title = "Honing and Lapping for Fine Surface Finish"

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Honing and lapping are precision finishing processes that achieve surface finishes and dimensional accuracies beyond what grinding alone can produce. Both processes use abrasives but work differently and are applied for different purposes at Ridgecrest Metalworks.

Honing is used primarily to finish the internal diameter of bores—cylinder bores in engine blocks, hydraulic cylinders, and precision bushings are typical applications. A honing tool consists of abrasive stones mounted on an expanding mandrel. The tool rotates and simultaneously reciprocates axially inside the bore. This combined motion produces a characteristic crosshatch pattern of fine scratches on the bore surface, which is important for oil retention in engine cylinders. The crosshatch angle (typically 30–60 degrees from the bore axis) is controlled by the ratio of rotational speed to stroke speed.

Honing stones are fed radially outward by a cone mechanism at a controlled force or displacement rate as material is removed. This self-correcting nature means honing improves the cylindricity (straightness and roundness) of a bore as well as the size. Honing removes only small amounts of material (0.001–0.010 inch on diameter), so the bore must be pre-machined (drilled and bored or reamed) to near-final size. Honing achieves bore tolerances of ±0.0001–0.0005 inch and surface finishes of Ra 4–32 microinch. Plateau honing—a two-step process using coarse grit followed by very fine grit—creates a flat-topped crosshatch ideal for fast break-in of engine cylinders.

Lapping uses a soft lapping plate (cast iron, copper, or hard lead) and a free abrasive slurry (aluminum oxide, silicon carbide, or diamond lapping compound) to remove material from a flat or spherical surface by a controlled rubbing action. Lapping is extremely gentle and slow—used to achieve surface finishes below Ra 4 microinch (4 micro-inch is the threshold of lapping; Ra 1 microinch and flatness within light bands are achievable). Gauge blocks, valve seats, flat-sealing surfaces, and optical components are lapped.

Superfinishing (microfinishing) uses oscillating abrasive stones on external cylindrical surfaces to remove surface peaks and create very low Ra surfaces for bearing and seal contact areas.
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