Aerospace Embraces In-Process Metrology
While metrology assisting machining, manufacturing, and assembly is not a new concept, its acceptance is becoming more widespread in aerospace plants
By Bruce Morey
While metrology assisting machining, manufacturing, and assembly is not a new concept, its acceptance is becoming more widespread in aerospace plants.
The need for faster throughput at lower cost is spurring tighter integration of metrology equipment to assist machining, manufacturing, and assembly. Aerospace programs may be leading this move, especially as current programs move into production. For example, the F-35 program purchased a large MMZ-model CMM made by Zeiss (Minneapolis) not for an off-line quality function, but rather integrating it directly into the manufacturing process. "Aircraft outer skin components are machined from digital design data on production trim tools," explains Fred Child, senior manager for quality assurance for Lockheed Martin Aeronautics (Ft. Worth, TX). The Zeiss MMZ is connected to a Flexible Overhead Gantry (FOG) machining center via a moveable pallet system. The entire pallet, with several other composite parts, is measured near the point of machining by shuttling them from the machining center into the Zeiss CMM, according to Child. Other CMMs in the same facility include a G80K model made by LK Group (Fairlie, UK) with a measuring envelope of 3.8 m in the X axis and 4.5 x 2m in Y, a Brown and Sharpe (North Kingstown, RI) DEA with a measuring envelope of 2 x 6 x 1.5 m, and two smaller LK models.
Child notes a significant increase in metrology applications in recent years at Lockheed Martin. "In the past, CMM's and laser-measurement technologies were limited to first-article inspections, tooling, and a few very specialized applications." Now, he sees such technologies used throughout manufacturing and assembly for inspection, process control, assembly control, and troubleshooting. "We are becoming more data-driven at the floor level as a tool for controlling processes, instead of as an accept-or-reject step for controlling product."
While fixed CMMs are important, portable systems like laser trackers, laser radar, and portable-arm CMMs are a growing component of this trend, according to Rina Molari-Korgel, application engineer for Hexagon Metrology Services (Ft. Worth, TX). Her experience base spans 25 years and multiple military aerospace programs, from fighters to helicopters. Among other trends, the effect of carbon fiber reinforced plastic (CFRP) becoming a mainstream material is profound. This is as true for commercial aircraft such as the 787 as it is for military aircraft like the F-35. "CFRP is unforgiving compared to metal," says Molari-Korgel. "That is one reason you need to do it right the first time, driving the need for metrology in production. If you do not, you end up scrapping an expensive part where before in metal you might have been able to rework it."
She is also seeing hard-tooled fixtures replaced by metrology, a trend that has accelerated from its beginnings 15 years ago. This is true for both machining fixtures (think drilling templates) as well as assembly fixturing. "Using portable metrology equipment cuts both time and capital expense compared to hard tooling." Why portable systems versus fixed CMMs? "Portable systems are often the only practical metrology systems to use for many of these large parts." With laser trackers delivering accuracies as good as 0.0015" over a 20' span (0.04 mm over 6.1 m; two sigma), they can meet the accuracies of many aerospace programs which in many cases are in the 0.005–0.010" (0.127–0.254 mm) range. For example, she points out ease of use as a key factor for using laser trackers in production. "Once I download my CAD model, I can start measuring within an hour of having the laser tracker out of its box on the factory floor." She explains why CAD and digital data is so important in aerospace, along with off-line programming (OLP). "Unlike the situation in other industries, in aerospace often the only copy you have is in a CAD model, there is no physical prototype to refer measurements to."
This is not necessarily a disadvantage. For manufacturers that produce many different parts, or wish to reuse the same metrology equipment in a different way, reliance on a CAD model merely requires developing a new inspection program. This characteristic converts the virtual tooling of metrology-assisted production into flexible tooling as well.
That is the case with Carbon Carbon Advanced Technologies (CCAT, Ft. Worth, TX). "We are an engineering-development company that builds prototypes and engineering development models with our advanced carbon-carbon composite process," explains Raj Narayanan, the company's quality assurance representative. CCAT builds aerodynamic structures up to 14' (4.3-m) long, or small, complex rocket nozzles only a few feet in diameter but composed of approximately 63 individual pieces. They manufacture only small volumes. "If we build five of anything, that's a lot for us." They use two Leica laser tracker models to measure critical features of the parts they make. This approach "helps you build a complex structure and assemble it without having expensive tooling jigs," he explains. "It also allows you to quickly respond to an engineering change," a frequent occurrence in engineering development programs. While using 3-D models imported into software for OLP of the trackers, they rely on separate 2-D drawings on paper for GD&T information for quality control. "Most of what we are building now is flight-grade hardware. So, the only way to certify it as such is to certify it back to 3-D data and 2-D tolerance requirements. The laser tracker is invaluable in that regard."
The history of Nikon Metrology (formerly Metris, Waterloo, ON, Canada) with metrology-assisted production goes back seven years. "We first developed integrated laser systems that included metrology for composite material layup and verification. That system grew into a guidance tool for manual assembly processes," says Jarrad Morden, general manager of Nikon Metrology Canada. Their custom-built, integrated systems use various metrology instruments, including Nikon Metrology's indoor GPS (iGPS) technology, to align one or more laser projectors precisely to a part, tool, or surface.
