Aerospace Demands Precision Inspection
With the latest metrology gear, aerospace/defense builders meet tougher tolerance requirements
By Patrick Waurzyniak
Aerospace manufacturers are facing ever-more stringent tolerance demands on critical defense programs, including the manufacturing of components and final assembly of the next-generation F-35 Joint Strike Fighter (JSF) aircraft.
By deploying the latest metrology tools, aerospace/defense builders can meet the high-tolerance demands of the JSF program, which can be much greater than more typical aircraft requirements, as well as key civilian programs such as Boeing's 787 Dreamliner passenger airplane. Today's technology includes inspection systems using lasers, vision, portable arms, and gantry-based CMMs that can handle a wide variety of metrology tasks.
Huge aircraft components can often require larger measurement systems, like the gantry CMMs currently used by Lockheed Martin (Bethedsa, MD) to obtain highly accurate measurements of the components used in building the F-35 fighter. Lockheed, which performs final assembly of the JSF at its Dallas/Fort Worth manufacturing facilities, needed a high-accuracy, large-scale CMM to measure the jet fighter's wing skins and subassemblies when it selected a gantry-style CMM from Carl Zeiss IMT (Maple Grove, MN).
"We were awarded the contract two years ago by Lockheed Martin to provide a turnkey, state-of-the-art measuring solution that is arguably one of the most accurate CMMs in the world for its size," notes Kevin Santilli of Carl Zeiss IMT. "This machine was selected by Lockheed Martin to measure the wing skins of the new F-35 that Lockheed is building in Dallas. This wing skin is unbelievably large. It forms the whole skin for the fighter's wing, and it requires some incredibly tight tolerances over the entire length of the wing."
The Zeiss MMZ gantry CMM used to measure the F-35 wing skin has a measurement range of 5-m wide x 16-m long x 2.5 m in the Z axis, Santilli notes. "The new technology in the aircraft industry that really made this machine attractive to Lockheed Martin was the scanning capability of the system. In this case, Lockheed had thousands of bores on this wing skin that they had to inspect, not only for the physical size, but they also needed to know the form and location of each bore. The scanning functionality of the MMZ, having the capability to scan hundreds of points inside the bores' diameter, versus just taking individual touch points on a traditional CMM, was a huge advantage to Lockheed Martin, and it also gave them the additional accuracy and throughput that they were seeking. The wing skins' profile tolerances are very tight over the entire length of the wing, making scanning the profile the only viable inspection option from an accuracy and throughput perspective."
The fixture and table group that support the F-35 wing skin and sub-assemblies weigh in excess of 60 tons. With these large gantry CMMs, the equipment can be used on the factory floor, or in climate-controlled lab environments, Santilli notes. "When you talk about aerospace companies, most of them use traditional bridge-type CMMs. Data density is so important to these companies, because they don't just want to know the size of a bore, but also the form of the feature and its location. They want to feed back information to the machine tools, to provide real-time correction to manufacturing."
On-machine inspection in aerospace also is growing in importance, Santilli observes. "We see more and more of a shop-floor, right-next-to-the-machine-tool type of inspection, even those types of applications that in many cases require the very same tolerances that you would find in a metrology lab," he says. "We typically see tolerances from the 20-µm range for true positions, bore diameters and profiles in the aerospace industry, especially on machined engine components. If you look at stampings or molds, you'll see tolerances in the 0.005" [0.127-mm] range."
Laser-based inspection measurement systems from Metris USA Inc. (Brighton, MI, and Leuven, Belgium) include the company's Laser Radar and Indoor GPS (iGPS) metrology systems used to measure large aircraft workpieces. Metris recently added an updated software package with its Laser Radar Driver version 5.1 software (see Tech Front, "Laser Radar Automates Hole Inspection," on page 35) used to perform automated, noncontact inspection of holes said to reach accuracy and repeatability levels 30 times higher than previously possible.
Both the laser radar and the iGPS systems can measure large aerospace/defense workpieces accurately, according to James Gardner, Metris USA director, business development, large-scale metrology. "Our customers are looking for that noncontact, fully automated type of solution, and Metris offers these solutions in our Laser Radar, iSpace (indoor GPS), and K-series optical CMMs—those three are the most portable, automated solutions that we have in large-volume applications," he says. "Our Laser Radar has been enhanced this year into a fully automated solution. A great example is a system that we built for Boeing, working with New River Kinematics on software and integration. Boeing uses our Laser Radar for a wing-to-body joining system. In the past, these applications were often done using laser trackers, with an operator manipulating a corner-cube reflector. With our Laser Radar, it is a fully fully noncontact, automated process, therefore reducing the cost of equipment, personnel, and time."
