Precision Ground
Valve Spools

h5 tolerance is standard for hydraulic and pneumatic valve spools — this gives a diametral clearance of 3–8 µm with an H5 or H6 valve bore. For servo and proportional valve spools: h4 tolerance — diametral clearance 1–3 µm. HXC achieves both by cylindrical grinding between centres after turning, verified by CMM.

Cylindricity ≤ 0.002 mm is achieved by cylindrical grinding between centres: the spool is mounted between precision centres (not chuck-gripped) to eliminate clamping distortion; multiple fine dressing passes prepare the wheel; and multiple spark-out passes are made at the end of grinding until the sparking ceases. The entire spool OD is measured at 5 cross-sections and 4 angular positions per section by CMM.

HXC machines all standard groove types: control grooves (rectangular, triangular, notched); undercut grooves for O-ring or Teflon seals; internal porting channels; and flow metering notches. All groove dimensions (width, depth, position, corner radii) are machined in a single turning setup referenced to the OD centreline and verified by CMM.

Yes — matched pairs are HXC's standard approach for servo and proportional valves. The valve body bore is finished first and measured. The spool is then ground to fit the specific bore — achieving the specified diametral clearance (typically 1–3 µm) regardless of tolerance stack. CMM reports for both body and spool are provided as a matched set.

Gas nitriding is the preferred surface treatment for steel hydraulic valve spools: 0.1–0.2 mm nitride case depth, surface hardness HV 700–900, very low distortion (< 0.005 mm OD change). Post-nitriding cylindrical grinding restores OD to final tolerance. Nitrided spools have 3–5× longer wear life than through-hardened spools.

Ra 0.2 µm on all spool land surfaces — achieved by grinding with a fine-dressed vitrified wheel (G40 or finer) and verified by contact profilometer. For servo valves requiring minimum stick-slip: Ra 0.1 µm with a final lapping or superfinishing pass.

Yes — for systems where multiple identical spools are used, HXC sorts finished spools into ±0.001 mm diameter bins and supplies matched sets. This allows the customer to select spools that are interchangeable within the tightest clearance tolerance class.

Prototype: 6–9 days. Batch (50–500 pcs): 10–16 days. Servo matched pairs: add 2–3 days for matching and documentation.