Invented more than 70 years ago to de-glaze cylinder bores in early automobile engines, conventional honing today is meeting the most advanced aerospace requirements for ultra-precise machined parts. Computer controls, new tool designs, new abrasives, integrated air-gage part measurement, and servo-driven tool feed systems and spindles enable new honing machines to produce part bores with 0.000010-inch accuracy and crosshatched surface finishes targeted to a very narrow range. Honing can even correct bore geometry distortion from upstream machining processes, welding or heat treating. And while most users want a uniform round shape in a bore, honing can also impart a shape, such as a barrel, in a selected region of a bore if desired.
This new generation of machines is 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, less hysteresis, noise and vibration. Flight control systems are a good example. The ultra-high performance hydraulic valves in these systems are about 125 to 250 mm (5 to 10 inches) long, with a bore of about 12 mm (0.5 inch) diameter, including numerous lands and crossholes. Honing is used to produce bore diameter tolerances of 0.00025 mm to 0.0005 mm (0.000010 to 0.000020 inches). In fact, some parts are produced to tolerances beyond the measuring capability of many gages. In addition to sizing and finishing the bore, honing perfects the roundness, straightness and finish of the bore. These valves operate with a clearance of 0.005 mm (0.0002 inch) 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. With the resolution on the tool-feed systems of today's machines, process variability is small and process capability (Cpk) is quite high.