Laser measurement has pros and cons

  • 23-Apr-2009 08:56 EDT
GKS CMM (1)  and point cloud.jpg

A coordinate measuring machine fitted with a laser sensor shows the output as a "point cloud" on the screen. Laser Design

There are a number of measurement tools used for manufacturing in aerospace, including CMM machines, that have long been one of the main tools for this type of work, as well as laser or optical scanning.

CMM is most valuable on new machine parts made from CAD files, where it is used to verify the part against its original criteria. The technology, however, is slow, so there is a trade-off between speed and the amount of data that can be collected.

Regarding the next generation of CMM machines, Giles Gaskell, a mechanical engineer and Business Development Manager for Laser Design’s GKS Inspection Services division, said he does not think there will be any breakthroughs to solve the technology’s problems of weight, expense, and lifespan. Rather, the next-generation measurement technology is already here and is optically based.

Lasers such as the ones used by Vought on the 747-8 program are particularly effective at measuring components for reverse engineering, and are also adept at measuring large components such as wings that are too big to fit on a CMM machine.

The primary drawback with laser systems is that they cannot scan shiny parts because the beam scatters and prevents precise measurements from being made. (Wings are shiny, too, but for their measurement the lasers are fired at small, marble-sized targets attached to the wing rather than at the wings themselves.) They cannot scan parts made from composite materials, either, because the resin and numerous fabric layers also scatter the beam.

CMM technology will always have a place in manufacturing measurement of both metal and composite components until that limitation with optical methods can be surmounted.

"The step change in the industry will be when people can produce a reliable optical measurement system that can scan shiny parts," said Gaskell.

A third measurement technique, computed tomography, solves the issue of laser beam scattering, but is limited in the size of components it can measure to those that are about 1.0 ft2, and is also very expensive. Just as medical CT scans provide a 3-D image of the inside of the body, computed tomography is especially proficient at providing accurate scans of aerospace components with internal geometries such as valves and connectors. Unlike laser scans, CT scans are unaffected by surface finishes.

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