Quick answer: Ra describes average profile roughness, not a guarantee of sealing, wear life or appearance. On a CNC-turned part, Ra 0.8–1.6 μm can be a useful quotation starting point for a finish pass. Ra 0.4 μm needs a more controlled setup; Ra 0.2 μm or 0.05 μm should be treated as a special requirement and reviewed with the material, geometry, finishing route and measurement method. No single range applies to every part.
Need a specified Ra on a turned component?
Send a 3D CAD model, a 2D drawing marking the critical surfaces, material and condition, quantity, target Ra, dimensional tolerances and inspection requirements. Upload your files for a project-specific CNC turning quote.
What Ra Means for CNC Turning
Ra is the arithmetic mean height of a roughness profile, usually stated in micrometres (μm). A lower value indicates a smoother measured profile, but it does not describe every feature of the surface. Two surfaces with the same Ra can behave differently if their peak shape, waviness, lay or defects differ. For seal or bearing interfaces, the drawing may therefore need other parameters as well as Ra.
Specify the requirement on the functional surface, not across an entire part by default. A cosmetic outer diameter, a bearing journal and an O-ring groove may need different acceptance criteria. The ISO 21920-2:2021 surface-texture standard defines profile parameters; the customer drawing and agreed inspection plan govern acceptance.
Indicative Ra Ranges and Process Review
The ranges below are discussion points for an RFQ, not Lauens production guarantees or universal design rules. Material, diameter, interrupted cuts, rigidity, tool condition and measurement settings can change the result.
| Requisitos de dibujo | Typical route to evaluate | Question before quotation |
|---|---|---|
| Ra 3.2–6.3 μm | General turning where function permits | Is the finish cosmetic or functional? |
| Ra 0.8–1.6 μm | Controlled finish-turning pass | Which diameters and materials need this limit? |
| Ra 0.4 μm | Fine turning, with a trial cut or secondary finish if needed | Can the part be held rigidly and measured accessibly? |
| Ra 0.2 μm | Process validation; grinding, honing or lapping may be needed | What are the roundness, size and inspection limits? |
| Ra 0.05 μm | Specialist superfinishing or polishing route, subject to feasibility review | Is this value essential, and how will it be verified? |
Do not select a lower Ra only because it looks “higher quality.” Extra passes, secondary processing and inspection can materially affect price and lead time. If the mating seal or bearing supplier specifies a texture range, include that specification in the RFQ.
Why the Same Lathe Can Produce Different Results
Feed and tool geometry. A lower feed and suitable tool nose radius can reduce the theoretical turning marks. The simplified geometric estimate Ra ≈ f²/(32r) uses feed per revolution f and tool nose radius r in millimetres; convert the result from mm to μm. For f = 0.10 mm/rev and r = 0.8 mm, the idealized result is approximately 0.39 μm. This is a planning estimate, not a measured finish or a machining promise.
Material and tool condition. Built-up edge, tool wear, material condition and chip control can dominate the actual profile. An alloy name by itself is not enough to promise a Ra value. Specify the grade and condition, and allow the machinist to confirm the tooling and finishing route.
Rigidity and geometry. Slender shafts, thin walls, long overhangs and interrupted cuts can chatter or deflect. A finishing operation may improve roughness while changing size or roundness, so these requirements must be reviewed together.
Ra 0.2 μm and Ra 0.05 μm: Can Turning Alone Meet Them?
There is no reliable yes-or-no answer without a part drawing. A stable precision-turning process may achieve a very fine finish on a favourable material and geometry, but Ra 0.2 μm should trigger a feasibility and inspection review. Ra 0.05 μm generally calls for a specialist process chain rather than a standard CNC turning promise. Grinding, honing, lapping or superfinishing may be considered according to the surface function and dimensional tolerances.
Before quoting a low Ra target, identify whether the requirement applies to one journal, every external diameter, a bore, or a sealing face. Include any specified Rz, waviness, roundness, lay direction and inspection report requirements. See our CNC turning service for the broader machining workflow.
How to Specify and Inspect Surface Texture
ISO 2768-1 concerns general tolerances for linear and angular dimensions; it does not supply a default Ra value. Put the surface-texture callout directly on the drawing where the function requires it. ISO 21920-1:2021 addresses how profile surface texture is indicated on technical drawings, while ISO 21920-3:2021 covers specification operators. If your contract names an older standard, confirm the agreed interpretation rather than silently substituting a new one.
A stylus profilometer is a common way to assess a profile roughness value. The measurement location, direction, filtering, evaluation length and number of traces can affect the result. A visual comparator can help shop-floor screening but should not replace a specified measurement report for a critical Ra limit. Agree the acceptance method before production.
RFQ Checklist for Turned Parts With an Ra Requirement
- 3D CAD model (STEP or IGES) and a 2D drawing with the Ra callout on each critical surface
- Material grade and condition, annual and batch quantities, and target delivery date
- Dimensional tolerances, roundness or concentricity requirements tied to the finished surface
- Any seal, bearing or mating-part specification that drives the finish
- Required inspection standard, trace location, report and material documentation
- Whether secondary grinding, honing, lapping or polishing is acceptable
Get a manufacturability review before fixing your process route.
Lauens can review the drawing, identify questions about finish and tolerances, and confirm an appropriate quotation scope. Request a CNC turned-part quote.
Preguntas frecuentes
Is Ra the same as Rz?
No. They describe different aspects of a measured roughness profile. A fixed Rz-to-Ra conversion is not reliable for every surface, so do not replace one drawing requirement with the other without engineering approval.
Does a tighter Ra automatically improve a seal?
No. Seal performance also depends on the seal material and design, contact pressure, lay, waviness and mating geometry. Use the seal supplier’s specification and identify the sealing surface on the drawing.
Can I specify one Ra value for an entire turned part?
You can, but it may add cost without improving function. Mark the surfaces that truly need the tighter value and leave non-critical surfaces to an appropriate general finish.
What should I send for a fast quote?
Send the CAD model, marked drawing, material condition, quantities and target delivery date. Flag any Ra 0.4 μm or lower requirement and the required measurement method. Check the accepted CNC file formats, entonces submit your RFQ.
