Metrology Turns to Jet Engines
By Bruce Morey
The pressures on today’s aircraft engine manufacturers are both familiar and intense—deliver engines with more power that use less fuel, last longer, and cost less. In response, aircraft engines are incorporating some new twists in their designs as well as using ever-tighter tolerances and lighter materials. These include titanium alloys in hot parts of the engine and composites in cooler parts. Individual parts are also more carefully engineered. Designers craft turbine and fan blade designs into organic looking shapes with advanced design tools such as computational fluid dynamics codes.
Ensuring these parts meet complex specifications is the job of metrology professionals like Eric Hogarth, Digital Metrology Leader for GE Aviation (Evendale, OH). "New engines are getting ever more complex, which requires ever more complex inspection," he explains. He also notes that manufacturing engineers today like digital metrology data. He sees even ‘hard gaging’—purpose built tooling for specific measurements—delivering digital data rather than the pen-and-paper methods of the past, in many cases replacing hard gages with coordinate measuring machines (CMMs). These are now common on the shop floor, hardened and ruggedized to operate in vibration-prone and dirty areas like forging shops. "We have a couple of blade shops that have 75 CMMs out on the shop floor," remarks Hogarth.
Along with digital data, manufacturing engineers would like it faster. Lean techniques now used in engine manufacturing have in part spiked this demand, according to Hogarth. "Some of the better inspection methods can be quite slow, so we see speed as a big issue as well as capability [in measuring]." One approach to faster measurements is using scanning analogue probes over single point touch probes. GE Aviation has equipped many of its CMMs with these.
The company is also using noncontact sensing as well. In one particular application, they use a structured white-light system to measure compressor blades made of titanium. They are currently evaluating it for measuring larger composite blades. He explains that GE Aviation developed its own patented technique to eliminate the coating usually required to get accurate data from structured white-light systems while improving accuracy as well. "The accuracy is still not as good as a CMM, but there are some parts with an open enough tolerance [where these are useful]," he explains. These include airfoils and inlet guide vanes. Exact tolerance surfaces such as dovetails at the mating end of turbine blades remain for CMMs.
An even more exotic sensing method is Computed Tomography X-ray systems. They are combining data from a CMM with internal geometry obtained through CT X-Ray. This technology is somewhat limited in the materials penetrate. However, the composite parts increasingly used on engines are ideal for this. Taylor reports that the CT systems they use were internally custom-built. They use detectors similar to the CT products that GE develops and markets.
The future? While advancing sensor technology, especially noncontact sensors, to measure faster, more accurately—and less expensively—will certainly continue, another problem is looming. "Post-processing and resolving that increased amount of data is still a bit of a challenge for us," admits Tim Taylor, team leader in Advanced Metrology for GE Aviation. He sees the near future with engineers focusing on developing software and processes to absorb that data and convert it into information for action.
"We are still at the beginning of our understanding of how best to use multi-sensor technology," says Taylor. Combining touch probes, scanning analogue probes, noncontact data, or CT scans and make sense of it remains a goal, according to him. There may be too much of one kind of data, not enough of another. "I think people who are testing and experimenting with multi-sensing are still struggling with deciding when or how to use multisensing in selection of one sensor over another," he says.
Gwyn Carter, senior application specialist for Carl Zeiss Industrial Metrology LLC (Brighton, MI) agrees CMMs have arrived on the shop floor. This started over a decade ago when he helped a major aircraft engine manufacturer replace ‘guillotine’ hard gages for measuring turbine blades. "They wanted process data to make adjustments and do a better job of monitoring," explained Carter. "They really mastered the art of using CMMs to measure their blades." In this first application, they placed five Zeiss CMMs in a cell around a grinder that machined the blades. The CMMs measured the blades and the system fed data back to the grinder controllers for in-process adjustments. "This kind of in-process control has pretty much become the standard with all of my aircraft engine manufacturers now," he remarks.
"We are still at the beginning of our understanding of how best to use multi-sensor technology," says Tim Taylor, team leader in Advanced Metrology for GE Aviation.
Turbine blades and blisks (blades and disks in a single, integrated unit) are the parts most often measured with their equipment. He also notes that these parts are ideal for analogue scanning probes, which Carl Zeiss calls active scanning. This is offered in their VAST and VAST XT scanning probes. For example, he reports such probes collecting 8–10,000 data points on blades that range from 1 to 4" (25.4–102 mm)in width. These are mounted on articulating heads to satisfy some of the challenging reach requirements for blades and blisks.
He also emphasizes the usefulness of metrology programming software that imports CAD models, such as Calypso. "By importing the CAD model, we are able to perform a section cut on a turbine blade or blisk and determine the form the blade should look like," he explains. "In Calypso, we will "cut" the CAD model of the blade in software and create the nominal data and use that to program the part. Then, our stylus on the actual CMM will be able to follow that path." While able to use the measurement data as the users wish, Carl Zeiss also offers Blade Pro, an analysis package tailored for turbine blades. Unlike other CMMs that might replace hard gaging, turbine blades and blisks require both precision and at least four axes. "Generally we deliver an Accura bridge-style machine with a rotary table for the fourth axis," he reports.
He agrees the industry continues to demand quality data faster. "That is where the biggest changes have occurred over the years," he says. In response, Carl Zeiss recently improved its active scanning methods. Normally, an operation indexes the rotary table without the scanning probe engaged on the part. This is dead time in the cycle. In their new mode under development, they measure the part with an active probe while the part is indexing on the rotary table. "It is an active-active scanning technique and improves speed," he says.
