says Mark Saberton, chief engineer, Flow International. “In addition, the cuts are clean because of the speed of the grit. Each grain functions as a miniature cutting tool, and the cutting action is a fracture, not a shear as in conventional machining. The grains travel at 2100 mph (3400 kph) usually 300× the surface speed of a conventional tool. The grain ‘cutting tool’ costs only a few cents a pound, and waterjet machines use about one pound per minute.

“Cutting force is minimal, as is vibration, so fixturing is much simpler. Parts are usually held down by vacuum cups, which can be a major benefit when working with large aircraft components.

“The process also generates no dust, which can be toxic, and sometimes highly conductive, which can cause problems with adjacent electric equipment.

“In the near future we plan to offer higher pressure pumps that will provide streams at significantly higher pressure than the 60,000 psi [414 MPa] which is now common in much of the industry,” says Ruppenthal. “This should give a 50% increase in cutting speed.

“When drilling small holes, there is a lot of bounce-back from a 50,000 psi [345 MPa] stream that can damage the composite,” says Hashish. “Stutter stepping or gyrating the stream can prevent that, so there is piercing without delamination.”

“It costs the operator about $20/hr, not counting labor, to operate one of our waterjet machines,” observes Saberton. “With conventional machining, the price is around $200/hr. It pays for itself very quickly if you can keep the work flowing.

“Another advantage for aerospace applications is the ability to consistently cut angle holes, which is important in many designs,” says Saberton.

“We are also looking at competing for jobs requiring conventional machining, as are our competitors. Cleaning and descaling are other areas that look promising, particularly in the jet engine rebuild market,” he concludes.

Product improvements are evident in most waterjet systems. Manufacturers are working to expand the type of products waterjet cutting can handle and to take over from more conventional machining operations.

“PaR Systems, Inc. (Shoreview, Minnesota) designs and manufactures integrated motion five-axis machines with taper control on contoured surfaces and we work with both composites and metals,” explains PaR’s Dennis DesMarais “In both cases, motion control is the key to the operation.

“We offer advanced programming and compensation algorithms that allow more accurate throughput, Our controls also dynamically compensate for true deflection of the waterjet stream in the cut material to increase throughput while maintaining cut quality. Overall accuracy depends on the size of the part and the machine. They have to be well matched. System accuracy depends on software and machine structure, particularly the bridge assembly. Our waterjets can hold an accuracy of 0.002" [0.05mm].

“Often an operation requires a combination of cutting and drilling, so we incorporate a conventional spindle in our machines. This is a space and time saver since a second machine isn’t needed. We have our own controller which we use on our six-axis machines. Fixturing, particularly for aerospace applications, has to be unique because of the sizes and shapes involved. We have designed our own configurable fixturing system.

“This cold cutting process can be very precise and is not material sensitive, as long as the material is solid and of a consistent density. Two ways to control the cut are by having contoured surface taper-control software, or adjusting the machine speed.

“Two key variables the software has to take into account are jewel wear, and focusing tube growth. All are expendable and wear with time. These two features must be blended for the best combination of system life, cutting speed, and accuracy. For example, if you cut faster you will get faster wear of the focusing tube. You must also factor in volume and precision.

“We manufacture, sell and service the entire system, not just the pump, abrasive feed, cutting head, and control. A KMT hydraulically driven intensifier pump is used with the PaR Systems processing equipment for aerospace work because of its greater reliability and the ability to deliver ultra high pressure of 60,000 psi with low maintenance, as opposed to the direct drive pump design. You don’t want the machine to go down in the middle of a critical cut,” DesMarais concludes.

“Cleanliness is essential to reliability in the aerospace market, so the preference in some cuts where cleaning or contamination might be a problem is to use a very-high-pressure, water-only system,” explains Lou Luedtke, president/CEO of the National Composite Center (Dayton, OH). “Pressures can be as high as 90,000 psi [620 MPa] or more.

