Guide
What surface finish and defect limits should a liquid-cooling quick-disconnect sealing surface meet?
A quick-disconnect sealing surface is the machined area of a coupling plug or socket that a seal, a valve or a mating part closes against, and the OCP UQD Specification v2.0.0 (2026) marks it “machine to 32Ra MAX” without printing a unit. Short answer: write the roughness on the drawing with its unit spelled out and confirmed by the customer, adopt the visual limits of the OCP UQD Specification v2.0.0 (2026) (0.3 mm² or 1.0 mm² for a surface imperfection, depending on the zone, plus no debris and no corrosion) by reference, and turn them into an incoming-inspection checklist that says what is checked on every part.
By Saola Manufacturing engineering team · Published · Updated
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What roughness does the UQD specification put on the sealing surface?
The notes under Tables 1 to 4 of the OCP UQD Specification v2.0.0 mark the sealing and interoperability surface of the UQD and UQDB socket and plug “machine to 32Ra MAX”. It is a maximum only, with no lower limit, no stated lay direction and no measurement method.
The same notes say that seal glands are located on this surface and that the gland geometry is defined by the coupling supplier. The roughness limit is therefore a common floor, and the gland shape and the seal choice sit on top of it in the supplier’s drawing. The surface is called an interoperability surface because suppliers are expected to cross-verify their parts against at least three other suppliers (Section 10 of the specification), so a part that seals against its own seal but not a neighbor’s is not a conforming surface.
The visual table separately treats machining marks kindly: base material and as-manufactured characteristics such as tool marks are acceptable provided the prevailing Ra requirement is met. In other words the roughness number, not the look of the tool marks, is what accepts or rejects the finish.
What unit does 32Ra use, and why confirm it before it goes on a drawing?
The specification prints “32Ra MAX” without a unit. The working reading is 32 microinches, about 0.8 micrometers, but the customer should confirm it and every drawing note should print the unit.
The conversion is derived, not quoted: 32 microinches × 0.0254 micrometers per microinch = 0.81 micrometers. The reading rests on convention, since Ra in microinches is the customary unit on drawings from US-based sources and the number 32 is a familiar value in that system, and on engineering judgment, since 32 micrometers would be far too coarse for a sealing face. Neither is a statement in the specification, and the unit is a factor of 1000 apart in the two readings, which is exactly the kind of error that a drawing note should rule out.
Print both units on the drawing, state that the value is a maximum, and have the buyer confirm the unit with the coupling designer before the first quote. The Ra on machined parts guide and the Ra vs Rz guide cover how to write the callout itself.
What surface defects does the specification allow?
Table 10 of the OCP UQD Specification v2.0.0 (2026) recommends that surface imperfections stay within 0.3 mm² in zones 1, 2, 4a, 4b and 7 and within 1.0 mm² in zones 3, 4, 5 and 6. It also recommends no residual debris, no foreign material, rust or corrosion, and no thread damage that prevents installation.
These criteria sit in Section 15.2, an appendix on quality guidelines, and the specification states that information in its appendices is for reference only and non-normative. They bind a supplier only when the drawing or purchase order adopts them, which is why the note must say so. The specification also says that where no guidance is given, the criterion is left to the coupling supplier’s discretion.
Defect area is easier to judge as a size. Treated as a circle, the 0.3 mm² limit in the specification is about 0.62 mm across (derived from d = √(4A/π)). Treated as a rectangle, a scratch reaches that same specification limit at 3 mm long when it is 0.1 mm wide (derived from area = length × width). Figure 16 of the specification divides plugs and sockets into numbered zones, and the drawing should reproduce or reference that figure so that “zone 3” means the same thing to the shop and to the buyer.
| Attribute | Zones (Figure 16) | Recommended criterion |
|---|---|---|
| Surface imperfections: minor notch, impression or deformation | 1, 2, 4a, 4b and 7 | Every defect up to 0.3 mm² (a circle of about 0.62 mm diameter, derived) |
| Surface imperfections: same description | 3, 4, 5 and 6 | Every defect up to 1.0 mm² (a circle of about 1.13 mm diameter, derived) |
| Manufacturing artifacts: tool marks | All | Acceptable if the prevailing surface (Ra) requirement is met |
| Manufacturing artifacts: shavings, burrs, remnants | All | No residual debris from manufacturing, loose or attached |
| Compromised material: foreign particulate, oxidation | All | No foreign material, rust or other evidence of corrosion; an intentional treatment such as passivation may leave a yellowing tint |
| Thread profile: damage that prevents installation | 1 and 7 | Evaluate with the applicable thread or plug gage, or the mating port, using the termination thread class |
How is the surface inspected?
The specification’s general guidance is to rotate the part through one full 360-degree turn under magnification, under typical office lighting, after removing loose particles with filtered compressed air. A TAPPI/ANSI T 564 chart may be used to estimate defect size.
