EDM and wire EDM

Why it matters

Hardened die steels, cemented carbides and superalloys are too hard or too tough to cut economically, and many die cavities, sharp internal corners and narrow slots cannot be reached by a rotating cutter. EDM removes metal by sparks, whatever its hardness, as long as it conducts electricity — which is why nearly every press tool, injection mould and turbine-blade cooling hole depends on it.

Key ideas

Principle

• Tool and workpiece are both electrodes, separated by a small spark gap (typically a few µm to some tens of µm) filled with a dielectric liquid (hydrocarbon oil for die-sinking, deionised water for wire EDM).
• A voltage pulse ionises the dielectric at the point of smallest gap; a plasma channel forms and a spark of very high current density melts and vaporises a tiny crater on both electrodes. When the pulse ends the channel collapses, the dielectric de-ionises, and the molten metal is flushed away as debris.
• Material removal is thermal: it depends on melting point and thermal conductivity, not on hardness. Work must be electrically conductive.
• Usually the work is the anode (+) and the tool the cathode (−) for roughing with short pulses; polarity is chosen to put more heat into the work and less into the tool.
• A servo keeps the gap constant as material is removed; short circuits or arcing are prevented by retracting the tool.

Functions of the dielectric: insulate until breakdown, concentrate the discharge, de-ionise quickly after each spark, cool, and flush debris. Poor flushing causes arcing, unstable cutting and poor finish.

Pulse generators

• RC (relaxation) circuit: a capacitor charges through a resistor from supply voltage V0 and discharges across the gap when its voltage reaches the breakdown voltage Vd. Simple and gives very fine finishes at low energy, but slow because charging is slow.
• Transistorised (controlled pulse) generators: on-time, off-time and peak current are set independently; used on all modern machines.

Process parameters and results

• Higher discharge energy (current × on-time) → larger craters → higher MRR but rougher surface and larger overcut. Finishing uses low energy and high frequency.
• Overcut: the cavity is larger than the tool by about one spark gap on each side.
• Tool wear: the tool also erodes; the wear ratio (work removed/tool worn) depends on tool material — graphite and copper are common; copper–tungsten for fine detail. Tools wear most at corners, so roughing and finishing electrodes are often used.
• Surface integrity: a re-solidified recast (white) layer with micro-cracks and tensile residual stress, over a heat-affected zone; critical parts are polished or etched afterwards.

Die-sinking EDM reproduces the tool shape as a cavity (moulds, forging dies, blind pockets). EDM drilling uses a rotating tube electrode for small deep holes.

Wire EDM (WEDM)

• A continuously fed wire (brass or coated, about 0.1–0.3 mm) cuts a through-profile under CNC control, in deionised water. Fresh wire is always used, so wire wear does not affect accuracy.
• Kerf width = wire diameter + 2 × spark gap (overcut); the CNC applies a wire offset of half the kerf.
• Cutting rate is quoted as area rate (mm²/min) = path speed × work thickness, since a thicker plate cuts proportionally slower along the path.
• Used for punch and die profiles, extrusion dies, gears and splines in hardened steel, and taper cuts by tilting the wire guides.

Formulas

• V(t) = V0·(1 − e^(−t/RC)) — capacitor voltage in an RC circuit, V; R resistance (Ω), C capacitance (F), t time (s).
• tc = RC·ln[V0 / (V0 − Vd)] — charging time to breakdown voltage Vd, s. Spark frequency f ≈ 1/tc when discharge time is negligible.
• E = ½·C·Vd² — energy per spark, J.
• P = E·f — average discharge power, W. P is maximum when Vd ≈ 0.715·V0.
• MRR ≈ 4×10⁴ · I · Tw^(−1.23) — empirical die-sinking MRR, mm³/min; I current (A), Tw melting point of the work (°C). It is a textbook empirical fit; use your own data book's constants.
• w = d_w + 2·s — wire EDM kerf, mm; d_w wire diameter, s spark gap (overcut) per side.
• t = L·h / A_r — wire EDM cutting time, min; L path length (mm), h thickness (mm), A_r area cutting rate (mm²/min).

Worked examples

Example 1 (standard) — RC generator. V0 = 200 V, R = 50 Ω, C = 10 µF, breakdown voltage Vd = 150 V. Find the charging time, spark energy, frequency and average power (neglect discharge time).

1. RC = 50 × 10 × 10⁻⁶ = 5 × 10⁻⁴ s.
2. tc = RC·ln[V0/(V0 − Vd)] = 5 × 10⁻⁴ × ln(200/50) = 5 × 10⁻⁴ × 1.386 = 0.693 ms.
3. E = ½·C·Vd² = 0.5 × 10 × 10⁻⁶ × 150² = 0.1125 J.
4. f = 1/tc = 1443 Hz; P = E·f = 0.1125 × 1443 = 162 W.

Example 2 (GATE level) — maximum power setting and wire EDM time. (a) An RC circuit has V0 = 250 V, R = 25 Ω, C = 8 µF. Find the breakdown voltage for maximum power and the corresponding power. (b) A wire of 0.25 mm diameter with a spark gap of 0.03 mm cuts a 200 mm long profile in a 25 mm thick die plate at an area rate of 100 mm²/min. Find the kerf and cutting time.

1. Power ∝ Vd²/ln[V0/(V0 − Vd)]; its maximum is at Vd/V0 = 0.7153, so Vd = 178.8 V.
2. RC = 25 × 8 × 10⁻⁶ = 2 × 10⁻⁴ s; tc = 2 × 10⁻⁴ × ln[1/(1 − 0.7153)] = 2 × 10⁻⁴ × 1.2563 = 0.2513 ms.
3. E = 0.5 × 8 × 10⁻⁶ × 178.8² = 0.1279 J; f = 3980 Hz; P = 509 W.
4. (b) w = d_w + 2·s = 0.25 + 2 × 0.03 = 0.31 mm.
5. t = L·h/A_r = 200 × 25/100 = 50 min.

Common mistakes

• Believing EDM removal depends on hardness — it depends on melting point and conductivity; but EDM fails on non-conductors (most ceramics, glass, plastics).
• Using ½CV0² instead of ½CVd² for spark energy.
• Taking wire EDM speed as a linear feed independent of thickness; the area rate is the stable quantity.
• Forgetting overcut when sizing an electrode or setting the wire offset.
• Ignoring the recast layer on fatigue-critical parts.
• Treating MRR as "power" — energy per second must be divided by the specific energy of removal to give volume per second.

For GATE PI

• NAT on RC circuits: charging time, spark energy, frequency, power, the 0.715·V0 condition; on wire EDM kerf and cutting time; on empirical MRR.
• MCQs on the mechanism (melting and vaporisation), role of the dielectric, polarity, recast layer, electrode materials and overcut.
• Comparisons with ECM, USM, LBM and EBM (next topics).

Quick check

1. Does EDM removal rate depend on the hardness of the work?
2. Capacitor 5 µF discharges at 100 V. Energy per spark?
3. Wire 0.2 mm, spark gap 0.025 mm: kerf?
4. Which dielectric is used in wire EDM?
5. For maximum power in an RC circuit, Vd ≈ ? × V0.

Answers: 1. No; it depends on melting point and conductivity. 2. 0.5 × 5 × 10⁻⁶ × 100² = 0.025 J. 3. 0.25 mm. 4. Deionised water. 5. About 0.72.