CNC Turned Stainless Steel Valve Seat Spools
Valve seat spools turned from 316L stainless bar, with sealing lands ground to 0.005 mm and 0.003 mm roundness, shoulders square to 0.01 mm, land widths held to ±0.02 mm and seat faces lapped. Cross ports are drilled and deburred from both ends so no chip is left inside the spool.
- Sealing Diameters Ground 0.005 mm
- Shoulders Square 0.01 mm
- Seat Faces Lapped
- Bore and Ports Deburred
*Send the sleeve bore with its actual measured size, not just the nominal. A spool ground to the nominal clearance will be tight in a sleeve that came in at the low limit.
Valve Seat Spool Overview
A spool is the moving part of a directional valve. Its lands seal against the sleeve bore, its shoulders set the stroke and its grooves route the flow. Clearance and roundness decide whether the valve creeps or sticks.
The spool is turned with grinding allowance on the lands, heat treated if required, then ground between centres so every land shares one axis. Seat faces are lapped last, and the cross ports are deburred before the final wash.
316L Stainless Steel Bar
316L is used for water and mild chemical valves because it resists corrosion in the fine clearance between spool and sleeve, where stagnant fluid sits. It also work-hardens during grinding, which helps the land hold its size.
17-4PH or Hard Chrome on 316L
17-4PH is chosen when the valve works at high pressure and the land has to resist scoring. Hard chrome on 316L gives a low-friction surface at lower cost, but the plating thickness must be allowed for before grinding.
For a valve that cycles millions of times, the surface treatment matters more than the base alloy; for a valve in seawater, the alloy decides.
Valve Seat Spool Specifications
Turning and grinding data for spools, with the land, shoulder and port values that decide whether the valve seals.
| Specification | Machining Data | Specification | Machining Data |
|---|---|---|---|
| Product | Valve Seat Spool / Spool Sleeve | Machining Process | CNC Turning, Grinding, Lapping |
| Material | 316L Stainless Steel Bar | Diameter Range | Ø 12 – 70 mm |
| Sealing Diameter | Ground to 0.005 mm, Roundness 0.003 mm | Surface Finish | Ra 0.2 µm on Sealing Lands |
| Shoulder Squareness | 0.01 mm to the Spool Axis | Land Width | ±0.02 mm |
| Groove Depth | ±0.02 mm | Concentricity | 0.005 mm Between Lands |
| Through Bore | Ø 4 – 20 mm, H8 | Cross Ports | Ø 2 – 8 mm, Deburred Both Ends |
| Customization | Hard Chrome, Nitriding, Metering Slots | Lead Time | 6 – 8 Days Sample, 12 – 16 Days Batch |
| Inspection | Micrometer and Air Gauge, Roundness Tester, CMM Shoulder | Packing | Individual Tube, Oil Coated, Carton |
Key Features of Our Valve Spools
Turned with grinding allowance, ground on one axis.
Lands Ground Between Centres
Every land is ground from the same centres, so the spool has one axis and slides in the sleeve without a tight spot at mid-stroke.
Shoulders Square to 0.01 mm
The shoulders that set the stroke are machined square, so the valve opens at the same position from either direction.
Cross Ports Deburred Inside
Ports are deburred from both ends with a back-chamfer tool, so no burr is left to score the sleeve or break off into the circuit.
Seat Faces Lapped
The seat face is lapped flat after grinding, which is what stops internal leakage when the valve is closed.
Spool that has to seal without creeping?
Surface Finishes for Valve Spools
A coating on a ground land changes the clearance, so the finish is agreed before the final grind size.
| Finish | Main Purpose | Finish | Main Purpose |
|---|---|---|---|
| Passivation | Removes free iron after grinding | Hard Chrome Plating | Wear resistance on the sealing lands |
| Electropolishing | Lowers Ra on the spool body | Masked Lands | Keeps the ground diameter unchanged |
| Lapping | Final seal on the seat face | Oil Coating | Corrosion protection during shipping |
*Hard chrome adds 10 to 25 µm per surface. Plated spools are ground undersize, plated, then ground to the finished land diameter and lapped.
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Turning Process for Valve Spools
The sealing lands are ground after turning, so the spool slides in its sleeve without a visible step and the valve does not creep.
01. Sleeve and Clearance Review
Check the measured sleeve bore, the required clearance and the stroke against the drawing, and confirm the port positions before turning.
02. Turn With Grinding Allowance
Turn the spool, leaving 0.15 to 0.25 mm on the sealing lands, and cut the grooves and shoulders to width in the same set-up.
03. Drill Cross Ports
Drill the cross ports on the mill or a driven tool, then deburr both ends and wash the bore before heat treatment or plating.
04. Grind the Lands
Grind between centres to the finished land diameter, checking roundness and taper on an air gauge at both ends of each land.
05. Lap Seats and Protect
Lap the seat faces, passivate, oil coat and pack each spool in its own tube so the lands do not touch during transport.
Send the sleeve bore size and the stroke, and we will quote spools with the lands ground to 0.005 mm and the seats lapped.
Valve Seat Spool FAQs
Typically 3 to 8 µm on a 20 mm spool for hydraulic duty, and 10 to 15 µm for water. The right figure depends on the fluid viscosity and the temperature range, and we machine to the value you specify.
0.003 mm, measured on a roundness tester at three sections per land. That is what keeps the spool from sticking when the valve is cold.
Yes, 10 to 25 µm of hard chrome, applied before the final grind. The land is turned undersize and ground after plating to the finished diameter.
With a back-chamfer tool from the bore side after drilling, then a wash and a borescope check. A burr left inside will score the sleeve within a few hundred cycles.
We can, if you send the sleeves or their measured bores. Matched sets are numbered and packed together so the clearance is not lost at assembly.
0.002 mm by lapping, with Ra 0.2 µm. That is the surface that controls internal leakage when the valve is closed.
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