The same section gives the lighting as about 700 to 1200 lux, recommends magnification without naming a power, and gives no sampling plan for visual checks. The magnification, the sample size and the stage at which the check happens (after final clean, before packing) therefore have to be agreed in the purchase order. Where parts have cross holes or recessed seats, add the viewing aid, such as a borescope, to the agreement, because a 360-degree rotation does not see inside a bore.
Roughness itself is a separate measurement. The specification gives the limit and no instrument or method, so the report should state the unit, the cutoff and the trace position. A narrow land may be too short for a standard trace length, which is a conversation to have before the first article; the surface roughness measurement guide explains the constraints.
What should the drawing note and incoming inspection checklist say?
Put the roughness on the sealing surface with its unit, adopt the Table 10 limits by reference to the zone figure, and list the checks in the order they are done. Say which checks are made on every part and which on a sample.
The table below turns the specification into drawing and inspection wording. For a part running at ten to twenty thousand a year, a practical split is a visual screen of the sealing faces for debris and damage on every part, with roughness and dimensions checked on a sample; that split is a suggestion for the buyer to accept or change, not something the specification says.
Cleanliness belongs on the same drawing, since the visual table rejects debris whether loose or attached. The technical cleanliness guide and the incoming inspection guide cover the broader checks, and this checklist is the coupling-specific layer on top.
| Element | Suggested wording or check | Where it comes from |
|---|---|---|
| Roughness note | Ra value with unit printed twice, for example micrometers and microinches, on every sealing or interoperability surface, stated as a maximum | Notes 2 of Tables 1 to 4; unit confirmed by the customer |
| Defect limits | Adopt the Table 10 limits by reference, with the zone figure attached to the drawing | Table 10 and Figure 16 |
| Debris and corrosion | No residual debris, loose or attached; no rust or corrosion on any surface | Table 10 |
| Passivation tint | State that a yellowing tint after passivation is acceptable, if the customer agrees | Table 10 footnote |
| Pre-inspection clean | Blow off loose particles with filtered compressed air before looking | Section 15.2.1 |
| Visual method | Rotate the part through one full turn at the agreed magnification under normal office lighting; size defects against a chart | Section 15.2.1 |
| Roughness check | Stylus measurement on the sealing surface, with the cutoff and the unit written on the report | The specification gives the limit but no method |
| Packaging | Protective caps on interfacing surfaces and terminations | Section 11 |
What can a CNC shop control, and what is left to the coupling maker?
A CNC shop controls cutting, deburring, cleaning and handling of the sealing surfaces; leak, force, flow and cycling performance are verified on assembled couplings by the coupling maker using ISO 18869 test methods. The finish and visual limits are the part of the specification a machined-part drawing can carry.
Section 9 of the specification has the coupling supplier run its verification tests with at least five samples per test, following ISO 18869, and report them on a template. The ISO 18869:2017 summary describes that standard as covering methods for testing and evaluating the performance of quick-action couplings and notes that a revision is in development. None of those tests is run on a bare machined body, so a machined-part supplier is accountable for the finish, the edge condition, the cleanliness and the dimensions on the drawing, not for coupling performance.
At Saola, CNC turning and milling of plug and socket bodies run at the lead plant (see CNC turning and CNC milling), and cleaning, passivation or plating steps are arranged through partner plants that Saola manages. Whichever route a step takes, the quote names the production site, and the inspection checklist above applies at the point where the parts are released to you. The specification also recommends protective caps on interfacing surfaces and terminations during shipping, so put capped packaging on the drawing.
Frequently asked
Is 32Ra in microinches or micrometers?
The specification does not say. The working reading is 32 microinches, about 0.8 micrometers, but the unit should be confirmed with the customer and printed on the drawing in both units.
Are the Table 10 visual limits mandatory?
They are in a non-normative appendix, so they bind a supplier only when the drawing or purchase order adopts them. State on the drawing that they apply, with the zone figure attached.
Do visible tool marks fail the visual check?
Not by themselves. The table accepts as-manufactured characteristics such as tool marks provided the prevailing Ra requirement is met.
Is the yellow tint after passivation a defect?
The footnote to the visual table says an intentional treatment such as passivation may leave a yellowing tint on stainless steel. Write on the drawing that the tint is acceptable so an inspector does not reject it as oxidation.
Does the specification give a sampling plan for visual inspection?
No. Its sample size of five applies to verification tests run on couplings per ISO 18869, not to visual inspection of machined parts, so agree the plan in the purchase order.
Sources
- Universal Quick Disconnect (UQD) Specification, Version 2.0.0, Open Compute Project, 2026
- ISO 18869:2017, Hydraulic fluid power: Test methods for couplings actuated with or without tools (summary page), Genorma, ISO publication date July 2017