Non-traditional machining: EDM, ECM, USM, LBM, AJM

Why it matters

Hardened die steels, carbides, superalloys, ceramics and glass are too hard, too brittle or too delicate for a conventional cutting tool, and many mechatronic parts need features no drill or end mill can make: fine cooling holes, sharp internal corners, micro-slots, stent patterns. Non-traditional (advanced) machining processes remove material with electrical, chemical, thermal or impact energy instead of a harder tool, and most of them are CNC-controlled servo systems.

Key ideas

Classify the processes by the energy that removes material – this tells you what they can and cannot cut.

Electrical discharge machining (EDM) – thermal.

• Tool (usually copper or graphite) and work are both electrical conductors, separated by a small gap (about 0.01–0.5 mm) filled with a dielectric (kerosene/EDM oil or deionised water).
• A pulsed voltage ionises the dielectric; each spark melts and vaporises a tiny crater on both electrodes. The dielectric de-ionises between pulses, cools the region and flushes out debris. A servo feed keeps the gap constant.
• MRR rises with spark energy and frequency; surface finish worsens with spark energy (larger craters). A recast layer and heat-affected zone are left on the surface.
• Hardness does not matter, but the work must conduct. Tool wear exists, so wear ratio (work removed / tool removed) is a figure of merit. Wire EDM uses a travelling brass wire for through-profiles.
• In the classic RC (relaxation) generator a capacitor charges through a resistor and discharges across the gap; power delivered is maximum when the discharge voltage is about 0.72 of the supply voltage.

Electrochemical machining (ECM) – electrochemical.

• Reverse of electroplating: work is the anode, tool is the cathode, and a fast-flowing electrolyte (NaCl or NaNO₃ solution) passes through a gap of about 0.1–0.6 mm under a low DC voltage (roughly 5–25 V) with very high current density.
• Metal dissolves atom by atom according to Faraday's laws, so there is no tool wear, no residual stress, no burr and no heat-affected zone (the electrolyte warms through Joule heating, but the work is not damaged). Hard and soft metals machine at the same rate.
• Used for turbine blades and complex forging dies. Limits: work must conduct, high power and electrolyte-handling costs, and sharp corners are hard to hold.
• Removal is set by current, not hardness: MRR depends on atomic mass, valency and density of the anode metal.

Ultrasonic machining (USM) – mechanical impact.

• A shaped tool vibrates axially at about 20 kHz with an amplitude of roughly 10–50 µm; abrasive grit (B₄C, SiC, Al₂O₃) in a water slurry is hammered into the work and chips it by brittle micro-fracture.
• Best for hard brittle materials (glass, ceramics, quartz, carbides); poor on ductile metals, which absorb the impacts plastically. Tool (soft steel, brass) wears.

Abrasive jet machining (AJM) – mechanical erosion.

• Dry abrasive (Al₂O₃ or SiC, about 10–50 µm) carried in a gas jet at roughly 150–300 m/s erodes brittle work. MRR rises with abrasive flow rate, particle velocity (nozzle pressure) and grit size, up to an optimum stand-off distance. Used for deburring, frosting, cutting thin glass and silicon. Not for ductile metals.

Laser beam machining (LBM) – thermal.

• A focused laser (CO₂, Nd:YAG or fibre) gives very high power density at a small spot, melting and vaporising material; an assist gas blows the melt out. Non-contact, no tool wear, cuts almost any material including non-conductors, fine kerf.
• Produces a small heat-affected zone, recast layer and taper; reflective, high-conductivity metals (copper, aluminium) are harder to cut; high capital cost.

Process selection. Conductive and hard with complex cavities: EDM or ECM. Brittle non-conductor: USM, AJM or LBM. Thin sheets with fine profiles: LBM or wire EDM. Stress-free surface needed: ECM.

Formulas

ṁ = η·I·A / (v·F)

• ECM mass removal rate (g/s). η = current efficiency (dimensionless); I = current (A); A = atomic mass of anode metal (g/mol); v = valency of dissolution (dimensionless); F = 96 485 C/mol (Faraday's constant).

Q = η·I·A / (v·F·ρ)

• ECM volumetric MRR (cm³/s) with ρ in g/cm³.

f = Q / A_t = η·J·A / (v·F·ρ)

• Equilibrium tool feed rate (cm/s). A_t = projected tool area (cm²); J = I/A_t = current density (A/cm²).

