# Surface Roughness (Ra) Quick Reference for Machined Parts

> 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.

https://www.sinbo-machining.com/wiki/reference/surface-finish-ra/

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## On This Page

- Surface roughness (Ra) values, appearance, cost and typical application
- Which Finish Reaches Which Ra
- How Much Material Each Finish Removes
- Features You Should Mask Before Finishing
- How to Use These Numbers
- Frequently Asked Questions
- Standards & Sources
- Related Pages
- Download This Table

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# 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.

### 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.

### 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.

### 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.

### 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.

## 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/](/wiki/surface/finishing/) — Bead blast, tumbling, polishing and lapping compared, with cost and removal per process
- [/wiki/surface/anodizing/](/wiki/surface/anodizing/) — How anodizing thickness and process interact with your machined surface
- [/wiki/surface/passivation/](/wiki/surface/passivation/) — Stainless steel passivation — what it does and does not change
- [/wiki/quality/en10204-material-certificate-guide/](/wiki/quality/en10204-material-certificate-guide/) — Material certificates and the documentation that travels with a finished part

## Download This Table

- CSV — /wiki/reference/download?slug=surface-finish-ra&format=csv&lang=en
- JSON — /wiki/reference/download?slug=surface-finish-ra&format=json&lang=en
- Markdown — /wiki/reference/surface-finish-ra/index.md