Nagel Z 20

Both honing and internal grinding improve bore diameter and finish, but by different mechanisms and to different quality levels. Internal grinding uses a high-speed rotating abrasive wheel to remove stock from the bore — it achieves good diameter accuracy and surface finish (Ra 0.1–0.4 µm) but is limited in bore roundness improvement because the wheel only contacts the bore at one point at a time. Honing uses multiple abrasive stones in simultaneous radial contact around the bore circumference — this multi-contact geometry averages out high spots and low spots, correcting roundness and cylindricity errors in the bore that grinding cannot correct. Honing also produces the crosshatch surface texture (from the combined rotation + stroke motion) that is essential for engine cylinder oil retention. For engine bores and hydraulic cylinders, honing is the standard finishing process; grinding is used for production stock removal before honing.

Plateau honing is a two-stage honing strategy that first hones the bore with a coarser abrasive to create the crosshatch pattern, then uses a fine abrasive (plateau stone) to remove the peaks of the crosshatch profile while leaving the valleys intact. This produces a 'plateau' surface — flat peaks for piston ring seating contact, with valleys for oil retention. The Rpk (reduced peak height), Rk (core roughness), and Rvk (reduced valley depth) parameters of the Abbott-Firestone curve describe the plateau surface mathematically. Engine OEMs specify plateau honing parameters to optimize piston ring break-in time and reduce oil consumption. The Nagel Z 20 programs plateau honing as a multi-stage automatic cycle: rough hone → intermediate → plateau finish, with in-process gauging maintaining bore diameter control throughout.

Air gauging (pneumatic gauging) is the standard bore measurement method in production honing. Compressed air is delivered through small orifices in the honing head that face the bore wall — as the honing head approaches target diameter and the clearance between the orifice and bore wall narrows, back pressure in the air circuit increases. This back pressure is calibrated to bore diameter, enabling real-time diameter measurement during the honing cycle without contact sensors touching the bore. The Z 20's CNC receives the air gauge signal and uses it to control honing stone expansion — when the target diameter back-pressure is reached, the CNC stops stone expansion and executes the programmed number of finish strokes, then retracts. Air gauging accuracy in production honing is typically ±0.002 mm or better.

Honing stones for the Z 20 are selected based on bore material, stock removal requirement, and target surface finish. For cast iron engine bores (cylinder blocks), silicon carbide (SiC) abrasive stones are standard for roughing, with aluminum oxide or CBN for finishing. For hardened steel bores (hydraulic cylinders, precision housings), CBN honing stones provide long life and consistent stock removal. For aluminum bores (modern engine blocks), silicon carbide with specific bond formulations suited to non-ferrous cutting are used. Nagel supplies tooling through its tooling division — mandrels, stones, and adapters are bore-size-specific and must be specified for each application. Stone grit progression (rough to finish) is part of the application engineering — Nagel applications engineers recommend the stone progression for each new bore honing application.

The crosshatch angle — the angle of the abrasive scratch lines relative to the bore axis — is determined by the ratio of stroking speed to rotational speed during honing. A 45° crosshatch (typical for gasoline engine passenger car cylinders) balances oil film retention and ring-to-bore seal. Diesel engines and heavy-duty applications often use shallower crosshatch angles (30°) to retain more oil for higher combustion pressures. High-performance and motorsport applications sometimes use steeper crosshatch (60°) to minimize friction. The Nagel Z 20 programs crosshatch angle by adjusting the stroke-to-rotation speed ratio in the CNC parameters — achieving a target crosshatch angle is part of the production process setup validated by measuring the actual crosshatch angle on a finished bore using a profilometer.