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CMMs Help Ensure Perfect Jet Engine Parts
LK Metrology Systems CMMs

Engineers at United Airlines' Maintenance Operations Center in San Francisco know that just as the quality of the jet engine parts they assemble is crucial, so are the devices used for measuring and inspecting these parts. As such, the company invested in LK Metrology Systems Inc.'s coordinate measuring machines (CMMs) to ensure that the design, maintenance and assembly of engine components are consistent and meet specifications and that no imperfect parts are used in the construction of the engines.

 "Our engineering department determines which parts have to be measured with the CMM, which is largely driven by the tolerances that need to be met in order to achieve optimal engine performance," says Art Mehlhaff, CMM inspection coordinator and team leader. "For instance, if the clearance between the compressor casing and the rotating compressor blades is greater than the set tolerances, your engine will run hotter, which can lead to poor performance, increased friction and possible engine failure. On the other hand, if you have optimum clearance--the clearance that the manufacturer has specified--then your engine will operate at the temperature and thrust rating it was originally designed for."

 United bought its first LK CMM nearly 20 years ago, but the addition of four new machines has helped standardize the company's measuring procedures. United's metrologists use the machines--two from the G80 series, one from the G90 series and a G80 upgrade--to inspect components ranging from landing gear to food service trays. "We found it beneficial to maintain consistency in the machines that we bring into the shop for support purposes and exchanging work between machines," notes Vic Cleland, United metrologist and CMM calibrator. "Compatibility is definitely a plus for both sides in the job that we do."

 Mehlhaff says that the versatility of LK's CMMs allows United to use the machines for inspection of nearly every part that ends up in a jet engine. "Of our five CMMs, the three in the inspection department are used primarily for final inspections of engines and component parts," he explains. "The other two machines reside in the metrology lab, where they're used for inspection of incoming tooling, calibration of fixtures, and research and development."

 United's General Electric CFM 56 engine is recognized as one of the leading commercial airline engines. It features the highest safety margins of any engine, running at nearly 80° F below the maximum allowable temperature. United attributes this to CMM pre-inspections and measurements of various engine components, including cases, compressors and rotors.

 United's metrologists use the CMMs to determine whether the parts have been remanufactured to the tolerances originally specified. If a part is out-of-tolerance, inspectors can document the part's exact degree of error and present that information to the engine manufacturer. The manufacturer can then determine the cause of the problem and improve its process to meet United's specifications. The airline estimates that it saves nearly $500,000 annually and prevents a great deal of waste by using the CMMs to detect out-of-tolerance parts prior to the assembly and testing stage. In addition, CMMs have eliminated the need for most of the expensive fixturing and other individual tooling for routine measurements of a number of production engines, which saved United millions of dollars in fixturing costs and made much more shop floor space available.

 United also saves money by using the CMMs to reprofile fan blades, a crucial procedure for blades that have sustained major damage due to harsh weather and impact with birds. After the vendor that used to provide hard gage fixturing for their fan blades ceased to offer this service, United's only option other than in-house fixturing is outsourcing this process to contractors at premium costs.

 LK's CMMs have also been instrumental for United's research and development. The CMM's measurements help verify the quality of vendor-supplied parts and aid in the development and certification of in-house repair processes.

 "Our CMMs enable us to complete a multitude of projects in one run or several small runs, which saves us a significant amount of research and development time and reduces costs," says Cleland. "We can check parts for defects and correct them before they make it into production. To top it off, our labor costs and cycle times have plummeted. Parts that used to take 2-3 days to measure with special fixturing can now be inspected in just a few hours."