Gardner, who previously worked as an aircraft toolmaker and managed a metrology group that Raytheon purchased, says at one time laser trackers and portable CMMs were among the few options available. "We used trackers and portable arms, that's all we had; before that, we had theodolites," he recalls. "Laser radar's not new technology, but it's now becoming accepted as the industry standard.
"Currently, Boeing has over a dozen Laser Radar units, and Lockheed has close to the same, so we're starting to see greater acceptance of this technology," Gardner adds. "Last year, Laser Radar was purchased not so much for use in tooling/engineering groups, but more for use in production. Applications such as measuring and pre-determining shim gaps, and monitoring large-assembly joining stations, are all new applications for Laser Radar."
After acquiring Metric Vision, Metris developed the Laser Radar technology into a system that has become common in large-scale production manufacturing, Gardner says. "In the last couple years, companies have been implementing Laser Radar more and more into production processes. In any production process, system reliability and proven service capability are key. There is no leeway for downtime in this type of application.
"Last year, we had two major applications put into production for Boeing and the Lockheed laminate control process that we did on the Joint Strike Fighter," he adds. "This is something they used in the manufacturing technology group. Last year, they moved it into production, where a worker can use Spatial Analyzer software from New River Kinematics to write a script. It's pretty much a green light/red light—more of a big button solution."
A large-volume tracking system, the laser-based iGPS uses multiple transmitters and sensors to derive highly accurate measurements of large aircraft workpieces. Since acquiring the former Arc Second Inc. and its indoor GPS technology and purchasing Vertek ILS Canada in 2006, Metris has improved the iGPS system. Last year it introduced iSpace, which activates a largescale metrology workspace in which objects can be tracked accurately. The iSpace system builds on Metris' iGPS global positioning that turns workspaces into scalable metrology work volumes using a network of iGPS transmitters. The company now bundles iGPS technology into seven off-the-shelf configurations with volumes ranging from 400 to 1200 m2, available in different classes to suit the accuracy requirements of the customer.
Metris' iGPS has had success in expanding applications for aerospace, notes Gardner. "We successfully installed an iSpace system last year in an aerospace manufacturing facility that replaced a competitor's system. With our system, they can walk around with an articulated arm and relocate the arm anywhere in the working volume," Gardner says. "This completely eliminates the previous problems of moving an instrument. There is no longer a need to 'leap-frog' an instrument through locating off common points, which is the greatest possibility for increased uncertainty in a measurement. The tolerances we've been able to get in this 4 x 6-m area are unheard of₃we're looking at 0.006" [0.15-mm] as a norm, and this is our customer coming up with these numbers. We've tested the numbers and stated under 200 microns, but customers are coming back and saying, 'We're seeing 150 microns or less in some areas.'"
The iGPS system's accuracy, transmitter robustness, and setup times have all been improved, Gardner notes. "This is a system that will do a great deal, because it will not only measure, it will monitor anything that needs tracking inside of a work envelope," he adds. "You can monitor a metrology device, or a nondestructive testing piece, an integrated torque wrench, or a projection unit. At one of our partner companies, Sensor Analytics, we're actually tracking a device that they have to test paint thicknesses, and we're working with Lockheed on an application to possibly use this thickness gage.
"Normally, a reading is taken with a gage, and the coordinate has to be logged as to where the reading was taken; iGPS can monitor that device, so you have to test the paint around an area, and you have to log where you took that reading. Indoor GPS is monitoring that test location, so as soon as you take a reading with your device, it records an X-Y-Z coordinate, and you know exactly where on the aircraft the reading was taken. This translates well into the rework of existing aircraft. In my past experience working on the P3 aircraft, workers would grind down the corrosion, you'd have to test and see what the thickness was, and then you'd have to mark the location. With the device integrated into the iSpace system, these locations are automatically coordinated."
On-machine verification software packages have made OMV more capable, as with the latest version of PowerInspect from Delcam plc (Birmingham, UK, and Windsor, ON, Canada). Delcam recently updated PowerInspect to include support for the Revo five-axis scanning probe from Renishaw plc (Wotton-under-Edge, Gloucestershire, UK, and Hoffman Estates, IL).
