Surface finish is a critical specification for CNC machined plastic parts, affecting friction, sealing performance, appearance, and cleanability. This guide explains Ra values, achievable finishes for common plastics, and how to specify surface finish correctly.
Understanding Ra Surface Finish Values
Ra (Roughness Average) is the most common surface finish parameter, measured in micrometers (µm) or microinches (µin). Lower Ra values indicate smoother surfaces.
| Ra (µm) | Ra (µin) | Description | Typical Method |
|---|---|---|---|
| 3.2 | 125 | Standard as-machined | CNC milling, turning |
| 1.6 | 63 | Smooth machined | Fine feed, sharp tools |
| 0.8 | 32 | Very smooth | Fine finishing pass |
| 0.4 | 16 | Excellent finish | Polishing, burnishing |
| 0.2 | 8 | Mirror-like | Vapor polishing (acrylic) |
Achievable Surface Finish by Plastic Material
| Material | Standard Ra (µm) | Best Ra (µm) | Notes |
|---|---|---|---|
| PEEK | 1.6-3.2 | 0.4 | Excellent machinability |
| PTFE | 3.2-6.3 | 1.6 | Soft, harder to get smooth |
| POM (Delrin) | 0.8-1.6 | 0.4 | Excellent as-machined finish |
| Acrylic (PMMA) | 1.6-3.2 | 0.2* | *With vapor polishing |
| Nylon | 1.6-3.2 | 0.8 | May absorb moisture |
| PBI | 1.6-3.2 | 0.8 | Hard, good finish |
How to Specify Surface Finish on Your Drawing
Include the Ra value with your callout, e.g., “Ra 0.8 µm on sealing surface.” Indicate which surfaces require finish control. For tight sealing or optical applications, specify the finish requirement clearly.
Explore our surface finishing capabilities for more options.
ISO 9001 certified. Upload your drawing for a quote with surface finish options. RFQ in 24hr.