Once a feature needs tighter control than standard milling or turning delivers, the next question isn't whether to switch processes — it's which one. Wire EDM and precision grinding aren't interchangeable options for the same job; they solve different geometry and material problems, and picking the wrong one adds a process step that doesn't actually fix what's driving the tolerance.

The tolerance ladder

Standard milling and turning holds tolerances to ±0.005 mm — the baseline capability most features don't need to exceed. Beyond that, EDM holds ±0.003 mm and precision grinding holds ±0.002 mm, the tightest band on the shop floor. The full breakdown of when a callout actually needs one of these tighter processes, rather than just reading as an aggressive spec, is covered in how to read a tolerance callout before you send an RFQ.

When EDM is the right call

Wire-cut, sinker, and small-hole EDM remove electrically conductive material without a conventional cutting edge, which makes them useful for fine internal geometry, small precision holes, and hardened alloys a cutting tool struggles with. If the feature is an internal profile, a small hole, or sits in hardened material — a keyed bore or a non-round profile on an implant or instrument component, for example — EDM is usually the route, not grinding.

When grinding is the right call

ID, OD, surface, and centerless grinding control final size and surface finish on shafts, bores, flats, and bearing features. Where EDM answers a geometry or hardness problem, grinding answers a size-and-surface problem on features that are already close to round or flat — bearing surfaces, precision shafts, and mating or alignment features on components like the pins, screws, and shafts covered in CNC turning for orthopedic implant components, and the drivers and guides typical of surgical instrument manufacturing.

Combining both on one part

Many components use all three tolerance bands in sequence: a shaft turned to ±0.005 mm, a keyway EDM'd to ±0.003 mm, and a bearing journal ground to ±0.002 mm on the same part. The finish that follows matters too — stainless features headed into a surgical or implant application typically move on to passivation, covered in what medical device surface finishing actually changes.

When a print shows a tolerance tighter than ±0.005 mm, identify which specific feature drives it and why — geometry, hardness, or surface — and note that on the RFQ. See what to include in an RFQ for medical device parts for how that fits alongside the rest of the request, use the RFQ checklist's dedicated field for critical tolerances, or send the drawing directly through the contact page.