| name | edm-machining |
| description | EDM (Electrical Discharge Machining) — sinker/wire EDM, material removal rate, surface roughness, recast layer, electrode design, dielectric fluid, accuracy, hard material machining, ASTM standards. |
| metadata | {"priority":7,"promptSignals":{"phrases":["EDM machining","electrical discharge machining","wire EDM","sinker EDM","EDM surface finish","EDM electrode"],"minScore":3}} |
Electrical Discharge Machining (EDM) — Complete Skill
EDM Fundamentals
Principle: electrical discharge (spark) erodes material from workpiece; non-contact; works on any electrically conductive material regardless of hardness
Mechanism:
- Tool (electrode) and workpiece separated by dielectric fluid (gap 0.01–0.5 mm)
- Voltage pulses applied → electric field breakdown → plasma channel forms
- Temperature in channel: 8000–20,000°C → melts and vaporizes material
- Dielectric flushes away molten material (debris)
- Process repeats at 1000–500,000 pulses/sec
Key parameters:
- Discharge current I_p [A]: higher → more material removed → rougher finish
- Pulse on-time t_on [μs]: higher → deeper craters → rougher finish
- Pulse off-time t_off [μs]: higher → slower MRR; lower → risk of wire break
- Voltage V [V]: 20–250V typical; determines discharge energy and gap
- Gap voltage V_g [V]: monitored to maintain spark gap
Sinker (Die) EDM
Applications: dies, molds, complex 3D cavities; any conductive material; hardened steel, carbide
Electrode material:
Graphite: highest MRR; good EDM-ability; low wear; most common for steel
Copper: finer finish; lower MRR; for intricate detail; better surface than graphite
Copper-tungsten: for carbide machining; very high wear resistance; expensive
Copper-graphite: compromise
Electrode undersize:
Electrode smaller than cavity by gap G (per side): G = 0.01–0.3 mm depending on current
Higher current → larger gap → more undersize needed
Electrode wear ratio:
EWR = (volume electrode worn) / (volume workpiece removed) × 100% [%]
Graphite on steel: EWR = 0.1–5% (excellent); copper: EWR = 5–15%; Cu-W on carbide: EWR = 1–3%
Material Removal Rate (MRR):
MRR = I_p² × t_on × k / ρ_workpiece [approximation; k = material constant]
Practical range: 1–100 mm³/min (graphite electrode, steel workpiece, high current)
MRR for steel (rough): 30–50 mm³/min; (finish): 0.1–5 mm³/min
Wire EDM (WEDM)
Applications: 2D + taper cuts through conductive material; punches, dies, complex profiles; exotic alloys
Wire material:
Brass wire (0.25 mm): standard; most common; best balance of MRR/accuracy/cost
Copper wire: higher conductivity; slightly higher MRR; less stable
Zinc-coated brass: improved flushing; higher cutting speed; premium
Wire tensile strength: 700–900 MPa; wire tension: 0.08–0.5 N
Cutting speed (WEDM):
v_cut = MRR / (thickness × wire_dia) [approximate]
Typical: 100–400 mm²/min (brass wire, steel workpiece, 25 mm thick)
Accuracy:
Wire WEDM: ±0.002–0.005 mm (positioning); ±0.003–0.010 mm (form)
Achievable with multiple skim cuts (rough + 1–3 skim passes)
Taper cutting:
Wire tilted (upper/lower guide offset): taper angles to ±30° typical; ±45° maximum
Surface Finish
Ra vs. discharge parameters:
| Finish | I_p [A] | t_on [μs] | Ra [μm] | Rc [μm] |
|---|
| Rough | 30–100 | 100–500 | 3–10 | 15–50 |
| Medium | 10–30 | 20–100 | 1–3 | 6–15 |
| Fine | 1–10 | 5–20 | 0.4–1.5 | 2–6 |
| Mirror | 0.1–1 | 1–5 | 0.1–0.4 | 0.5–2 |
Surface texture: EDM leaves characteristic crater pattern (non-directional); isotropic surface
EDM vs. ground surface: EDM is non-directional (better for tribological applications); grinding is directional
Recast Layer (White Layer)
Recast layer: rapidly solidified material that did not flush away; remains on surface
Properties: higher hardness than bulk (due to rapid quenching); brittle; contains cracks/microcracks
Typical thickness: 5–50 μm (proportional to discharge energy)
Hardness: 60–70 HRC (steel workpiece; white layer re-hardened martensite)
Cracking in recast:
Thermal shock → microcracks in white layer; depth = 0.5–2× white layer thickness
Fatigue cracks initiate at recast layer → important for dynamic parts
White layer removal:
Chemical etching: HCl/HNO₃ mixture; removes 25–50 μm layer uniformly
Electrochemical polishing: removes white layer without mechanical stress
Gentle grinding: 0.01–0.05 mm stock removal after EDM (for fatigue-critical parts)
Residual stress from EDM:
Near surface: tensile RS (from rapid solidification of recast) → fatigue debit
Heat affected zone: compressive RS below (small) → complex RS profile
After white layer removal: net compressive possible if only base material remains
Hard Material EDM
Tungsten carbide (WC-Co):
EDM-able (Co binder is conductive); MRR lower than steel; electrode: copper-tungsten
Surface quality: good; but cracking more likely in recast due to thermal shock
Roughing: graphite electrode; finishing: copper electrode
Titanium alloys (Ti-6Al-4V):
EDM in deionized water preferred (prevents fire risk); lower MRR than steel
Recast on Ti: α-case (oxygen-enriched) → very brittle → MUST be removed for aerospace parts
Ceramics:
ONLY if conductive (e.g., Si₃N₄ + TiN, ZrO₂/TiN, Si-SiC composites); pure alumina: NOT EDMable
For insulators: assisting electrode method or reverse EDM possible
EDM Process Planning
Sequence for precision die:
- Rough EDM (high I_p): achieve rough form; Ra 5–10 μm; leave 0.1–0.3 mm for finish
- Finish EDM (medium I_p): improve form; Ra 1–3 μm
- Mirror EDM (low I_p): Ra 0.2–0.5 μm (multiple passes)
- Optional: hand polish Ra 0.05–0.1 μm if needed
Flush strategy:
Flood flushing: simple; lower MRR; for shallow cavities
Injection flushing (through electrode): best for deep cavities; highest MRR
Suction flushing: for blind holes
No flush → secondary discharges → arcing → surface damage → scrap
Dielectric Fluid
Sinker EDM: mineral oil (most common); silicon oil for fine finish; petroleum-based
Wire EDM: deionized water; conductivity 1–20 μS/cm; pH 7–8; continuous ion exchange
Standards
| Standard | Scope |
|---|
| ASTM E3 | Metallographic specimen preparation (recast layer examination) |
| ISO 4288 | Surface roughness measurement |
| ISO 1101 | Geometric tolerancing (applies to EDM parts) |
| AMS 2750 | Pyrometry (furnace calibration for recast measurement) |
Output
Provide: EDM type (sinker/wire), workpiece material and hardness [HRC], electrode material and EWR [%], discharge parameters (I_p [A], t_on [μs], t_off [μs]), MRR [mm³/min], surface roughness Ra [μm] and Rc [μm], recast layer thickness [μm] and hardness [HRC], residual stress at surface [MPa] (tensile/compressive), accuracy (dimensional ±[mm], form [mm]), white layer removal method if required, cutting speed [mm²/min] (WEDM), taper angle [°] (if WEDM), and applicable standard (ISO 4288, ASTM E3 for recast examination).
