Make it Bigger, Lighter, and Faster
Aerospace manufacturing has a new generation of materials and processes
By Robert B. Aronson
In addition to the usual challenges faced by any manufacturing operation, those in aerospace face stronger constraints on weight, strength, and above all, safety.
To meet the increasing need for complex monolithic parts manufacturing in the aerospace industry, MAG Cincinnati (Hebron, KY) offers a line of vertical and horizontal profilers in single or multispindle five-axis models.
"One of our newest machines is the five-axis HyperMach H-Series horizontal profiler that cuts aluminum at up to 50 m/min," says MAG Cincinnati Manager of Aerospace Product Development Randy Von Moll. HyperMach HSeries profilers are said to be the industry's highest output five-axis, plate-machining centers. The design adapts modular components from the high-performance vertical profiler family to provide five-axis contouring machining with the chip and coolant management of a horizontal machining center. The unit has drop-through chip evacuation to an integrated chip conveyor.
Toshiba Machine Co. (Elk Grove Village, IL) offers three units designed specifically for aerospace applications. The largest is the five-axis MGF-40135, which has a 4 x 13.9-m table and two independent heads. A 37-kW motor drives the spindles at 4000 rpm.
For the production of monolithic aluminum parts, Ingersoll Machine Tools (Rockford, IL) offers its five-axis High-Velocity Profiler (HVP). The design features a maximum cutting speed of 50 m/min and fluid-bearing spindles to provide dampening at high material removal rates.
A flexible machining process developed by Wera Profilator GmbH (Wuppertal, Germany and Ann Arbor, MI) may find an aerospace application in gear cutting. The unit cuts external and internal splines and gears that may be used for applications such as making the planetary gear sets of a turbofan engine.
A process called "scudding" uses a tool, similar to a gear-shaper cutter, to make internal and external spur and helical gears, splines, and nonsymmetrical gear forms. In this process, the cutter is fed directly through the workpiece as both the cutter and workpiece rotate on synchronized spindles.
In this dry process, cutting is done with a resharpenable, HSS tool with a TiAlN coating. Test results indicate a surface finish of 0.8 Ra/2.5 Rz is typical with a gear quality level of DIN 4. Lead modification is made through axial movements without redesign of cutting tools. The process is reportedly five times faster than gear shaping and comparable to gear hobbing and broaching, but with higher-quality results.
The aerospace industry has an ever-increasing demand for titanium and other hard-metal-alloy components. SNK America (Elk Grove Village, IL) has addressed this need with two five-axis profilers designed to efficiently machine hard metals. The FSP-80V vertical profiler features a ±30° five-axis A/B head and high torque spindle motor. to rough and fine-finish titanium in a single fixturing. A 90 x 33" (2290 x 838-mm) worktable handles long parts as well as multiple work fixturing. Speed range of the unit's 40 hp (30-kW) spindle motor is 20–4000 rpm.
The horizontal version HPS-120A has a ±270° C axis and ±90° A axis. Table capacity is 120 x 50" (3050 1270 mm). The unit is available in single and double-pallet models.
Some machine-tool builders offer units designed to encourage shops to take on complex aerospace machining tasks. For example, the Matsuura five-axis VMCs from Methods Machine Tools Inc. (Sudbury, MA) offers multitasking Cublex technology. VP of National Distribution at Methods, Dave Lucius, reports that customers want a single workcenter tailored for the small-lot production of highly complex parts. "Think of this machine as a toolchanging B-axis lathe that can automatically change up to 40 different chucks, with expandable tool capacity and five-axis, high-speed machining capability," he says.
The multifunction unit can mill and turn, as well as provide limited surface grinding for cosmetic surface-finish requirements.
Titanium is taking over from aluminum in many aerospace applications, and that presents some added challenges to cutting-tool manufacturers. In one instance, Seco Tools Inc. (Troy, MI) provided an answer with its Jetstream tooling.
According to the Turning Programs Manager Don Graham, "The main goal was to find a way to eliminate the exceptionally long chips, some 1-m long. These chips were generated when cutting high-strength materials, particularly titanium. They not only interfered with production, they were a safety hazard for the operators who had to manually remove them."
