Machining and cutting
EDM, wire and sinker: what goes wrong and where to inspect
Electrical discharge machining removes material by sparks between an electrode and the workpiece, cutting hardened steel and carbide into shapes that cutters cannot reach.
01How the process works
Wire EDM cuts profiles with a thin travelling wire; sinker EDM burns a cavity with a shaped graphite or copper electrode. Both work in dielectric fluid and produce no cutting forces, ideal for dies, punches, moulds and aerospace features.
Each spark melts a thin surface layer that resolidifies as a brittle recast or white layer, sometimes with micro cracks. Skim cuts reduce it. For fatigue loaded parts the allowed recast thickness must be specified and verified by metallography.
- Electrode or wire set-up. Electrode made or wire threaded.
- Roughing. High energy cut.
- Skim cuts. Low energy passes for accuracy and finish.
- Cleaning. Dielectric and debris removed.
- Inspection. Dimensions, finish, recast check.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Recast layer too thick | Insufficient skim cuts | Metallographic section |
| Micro cracks | High energy on hardened steel | Metallography, PT |
| Dimensional error | Wire lag, electrode wear | CMM, optical measurement |
| Surface roughness | Generator settings | Roughness tester |
| Corner radius | Wire diameter | Optical comparator |
| Wire marks and steps | Restarts | Visual |
03Where an inspector should look
Before production
- Recast limit specified
- Material and hardness
- Electrode design
During production
- Number of skim cuts
- Generator parameters
- Wire tension
After production
- Dimensional report
- Sample metallography if required
- Surface finish
04Standards and references
Aerospace specifications such as AMS 2650 limit recast layer; customer surface integrity requirements.
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.