ECM, chemical machining and USM

Why it matters

Turbine blades, gun-barrel rifling, aerospace skin panels, fine meshes, and holes in glass or ceramics all need processes that neither depend on tool hardness nor leave heat damage or burrs. ECM dissolves metal atom by atom with no tool wear, chemical machining etches large thin areas, and USM chips away hard, brittle non-conductors. Each has a simple quantitative model that GATE uses heavily for ECM.

Key ideas

Electrochemical machining (ECM)

• Reverse of electroplating: the work is the anode (+), the shaped tool the cathode (−), separated by a small gap (about 0.1–0.6 mm) through which electrolyte (NaCl or NaNO3 solution) is pumped at high speed. A low voltage (about 8–20 V) and very high current density (of the order of 0.1–2 A/mm²) dissolve the anode by Faraday's law; hydrogen is released at the cathode and metal hydroxides form as sludge.
• No tool wear (nothing deposits on the cathode), no heat-affected zone, no residual stress, no burrs; removal is independent of hardness, needs only electrical conductivity.
• The tool is fed at constant rate f; the gap settles to an equilibrium gap where dissolution rate equals feed rate. The shape copies the tool, allowing for this gap (tool design is iterative).
• Limitations: high power and capital cost, electrolyte handling and corrosion, sludge disposal, not for non-conductors, sharp internal corners difficult, accuracy around ±0.02–0.1 mm. Uses: turbine blades, deburring, rifling, cavities in superalloys. Electrochemical grinding (ECG) combines it with an abrasive wheel.

Chemical machining (CHM)

• Material is dissolved by an etchant (acid or alkali, e.g. ferric chloride for steels and copper) on areas not protected by a maskant. Variants: chemical milling (deep pockets on large panels, scribed and peeled maskant) and photochemical machining / blanking (thin sheets, photoresist mask — printed-circuit boards, shadow masks, fine screens).
• Etching proceeds sideways as well as downwards, producing undercut. The etch factor relates undercut to depth — check which way your textbook defines it (undercut/depth or depth/undercut).
• Low tooling cost, no burrs or stress, any shape on any area; but slow (removal rates of the order of 0.01–0.1 mm/min depth), limited depth and hazardous chemicals.

Ultrasonic machining (USM)

• A tool of soft, tough material (low-carbon steel, stainless steel) vibrates axially at about 20 kHz with an amplitude of 10–50 µm, driven by a magnetostrictive or piezoelectric transducer and a concentrator (horn). An abrasive slurry (boron carbide, SiC or Al2O3 in water) is fed under it under a small static load.
• Grains are hammered into the work and chip it by brittle micro-fracture; also cavitation and some free impacts. Works best on hard, brittle materials (glass, ceramics, quartz, carbides, gemstones) whether conductive or not; poor for ductile metals, which deform instead of chipping.
• MRR rises with frequency, amplitude and grit size (up to about the amplitude), abrasive concentration, and has an optimum static force; it falls with the work's fracture toughness. Tool wear is significant, and the hole is oversize by about twice the grit diameter.

Formulas

• m = I·t·A / (v·F) — mass dissolved by Faraday's law, g; I current (A), t time (s), A atomic weight (g/mol), v valency, F = 96 485 C/mol.
• Q = η·I·E / (ρ·F) — volumetric MRR, cm³/s (ρ in g/cm³) or mm³/s (ρ in g/mm³); E = A/v gram-equivalent weight; η current efficiency.
• E_alloy = 100 / Σ(x_i·v_i / A_i) — equivalent weight of an alloy, x_i = weight percent of element i.
• f = Q / A_t — tool feed rate at equilibrium, mm/s; A_t = frontal area of the tool (mm²).
• J = κ·(V − ΔV) / y — current density across gap y; κ electrolyte conductivity (S/mm), ΔV overvoltage (V).
• y_e = η·κ·(V − ΔV)·E / (ρ·F·f) — equilibrium gap, mm (consistent units: κ in S/mm, ρ in g/mm³, f in mm/s).
• d = r_e·t, u = E_f·d — chemical milling depth from etch rate r_e (mm/min) and time t (min); undercut u from etch factor E_f defined as undercut/depth.
• v_max = 2·π·f·a — peak tool velocity in USM, m/s; f frequency (Hz), a amplitude (m).

