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When a drawing calls for tolerances tighter than about ±0.01 mm or a surface finish better than Ra 0.8 µm, conventional CNC milling and turning begin to run out of room. Two processes usually take over at that point: wire EDM and surface grinding. Both are finishing operations, and both can hold accuracies that no cutting tool can match — but they remove metal in fundamentally different ways, behave differently on different materials, and solve very different geometric problems. Understanding those differences helps you specify the right process on the drawing instead of paying for capability the part does not need.
Wire EDM machining cuts with sparks, not a tool. A thin brass or coated wire, typically 0.1–0.33 mm in diameter, is fed continuously through the workpiece while electrical discharges erode the material along a programmed 2D path. The wire never touches the part, so there is no cutting force, no chatter, and no tool pressure — which is exactly why it can finish thin walls and delicate features without distorting them. The trade-off is that the workpiece must be electrically conductive, and the sparking action leaves a thin recast layer on the cut surface. Precision work therefore uses one or more low-energy skim passes after the main cut to clean that layer up and bring the surface to its final finish and accuracy.
Surface grinding is a mechanical, abrasive process. The part is held on a magnetic chuck — or a dedicated fixture for non-magnetic materials — while a rotating grinding wheel traverses across the surface, removing stock in increments measured in microns. Because the wheel makes contact, grinding does introduce heat and pressure, which the operator manages with coolant and light passes. In return, it delivers the best flatness, parallelism, and surface finish of any conventional finishing method.
| Aspect | Wire EDM | Surface Grinding |
|---|---|---|
| Material removal | Spark erosion (thermal, non-contact) | Abrasive wheel (mechanical contact) |
| Cutting force | Zero — no deflection or distortion | Contact pressure and heat, controlled with coolant |
| Material requirement | Electrically conductive materials only | Virtually any metal, conductive or not |
| Typical geometry | 2D through-profiles, sharp internal corners, thin walls, tall slots | Flat faces, parallel surfaces, grooves, angles, arcs |
| Achievable tolerance | Around ±0.003–0.005 mm with skim passes | ±0.002 mm, with matching parallelism |
| Surface finish (Ra) | 0.8–1.6 µm on a single cut; 0.2–0.4 µm with multiple skim passes | 0.2–0.4 µm standard; mirror finishes possible with polishing |
| Surface layer | Thin recast layer, removed by skim passes | No recast layer; grinding burn possible if parameters are wrong |
Wire EDM and surface grinding are complements, not competitors. A common tool-and-die workflow runs like this: mill the part to near-net shape with 0.1–0.3 mm of stock left per side, heat treat it, wire EDM the profile or die opening to final size, then surface grind the top and bottom faces for flatness, parallelism, and thickness. Each process does the job it is best suited for, and the part ends up more accurate than either process could make it alone. On the drawing, call out which surfaces get which process rather than using a blanket finish note — it prevents over-specification and keeps the quote honest.
If the feature is a profile through hard, conductive material — especially with sharp corners or thin walls — finish it with wire EDM. If the feature is a flat face that must be parallel, dimensionally exact, and mirror-smooth, finish it with surface grinding. Many precision parts need both, applied to different surfaces.
ANOK Precision Manufacturing provides both processes in-house as part of its one-stop precision CNC machining services in Shenzhen, China. Our wire EDM department holds tolerances as tight as 0.003 mm on hardened steel, titanium, and carbides, while our surface grinding team works to ±0.002 mm with surface roughness down to Ra 0.4 — and mirror finishes of Ra 0.2 through polishing. As an ISO 9001:2015 certified factory serving medical, aerospace, and automation customers since 2007, we can review your drawing and recommend the right finishing route for each feature. Send us your files for a free, no-obligation quote.