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Manufacturing Engineering March 2008
Scanning Speeds Reverse Engineering
RE tools quickly turn parts into data for aerospace manufacturers
By Michael Tolinski
While aircraft manufacturers struggle to keep up with orders for new planes, more aero component-makers are stepping in to supply parts they've never supplied before. But making parts requires data, and often current data or even prints don't exist, especially for older aerospace components that have undergone undocumented changes. In these cases, the only option is to reverse-engineer a current part and capture its dimensions, and for this work, 3-D laser scanning stands out as a particularly useful tool.
"There's a lot of reverse engineering [RE] that goes on in the aerospace industry," says Giles Gaskell, director of business development, NVision Inc. (Wixom, MI). A particular target area for RE has been replacement parts for aircraft engines. Engine manufacturers may sell an engine for not much more than it costs to make, and then look to profit from selling spare parts down the line, Gaskell explains. "People have seen that opportunity and thought, 'if we can make spare parts, then we can make all the profits without having to make the engine in the first place.'"
The kinds of parts these manufacturers are reproducing and selling do not fall under patent protection, he says. Thus, this kind of duplication is generally an open secret among aerospace manufacturers: "It's not something they can do under the table, because the parts do have to be approved by somebody—you can't just stick any old junk on an airplane and expect to get away with it."
Aero manufacturers of all sizes look carefully at the competitors' parts they might want to start producing, says Dan Jeanloz, application engineer for the Brown & Sharpe service division of Hexagon Metrology Inc. (North Kingstown, RI). "They buy the original manufacturer's part and ask us to reverseengineer it so they can create a model and start making that part as an aftermarket part." This is especially true for lucrative parts that are definite money-makers.
Part-duplication is just one of many reasons for performing RE, says Dave Armstrong, product manager for Hexagon's Leica Geosystems (Lake Forest, CA). Most reasons have to do with creating master CAD data when it doesn't exist or is not current. With modern metrology tools, "Aerospace companies now have the ability to create a digital master that can be produced in a fraction of the time and cost, as opposed to traditional hard tooling," says Armstrong. "They can quickly scan a good part that they know is within specification, and then use this data as their master CAD to machine from to create legacy parts.
"Not everyone's definition of reverse engineering is the same," he adds. "Some people would class reverse engineering as scanning a part and comparing the actual data, to a CAD model; some people would class reverse engineering as scanning a part and capturing point data; [and] some would say you reverse engineer by post-processing and creating an STL file. To truly reverse engineer a part, you need to scan the part, post-process the data [create a STL mesh], and then from that create NURB surfaces [an IGES CAD Model]."
Above the component level, often the goal is simply to obtain correct data for an as-built structure. "Plane assembly is very complex," says Marc Soucy, president of InnovMetric Software Inc. (Québec, Canada). Even if the manufacturer possesses up-to-date CAD data for each part and assembly, the resulting plane is not necessarily too close to what the combined data would predict. "So, for documentation [and] maintenance, it's valuable to use an RE process to get an as-built plane into CATIA." For example, Boeing digitizes its as-built planes, he says, and uses InnovMetric's PolyWorks software to prepare the digitized information to reconstruct a solid model in CATIA.
Recent improvements in 3-D scanning have enabled more of this RE to be done in aerospace and other industries. "On the hardware side, there's something for everyone," says consultant Todd Grimm (T.A. Grimm & Associates Inc.; Edgewood, KY). For instance, laser scanners range in price from $2500 to well over $100,000, depending on the user's needs. The key challenge now is educating more users, Grimm observes. "People have a lot of misperceptions about 3-D scanning, and many don't even know the right questions to ask."
In this process, scanners acquire massive amounts of point-cloud data quickly. By comparison, a CMM with an analog scanning-probe head allows less speed, though more accuracy. With a laser scanner, "you get a lot of data quickly, but your accuracy is 20–50 µm," says Jeanloz of Brown & Sharpe. "With the analog head on a CMM, your accuracy is 3–4 µm, so it's a big difference."
However, the speed of laser scanners can't be ignored, says Brian Gudauskas, application engineer for Hexagon's Romer Inc. (Wixom, MI). "Reverse engineering in the past was done with touch probes [roughly 1–2 points per second], and in the recent past with analog scanning probes [roughly 150–400 points per second]." At 20,000 points per second or more, "laser scanning now offers the ability to get mass quantities of points for a low cost in time and money."
