Surface Roughness (Ra) Quick Reference
Surface roughness Ra chart in µm and µin: what each value looks like, what it costs versus as-machined, how to achieve it, and the material each finish removes.
Surface roughness (Ra) values, appearance, cost and typical application
| Ra (µm) | Ra (µin) | Visual description | How it feels | Cost multiplier (vs as-machined) | Typical application | Achievable by |
|---|---|---|---|---|---|---|
| 0.05 | 2 | True mirror — sharp, undistorted reflection of objects | Perfectly smooth, glass-like | 4–6x | Optical mirrors, precision optical components | Multi-stage polish and lap |
| 0.1 | 4 | Very smooth, hazy reflection — objects are visible but blurred | Very smooth, barely detectable texture | 3–4x | Medical instruments, hydraulic components, mould faces | Precision polish |
| 0.2 | 8 | Semi-reflective, faint lines visible under close inspection | Smooth, very slight texture detectable with a fingernail | 2–3x | Bearings, sealing faces, precision mating surfaces | Fine polish, tumble polish |
| 0.4 | 16 | Smooth, fine directional lines visible. Low reflectivity. | Smooth, slight texture under a fingertip | 1.5–2x | General precision parts, cosmetic stainless | Hairline finish, fine polish |
| 0.8 | 32 | Smooth but machining marks visible on close inspection | Even texture, slight roughness | 1.2–1.5x | CNC finishing pass, light-duty sealing surfaces | CNC finishing pass, light bead blast |
| 1.6 | 63 | Visible tool marks. Standard machined appearance. | Clearly rough, machining marks felt | 1x | Non-critical machined faces, standard finish | As-machined |
| 3.2 | 125 | Rougher, distinct machining lines. Common on roughing passes. | Rough, clearly felt | Included | Non-critical surfaces, hidden features, pre-coating | As-machined (roughing) |
| 6.3 | 250 | Clearly rough, deep machining marks, saw-cut appearance | Very rough, catches a fingernail | Included | Hidden surfaces, pre-coating, weld preparation | Sawing, rough milling |
How to read this table. Ra is the arithmetic mean of the deviation from the mean line. It does not describe the texture itself — the same Ra can be a fine spiral feed mark or a coarse random scratch. Where the texture matters, add a note about the process.
Which Finish Reaches Which Ra
Same Ra, very different price. Match the process to the value you actually need — and remember that a cosmetic surface and a hidden surface rarely need the same number.
| If your part needs… | Finish | Typical Ra target (µm) | Cost factor |
|---|---|---|---|
| No cosmetic requirement, hidden or internal surface | As-machined | 0.8–3.2 µm | Included |
| Uniform matte appearance, hide machining marks | Bead blast | 1.6–6.3 µm | 1x (baseline) |
| Uniform matte plus consistent colour before anodize or coating | Bead blast + anodize/coat | 1.6–3.2 µm | 1x + coating cost |
| Directional grain appearance (stainless panels, decorative trim) | Hairline / brush finish | 0.4–1.6 µm | 1.2–1.5x |
| Deburring and slight smoothing on many small parts at once | Vibratory tumbling | 0.8–3.2 µm | 0.5–1x (batch) |
How Much Material Each Finish Removes
Finishing removes material, and removing material changes size. On a diameter, the removal happens on both sides, so the figure in the third column is twice the per-surface figure.
| Process | Material removal per surface | Removal on a diameter (both sides) | Can you control it? | Recommendation |
|---|---|---|---|---|
| As-machined | 0 mm | 0 mm | N/A | No allowance needed. |
| Bead blasting | 0.002–0.005 mm | 0.004–0.01 mm | Limited — depends on media, pressure, time | Negligible for most tolerances. Mask threads and bores. |
| Hairline / brush | 0.005–0.02 mm | 0.01–0.04 mm | Moderate — controlled by grit and passes | Allow 0.02 mm on brushed faces. |
| Vibratory tumbling | 0.01–0.05 mm | 0.02–0.1 mm | Low — batch process, hard to control precisely | Allow 0.02–0.05 mm over the batch range. |
| Sanding / grinding | 0.01–0.1 mm | 0.02–0.2 mm | Good — controlled by grit progression | Allow 0.02–0.05 mm for fine grinding. |
| Polishing | 0.01–0.1 mm | 0.02–0.2 mm | Moderate — depends on starting surface | Allow 0.02–0.05 mm for polish from machined. |
| Mirror polishing | 0.02–0.1 mm | 0.04–0.2 mm | Moderate — multi-stage, cumulative | Allow 0.05 mm minimum on mirror-polished surfaces. |
| Lapping | 0.005–0.05 mm | 0.01–0.1 mm | Very good — controlled by time and abrasive | Lapping is often used to achieve final dimensional accuracy. |
Features You Should Mask Before Finishing
Finishing on a feature that already has a tight tolerance is how parts come back out of specification. If the part has one of these, mask it or leave it as-machined.
