What is Surface Finish?
Surface finish (or surface roughness) describes the texture of a machined surface. In EDM, it's primarily measured as Ra (arithmetic average roughness) in micrometres (µm).
EDM Surface Finish Ranges
| Finish Level | Ra Value | Typical Use |
|---|---|---|
| Rough cut | 3.2–6.3 µm | Material removal, roughing |
| Standard | 1.6–3.2 µm | General tool room work |
| Fine | 0.4–1.6 µm | Precision components |
| Very fine | 0.15–0.4 µm | Die faces, critical surfaces |
| Mirror | ≤ 0.1 µm | Optical, medical, aesthetic |
Factors Affecting EDM Surface Finish
1. Discharge Energy
Lower discharge energy = finer surface finish but slower cutting. Multi-cut strategies use high energy for roughing then progressively lower energy for finishing.
2. Number of Cuts (Wire EDM)
- 1 cut: Ra 2.5–3.2 µm (roughing only)
- 2 cuts: Ra 1.2–1.6 µm
- 3 cuts: Ra 0.6–0.8 µm
- 4+ cuts: Ra 0.15–0.4 µm
3. Wire Type and Diameter
Coated wires (zinc-coated, diffusion-annealed) produce better surface finish than plain brass. Thinner wires allow finer cuts but are more fragile.
4. PIKA Circuit (Sinker EDM)
Berlin's CNC Sinker EDM machines feature the PIKA circuit — a specialised finishing circuit that achieves mirror finish Ra ≤ 0.1 µm. This is essential for injection mould cavities where surface quality directly affects part aesthetics.
Practical Tips for Better Surface Finish
- Use multi-cut strategies — Don't try to achieve fine finish in a single cut
- Maintain clean dielectric — Contaminated fluid degrades surface quality
- Control wire tension — Inconsistent tension causes waviness
- Choose appropriate flushing — Proper debris removal prevents secondary discharge
- Service your machine regularly — Worn guides and bearings affect finish quality
Conclusion
Surface finish capability is one of the key differentiators between machine grades. Berlin's Hyper and N Series machines achieve Ra 0.15 µm in wire EDM, while the CNC Sinker EDM with PIKA circuit reaches mirror finish Ra 0.1 µm — meeting the most demanding requirements in aerospace, medical and optical applications.