Adding Laser Probe to CMM Helps Aerospace Supplier Provide More Complete Inspection

By adding a laser probe to a coordinate measuring machine (CMM), Sonaca, a Belgian aerospace structures supplier, has greatly increased the completeness of its inspections and obtains full documentation of the geometry of every component. When the company inspected sheet metal components in the past, 3D shape was validated by using two complementary methods : base shape was controlled by comparing them to mylar patterns on a light table, and 3D shape was controlled via manual methods. Using CMMs to inspect the complex sheet metal parts would have required a long programming process for each of the many different parts produced by the company. "We found the ideal solution to this problem by adapting a laser probe on a CMM," said Anne-Laure Schurins, Quality Control Manager for Sonaca, Gosselies, Belgium. "In a few minutes, the laser probe generates millions of points that completely describe the geometry of the part. We import the point cloud into a software package that compares it to the actual part geometry and highlights any discrepancies. Finally, we save an electronic file that provides complete documentation of the part geometry."

Sonaca has fabricated the leading-edge slats from aluminum alloy for every Airbus aircraft made since the A310. The company produces most parts in-house on numerically controlled milling machines or by stretching in combination with chemical milling. Sonaca also produces structural bonding and composite parts for the A320 and A340 series planes. Sonaca assembles the Airbus leading edges with the most advanced technologies, such as automatic riveting and flexible assembly lines like those used in the automotive industry. As a risk sharing partner on the Embraer ERJ 135/145, Sonaca is responsible for the detailed design of the center fuselage section and also the wing fixed leading edges, including the anti-icing system. In the Embraer program, Sonaca fabricates a tremendous number of elementary references, mainly sheet metal parts by stretching, fluid cell stamping and chemical milling. The company also uses graphite fiber composites in the nacelle fairings and supports that contribute to local stiffening of the cabin floor. Sonaca assembles two fuselage sections for the ERJ 135/145, demonstrating its ability to succeed in a very cost-competitive environment.

Previous inspection method

Sonaca produces sheet metal components for both Airbus and Embraer using two different technologies, one involving forming on a stamping press and the other folding with a press brake. Tolerances for these parts are typically 0.2 mm on the contours and hole positions. A thorough inspection of these parts is required on a regular basis, such as the first parts produced at the beginning of every shift or after a design change. In the past, this type of inspection was performed by inscribing the part geometry onto a mylar sheet, laying the mylar sheet on a light table and placing the part on top of it. Then highly experienced inspectors examined the parts and were able to easily identify out of tolerance errors. This process took about 15 minutes. One problem is that this method was difficultly applicable with more complicated 3D shapes. Another problem is that producing the mylar patterns is an expensive task that had to be performed partially by an outside supplier. Another problem is that this method provides little documentation or traceability besides the confidence that the company holds in the ability of its inspectors. The objective of Sonaca was to digitalized the inspection within a full numeric manufacturing process.

Schurins looked for ways to improve the existing inspection process. She considered the idea of moving the inspection to a CMM. Programming the CMM to move its probe automatically to the points needed to inspect the part would have allowed for inspection of most 3D parts and would have provided electronic documentation of each part. However, it would have been a long and involved process to program the CMM for each of the many different parts produced by the company and the programs would have to be modified for every design change. In addition, Schurins expected the trend towards more complex 3D designs to continue and she was concerned that the company would soon begin to produce contours that were too complicated to be inspected in a reasonable amount of time using the CMM's point-by-point approach. "When I heard about the relatively recent innovation of laser scanning I immediately decided that it deserved further investigation as a possible solution to this problem," she said.

Adapting a laser probe on a CMM

Rather than absorbing the expense of purchasing a complete laser scanning machine, Sonaca purchased a relatively inexpensive laser probe from Laser Design, Minneapolis, Minnesota and installed it on a Mitutoyo CMM machine. The laser probe is mounted to the CMM in place of the traditional contact probe. Integrating the laser probe with the CMM is relatively simple because a laser probe, unlike typical touch probes, does not need to be in an exact location to measure because of its large field of view. It just needs to know exactly where it was when the data was collected so that the scan data can be accurately positioned in space. With a depth of field ranging from one to several inches, all that matters is that the laser probe passes through the area of interest on the part. Sonaca used the method of integration in which the CMM's motion system guides the probe while probe computer monitors the encoders to track position. Another option is for the laser probe's computer to actively control the CMM position, even to the point of using feedback from the probe to keep the part surface in its field of view.

The laser scanner can be operated in either joystick mode or programmed for automated inspection of volume parts. In the joystick mode, the laser probe is attached to the CMM and is controlled by its own computer which is independent of the CMM's control system. The operator observes the data captured on the probe computer screen, plans and executes the next move, and continues until the desired coverage is achieved. Alternatively, in an installation that includes a communication link between the probe and CMM controller, the operator can set up moves on the probe computer that are sent to the CMM controller and even modified on the fly to track surfaces in response to feedback from the laser probe. This mode also allows for entire part inspection sequences to be stored, recalled and repeated.

Additional information improves process

The laser probe comes with a computer that collects the laser scan data and converts it to a 3D point cloud. Instead of collecting points one by one, the laser scanner picks up tens of thousands of points every second. This means that reverse engineering of the most complicated parts can often accomplished 5 to 10 times faster than with touch probe digitization of the part. Laser scanning can reverse engineer parts that are so complex that they would be practically impossible one point at a time. The Geomagic Studio software provided with the scanner greatly simplifies the process of moving from point cloud to computer aided design (CAD) model, making it possible in minimal time to generate a CAD Model of the scanned part that faithfully duplicates the original part. Sonaca also uses Geomagic Qualify software to compare original design geometry to the actual physical part, generating an overall graduated color error plot that shows in a glance where and by how much surfaces deviate from the original design.

"It takes about 15 minutes to set up the CMM and scan the part and another 15 minutes to analyze the data and generate a plot that shows the complete geometry of the part in comparison to our specifications (for our smallest and easiest parts)," Schurins said. "This is only a little bit more time than was required in the past but we generate far more digitalized information now. we now obtain a complete 3D model of the part that is annotated so that it shows any discrepancies from the customer's specifications. The part of the model that is within tolerances is colored green. The areas of the part that are out of tolerance are colored on a scale that depends on how far from the tolerance band they are. This provides solid electronic documentation that we can show to our customer to demonstrate that we have met their specifications. In case there is a problem, these charts provide detailed diagnostic information to our manufacturing people that helps them resolve it.

SOURCE: Structured Information