Worked examples

Example 1 (standard) — ECM of iron. Iron (A = 55.85 g/mol, v = 2, ρ = 7.86 g/cm³) is machined at I = 1000 A with 100 % efficiency, using a tool of frontal area 500 mm². Find the MRR and the tool feed rate.

1. E = A/v = 55.85/2 = 27.93 g.
2. Mass rate = I·E/F = 1000 × 27.93/96 485 = 0.2894 g/s.
3. Q = 0.2894/7.86 = 0.03682 cm³/s = 2.21 cm³/min (36.82 mm³/s).
4. f = Q/A_t = 36.82/500 = 0.0736 mm/s = 4.42 mm/min.

Example 2 (GATE level) — ECM of an alloy and the equilibrium gap. A nickel alloy contains (by weight) 72 % Ni (A = 58.71, v = 2), 20 % Cr (A = 52.00, v = 3) and 8 % Fe (A = 55.85, v = 2); ρ = 8.3 g/cm³. It is machined at 2000 A with a tool of frontal area 1000 mm², η = 1. Electrolyte conductivity κ = 0.02 S/mm, applied voltage 14 V with 2 V overvoltage. Find the equivalent weight, MRR, feed rate and equilibrium gap.

1. Σ(x·v/A) = 72 × 2/58.71 + 20 × 3/52.00 + 8 × 2/55.85 = 2.4527 + 1.1538 + 0.2865 = 3.893.
2. E_alloy = 100/3.893 = 25.69 g.
3. Q = I·E/(ρ·F) = 2000 × 25.69/(8.3 × 10⁻³ × 96 485) = 64.2 mm³/s.
4. f = Q/A_t = 64.2/1000 = 0.0642 mm/s = 3.85 mm/min.
5. y_e = κ·(V − ΔV)·E/(ρ·F·f) = 0.02 × 12 × 25.69/(8.3 × 10⁻³ × 96 485 × 0.0642) = 0.12 mm. Check: current density = 2000/1000 = 2 A/mm², and κ(V − ΔV)/y = 0.02 × 12/0.12 = 2 A/mm². ✓

Common mistakes

• Making the work the cathode in ECM — the work must be the anode, or metal plates onto it instead of dissolving.
• Using atomic weight instead of equivalent weight (A/v), or averaging equivalent weights linearly for an alloy instead of using the reciprocal rule.
• Mixing g/cm³ with mm³ — convert ρ to g/mm³ (divide by 1000) for MRR in mm³/s.
• Forgetting the overvoltage when computing the gap.
• Using USM on ductile metals; it works on hard, brittle materials.
• Assuming chemical etching is purely downward — undercut must be allowed for in the mask.

For GATE PI

• NAT on Faraday's law: MRR for pure metals and alloys, feed rate, equilibrium gap, current density, machining time.
• MCQs comparing ECM, CHM, USM, EDM, LBM: mechanism, suitable materials, tool wear, surface damage.
• Etch factor and maskant questions; USM parameter effects (amplitude, frequency, grit size, static load).

Quick check

1. In ECM, is the tool anodic or cathodic?
2. Copper (A = 63.5, v = 2) at 500 A: mass removal rate in g/s?
3. Why is tool wear zero in ECM?
4. Which material suits USM best: mild steel or glass?
5. Peak tool velocity for f = 20 kHz and amplitude 25 µm?

Answers: 1. Cathodic. 2. 500 × 31.75/96 485 = 0.1645 g/s. 3. Only hydrogen evolves at the cathode; metal does not deposit on it. 4. Glass. 5. 2π × 20 000 × 25 × 10⁻⁶ ≈ 3.14 m/s.