| Feature type | Risk level | Recommendation |
|---|---|---|
| Press-fit bores (H7/p6) | High | Mask or do not finish these surfaces. Even 0.01 mm removal changes the interference fit. |
| Threaded holes | Medium | Bead blasting is usually fine (minimal removal). Polishing removes thread peaks and may cause a loose fit. Mask or avoid. |
| Shaft diameters in bearings | High | Do not polish or blast bearing journals unless you account for the removal in the tolerance stack. |
| Sealing surfaces (O-ring grooves) | Medium | Sanding or lapping sealing faces is fine if controlled. Avoid blasting (media embedment). |
| Flatness-critical faces | High | Hand polishing creates waviness on large flat surfaces. Use machine grinding or lapping for flatness requirements. |
| External cosmetic surfaces | Low | Dimensional removal is acceptable on appearance surfaces as long as it is within the profile tolerance. |
How to Use These Numbers
- State Ra, not “smooth” or “polished”. “Smooth” means something different in every shop; Ra 0.8 µm does not.
- Ra is an average, not a texture. If the direction or character of the lay matters — cosmetic stainless, brushed panels, sealing faces — add a note describing the required process, not just the number.
- Do not put the same Ra on every face. Hidden and non-critical surfaces can stay at the as-machined value (1.6–3.2 µm) at no cost, while a visible sealing face may need 0.2 µm. Splitting the callout usually costs less than the finishing.
- Say whether the Ra applies before or after coating. Anodizing, plating and powder coating each add their own layer and roughness on top of the machined surface.
- Finishing removes material. If a feature has a tight tolerance, mask it — or specify the finishing allowance in the tolerance itself.
Frequently Asked Questions
What Ra value should I specify for a machined part?
Ra 1.6 µm is the standard machined appearance and is what most parts get with no extra cost. Ra 0.8 µm is the usual choice for a general precision part, and Ra 0.4 µm or better is needed where the surface is a sealing or cosmetic face. Specify per surface, not per part.
Ra 1.6 µm is the standard machined appearance and is what most parts get with no extra cost. Ra 0.8 µm is the usual choice for a general precision part, and Ra 0.4 µm or better is needed where the surface is a sealing or cosmetic face. Specify per surface, not per part.
Is Ra 0.8 the same as Ra 0.8 µm?
Ra is always a length, and the unit has to be stated. Ra 0.8 µm is roughly Ra 32 µin, which is a visibly smoother surface than the same number quoted in a different unit. Always write the unit on the drawing.
Ra is always a length, and the unit has to be stated. Ra 0.8 µm is roughly Ra 32 µin, which is a visibly smoother surface than the same number quoted in a different unit. Always write the unit on the drawing.
Does polishing change the part size?
Yes. Polishing removes roughly 0.01–0.1 mm per surface, and on a diameter it removes that much on each side. On a bearing seat, a seal groove or a press-fit bore, that removal can push the feature out of tolerance. Mask those surfaces.
Yes. Polishing removes roughly 0.01–0.1 mm per surface, and on a diameter it removes that much on each side. On a bearing seat, a seal groove or a press-fit bore, that removal can push the feature out of tolerance. Mask those surfaces.
What is the cheapest way to improve the look of a machined surface?
A fine bead blast. It is consistent across many parts, it is a batch operation so it is cheap at volume, and it hides machining marks without the dimensional risk of polishing.
A fine bead blast. It is consistent across many parts, it is a batch operation so it is cheap at volume, and it hides machining marks without the dimensional risk of polishing.
Can you hold Ra 0.05 on a machined part?
Not on the machined surface alone. Ra 0.05 µm is a mirror finish that needs multi-stage polishing and lapping, and it costs several times the part's machining. It is worth it only where the surface is functional — an optical face or a precision seal.
Not on the machined surface alone. Ra 0.05 µm is a mirror finish that needs multi-stage polishing and lapping, and it costs several times the part's machining. It is worth it only where the surface is functional — an optical face or a precision seal.
Standards & Sources
- ISO 4287-1 / ISO 4288 — Geometrical Product Specifications (GPS) — Surface texture: profile method
- Site page: /wiki/surface/finishing/ — bead blast, tumbling, polishing and masking in detail
- Site page: /wiki/surface/anodizing/ — how anodizing changes the surface after machining
Related Pages
- /wiki/surface/finishing/ — Bead blast, tumbling, polishing and lapping compared, with cost and removal per process
- /wiki/surface/anodizing/ — How anodizing thickness and process interact with your machined surface
- /wiki/surface/passivation/ — Stainless steel passivation — what it does and does not change
- /wiki/quality/en10204-material-certificate-guide/ — Material certificates and the documentation that travels with a finished part