SurfaceFinish—Ra,Rz,andRoughnessValues
Roughness, measurement methods, and their impact on function and cost.
What is Surface Finish?
Surface finish (also: surface roughness) describes the fine texture of a machined surface. No machined surface is perfectly smooth — under the microscope, grooves, ridges, and irregularities from the manufacturing process are always visible.
Roughness affects:
- Sealing function — Rough surfaces seal poorly
- Friction and wear — Smooth surfaces = less friction
- Fatigue strength — Grooves act as stress concentrators
- Appearance — Glossy vs. matte surface
- Coatability — Adhesion of paints, galvanic coatings
Roughness Parameters — Ra and Rz
The two most important roughness parameters in practice:
| Parameter | Name | Definition |
|---|---|---|
| Ra | Arithmetic mean roughness | Average deviation of the profile from the centerline. The most commonly used value. |
| Rz | Mean peak-to-valley height | Average of the 5 highest profile peaks and 5 deepest profile valleys. More sensitive to outliers. |
Rule of thumb: Rz ≈ 4 × Ra (varies by process and material).
Roughness Values in CNC Practice
| Ra (µm) | Rz (µm) | Description | Achievable By |
|---|---|---|---|
| 6.3 | 25 | Roughed — visible machining marks | Roughing, rough turning |
| 3.2 | 12.5 | Standard machined surface | Standard CNC milling / turning |
| 1.6 | 6.3 | Finished surface — barely visible marks | Finishing pass, fine cutting data |
| 0.8 | 3.2 | Fine surface — smooth, glossy | Fine finishing, polishing |
| 0.4 | 1.6 | Very fine surface — near mirror-like | Grinding, superfinishing |
| 0.1 | 0.4 | Mirror finish — optical quality | Polishing, lapping |
At Strobel Industry, we deliver Ra 3.2 as standard. Finer surfaces down to Ra 0.4 are available on request.
Impact on CNC Costs
As with tolerances: smoother surfaces = higher costs.
- Ra 3.2 — Standard machining, no extra effort → Baseline
- Ra 1.6 — Additional finishing pass, ~10-20% extra cost
- Ra 0.8 — Fine finishing or grinding, ~30-50% extra cost
- Ra 0.4 — Grinding mandatory, ~80-150% extra cost
Practical tip: Specify roughness only where it is functionally necessary (sealing surfaces, bearing seats, guides). All other surfaces remain at the standard machining roughness.
Frequently Asked Questions
What does Ra 3.2 mean?
Ra 3.2 means an arithmetic mean roughness of 3.2 micrometers (µm). This is the standard machining quality for CNC turning and milling. The surface shows slight machining marks that are barely perceptible with a fingernail.
What is the difference between Ra and Rz?
Ra is the arithmetic mean of all deviations from the centerline — a "smoothed" value. Rz is the mean peak-to-valley height from the 5 largest profile peaks and valleys — more sensitive to individual deep grooves. Rz ≈ 4 × Ra.
What is the best surface finish achievable with CNC milling?
CNC milling can achieve Ra 0.4 to Ra 0.8 µm (with ultra-fine finishing strategies and solid carbide ball nose cutters). For even smoother surfaces (Ra < 0.4), grinding or polishing is required.
Further Topics
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