Surface finish is specified on almost every machined part drawing and understood on almost none. The result is predictable: sealing faces that leak because the finish was never called out, and cosmetic parts that cost three times what they should because someone specified Ra 0.4 out of habit on every surface.
This guide covers what Ra actually measures, what each process delivers, and how to specify finish so the part works and the quote stays sane.
1. What Ra and Rz Actually Measure
Surface roughness parameters describe the microscopic texture left by the tool:
- Ra (arithmetic average), the average absolute deviation of the profile from its mean line, in micrometers. Forgiving of isolated peaks; the default parameter on most drawings.
- Rz (average maximum height), the average distance between the five highest peaks and five deepest valleys over the sampling length. Captures extremes that Ra smooths over; correlates better with sealing and fatigue performance.
- Roughness vs waviness vs form error, roughness is the tool-mark texture, waviness is longer-period variation (machine vibration, tool pressure), form error is gross deviation (flatness, roundness). A drawing that only specifies Ra leaves waviness and form to default standards, which is usually correct, but worth knowing.
2. The Process-to-Roughness Reference
| Process | Typical Ra | Notes |
|---|---|---|
| Rough milling / profiling | Ra 6.3 – 12.5 μm | Stock removal, non-functional surfaces |
| Conventional milling (finish pass) | Ra 3.2 μm | The standard as-machined default |
| CNC turning (standard) | Ra 3.2 μm | Sharp tooling, appropriate feed |
| Fine turning / milling | Ra 1.6 μm | Good general-purpose finish |
| Turn-mill / finish boring | Ra 0.8 μm | Common requirement for functional bores |
| Bead blasting (from machined) | Ra 0.8 – 0.4 μm | Uniform matte; hides tool marks |
| Surface grinding | Ra 0.4 – 0.2 μm | Flatness-critical faces, seal surfaces |
| Precision grinding | Ra 0.2 – 0.1 μm | Bearing seats, gauge surfaces |
| Polishing / electropolishing | Ra 0.1 μm and below | Optical, cosmetic, hygienic surfaces |
3. The Cost Ladder
Each step down the roughness ladder is a separate operation, and the cost steps are steep:
| From → To | What It Takes | Cost Impact |
|---|---|---|
| Ra 6.3 → 3.2 | Finish pass with sharp tooling | Marginal |
| Ra 3.2 → 1.6 | Finer feed, sharper tools, possibly second pass | Moderate |
| Ra 1.6 → 0.8 | Finishing tools, controlled parameters | Noticeable |
| Ra 0.8 → 0.4 | Grinding operation (new setup) | Large step |
| Ra 0.4 → 0.1 | Precision grinding or polishing (skilled manual work) | Largest step |
Rule of thumb: every halving of Ra adds cost, and the jumps between process families (machining → grinding → polishing) add the most. Specify the smoothest surface your function requires, not the smoothest surface you have seen on a drawing.
4. How to Specify Finish Correctly
- Apply a general default across the drawing, e.g. "Ra 3.2 unless otherwise specified"
- Call out individual surfaces that need something specific: sealing faces, bearing seats, sliding surfaces, optical surfaces
- Specify lay direction (machining direction) where it matters, it affects sealing behavior and wear
- Reference the assessment standard (ISO 4287/ISO 4288 family) rather than leaving interpretation open
- For coated or plated parts, specify both the base Ra and the coating requirement, base roughness shows through
5. Which Surfaces Actually Need Tight Finish
| Surface | Why Finish Matters |
|---|---|
| Bearing seats | Finish affects fit, fretting and fatigue life |
| Sealing / gasket faces | Roughness creates leak paths; Rz often more relevant than Ra |
| Sliding / wear surfaces | Finish affects friction, wear rate and lubrication film |
| Optical / cosmetic | Specular requirements; texture grade specified separately |
| Pre-plating / pre-coating | Base roughness reproduces through the coating |
| Everything else | Standard machined roughness is almost always sufficient |
6. The Verification Question
Roughness is measured, not judged. A surface comparator or profilometer gives the number; "it looks smooth" does not. For critical surfaces, ask for the measurement with the first-article report, a CMM report covers dimensions, and a roughness measurement covers the finish, and they are different documents.
One last honest note on anodizing: die-cast aluminum with high silicon content (ADC12 and similar) can produce uneven color under conventional decorative anodizing, because the silicon distributes unevenly through the structure. If your part needs a high-quality anodized finish, decide the finish plan before the part is made, switching to a wrought alloy or a coating process later is a material change, not a parameter change.
If you are unsure what your part needs, the fastest path is to mark the functional surfaces on the drawing and let the quote come back with a finish plan. Send us the drawing with the critical surfaces flagged, we will specify the rest to standard, and your part will neither leak nor overpay.