The Jetstream tooling design solved the problem, by delivering a stream of high-pressure coolant directly to the workpiece interface.
"Our aerospace customers commonly face challenges that involve working with the new, harder alloys, particularly those of titanium and nickel," explains Aerospace Industry Specialist Sean Holt of Sandvik Coromant (Fairlawn, NJ). "These materials have low machinability, where the most cost-efficient tools are naturally required, but it is the programming methods that can have a major impact. Another challenge is meeting the requirements for thinner, more complex shapes. Rather than suggest a tool or tool family to meet some need, we start by looking at the specific features required to manufacture the customer's product.
Although a number of new tool coatings are available, Sandvik engineers recommend no coating be used when cutting titanium. "So much heat is generated at the cutting point that the coating is destroyed in microseconds," explains Holt.
To meet the aerospace industry's growing need for largetravel machining centers to handle monolithic airframe components, Haas Automation Inc. (Oxnard, CA) offers the VF-12 VMC. It features travels of 150 x 32 x 30" (2810 x 813 x 762 mm), and is available in both 40 and 50-taper configurations, with spindle speeds to 10,000 rpm. Standard equipment includes a 24+1 tool side-mount tool changer, with a 40+1 tool side-mount optional, chip auger system, programmable coolant nozzle, rigid tapping, and 95-gal (190-L) flood-coolant system.
Composites are now the major aircraft building material. For the 787 Dreamliner, one of the major commercial aircraft projects at Boeing (Chicago), more than 50% is made of composite materials, and it is expected that even more will be used in future aircraft.
For the 787, the composite materials are generally the same as those used for earlier aircraft, but when laying up flat or slightly contoured aircraft sections, such as a wing, the composite prepregs are cut into smaller sections, so automatic equipment more easily places it.
For more-complex surfaces, an automatic machine is used.
Only the most complex shapes are formed by hand layup, and that operation is partially automated. In a process called drape forming, a template projected on the tool shows the technician where to fit a precut composite section.
"The goal is to form the material in the shape we need as quickly as possible. We need to shave time from all the steps: getting material onto the tool, curing the part, removing it, and getting the tool ready for the next cycle."
Innovations in manufacturing processes are also flowing from Boeing. "We handle all aspects of machining from research through production and maintenance," explains Kimberly Smith, director, Auburn Machining, Emergent Operations and Advanced Metal Structures. "A major part of the work is keeping ahead of requests.
"We are also following cost-cutting techniques such as advance forming. In this process, a number of thin titanium sheets are diffusion-bonded together to form a monolithic part that no longer requires subassembly operations."
For some time, aircraft manufacture was based chiefly on aluminum construction. Now there is greater emphasis on composites and titanium," says Technical Fellow, Material and Process Technology, Mike Watts.
One benefit of titanium is its weldability, which is now being researched. Titanium does not experience significant reductions in strength when compared to welded aluminum joints.
As greater amounts of titanium are utilized for airframe construction, the need to improve machining rates is receiving greater attention. One of the major obstacles is the poor thermal conductivity of titanium, which does not allow heat to travel well through the material. This means that much of the heat produced during cutting stays on the cutter. With increasing cutter velocity, there is an increase in heat produced, limiting how fast a cutter can go before it starts to melt.
"We would like to see cutting tools with more heat resistance for high-performance cutting of titanium," says Watts. "Titanium doesn't soften until 1650°C, but the cutting tools can start to soften at 760°C, which puts a limit on the practical cutting speed.
"The interesting thing about titanium is it doesn't take much power to machine it. A lower-power machine, one with a 30-hp [22.5-kW] spindle, is usually adequate. Instead of high rpm and high-power machine spindles, we want high torque from low-rpm spindles. This can be contrasted against aluminum cutting machines, where we prefer high-power spindles rated at 100 hp [75 kW] and high rpm to provide high material removal rates," says Watts.
