Sunnen SL-5000

Engine cylinder bores use a specific crosshatch pattern (Ra 0.4–0.8 µm) optimized for piston ring seating and oil retention during ring break-in — the rough crosshatch valleys hold oil and the peaks wear during break-in until rings are fully seated. Hydraulic cylinder bores use dynamic piston seals (O-rings, U-cups, lip seals) rather than piston rings, and these seals require a smoother surface. Too rough (Ra > 1.0 µm) causes rapid seal wear from abrasion by surface peaks. Too smooth (Ra < 0.1 µm) causes inadequate hydraulic fluid film at the seal interface, also causing wear (adhesive wear). Ra 0.2–0.8 µm with low Rpk (peak height) is the hydraulic cylinder sweet spot — smooth enough to not abrade seals, rough enough to maintain the fluid film.

Long hydraulic cylinder bores are typically gun-drilled and then boring-bar finished. Boring bar deflection — the boring bar bends slightly under cutting force, especially at the far end of a long deep bore — introduces taper: the bore entry (near the chuck) is typically at target diameter while the bore exit (far end) is slightly undersize or has a slight diameter variation along its length. The SL-5000's multi-point in-process gauging detects this taper profile by measuring diameter at multiple positions along the bore. The CNC then adjusts the stone expansion profile along the bore length — expanding the stones more in the tapered undersize regions and maintaining standard expansion elsewhere — to achieve consistent diameter and < 0.01 mm cylindricity over the full bore length.

Hard chrome-plated hydraulic cylinder bores (typically 0.1–0.5 mm chrome on bore ID) require honing after plating to reach final bore size and surface finish. Chrome plating deposits at uneven thickness around the bore circumference and along its length, making post-plate honing mandatory for dimensional and finish compliance. Diamond honing stones are required for chrome (aluminum oxide and silicon carbide stones load up on chrome). The SL-5000 handles chrome-plated bore honing with diamond superabrasive stone tooling — Sunnen supplies chrome-specific diamond mandrel assemblies. Post-plate honing to Ra 0.2–0.4 µm is typical for chrome hydraulic cylinders used in agricultural and construction equipment.

For aerospace, subsea, and safety-critical hydraulic systems, bore quality documentation typically must include: bore diameter at multiple measurement points (per drawing tolerance), bore cylindricity (taper and roundness measured per ISO 1101), surface finish (Ra, Rz, Rpk per ISO 4287/4288), and optionally a surface texture Abbott-Firestone curve analysis. The SL-5000's in-process gauging and SPC output system captures bore diameter and cylindricity data on every bore. Final surface finish measurement (Ra, Rz) is typically performed on a post-process profilometer and associated with the bore's dimensional data record. Sunnen can configure the SL-5000 data output for aerospace first article documentation and AS9100/NADCAP bore quality requirements.

Cycle time for hydraulic cylinder bore honing on the SL-5000 depends on bore diameter, bore length, and the amount of material to be removed. For a typical industrial cylinder (150 mm diameter, 2,000 mm long, removing 0.05–0.10 mm from a rough-bored surface), typical cycle time is 20–45 minutes including rough honing, finish honing, and plateau honing steps. Very large bores (300–500 mm diameter) with more material removal or tighter cylindricity correction requirements may require 60–120 minutes per bore. Aerospace or certification-required bores with mandatory intermediate gauging stops add time. Long-bore honing is inherently a slower process than engine bore honing — the value is surface quality and geometry control over the full bore length.