Concentrated precision

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END USER: Delta TechOps, (866) 676-3358, www.deltatechops.com. CHALLENGE: Maximize accuracy and efficiency in inspecting complex turbine engine blades and vanes. SOLUTION: Measurement system utilizing concentrated light. SOLUTION PROVIDER: NVision Inc., (972) 393-8000, www.nvision3d.com

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Delta TechOps, the technical operations division of Delta Air Lines, is the largest airline maintenance, repair and overhaul provider in North America and the third largest worldwide. In addition to maintaining the Delta aircraft fleet, Delta TechOps serves more than 150 aviation customers, specializing in high-skill work such as engine, component, airframe and line maintenance. 

Delta TechOps maintains a variety of turbine aircraft engines as well as turbine-driven, ground-based auxiliary power units. The company disassembles engines and routes components to different parts of the organization for inspection and repair. 

The area that handles turbine blades and vanes deals with a vast number of complex components. According to Cameron Leonard, lead aviation maintenance technician, an engine can contain 1,300 to 3,000 blades and vanes. “Depending on the engine, how long it has run and other parameters, we have to measure features such as the chord, axial chord and root of a component to make sure it is airworthy or if it needs repair,” he said. 

Tolerances are commonly within 0.001 ", and various measurement technologies are employed, including micrometers, calipers and nondestructive testing methods such as fluorescent penetrant, X-ray and ultrasound. “It all depends on what the manufacturer requires,” Leonard said.

Some of the complex 3-D contours of the blades are challenging to consistently and quickly measure. Manual measurement is time consuming, and results can vary from technician to technician. 

Delta TechOps sought a way to increase the speed and consistency of the measuring process by examining measuring systems that were more applicable to its parts. Laser-based measuring was one alternative, but the shiny surfaces of the blades can cause a laser beam to speckle, or scatter. As a result, laser measurement required that the component be sprayed with a nonreflective coating that, Leonard, said, “is more trouble than it is worth.” 

Another possibility, automated touch-probe technology, suffered in precision when measuring the sharp edges of the blades, because accuracy is dependent on ball-offset geometry related to the size of the touch stylus vs. that of the edge being measured.

In the noncontact Maxos inspection system from NVision Inc., Coppell, Texas, Delta TechOps found an accurate and efficient way to inspect the complex parts. The system employs a proprietary concentrated light source that enables accurate measurement of reflective surfaces without the need for coating. Because there is no ball probe involved, the system can measure radii of less than 0.1mm (0.004 "). According to NVision, accuracy is ±2µm (0.00008 ") on matte surfaces and ±10µm (0.0004 ") on shiny surfaces. 

Courtesy of NVision

The noncontact Maxos inspection system from NVision employs a proprietary, concentrated light source that enables accurate measurement of reflective surfaces, such as turbine blades and vanes, without the need for coating.

Steve Kersen, NVision vice president of sales and marketing, noted that unlike the “point-move-point-move” motion of a touch probe, the Maxos system can collect 100 points a second at a resolution of 0.2µm without pause. He admitted that laser systems, which NVision also provides, can collect as many as 30,000 points per second, “but that does you no good if the points aren’t accurate” due to reflectivity issues. He said end users report that the system is at least four times faster than touch-probe measurement methods. 

Colin Ellis, NVision engineering manager, noted that eliminating the need to coat the part surface saves time, which might be as much as 5 minutes for application and 15 minutes for removal. It also improves accuracy because the spray can change a measured part dimension. “The problem is that the spray can add anywhere from 0.0005 " to 0.003 ",” he said. “You just don’t know because nobody sprays perfectly.” 

Ellis added that a part-specific automatic inspection program can be written in the Maxos system’s software based on a 3-D model of the part, manually entered parameters or a teaching routine using a new component. 

The system sensor has two axes of motion, which, combined with the three axes typical of a CMM and two optional turntables, provides a total of seven axes of movement. The axes are integrated and can be moved simultaneously, compared to the individual movement of axes in a traditional CMM, Kersen noted.

Maxos is one part of the overall maintenance and repair effort at Delta TechOps. “This machine is not going to do every inspection for all 3,000 blades and vanes in an engine,” he said. “We do have GO/NO-GO gauges, for example, that are quick, easy and accurate and are not looking to replace them.” 

The speed and accuracy of Maxos is used for “some measurements we couldn’t do before and that required us to send the part to an outside party for processing,” he said. Overhauling those parts in-house saves the shop time and money.