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

Why it matters

Forging dies, injection-mould cavities, diesel fuel-injector spray holes, turbine blades and hardened gears are often too hard, too intricate or too small for conventional cutting tools. Non-traditional (advanced) machining processes remove material with sparks, electrolysis, light, vibration or abrasive jets, so the tool need not be harder than the work. Knowing which process suits which material and feature – and the few equations that size them – is standard in exams and in tool-room practice.

Key ideas

Classification by energy.

• Mechanical: abrasive jet (AJM), ultrasonic (USM), water jet and abrasive water jet (WJM/AWJM).
• Electrochemical: electrochemical machining (ECM), electrochemical grinding (ECG).
• Thermal: electric discharge machining (EDM, wire-EDM), laser beam (LBM), electron beam (EBM), plasma arc machining (PAM).
• Chemical: chemical milling and blanking (etching).

EDM. A shaped tool electrode (graphite or copper) and a conductive workpiece are separated by a small gap (tens of micrometres) filled with a dielectric (kerosene/hydrocarbon oil, or deionised water for wire-EDM). Repeated sparks, each lasting microseconds, melt and vaporise tiny craters on both electrodes; the collapsing plasma channel and dielectric flushing remove the debris.

• Work must be electrically conductive, but its hardness is irrelevant – ideal for hardened die steels and carbides.
• Tool wear occurs, so polarity and electrode material are chosen to minimise it.
• Higher spark energy raises MRR but gives deeper craters, rougher surfaces and a thicker recast (white) layer with microcracks.
• Power supplies: RC relaxation circuits (classic exam model) and modern transistor pulse generators.
• Wire-EDM uses a travelling brass wire to cut through profiles – punch–die sets, extrusion dies, gear profiles.

ECM. The reverse of electroplating: the workpiece is the anode, the tool the cathode; a high-velocity electrolyte (NaCl or NaNO₃ solution) flows through a gap of about 0.1–1 mm at high current density and low voltage (about 5–30 V). Metal dissolves anodically following Faraday's laws.

• No tool wear, no heat-affected zone, no residual stress or burrs, and MRR independent of hardness.
• Accuracy is limited by stray machining at the side gaps; electrolyte handling, corrosion and disposal are costly.
• Used for turbine blades, rifling, deburring of intersecting holes (e.g. fuel systems), and die sinking.
• ECG combines electrolytic dissolution (most of the removal) with light grinding.

USM. A tool (horn) vibrates axially at about 20 kHz with an amplitude of a few tens of micrometres, pressing an abrasive slurry (B₄C, SiC, Al₂O₃ in water) against the work. Abrasive grains hammer the surface and chip it by brittle fracture. Best for hard, brittle materials – glass, ceramics, quartz, carbides – including non-conductors; ductile metals absorb the impacts and machine poorly. Tool wears; low MRR.

AJM / WJM. AJM: a fine abrasive (Al₂O₃ or SiC, tens of micrometres) carried by dry gas at high velocity (around 150–300 m/s) erodes brittle and heat-sensitive materials – frosting glass, deburring, cleaning, cutting thin brittle sheets. MRR rises with abrasive flow rate, particle size and velocity. WJM uses a very-high-pressure water jet (several hundred MPa) to cut soft materials (rubber, foam, gaskets, food); AWJM adds garnet abrasive to cut metals, stone and composites with no heat-affected zone.

LBM and EBM. LBM focuses a coherent beam (CO₂, Nd:YAG, fibre lasers) to a tiny spot, reaching power densities that melt and vaporise any material; assist gas (O₂ for steel, N₂ or Ar for clean edges) ejects the melt. Used for cutting sheet, drilling small holes, marking. Limits: reflective and highly conductive metals (Al, Cu) are harder to cut with CO₂ lasers; a small heat-affected zone and recast layer; taper in thick sections. EBM uses a beam of electrons in vacuum for very fine holes in any material.

Choosing a process. Hard conductive die steel cavity → EDM; complex profile through hardened plate → wire-EDM; contoured superalloy blade with no surface damage → ECM; small holes in glass or ceramic → USM; thin sheet profiles → LBM; brittle surface frosting/deburring → AJM; non-metallic soft sheet → WJM.