"The main benefits of five-axis inspection come in the measurement of complex, doubly curved surfaces, including those found in aerofoils and turbine blades," notes Glenn McMinn, president, Delcam North America, regarding using OMV capabilities in aerospace. "For example, the time taken on a measurement application involving a range of aero-engine blisk features was reduced from 46 mins to 4 mins 30 sec. The key to the time savings with five-axis inspection is the ability to overcome this limitation. The Revo system uses synchronized head and machine motion when scanning, allowing the probe to follow changes in part geometry rapidly without introducing dynamic errors. As a result, the CMM is able to move at a constant velocity while measurements are being taken, without impacting accuracy."
Another OMV software package comes from Hexagon Metrology Inc. (North Kingstown, RI, and Stockholm) with the newly updated PC-DMIS NC package, which can be used by aerospace manufacturers to accurately measure manufacturing processes involving large wing spars, or in waterjet-cutting operations used to efficiently cut large composite aircraft workpieces, notes Ken Woodbine, president of Wilcox and Associates Inc. (North Kingstown, RI), Hexagon's software developer.
With OMV technology, aerospace manufacturers have more ways to accurately measure for such applications. "They needed a way to solve this problem, and OMV has been introduced as a solution that's now well-received and respected," he adds. "It's a specific technique, targeting an industry issue for which we produced a solution."
With its acquisition of German probe maker m&h Group, Hexagon also now bundles PC-DMIS NC with onmachine probes for temperature sensing. The new temperature sensors allow a machinist to wait until the workpiece hits an optimal temperature before starting the cut. "That gives you greater dimensional traceability and process information," Woodbine says. "That's how you can look at your process statistically and understand the impact of temperature on the process."
"You're trying to minimize the variables that you have whenever you machine different types of materials," adds Gary Hobart, Hexagon product line manager for vision. "Materials often contract and expand as a result of temperature, but now we can know what's actually happening with regards to the speeds and the feeds that are used to machine the workpiece. You can then get a better check on reality when you ask, 'Can I manufacture this component in a faster manner, without any deformation to it as a result of temperature change? Can I make a more accurate cut by physically changing the temperature within the machine tool? Do I actually improve that process because I can maximize the expansion of the material prior to cutting, knowing that once the cutting is achieved, it will then contract back down to form?'"
By using PC-DMIS NC, manufacturers can not only gain dimensional information, but also better understand the material and manufacturing processes, Hobart notes. "You can improve efficiency, make sure the expansion rates and feed rates are covered, and determine that your process is wholly reliable, even from a material-expansion viewpoint."
Multisensor metrology combined with the PC-DMIS software adds to the inspection toolbox available to aerospace builders. "The other aspect that is more paramount is the connectors between electronic components on aircraft," Hobart says. "A lot of these components will be a combination of metal, rubber, or plastic; you would conventionally use a CMM, vision machine, or an optical comparator to inspect them. With multisensor technology and PC DMIS, we mix a vision machine and optical comparator, combine them into a 3-D machine, and by using PC-DMIS CAD technology, you can inspect these components direct back to the master artifact."
In the past, some multisensor offerings were lacking in capabilities, Hobart says. "Nobody had a really good methodology of bringing 3-D measurement and crossprobe calibration information together. With PC-DMIS Vision, we've actually combined technology—from laser, white-light sensor, which is the 'new laser,' so to speak—to touch probe, vision, and rotary axis. We have a five-axis machine capability with multisensor technology that can be used to inspect back to the master CAD artifact."
Multisensor systems can be useful for applications such as inspecting jet-engine turbine blades. "We're taking a vision machine and adding a touch probe, or combining a tactile machine and adding vision with a video probe—that's our CMM-V," Hobart says. "We can articulate the head in order to see the component at different angles.
"The predominant factor is that the CAD model is the master," Hobart states. "It's perfect. It's what the designer wants to achieve. It's linked to the next component in the food chain. Because I've used that in our sensor and machine technology, comparing back to the CAD model, we can give the customer faster information regarding not only, 'Is the component good or bad?' but also what's happening with their manufacturing process. Whether that information is for an NC machine tool, a video measuring device, a multisensor machine, a CMM, a laser tracker, or an articulating arm, it's offering the customer what they need to reduce manufacturing costs, improve reliability of their products, and create stability within their process."
This article was first published in the March 2009 edition of Manufacturing Engineering magazine.