Call out Ra where it matters—not everywhere
Surface roughness is one of the fastest ways a drawing inflates cost without improving function. Buyers often paste a fine Ra on every face because a handbook table looked “safer.” Shops then slow feeds, add polishing or grinding steps, and fill inspection reports with measurements that never affected assembly. This article explains typical as-machined Ra ranges for milled and turned faces, when a finer finish is worth paying for, how Ra relates briefly to Rz, and what to put in inspection notes—using only standard, typical guidance, not invented certifications or customer stories.
What Ra actually measures
Ra (roughness average) is the arithmetic average of absolute deviations of the roughness profile from the mean line over a sampling length. It is the most common roughness callout on machining drawings in many markets. It does not fully describe lay, waviness, scratches or sealing behavior by itself. Two surfaces with the same Ra can perform differently if one has deep isolated valleys (sometimes better captured by Rz or other parameters).
Ra versus Rz (short version)
- Ra averages the profile; it is forgiving of occasional peaks/valleys in the average
- Rz is related to peak-to-valley height over sampling lengths (definitions vary slightly by standard edition); it is often larger than Ra for the same surface
- If your sealing, coating or fatigue requirement came from a standard that specifies Rz (or Rz ISO / Rmax legacy language), do not silently convert to a much tighter Ra without engineering review
For most general CNC buyer callouts, Ra is enough. Add Rz only when a governing standard or sealing spec requires it.
Typical as-machined ranges (guidance only)
Actual results depend on material, tool wear, rigidity, coolant and cut strategy. The bands below are typical shop expectations for discussion—not guarantees and not a substitute for a process capability study on a critical face.
Milled faces
- Rough milling / heavy stock removal: often much coarser; usually not left as a final cosmetic face
- Standard finish milling on aluminum and many steels: commonly in a moderate Ra band suitable for non-sealing structural faces (often discussed in the neighborhood of roughly Ra 1.6–3.2 µm / 63–125 µin class, with spread by setup)
- Careful finish passes with sharp tools: finer bands are achievable when specified
- Very fine milled cosmetics may still need secondary abrasive steps
Turned diameters
- Standard turning with appropriate feed: often similar moderate Ra class for non-critical ODs
- Fine feed finish turns: finer Ra without necessarily going to grind
- Bearing seats, seal journals and press fits: frequently need a specified finer Ra and controlled lay; grinding or polishing may be required beyond what turning alone should promise
After secondary operations
Bead blast, tumble and brush change both Ra and appearance. Anodize and plating follow the substrate texture. If cosmetics are contractual, approve a sample rather than chasing a single Ra number alone. Always treat handbook “typical” values as starting points. Lock contractual finishes to the drawing and the agreed inspection method.
When finer finish adds real cost
Expect cost and lead time to rise when you:
- Tighten Ra across large areas instead of only functional bands
- Require fine Ra on deep pockets, thin walls or hard-to-reach faces where tools chatter
- Demand fine finish on tough stainless or titanium without allowing realistic feeds
- Mix fine Ra with aggressive geometric tolerances that force extra skim passes
- Require grinding, lapping or polishing after CNC
- Call out fine finish on as-cast or as-forged faces that still need machining stock
Finer is not always better. Soft gasket seals sometimes prefer a specified texture band; optical and bearing surfaces need controlled fine finishes; paint and powder may hide moderate tool marks that anodize would show.
Where buyers should actually specify Ra
Prioritize callouts on:
- Seal journals and O-ring grooves (and note if a max Ra or a band is required)
- Bearing seats and precision press fits
- Faces that remain visible after anodize or powder coat when cosmetics matter
- Mating pads where friction or wear is sensitive to texture
- Surfaces called out by a customer or industry specification you must meet
Leave standard mill and turn faces without an ultra-fine blanket note when function is structural only. A general note such as “All unspecified machined surfaces: as-machined commercial finish unless noted” is clearer than scattering identical tight Ra balloons everywhere.
Drawing practices that reduce ambiguity
Use balloons only on functional faces
Point the roughness symbol at the faces that matter. Avoid a title-block Ra that is tighter than most faces need unless the part truly is a fine-finish component.
State the parameter and units
Write Ra (and units µm or µin) explicitly. If you also need Rz, say Rz. Do not assume the shop converts your habit from one market’s units to another without looking.
Lay and process notes when needed
For seal diameters, grinding lay or a ban on certain abrasive directions may matter. Note “turned finish OK” or “grind OK” if either is acceptable. If plating thickness will change the final texture, specify whether Ra applies before or after coating.
Sampling and inspection
If only a few faces are critical, say which are inspected and whether a portable profilometer trace is required versus visual comparison specimens. Over-inspecting every pocket floor adds cost.
Milled versus turned texture (what buyers see)
Milled faces show end-mill or face-mill lay; pocket floors may show stepovers. Turned diameters show circumferential lay from feed marks. Both can meet the same Ra numerically and still look different. For cosmetic aluminum parts headed to anodize, discuss whether a bead-blast or brush step is part of the finish stack—Ra alone will not capture the look you saw on a competitor’s brochure (and you should not copy competitor imagery; approve your own sample).
Relation to tolerance and machining strategy
Tight diameter tolerance plus very fine Ra on a long slender shaft may push grinding. Generous tolerance with moderate Ra often stays on the CNC cell. Thin walls chatter; chatter ruins both geometry and roughness—so finish callouts on fragile geometries should be reviewed with the process engineer early.
Hard materials and interrupted cuts also limit what “free” finish you can expect from a standard pass. If the part is hardened before final machining, say so; soft-mill then harden then grind is a different plan than finish-mill soft.
Inspection notes that help both sides
Useful PO / drawing language examples (adapt to your QMS):
- Critical seal OD: Ra X µm max, measure per agreed method, report on FAIR if required
- Non-critical faces: as-machined; no Ra report required
- After bead blast: appearance per approved sample; Ra only if still contractual
- Post-plate: confirm whether roughness applies to substrate or finished condition
Do not invent accreditation claims on the drawing. State the measurement need; let the quality plan name the equipment and sampling.
Common mistakes
- Title-block Ra 0.4 µm (or similar fine values) on every face of a large housing
- Specifying Ra without units or mixing µm and µin silently
- Converting Rz requirements to an arbitrary Ra that is far tighter than needed
- Calling fine finish on surfaces that will be fully covered by thick powder coat with no cosmetic need
- Forgetting that blast and tumble alter Ra after machining
- Asking for profilometer reports on dozens of non-functional faces
A one-page buyer rule set
1. Default: as-machined commercial finish on unspecified faces
2. Specify Ra only on seals, bearings, critical cosmetics and spec-driven faces
3. Use Rz only when a standard requires it
4. Say before/after coating when coatings are present
5. Approve a cosmetic sample when look matters more than a number
6. Align inspection effort with risk—measure what can fail the product
Next steps for an RFQ
Send the model and drawing with roughness symbols only where function demands them. Note material, coating and whether a first-article roughness report is required on listed faces. Reference quality and CNC service expectations on your RFQ form, and request a quote with process notes if grinding or polishing is implied by the callouts. Clear, minimal Ra strategy almost always yields faster machining and cleaner inspection than a blanket fine finish on every surface.