"Production of thin sections of aluminum down to 0.030" [0.76 mm] is not uncommon. Thin gage parts can also pose issues with follow-on processes, such as shotpeening where the risk of part distortion is increased. Additionally, more careful handling during follow-on processes and assembly is required," Watts concludes.
Northrop-Grumman (Los Angeles) shares many of the manufacturing challenges of other aerospace companies. But because of their production of military aircraft, their designs strongly emphasize aircraft survivability in a hostile environment and the need to incorporate a vast array of electronic systems in the aircraft.
Vice President for Engineering and Programs Frank Flores notes these trends:
- The use of simulations and demonstration systems before we get into actual development. We can evaluate the processes and approaches first, so that the overall design is faster and less costly. Developments in solid-modeling programs and rapid prototyping have greatly helped these efforts.
- In the composites area, we are going to the use of more carbon-foam tools. That is, the mold for a composite section is a special composite foam, instead of a hard metal section. The carbon foam offers a considerable cost savings because there is less machining of a metal mold, less coefficient of expansion, and faster tool heatup during cure.
- There is a strong push for more, and larger, monolithic parts, because they can be more accurate and much less costly than multipart sections. But holding the tolerances is a challenge. Warping of monolithic parts is a problem, but it's solvable. The fix used depends on the design. We can "pre-distort" the material so that it actually warps into shape, or stress-relieve it after the cutting.
A major part of the work at Northrop Grumman is aircraft for intelligence, surveillance, and reconnaissance (ISR). For these missions, each aircraft has a lot of electronics on board, including elaborate sensors, radar systems, and jammers. This greatly influences the airframe design.
Drones have been used for some time as reconnaissance craft. The official designation is unmanned aerial vehicle (UAV). The ultimate spinoff of this technology is the elimination of the on-board pilot. Prototypes are already under test by the US Navy. When pilot survival need not be considered, the design significantly changes. Eliminating the pilot would not only reduce aircraft manufacturing costs, but minimize pilot training expenses. It also makes the plane more versatile, particularly in a combat role, because there are far fewer limitations on maneuvers the UAV can perform.
Technical and cost challenges to the aerospace industry require a constant pushing of the envelope. And, this is particularly true where globalization influences the design and manufacture of aircraft. One of the main challenges with this kind of distributed manufacturing is maintaining a common standard. For example, with the F-35, the Joint Strike Fighter (JSF), the machine tool control scenario had to "push the envelope" on CNC design, engineering, and application technology to meet the project's unique manufacturing needs. Making more than 15,000 parts for the F-35 involves dozens of suppliers on several continents.
This challenge was met by Siemens in the aerospace version of its Sinumerik 840D CNC and compatible Simodrive 611D motor and drives packages. "Unique among the features of this control package is its ability to calculate the complex five-axis transformations in real time versus relying on an upstream post processor with its inherent guessing on machine kinematics, that is, the motion and resulting distortion that can corrupt the toolpath," explains Tim Shafer, director, Siemens Aerospace Center of Competence (Mason, OH). "Using this capability in conjunction with advanced probing routines eliminates the need for time-consuming finite part alignment, resulting in a substantial reduction in part setup time. This advancement is achieved using the onboard Traori (Siemens acronym for transformation orientation). It is a high-level language for kinematic machine transformations, and a verification software, which prevents any deviation in machining from the simulation modeling."
With this control package design, the same part program can be executed on different machine tools anywhere in the world. It's no longer a necessity that the postprocessor needs to consider manufacturing tolerances down to tiny misalignments of a rotary axis. That is compensated by the internal 840D CNC machine transformations, which are based on known algorithms in the CNC kernel. The controller corrects for any part movement within the work envelope, to ensure the optimum efficiency of the operations. The toolpath becomes a function of the tool tip orientation to the part, rather than a predetermined set of step motions.
The length and diameter of the cutting tools are likewise continuously monitored, again assuring accurate positioning and creating a tool wear monitoring system for controller reference.
"All JSF variants have common outer mold lines with common structural geometries. This high degree of commonality results in significant cost savings for the various armed services, which will each utilize modified versions of the JSF," he concludes.
This article was first published in the March 2009 edition of Manufacturing Engineering magazine.