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Deep boring for happy landings.

Deep boring for happy landings

When you're reconditioning the cavernous bores of aircraft landing gear, you just don't take chances. The workpieces you're re-building can cost $300,000 each, and the aspect ratios run up to 10 to 1. More important, lives depend on these "legs" as they cushion the touch-down impact for jumbo jets.

That's why Cleveland Pneumatics' Product Service Div (CPPSD) is so happy with their method for machining the bores of the giant legs brought in for scheduled overhaul. The Miami-based FAA repair station rebuilds landing gears for virtually all commercial aircraft.

By single-point boring rather than internal grinding or hand work, they've tripled the production rate and saved about $30,000/yr in production cost. As a result, they recovered their $40,000 retooling charge within the first 18 months. The bulk of the investment went for four huge Sandvik TNS antivibration boring bars. And the bulk of the economic return stemmed from the bars' ability to produce as-machined finishes in the 40 to 60 microinch range.

Typical Job?

A typical job is to bore out the 12"-diameter by 70"-deep ID of an L1011 landing gear made of 4330 alloy steel. There's only 0.030" on diameter of material available to remove. Worse yet, the surface may have fine cracks, a taper caused by wear, and uneven hardness resulting from Martensitic formations caused by friction heat. "Every job is a little different," says Rick Rosenik, manufacturing engineer. "That's what separates overhaul work from new production."

After initial machining of the bore to remove defects, sulfimate nickel is put back by electroplating, and the bore is machined to size. A final 0.008" electroplated chrome layer brings the part back to original specifications. Tolerance on the final bore is +0.002"/-0.000" on diameter and straightness. The finished piece is inspected for dimension and surface finish.

Depending on the workpiece at hand, the foreman may select a 3", 4", 5", or 7" Sandvik antivibration boring bar from CPPSD's tooling inventory. "You might call a $40,000 investment in four boring bars pretty steep," Mr Rosenik says. "But they're a far-cry more affordable than the all-carbide boring bars, which can run $40,000 to $90,000 apiece."

Find a better way

Mr Rosenik's search for an alternative to grinding or hand work started in 1987, mainly to avoid the negatives of grinding. "We knew boring would be faster and less hazardous to these $300,000 workpieces."

Specifically, he wanted to avoid the potential for grinding cracks and over-tempering or under-tempering caused by grinding heat. "There is simply less risk of process-induced failures with single-point machining," he explains. He adds that overhaul procedures on some landing gears specifically prohibited grinding.

"Because of the massive size, difficulty, and stakes involved, we knew we were into the class of hole requiring antivibration boring bars," Mr Rosenik says. "The question was, did such a bar exist that was up to the task both technically and economically."

Carbide ruled out by ROI

Analysis of all-carbide boring bars revealed that they could work, but wouldn't earn back their cost for at least five years. By contrast, the Sandvik TNS "tuned" antivibration boring bars appeared to offer the same rigidity and range, but at one-fourth the cost and with a 24-month ROI.

"Our ROI analysis hinged mainly on anticipated production cost savings," he added. "However, we also factored in the reduced risk of ruining or excessively reworking a $300,000 workpiece."

Of course, CPPSD doesn't ruin landing gears that arrive at their receiving dock. However, the specter of doing so has always injected a certain caution into establishing material-removal rates, whether it be by grinding or boring. Accordingly, the 24-month ROI projection with the boring arrangement was based on very conservative assumed machining rates.

Projections prove out

As it turns out, experience since the changeover has been better than expected. "By and large, we've cut machining time for the steel by two-thirds and for the nickel by one-third," Mr Rosenik states. Both operations are done with the same small-radius tool and water-soluble coolant. Only the feeds and speeds change with material being cut. "The electroplated sulfimate nickel is just as tough to machine as cast or forged nickel machining stock," says Mr Rosenik. "It's gummy and smeary, yet as abrasive to carbide as a honing stone. And it tends to stick to the cutting edge. So naturally we have to change our machine settings for the nickel plating."

PHOTO : Boring cut begins, with workpiece (in rear) supported in steadyrest. Because of builtin vibration damping in the TNS boring bar, the finished part will have 40 to 60 micro surface finish at depth-to-diameter ratios beyond 6 to 1.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:aircraft landing gear machining
Publication:Tooling & Production
Date:Sep 1, 1990
Words:785
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