Building on that experience, Morden sees three distinct areas for future growth of advanced metrology applications in aerospace: metrology-assisted production, metrology-assisted assembly, and fully automated inspection solutions. "Automatedinspection solutions offer significant benefits in reducing inspection time on part and assembly conformance checks," says Morden. The traditional manufacturing process—still relevant despite gains in metrology-assisted production—means adding value to a part, then inspecting it before shipping. "In the large-scale, aerospace world, that end-inspection process has typically been a manual process using products like tracking interferometers, photogrammetry, theodolites, or total station networks," explains Morden. These precision metrology tools require human operators and, of course, humans introduce variability. He points to automated measurement tools like Nikon Metrology's laser radar and iGPS localized instruments as a way to mitigate human variability. "We are able to fully automate a measurement plan that can be run lights-out. The trend in the industry, of course, is to move as much of that inspection upstream into the manufacturing process as possible, so that dimensional quality is built in, reducing or eliminating end-process inspection." He describes this as a clear industry trend that will evolve slowly over time.
Coupled with this development is the growing interest in using robotics for aerospace manufacturing applications, asserts Morden. Robots are repeatable, reliable, and cost-effective when compared to custom or hard automation. While often repeatable to submillimeters, they are not necessarily accurate enough to program to drill or machine to aerospace tolerances using standard off-line programming tools. Other variables matter as well. "Under varying load conditions and clamping forces, which often happens with tool changes, end effectors [on a robot] can lose accuracy relative to the workpiece," Morden explains.
Enter the Adaptive Robotic Control (ARC) system, a result of a two-year research program that involved Airbus UK (Broughton, UK) and Kuka Robotics (Augsburg, Germany). It combines a Nikon Metrology K-Series Optical CMM device with a Kuka robot. The metrology system feeds position data directly into the robot controller. Morden points out three important capabilities of this system: it verifies each position before a drilling or trimming operation; it's programmed off-line with standard robot programming packages using Catia CAD models of the parts; and it delivers accuracy independent of robot wear, temperature variations, or load variations. The system has been used at Airbus for two years, and Nikon Metrology reports no rejected parts in that time. A system delivered to Bombardier (Montreal, QC) also has had no reports of rejected parts after a year of use. Integration of the many, sometimes disparate elements of metrology available is the key to growth of metrology-assisted manufacturing, according to Morden.
In many cases, software is the key to that integration, especially when a manufacturer would like to use equipment from different companies. Because the software that comes with a particular metrology device tends to work only on that device, companies such as New River Kinematics (Williamsburg, VA), with its Spatial Analyzer code, play a vital role. Spatial Analyzer processes metrology data independent of the make or brand of the metrology instrument providing the data. Offering the user a single interface, Spatial Analyzer communicates with metrology hardware through specially developed instrument interfaces.
The manufacturers cooperate in developing these interfaces, which connect directly to each hardware controller. "Our philosophy is that we want to control that device, not simply download the position data it records," explains James Gardner of New River Kinematics. Operators simultaneously acquire data from multiple instruments, hosted on one or several networked computers, including portable arm CMMs, laser trackers, theodolites, local GPS networks, area scanners, or videogrammetry. A module within Spatial Analyzer calculates an overall system accuracy by combining the uncertainties of each instrument. "We interface with over a hundred different portable metrology instruments. A single operator or program can run multiple instruments," says Gardner. Spatial Analyzer imports CAD models to use in off-line programming of the metrology set available. "We now import virtually all available CAD formats."
As he sees it, accomplishing some of the ambitious goals the industry has set requires metrology embedded in a production system that is as easy to use as pushing a button. Production engineers and workers not necessarily schooled in the details of tolerance measurement will need to operate these systems on the floor. It also has to be predictive—there will be no time for carefully mating pieces through an involved process of trial and error. "Some programs are calling for producing an aircraft a day. They won't have time to stop, they need to know a problem and correct for it before it interrupts the process."
Because the CAD model description is such an important element in metrology-assisted production, some in the industry are pushing for improvements. Eric Tingle of Mitutoyo (Aurora, IL) also notes that comparing measurements of a part to a CAD model is the trend of the moment in aerospace. This is especially true now among smaller supplier companies that need to respond to a requirement an OEM places on them. What may help is the fact that many 3-D CAD descriptions allow for an annotated layer that describes GD&T. For example, he notes it's now standard on Boeing projects for that layer to accompany CAD files. Mitutoyo has responded by offering a graphical interpretation of GD&T symbols when an operator creates measurement programs in its MCOSMOS software, which is used on the company's CMM machines. "In our latest version of this software, the operator not only sees the CAD model, but also sees the GD&T call-outs on the screen," says Tingle. "Now he does not need a blueprint to guide him anymore. The operator uses the CAD model to get the job done, and can also add GD&T to a model that does not have it."
The next step in the evolution is to include more than simple GD&T, but also actual machining and inspection instructions along with the 3-D CAD model. Mitutoyo belongs to a consortium of companies that are developing the STEP AP238 protocol. "The STEP model with the AP238 extension gives our CMM machine instructions on how to set up a datum reference, and also gives our machine the tolerance and the GD&T standards," explains Tingle. When a customer outputs a STEP AP238 model, the CMM will understand how to align the part and what to inspect. The goal might be thought of as a broader definition of metrology-assisted production, because so many production cost constraints begin in design. "Some engineers over-engineer just to be safe. But an extra 0.001" (0.03 mm) of tolerance can cost a lot."
This article was first published in the March 2010 edition of Manufacturing Engineering magazine.