Hexagon metrology also delivers CMMs tailored for airfoils, blade, and especially blisk measurements, according to Dan Jeanloz applications engineering manager, northeast of Hexagon Metrology (North Kingstown, RI). He notes that their most popular CMM is also a bridge-type CMM equipped with a rotary table, such as the Brown and Sharpe Global Advantage equipped with motorized or articulated wrists that uses a Leitz LSP-x1 scanning probe. "The industry trend is more blisk or integrally bladed rotors measurements. These can get tricky to get probe access spaces between airfoils," he says.
Another challenging application unique to aircraft engine turbine blades is the many, precise cooling holes needed along their edges. "Turbine blades need to run in environments that are hotter than the melting point of the metal used to make the blades," he explains. "These cooling holes are in optimum location and size used to cool the surface of the blades." These passages are between 0.008–0.040" (0.203–1.02-mm) diameter and are difficult to measure. Hexagon has found vision systems to be ideal for this, equipped with a dual-axis rotary table and zoom camera (more details on type of camera). "We orient the hole perpendicular to the camera to measure it," he explains. Using vision solves many problems over attempting to measure with a touch or scanning probes. "First you have to find the hole. If you have a hole that is 0.010" [0.254 mm] in diameter with a position tolerance of 0.010", you may never find it even though it may be a borderline good hole," he explains. To measure with a contact method, the operator would place a pin in the hole and then measure the OD of the pin. "To do that for 100 holes per blade would be much too time consuming," says Jeanloz. He reports using an Optiv Advantage with a dual axis rotary and a zoom CCD vision camera.
Another device that might satisfy both the need for speed with exacting tolerance is the VisionGauge digital optical comparator offered by VISIONx (Pointe-Claire, Quebec, Canada.) These vision-based comparators combine the capabilities of an optical comparator, video coordinate measurement machine, and machine vision system. Unlike optical comparators that require Mylar overlay charts for part comparisons, the VisionGauge system simply reads in CAD and with minimal programming is ready to inspect parts on a ‘pass/fail’ basis. "While our systems are general purpose, aerospace seems to be an early adopter," remarks Patrick Beuchemin, President of VISIONx. VISIONx VisionGauge products are exclusively sold, distributed and supported by Methods Machine Tools Inc. (Sudbury, MA).
Making each individual component of a jet engine to spec does not mean the job of careful measurement is finished. Jet engines are dynamic and operate at high speeds. If assemblers do not balance rotating elements perfectly, noise, excessive wear, and less than optimum fuel economy will result. "Our equipment measures the surfaces of individual rotating components and then a software package determines the best way to mate and stack each of these components in order to deliver the straightest, best balanced, most true running engine," explains Neill Fleeman, technical director and president of Turbine Metrology (Kansas City, MO). The unique hardware they offer is their Paragon Circular Geometry Inspection (CGI), designed especially for jet engine turbine rotor disks. The heart of their system is table built to their own design. Using purpose-built mechanical measuring heads, the Paragon V2 measures each surface of a rotor over 500,000 times to a resolution of about 0.000008" (~0.2 µm). The surfaces are automatically related to one another, eliminating post-processing. The newly release Paragon V3 measures to 0.000005" (~0.125 µm).
Why not simply use a general-purpose CMM versus a special purpose machine? "The Paragon is about 1000 times faster," states Fleeman. "Also, on a CMM, once all the data is gathered, it still has to be analyzed to determine flatness, parallelism, concentricity, and eccentricity among other measurements." The Paragon device performs this automatically. It is also cost-effective compared to CMMs with similar measuring capability.
The company’s TrueBuild software accepts data from the Paragon to assemble wheels, rotors, and drums, while its BalancePoint software will predict how best to place blades. "It will compensate for machining variations, it will tell you how to arrange the blades to compensate for the best balance of the rotor," he says.
They tailor each system for the end user. "We have only delivered two identical systems," says Fleeman. "Each customer always needs something different." He also notes that maintenance, repair, and overhaul (MRO) facilities are showing greater interest of late than the OEMs. The challenge to MROs may be greater than an original manufacturer. "MROs have much the wider surface variations in their components, they may have some surface damage, and more erosion in mating surfaces, for example the rabbets." In response, systems for MROs use gage heads with more travel than their standard gage heads, with slightly less resolution.
"Metrology offerings in the last five years have changed significantly, which has also influenced how or what measurements are performed on all of our engines," says Jesse Boyer, Core Manufacturing Engineering Manager, Advanced Manufacturing Metrology for Pratt and Whitney (East Hartford, CT). His list includes: more advanced optics; 3-D inspection—both contact and noncontact; the ability to process large amounts of data; more shop-floor readiness; and easier to use traditional gaging.
Not only have metrology technology advanced, so have the engines. For Pratt and Whitney, that means the revolutionary PurePower Geared Turbofan (GTF.) "The technologies in the GTF allow us to better integrate manufacturing metrology and the inspection data or methods used for development. The GTF also has helped us mature inspection technologies into manufacturing inspection systems." According to him, measuring airfoils remains a particular focus even while new parts, like gears, are now part of an engine. ME
This article was first published in the March 2012 edition of Manufacturing Engineering magazine. Click here for PDF.