Jet Engine Repair Flies High With CNC

CASE HISTORIES OF MANUFACTURING PROBLEM SOLVING

Engine components for Air Force One, aircraft parts riddled with bullet holes, jet engine cases ripped open by ice and other projectiles-these are just some of the repair and overhaul jobs that Component Repair Technologies Inc. (Mentor, OH) has handled in its more than 20 years in business.

CRT was founded in 1985 by Tom Wheeler and Chuck Bart as an independent turbine engine overhaul shop. Over the years, the company has expanded its facility and process capabilities to perform all repair work inhouse. Messrs. Wheeler and Bart pride themselves on their extensive capabilities that enable them to control costs, minimize turn times, and deliver quality work to a customer list that includes major OEMs, commercial air carriers, and large engine shops.

The company, which occupies a 115,000-ft2 (10,683-m2) facility, is segmented into three business units according to product category: cases (turbine sections of engines), rotating workpieces (shafts, disks, and spools), and small parts for both small and large engines. Each unit is set up in cellular configuration with its own machining, assembly, and inspection capabilities.

CRT initially depended on large manual boring machines and manual lathes for its machining. As the need for more complex machining capability became apparent, the company has moved to adopt the latest in CNC technology to improve productivity, quality, and production flexibility.

In 1999 the company expanded its capability to handle large duct replacements and flange replacements in the cases unit. "We needed the ability to contour using CNC technology," explains General Manager Andrew LaTourette.

"You can contour manually, but it's very time-consuming if you don't have the capability to do pick feeds and repetitive back and forth necessary for stock removal." For CRT, this work is especially challenging because the cases are thin-walled and made of high nickel 718 and 625 Inconel. "These are gummy and unforgiving materials," LaTourette points out.

Most of CRT's jobs don't lend themselves to pushing off a lot of stock. Instead, the company does a lot of skim-and-trim finesse machining. "When you are doing flange replacements or replacing large duct segments, you have to keep things round and parallel," LaTourette explains. "The problems crop up when you get to the finish size because the parts will have a residual stress, and that's tricky to machine and keep round-especially when you have �0.003'' [0.08-mm] tolerances on a 43'' [1.1-m] diam."

The need for the capabilities that CNC machining could provide was apparent to CRT. "In the aircraft repair business, we have to stay sharp, offer excellent quality, and really keep pushing ourselves on our process capabilities so we can compete and stay in the game," says LaTourette.

"We're both a job shop and a custom manufacturer. Even though 65% of the parts go through the same work scope, all bets are off because you never know how someone is going to treat an airplane engine. We think of everything as a one-off. Our turnarounds are tight, so the flexibility of our CNC machines is critical," he explains.

Over the last six years, CRT has invested in four CNC machines from Absolute Machine Tools Inc. (Lorain, OH): two You Ji YV-1200ATC vertical turning lathes; a Johnford VMC-1600SHD for its cases unit; and a Johnford DMC-1500H bridge mill, which was recently installed in its small parts unit.

The YV-1200ATCs turn all the angular geometry on duct and flange replacements. "The grooves are whittled out on the You Ji VTLs," says LaTourette. "You plunge the slots, cut to size on the drop, cut the shelves, and once all that cutting is finished, a right-angle head takes over. Then we machine the slots, the scallops with a straight mill, and the hole patterns. We also do the geometry on the other side of the case."

All this work could be done on manual machines with form tools, LaTourette admits, "but not economically or even competitively."

The You Ji machines also helped CRT move certain parts out of the grind room. The company formerly ground a particular hub component to tight tolerances on its ID/OD grinders. Now it single-points the part to size on the You Ji VTLs, reducing grinding time by about 12 hr. The two You Ji VTLs are set up so that only one operator is needed to run both machines.

Grouped next to the two You Ji VTLs is the Johnford VMC-1600SHD. The VMC is equipped with a 31.5'' (800-mm) precision turntable that provides a fourth axis. It also includes right-angle heads that supply a manual fifth axis. The additional axes allow CRT to do bolt circles without changing tools, and the VMC also has probing capability.

The flexibility of the Johnford VMC has allowed CRT to pursue additional duct and flange replacement work. One such job calls for the Johnford to drill 102 holes in a flange for rotation pins and tackle all the scallop work on the part.

"You can do a large flange replacement like this manually and drill all the holes, but not competitively," says LaTourette. "You really have to push the envelope today if you want to stay in business."