A/v (alloy) = 1 / Σ(x_i·v_i/A_i)

• Effective electrochemical equivalent for an alloy; x_i = mass fraction of element i.

V_c = V₀·(1 − e^(−t/RC))

• EDM RC circuit capacitor voltage during charging. V₀ = supply voltage (V); R = charging resistance (Ω); C = capacitance (F); t = time (s).

E_s = ½·C·V_d²

• Energy per spark (J); V_d = discharge (breakdown) voltage (V).

P = E_s / t_c

• Average spark power (W) when charging time t_c (s) dominates the cycle.

v_max = 2·π·f·a

• USM peak tool velocity (m/s); f = frequency (Hz); a = amplitude (m).

Power density = P / (π·d²/4)

• LBM (W/m²); P = beam power (W); d = focused spot diameter (m).

Worked examples

Example 1 (ECM, standard). Pure iron (A = 55.85 g/mol, v = 2, ρ = 7.86 g/cm³) is machined by ECM at I = 1000 A, η = 100 %, with a tool of projected area 10 cm². Find the mass and volume removal rates and the equilibrium feed rate.

1. ṁ = I·A/(v·F) = 1000 × 55.85 / (2 × 96 485) = 0.2894 g/s
2. Q = ṁ/ρ = 0.2894 / 7.86 = 0.03682 cm³/s = 36.8 mm³/s
3. f = Q/A_t = 36.82 mm³/s / 1000 mm² = 0.0368 mm/s = 2.21 mm/min Answer: ṁ = 0.289 g/s, Q = 36.8 mm³/s, feed ≈ 2.21 mm/min.

Example 2 (EDM RC generator, GATE level). An RC relaxation circuit has V₀ = 200 V, R = 50 Ω and C = 10 µF. The gap breaks down at V_d = 150 V. Assume the capacitor discharges fully and the discharge time is negligible. Find the energy per spark, spark frequency and average power.

1. Charging time: V_d = V₀(1 − e^(−t/RC)) gives t_c = −RC·ln(1 − V_d/V₀)
2. RC = 50 × 10 × 10⁻⁶ = 5 × 10⁻⁴ s; t_c = −5 × 10⁻⁴ × ln(0.25) = 5 × 10⁻⁴ × 1.386 = 6.93 × 10⁻⁴ s
3. E_s = ½·C·V_d² = 0.5 × 10 × 10⁻⁶ × 150² = 0.1125 J
4. Frequency = 1/t_c = 1443 Hz; power P = E_s/t_c = 0.1125/6.93 × 10⁻⁴ = 162 W Answer: E_s = 0.1125 J, about 1443 sparks/s, P ≈ 162 W. A larger C raises energy per spark (faster removal, rougher surface); a smaller C gives a finer finish.

Common mistakes

• Swapping ECM polarity: the workpiece is the anode (+); if it were the cathode, metal would plate on to it instead of dissolving.
• Leaving out atomic mass and valency in the ECM formula, or mixing g and kg, cm³ and m³.
• Believing EDM or ECM can machine glass or ceramics – both need an electrically conductive workpiece.
• Choosing USM or AJM for ductile metals; they rely on brittle fracture.
• Calling LBM "free of heat damage" – it leaves a small heat-affected zone and recast layer.
• Using the supply voltage V₀ instead of the discharge voltage V_d in ½CV².

For GATE ME

• ECM numericals: MRR for a pure metal or alloy, equilibrium feed rate, time to machine a cavity.
• EDM RC circuit: charging time, spark energy, power, and the effect of C and R on MRR and finish.
• USM tool velocity, AJM and LBM parameters, and matching process to material (conductor or not, brittle or ductile).
• Mechanism-based MCQs: which energy form each process uses, and which process has no tool wear. Practise unit conversion between g/s, cm³/s and mm³/min.

Quick check

1. In ECM, which electrode is the workpiece connected to?
2. A USM tool vibrates at 25 kHz with 20 µm amplitude. What is its peak velocity?
3. Name two processes in this topic that can machine glass.
4. What happens to surface finish in EDM if the spark energy is increased?
5. Why is there no tool wear in ECM?

Answers: 1. The positive terminal (anode). 2. v = 2π × 25 000 × 20 × 10⁻⁶ ≈ 3.14 m/s. 3. Any two of USM, AJM and LBM. 4. It becomes rougher because each crater is larger. 5. Only anodic dissolution removes metal; the cathode tool only evolves hydrogen.