Formulas

MRR_m = A·I / (Z·F) MRR_v = A·I / (ρ·Z·F) ECM mass (g/s) and volume (cm³/s) removal rates with 100 % current efficiency; A = atomic mass (g/mol), I = current (A), Z = valency, F = 96 485 C/mol, ρ = density (g/cm³). Multiply by current efficiency if given.

f = MRR_v / A_t Equilibrium tool feed rate in ECM (mm/s); A_t = frontal area of tool (mm²).

V_c = V₀·(1 − e^(−t / RC)) t_c = R·C·ln[V₀ / (V₀ − V_d)] EDM relaxation circuit: capacitor voltage during charging, and charging time to reach the discharge voltage V_d; R in Ω, C in F, V in volts.

E = ½·C·V_d² P = E / t_c Energy per spark (J) and average spark power (W), neglecting discharge time. MRR in EDM is roughly proportional to spark energy × frequency, but the constant must come from data.

v_max = 2·π·f·a Peak tool velocity in USM (m/s); f = frequency (Hz), a = vibration amplitude (m).

q = P / (π·d² / 4) Laser power density (W/m²); P = beam power (W), d = spot diameter (m).

Worked examples

Example 1 (standard) – ECM of iron. Iron (A = 55.85 g/mol, Z = 2, ρ = 7.87 g/cm³) is machined at 1000 A, 100 % efficiency. Tool frontal area 2000 mm².

1. MRR_v = A·I/(ρ·Z·F) = 55.85 × 1000/(7.87 × 2 × 96 485) = 0.0368 cm³/s = 36.8 mm³/s.
2. Equilibrium feed: f = 36.8/2000 = 0.0184 mm/s = 1.10 mm/min.

Example 2 (GATE level) – EDM RC circuit. V₀ = 200 V, R = 50 Ω, C = 10 µF, discharge voltage V_d = 150 V. Neglect discharge time.

1. RC = 50 × 10 × 10⁻⁶ = 5 × 10⁻⁴ s.
2. t_c = RC·ln[V₀/(V₀ − V_d)] = 5 × 10⁻⁴ × ln(200/50) = 5 × 10⁻⁴ × 1.386 = 6.93 × 10⁻⁴ s ⇒ spark frequency ≈ 1443 Hz.
3. E = ½·C·V_d² = 0.5 × 10 × 10⁻⁶ × 150² = 0.1125 J per spark.
4. Average power P = 0.1125/6.93 × 10⁻⁴ = 162 W.

Example 3 – USM and LBM quick numbers. USM at 20 kHz, a = 15 µm: v_max = 2π × 20 000 × 15 × 10⁻⁶ = 1.88 m/s. A 1 kW laser focused to 0.2 mm: q = 1000/(π × 0.2²/4) = 31 830 W/mm² ≈ 3.2 × 10¹⁰ W/m².

Common mistakes

• Making the ECM workpiece the cathode – it is the anode.
• Forgetting valency Z or using density in kg/m³ with A in g/mol – keep consistent units.
• Treating spark energy × frequency as MRR – that product is power; MRR needs an empirical constant.
• Recommending USM for ductile metals or EDM for non-conductors (glass, ceramics).
• Claiming LBM has no heat-affected zone; it is small, not zero. ECM and AWJM are the ones with no HAZ.
• Using diameter instead of radius in the laser spot area (a factor of 4).

For GATE ME

Expect matching (process ↔ energy source ↔ medium ↔ typical application ↔ suitable material), statements on advantages and limitations, and numericals on ECM (Faraday's law MRR, feed rate, alloy removal), EDM (RC circuit charging time, energy per spark, power), USM tool velocity, and laser power density. Practise Faraday's law for alloys using the weighted equivalent.

Quick check

1. What is the workpiece polarity in ECM?
2. Which process suits drilling small holes in glass?
3. Energy per spark for C = 5 µF discharging at 100 V?
4. Which process leaves a recast (white) layer?
5. Why can EDM machine hardened tool steel easily?

Answers: 1. Anode (positive); 2. USM (or AJM for shallow features); 3. 0.025 J; 4. EDM (also LBM); 5. Removal is by spark erosion, which depends on thermal properties, not hardness.