The ten-millionths solution for bore accuracy

  • 27-Jan-2012 09:37 EST

Piston pumps used in hydraulic flight control systems have clearances of 0.005 mm between moving parts, requiring ultraprecise bore size, roundness, straightness, etc. The photo shows piston pump bodies in a multispindle Sunnen honing machine equipped with air gauging stations between spindles.

There is a new technology that enables makers of products with bored holes to achieve accuracies measured in millionths—even "fix" bore geometries. It also enables high process capabilities (Cpk), all in automated, high-throughput production.

The technology isn't really new, per se. Conventional honing has been hiding in plain sight for about 70 years, quietly and continuously being improved, refined, and automated to make it accessible and capable for 21st century part requirements.

It is often misunderstood as a messy, manual process, but today's honing is increasingly automated. A new generation of machines has become the "ten-millionths" solution for bore machining accuracy, enabling aerospace suppliers to meet a host of challenges for parts that include ram-air turbine components, fuse pins, turbine hubs/discs, landing gear, hydraulic valve sleeves, accumulators, and pumps.

The aerospace industry is constantly tightening the requirements for parts to achieve lighter weight and, particularly, greater performance from end products—higher power densities, more precise control, tighter sealing, and less hysteresis, noise, and vibration.

Flight control systems are a good example. The ultrahigh-performance hydraulic valves in these systems have bore diameter tolerances of 0.00025 to 0.0005 mm. The valves are generally about 125 to 250 mm long, with a bore of about 12 mm diameter, including numerous lands and crossholes. In addition to sizing and finishing the bore, honing perfects the roundness, straightness, and finish of the bore.

Some parts are produced to tolerances beyond the measuring capability of many gauges. These valves operate with a clearance of 0.005 mm or less between the valve body and match-ground plunger. The same holds true for the moving parts in the pumps that power these systems. As operating clearances between moving parts shrink, honing can tightly control the bore's surface finish to retain a lubricating film of oil.

Similarly, the bores of hydraulic accumulators are honed to eliminate any surface flaws that could propagate into cracks under stress. The bores of fuse pins, used at the attachment points of engine pylons, are honed to precise size and finish tolerances to ensure they shear under the correct level of stress. Multiple components in ram-air turbines are honed, as are the bolt holes in turbine hubs and discs. The bores of gears used by Airbus, Boeing, Cessna, and NASA are honed for similar reasons.

Overlaying the need for a bore's precision size and geometry is the requirement for high process capability, an area where servo-controlled honing shines. When holes produced satisfactorily on lathes suddenly have to meet a process capability of 1.67 or 2.0 Cpk turning operations may fall short. That kind of capability requires a process that is easy to "dial in" with high precision and very stable once it's established. A computer-controlled hone can easily get within 0.00025 mm of a specified size, and with the resolution on the tool-feed systems of today's machines the variability is small.

These new honing machines use a patented, closed-loop tool feed mechanism to automatically control hole size to accuracies of 0.00025 mm using air-gage probes adjacent to each spindle. These systems perform with minimal variability—the key to high process capability—and no operator intervention while producing thousands of parts. They optimize a bore's shape, size, and surface, all in one setup, with process capability of 1.67 Cpk or greater and downloadable SPC data.

Advanced computer-controlled honing systems can correct a multitude of errors in bore geometry, too, such as barrel, taper, and centerline bow, while optimizing the bore surface with a crosshatch finish to improve lubrication. Windows-based controls embed all the "art of honing" that has long been associated with the manual process. Even the crosshatch angle can now be dialed in at the control and then held constant from top to bottom in the bore thanks to servo control of spindle rotation and stroking motion. An air-gauge-equipped machine can sense a misshaped bore, such as barreled or tapered, and the machine will automatically correct the part. Integrated with other servo accessories—such as rotary tables, linear feeds, pick-and-place, or robot-handling systems—a single machine can now process 15 million or more parts per year, running unattended.

Honing is also unique in its ability to consistently produce a specific surface finish with a selectable crosshatch pattern. Makers of gears, hydraulic valves, pumps, and automotive parts use a wide range of surface finish parameters (e.g., Rpk, Rvk) to tweak the performance of their products. Honing before hobbing improves a gear blank's bore accuracy, making the hobbing operation more accurate and resulting in quieter, longer-lasting drives. The opposing helical patterns of the crosshatch also prevent gears from "walking" and binding on a shaft, while controlling the lubricating film of oil between mating surfaces. "Single point," unidirectional, rotary machining processes leave a faint "threaded" finish, which can lead to lubricating films being pushed out of the bore. Valve manufacturers have found that electrically actuated hydraulic valves with a conventionally honed bore operate more reliably at reduced voltages with lower hysteresis.

Dennis Westhoff, Manager, Global Business Development, Sunnen Products Co., wrote this article for Aerospace Engineering.

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