Surface Finish: Ra, Rz, and Roughness Values
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 |
|---|---|---|---|
| 25 | 100 | Sawn or flame-cut face | Sawing, flame cutting |
| 12.5 | 63 | Roughed, pronounced tool marks | Roughing |
| 6.3 | 25 | Roughed, visible tool marks | Rough turning, rough milling |
| 3.2 | 12.5 | Standard machined surface | Standard CNC milling and turning |
| 1.6 | 6.3 | Finished, barely visible marks | Finishing pass, fine cutting data |
| 0.8 | 4 | Fine, smooth and slightly glossy | Fine finishing |
| 0.4 | 2 | Very fine, near mirror | Grinding |
| 0.2 | 1 | Superfinished | Fine grinding |
| 0.1 | 0.63 | High gloss, optical quality | Polishing, lapping |
At Strobel Industry we deliver Ra 3.2 as standard (equals Rz 12.5, grade N8). Finer surfaces are available on request: down to Ra 0.8 for milling and Ra 0.4 for turning.
Note: The Rz values are guide figures. There is no exact conversion between Ra and Rz; the ratio depends on the profile shape (rule of thumb Rz ≈ 4 to 6 × Ra). → Ra/Rz converter with the full chart
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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