edm-machining

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:

1. Tool (electrode) and workpiece separated by dielectric fluid (gap 0.01–0.5 mm)
2. Voltage pulses applied → electric field breakdown → plasma channel forms
3. Temperature in channel: 8000–20,000°C → melts and vaporizes material
4. Dielectric flushes away molten material (debris)
5. 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:

1. Rough EDM (high I_p): achieve rough form; Ra 5–10 μm; leave 0.1–0.3 mm for finish
2. Finish EDM (medium I_p): improve form; Ra 1–3 μm
3. Mirror EDM (low I_p): Ra 0.2–0.5 μm (multiple passes)
4. 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).