CNC Surface Finish Standards: Ra vs Rz Roughness Chart
Surface roughness (Ra and Rz) quantifies microscopic deviations on machined surfaces, directly influencing friction, seal integrity, fatigue life, and cosmetic appearance.
Engineering Specifications & Parameters
| Conversion | 1 μm = 39.37 μin |
| Measurement Tool | Stylus Profilometer or Optical Interferometer |
| Typical Milling Limit | Ra 0.8 μm without post-polishing |
| Standard Unit (Metric) | Micrometer (μm) |
| Standard Unit (Imperial) | Microinch (μin) |
Understanding Ra vs Rz
- Ra (Roughness Average): The arithmetic average of profile height deviations across the evaluation length.
- Rz (Mean Roughness Depth): The average distance between the 5 highest peaks and 5 lowest valleys.
Machining Capability Chart
- As-Machined Standard: Ra 3.2 μm (125 μin)
- Fine CNC Milling/Turning: Ra 1.6 μm (63 μin)
- Precision Bearing Seats: Ra 0.8 μm (32 μin)
- Polished / Lapped: Ra 0.2 to 0.4 μm (8 to 16 μin)
Frequently Asked Questions
Why is Ra alone sometimes insufficient for sealing surfaces?
Ra averages all deviations and can hide sharp individual scratches or peaks that would cause O-ring fluid leaks; Rz catches peak-to-valley extremes.