“One problem is that those composites using agamid fibers, such as Kevlar, tend to stretch, so it’s important not to load the part while cutting or it will stretch. That’s why laser or waterjet is preferred for those machining operations.

“The effort to incorporate more composites into the aircraft structure has reportedly spawned attempts to use fasteners made of composite material instead of aluminum,” he concludes.

Boeing, one of the major users of waterjets, has been using these systems to cut composites for a number of years. Initially, these materials were used only for lightly loaded elements such as luggage storage areas or seats. Today they are used for major structural components.

“Our composites work uses both mechanical and hand layup depending on the complexity of the part and the loads involved,” explains Kirt Butler, MfgE, manufacturing and process engineering, Boeing Commercial Aircraft (Seattle, WA).

“Generally, the more complex contours use a woven-fabric composite as opposed to the tape-type composites. Waterjet is used to cut both partially and fully cured components.

“On two types of composites, you can’t use water: Aramid base materials have an affinity for water, and water can degrade the resins in polyester-based materials.

 
 

Angle cut of a composite aircraft stringer by a Flow Pulsar cutting head.
 

“When using standard machining processes, conventional tools are not suitable. They have to have unique flute designs and release angles. Conventional bits tend to tear the fiber. You have to accommodate the way the fibers are laid, either unidirectional or woven. This requires attention to the design of the tool and how it’s applied. Both these features influence the edge-cut quality.

“In some cases we use CO2 lasers to do the trimming, but you must be careful with feeds and speeds to avoid charring. We drill with polycrystalline diamond tools.

“We also don’t use waterjet for final assembly. In the confines of the assembly area there is no place to put the catch basin this process requires.

“We have to be careful of abrasive quality to maintain edge finish. Normally we use garnet or aluminum oxide. Because some systems can operate at pressures of 90,000 psi, we can cut with water only. This makes for a cleaner cut and fewer cleanup problems,” he concludes.

“Waterjet is the most versatile cutting system on the market,” said Dave Dumas of Jet Edge Inc. (Minneapolis, MN). “You can cut titanium one minute and rubber the next with minimal setup time. Waterjet has numerous advantages over traditional cutting systems. It cuts without heat, it minimizes dust and toxic fumes, it requires no finishing operation, it’s sanitary, has minimal lateral force, wastes very little material, and offers omnidirectional cutting.

“While we do compete with traditional cutting methods for many applications, we are finding that waterjet cutting often complements other cutting processes such as sawing and laser cutting. Our X-Y motion systems can be used with other technologies such as plasma, flame, routers, air drills, and part markers. Many of these technologies can be combined on a single system.

“Jet Edge’s cutting systems are known for large work areas and accuracy. We can hold an accuracy of 0.005" [0.13 mm] with a repeatability of 0.001" [0.025 mm] over the entire work envelope. Our cutting systems are powered by a hydraulic intensifier pump that can offer up to 40,000 hours of operation between maintenance cycles. This, of course, means greater productivity. The cutting systems are controlled by a state-of-the-art AquaVision DI motion controller that guides users through the project step-by-step.

“Most of our aerospace work involves trimming and hole making with some contour machining. In machining operations, waterjet can cut off blocks of material that often are large enough to be used for other projects. This can result in huge savings.

“If a company is looking into buying a waterjet system, I would advise them to look for the most accurate machine they can find because you never know what the next job will require. They also will want to have some tests cuts made to ensure that the machine will meet their specifications,” Dumas explains.

Aerospace use of waterjets on composites has given a boost to the acceptance of abrasive waterjet as a major manufacturing process.

Waterjet system suppliers note with some pleasure that many who buy their equipment to meet the needs of a specific aerospace job end up using their equipment more widely. Once the waterjet system is in place, and becomes more familiar to the user, operators find more applications for the process. It’s the old: “I didn’t know you could do that syndrome.”

 

This article was first published in the March 2006 edition of Manufacturing Engineering magazine. 


Published Date : 3/1/2006

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