Switching from a coordinate measuring machine (CMM) and height gauges to a laser scanner has substantially reduced the time required for reverse engineering while improving accuracy at Boeing's Phantom Works facility in St. Louis, according to David Skiles, Rapid Prototype Development Technician. Reverse engineering, producing a CAD model from a physical product, is important in cases where modifications are to be made to an older aircraft as well as other situations. In the past, the most common approach was using a CMM to measure data points on the surface of the aircraft, which was time-consuming and provided limited accuracy. Laser scanning, on the other hand, generates a far greater number of data points, which improves accuracy, in less time. " Using special surfacing software, scanned data can be used to create a CAD surface model of the part that is accurate to within 0.0012 inch to sheet metal surfaces and 0.0002 inch for hole position," Skiles said.

Phantom Works is an advanced research and development unit of Boeing that pursues breakthrough improvements in the affordability, quality and performance of aerospace systems. The staff of 4,500 engineers and scientists typically works in small, integrated teams that can be quickly formed and mobilized to help customers meet their toughest technical challenges. Their common challenge is to find better, faster and cheaper ways to design, develop, manufacture, test, operate and support both current and future systems.

A team developing 3-D modeling, simulation and virtual reality tools, for instance, has found ways to cut design cycle times in half, eliminating the need to build costly prototype hardware and creating more producible and supportable systems in the process. Such innovative technologies and processes are being used to save time and cost in the development of the Joint Strike Fighter (JSF) as well as for the improvement of such products as the AV-8B Harrier, F/A-18 Hornet and the F-15 Eagle.

The St. Louis facility has a frequent requirement to reverse engineer both old and new aircraft. The need usually arises for older aircraft which were designed without benefit of CAD when they are to be repaired or modified. In some cases, mylar drawings are available, which could potentially be scanned into a CAD model, but this process has some inherent drawbacks. For one things, the original drawings are in 2D, making it difficult or impossible to create a 3D CAD model which is required for many tasks, such as CNC machining or finite element analysis. Second, the drawings have often been changed many times so that there is often no way of knowing whether the physical aircraft is revision A or B or C or D.

Reverse engineering is also an important tool for current and even experimental aircraft that were designed using the latest CAD methods. In this case, it is necessary to compare the physical aircraft to the CAD model. An interference problem, for example, may arise when the aircraft is being assembled. In this case, the need arises to compare the physical aircraft to the original CAD design to determine the exact cause of the problem. The complexity of the geometry of state-of-the art aerospace structures makes this a very challenging task. Even when a specific problem doesn't exist, the need often arises to compare the aircraft to the model in order to confirm that the design intent is being met.

In the past, Phantom Works technicians used two primary methods to perform reverse engineering. The simplest was to use height gauges and other manual measuring instruments to measure discrete points on the surface of the aircraft. It is difficult to inspect a contoured part with manual measurements because there is no way to document the shape of curved surfaces. A technician can get only critical dimensions such as the location of hole centers, the diameters of holes, and wall thicknesses. In addition, this approach is time-consuming and only as accurate as the person taking the measurements. It is also difficult to inspect contoured parts with CMMs. A CMM has a probe that must physically touch the part. Although these machines can acquire data points more quickly than a person can, for a typical part it will capture only several thousand data points over several days. This relatively small number of points can not completely define a curved surface.

After researching the issue, Phantom Works came to the conclusion that laser scanners were the better option because they capture shape information without touching the model, making them fast and accurate. After evaluating the products on the market, they decided that the system that best fit their needs was the handheld scanner from NVision, Dallas, Texas. The major components of the system are a 3D laser sensor, a mechanical digitizer on which the sensor is attached, a PC, and software that extracts, displays, and manipulates the data. The sensor is a single viewpoint laser stripe sensor incorporating the illumination and sensing means to capture 3D data. Laser stripe sensors are significantly faster than simple laser point sensors. They work by projecting a line of laser light onto the object while a small CCD camera views the line as it appears on the surface. A dedicated interface card translates the video image of the line into more than 400 3D coordinates, allowing for a maximum data capture rate of 10,000 points per second.