Why Surface Finish Matters
Surface finish is more than cosmetic.
Surface finish is measured in micrometres (µm) and described using parameters defined in ISO 4287.
The Ra Parameter
Ra is the most common surface finish measurement used in engineering.
Ra 3.2 µmAs-machined finish with visible tool marks.
Achievable Ra by Machining Process
Typical ranges under normal production conditions.
| Process | Ra range (µm) | Typical | Notes |
|---|---|---|---|
| Rough CNC milling | 3.2 – 6.3 | Ra 3.2 µm | Heavy stock removal passes |
Factors That Affect Surface Finish
Feed rate: Higher feed rates produce a coarser finish.
Post-Machining Treatments
Anodising
An electrolytic process that grows a hard aluminium oxide layer.
Calling Up Surface Finish on Drawings
Surface finish is indicated on drawings using the ISO 1302 surface texture symbol.
- Only call up a specific surface finish where it is functionally important.