Recently, Romer's ScanShark 5V laser-scanning probe was updated to capture 458,000 points per second, making it nearly twenty times faster than the previous version. Increasing speeds this high could, at some point, be seen as overkill, but Gudauskas argues that higher numbers of points at higher densities are better to have than not. "When scanning first and post-processing later, you don't want to be missing anything."
Scanner accuracies are also expected to improve, says Michael Raphael, president and chief engineer, Direct Dimensions Inc. (Owings Mills, MD). "Some of the newer 3-D scanner systems can capture large, complex shapes such as aircraft wings, tails, and complex fairings—in single setups, with near-CMM accuracies and significantly higher point-spacing resolutions." Accuracy is improved by combining positioning systems with hand-held laser line scanners, allowing untethered contour scanning of larger shapes.
One specific project example was the scanning of all of the major structural components in the belly of a 40-yearold BAC 1-11 commercial airliner, says Raphael. To help the client incorporate a new radar system, technicians scanned the dimensions and positions of stringers, frames, brackets, clamps, skins, and fittings. "All of this raw information was then converted into a detailed 3-D CAD solid model in SolidWorks."
Turbine-blade scanning demonstrates the importance of choosing the right tool for good RE in aerospace. Blades can be lucrative parts to manufacture both in aerospace and in the power-generation industry, where they can be sold for tens of thousands of dollars, says Dan Jeanloz. In aircraft engines, blades need to be replaced relatively often, given the heat and wear they're exposed to.
Measuring certain areas of the blade may require the accuracy of a CMM with an analog probe. This is especially true at the blade's root, where there are usually ground surfaces and dovetail profiles requiring perfect mating surfaces to be held to extremely low tolerances. By contrast, along the airfoil of the turbine blade, looser tolerances of 50–100 µm are acceptable, well within the capabilities of a laser scanner.
Touch or analog probing is also sometimes a bit more adaptable to scanning certain kinds of turbine blades, adds Jeanloz. For a blisk—turbine blades and a disk combined into one part—it's very difficult to get a probe or laser access to hidden areas for scanning. Lasers work in line-of-sight, so "if you can't see it, you can't measure it." A probe is similar, but probes can also be modified to go around 90° bends. "A laser can't do that, so you have a little more flexibility with a probe."
Other examples of 3-D scanning in aerospace abound, reflecting the importance of digitizing aircraft surfaces and structures after they're manufactured. The examples reported below show how scanning supports RE in machining and assembly (note that given the competitive nature of the industry, few end-users chose to reveal their identities for publication).
"Putting scanning technology to work in reverse engineering in aerospace means solving the large-objects problem," says Rhex Edwards, product manager for Perceptron Inc. (Plymouth, MI). Large parts are the focus of one large aerospace machining plant that reportedly uses the company's V5 ScanWorks scanner to reverse-engineer airframe castings made from aluminum and titanium.
Edwards says that RE with scanning was performed after machine-tool programmers had discovered significant errors in some geometric data they were given when upgrading their solid-modeling CAD systems. "This made it very difficult to wring needed productivity gains from the CAM systems the programmers used."
During machining, cutters would occasionally smash into the expensive castings or "cut air" away from the workpiece surfaces for long periods. Root-cause analysis revealed that the geometry of the castings was out of date because of undocumented changes made in the past— including "hand-sanded" surface modifications in the foundry that were not, of course, recorded in the CAD/CAM data.
So the company reverse-engineered its own products. The castings and foundry tooling were scanned with the laser scanner, and its output was processed with PolyWorks point-cloud processing software from InnovMetric to generate correct geometric models. Edwards says the scanner's advantages for this application were its high scan rate and generous laser line width of 140 mm.
Another application used scanning to retrofit a new custom aircraft engine into the engine compartment of an existing plane. For this, technicians would have typically used tape measures and T-squares, inadequate for obtaining sufficiently accurate 3-D data. Instead, a Platinum ScanArm from Faro Technologies Inc. (Lake Mary, FL) was used to collect, store, and analyze digital 3-D data from the inside of the aircraft engine compartment.
The ScanArm integrates a laser probe with the company's FaroArm portable CMM. Software operators could then manipulate data to engineer alterations that allowed the positioning of the new engines, reports Faro. This reportedly resulted in dramatic time savings for the end-user.
Sometimes laser scanning is the only solution for accomplishing a manufacturer's goals. In this case, Bell Helicopter, maker of the famous UH-1Y "Huey," wanted to transfer production of the aircraft's cabin to another company. But Bell possessed only one set of about 500 templates that served as the physical masters for the cabin structures, and they could not be moved without interfering with current production.
Since there was no documentation that would allow new masters to be built, Bell consulted RE specialist 3DScanCo. Inc. (Atlanta, GA) about laser-scanning the templates. The contoured templates have multiple cut-outs and holes of various sizes and locations, making them tough to digitize.
Given its speed, "3-D scanning was the most cost-effective way of getting all the data into a digital format," says 3DScanCo President Karl Hatzilias. After scanning, Bell subsequently was able to transfer production by using the resulting digital masters to create the new tooling on-demand."
Other tools complement laser scanning for reverse engineering, including laser trackers for long-distance measurements, and white-light camera-based digitizing for capturing large surfaces. Even though laser scanners have improved dramatically in recent years, so has nearly every type of measurement and inspection tool, notes Michael Archer, engineering manager for Starrett Kinemetric Engineering Inc. (Laguna Hills, CA).
Machines that combine different measurement methods into one unit may serve some RE practices more flexibly. "For instance, vision machines are typically contact-probe equipped, with options for some type of laser scanner, all of which are tied to the same coordinate system. In this way, a part can be measured with the best tool for the given feature on it, and all the data are captured on the same machine."
This points to the idea that there's no "holy grail," or perfect tool or method, that will cover all reverse engineering situations, says Archer. "Parts can be large or small, complex or simple." Some of their materials may be compatible with contact inspection, while other parts can't be touched. "For every part there is a given technique that will be best, and usually a secondary method if the first proves to be too complicated [or] costly. I believe that there will always be a market for contact and noncontact types of measurements."
"I don't believe in an "ultimate" technology," agrees Marc Soucy of InnovMetric. His comment applies also to reverse engineering software. "There is a definite trend in the CAD world (CATIA, Unigraphics, and so forth) to improve RE capabilities. On the other hand, point-cloud processing software such as PolyWorks will always be needed to prepare the raw data for CAD. So, many hardware and software tools will always be necessary."
Scanning & RE Extends Beyond Aero
Aerospace is just one of many sectors in which 3-D laser scanning and related technologies are being used. A number of reverse engineering, rapid prototyping, and direct digital manufacturing applications—ranging from dinosaurs to dentistry—will be discussed May 20-22 at the SME's "3-D SCANNING: Reverse Engineering, Inspection and Analysis," which is co-located with the RAPID 2008 Conference & Exhibition near Disney World in Florida.
Given the conference's location, it's fitting that amusement park rides are the focus of one presentation. Doug Smith, manager of global applied technology for Walt Disney World Co., is scheduled to talk about how the amusement industry incorporates integrated 3-D design and analysis platforms to create and test rides, eliminating time and reducing costs. Using rapid prototyping, "Art directors with no engineering backgrounds can sit in a ride car and evaluate its look and feel. Engineers can see how it would fit on the track or make prototypes into existing assemblies to verify the design."
Another application to be discussed at the conference is more related to reverse engineering—which in this case re-created a dinosaur rather than a manufactured product. Art Andersen of Virtual Surfaces Inc. (Mt. Prospect, IL) will explain how he used a whitelight system to digitize a duckbill dinosaur's remains, so that tools for molding a replica could be created. Reportedly over 300 overlapping scan patches were required, totaling 17 GB of data.
With an aerospace focus, Direct Dimensions Inc. scanned a more recent kind of dinosaur, the US Navy's C-2 cargo aircraft. The company will tell how it performed 3D scanning on ten aging C-2 planes to capture their existing cargo-door geometry, which then helped the Navy re-engineer the doors.
The rapid manufacturing of ceramic implants will be discussed by Howard Kuhn of the ExOne Co. (Irwin, PA). "Ceramics represents the last material group to be exploited by rapid manufacturing technologies." Now ceramics are being 3-D-printed as dental crowns and as scaffolds for tissue engineering.