Non-traditional machining

In short

Non-traditional (unconventional) machining removes material by thermal, chemical, electrochemical or high-velocity mechanical action instead of a sharp tool shearing a chip. It became essential because modern parts use hard, brittle or tough materials (carbides, titanium, Inconel, ceramics, hardened steel) and shapes - deep small holes, complex cavities, thin slots - that no cutting tool can reach. The processes are classified by the energy used, and exams test MRR formulas, process selection and process capability. Heavily asked in GATE, ESE and university Manufacturing papers, and used daily at HAL, BHEL, ISRO/VSSC, BEL, Godrej & Boyce, Bharat Forge, tool rooms (Indo-German, CTTC) and die-mould shops in Pune, Rajkot, Coimbatore and Faridabad.

Explained

Why "non-traditional", and how the processes are classified

• In conventional machining a harder tool shears the work; forces are high and the tool must be harder than the work. Non-traditional processes avoid both limits - there is often no tool-work contact and no cutting force.
• Driving need: materials with hardness above ~45 HRC, low machinability (Ti, Inconel, Nimonic), brittleness (glass, ceramics), and features like 0.2 mm holes, aspect ratios over 20:1, sharp internal corners and complex 3-D die cavities.
• Classification by energy source:

• Hybrid processes combine two: ECG (ECM + grinding), EDG, ultrasonic-assisted EDM, laser-assisted turning.
• Common features: low MRR compared with turning/milling, high specific energy, higher operating cost per cm³, but the ability to do work no conventional machine can.

Electrical Discharge Machining (EDM) - the most examined process

• Principle: repeated sparks between a shaped tool (cathode) and workpiece (anode) submerged in a dielectric; each spark melts and vaporises a tiny crater. Work must be electrically conductive; hardness is irrelevant.
• Circuit: classic RC relaxation circuit (resistance-capacitance); modern machines use transistorised pulse generators giving independent control of pulse on-time, off-time, current and voltage.
• Spark gap 10-100 µm maintained by a servo feed; open-circuit voltage ~40-250 V, working gap voltage ~20-40 V for kerosene.
• Dielectric (kerosene, EDM oil, deionised water): insulates until breakdown, concentrates the spark, flushes debris, cools the gap. Supplied by injection, suction or side flushing.
• Overcut: the cavity is larger than the tool by roughly the spark gap all round, so tool size = cavity size − 2 × overcut.
• Tool (electrode) materials: electrolytic copper, graphite (best MRR, low wear), brass, copper-tungsten, tungsten for fine holes. Wear ratio = volume of tool worn / volume of work removed; graphite gives <1%, brass may exceed 50%.
• Surface: recast (white) layer of resolidified metal plus heat-affected zone - hard, micro-cracked, tensile residual stress; removed by polishing or a finish "trim" pass where fatigue matters.
• Polarity: straight polarity (work +ve) for most steels, as more heat goes to the anode in long pulses; reverse polarity in short-pulse finishing and for some materials.
• Duty cycle = Ton/(Ton + Toff). Long Ton → high MRR, rough surface, bigger HAZ; short Ton → fine finish (Ra down to ~0.2-0.4 µm).
• Variants: die-sinking EDM (3-D cavities); WEDM - a travelling 0.1-0.3 mm brass/zinc-coated wire cuts 2-D and taper profiles under CNC with deionised water, wire used once; EDM drilling / hole popping with rotating tube electrodes for 0.3-3 mm diameter deep holes (used for turbine-blade cooling holes and to make start holes for WEDM); EDG (grinding with a rotating conductive wheel); micro-EDM.
• Advantages: any conductive material irrespective of hardness, no cutting force (thin and fragile sections), sharp internal corners, intricate cavities, good accuracy (±5-25 µm).
• Limitations: conductive materials only, slow (MRR typically 1-400 mm³/min), tool wear, recast layer, taper in deep cavities, high power cost.

Electrochemical machining (ECM) and its family

• Principle: controlled anodic dissolution (reverse electroplating). Work = anode, shaped tool = cathode, electrolyte (NaCl or NaNO₃, 15-25% by weight) pumped through the gap at 20-60 m/s; 5-30 V DC, current density 20-200 A/cm².
• Removal obeys Faraday's law - material leaves atom by atom; no tool wear in theory (cathode is not consumed; H₂ evolves there) and no thermal damage, no residual stress, burr-free.
• Gap 0.1-0.5 mm, kept constant so dissolution rate = feed rate (equilibrium gap h = κ·(E − ΔV)·ε /(ρ·F·f), derived in exams).
• Hydroxide sludge (Fe(OH)₃ with steel) must be removed by centrifuge/settling - effluent handling is regulated, requiring consent from the State Pollution Control Board under the Water (Prevention and Control of Pollution) Act, 1974.
• Advantages: very high MRR for hard alloys, excellent finish (0.1-0.8 µm Ra), complex shapes in one pass, no stress or HAZ, suits thin and delicate parts.
• Limitations: conductive work only, cannot make sharp internal corners (radius ≈ gap), costly tooling and corrosion-resistant machine, electrolyte handling, poor for non-homogeneous alloys, high power (kW per cm³/min).
• ECG (electrochemical grinding): a metal-bonded diamond wheel acts as cathode; ~90% removal electrochemical, abrasive grains both cut the passive film and act as insulating spacers. Used for carbide tools and honeycomb edges.
• ECH (electrochemical honing), ECD (deburring), electropolishing, STEM (shaped tube electrolytic machining) for deep small holes in turbine blades.

Mechanical-energy processes: USM, AJM, WJM/AWJM

• Ultrasonic machining (USM): a tool (soft steel, brass) vibrates at 19-25 kHz, amplitude 10-50 µm, driven by a piezoelectric/magnetostrictive transducer through a horn (velocity transformer / concentrator); abrasive slurry (B₄C, SiC, Al₂O₃ in water, 20-30% by weight) between tool and work. Removal is by brittle fracture from hammering and free-impact abrasion.
• USM suits hard, brittle, non-conductive materials - glass, quartz, ceramics, ferrite, germanium, carbide, precious stones. Ductile metals absorb the impact and machine poorly. Tool wears and must be tapered-compensated; hole is oversize by about twice the grit size.
• Abrasive jet machining (AJM): dry abrasive (Al₂O₃, SiC, 10-50 µm) carried in air/N₂/CO₂ at 2-10 bar through a 0.2-0.8 mm sapphire/WC nozzle at 150-300 m/s; removal by erosion. For cleaning, deburring, etching, cutting thin brittle sections, trimming resistors. Nozzle life and dust control are the practical issues.
• Water jet machining (WJM): plain water at 150-400 MPa through a 0.1-0.4 mm orifice, speed ~900 m/s; cuts soft/non-metals - paper, leather, foam, plastics, food, insulation. No dust, no heat.
• Abrasive water jet machining (AWJM): garnet abrasive entrained into the jet - cuts almost anything, 100 mm+ thick steel, titanium, granite, glass, composites, with no HAZ; kerf tapers and the exit lags (striation) at high traverse. Widely used in Indian fabrication shops for stainless, Hardox and granite.

Thermal-beam processes: LBM, EBM, PAM, and chemical machining

• Laser beam machining (LBM): a coherent monochromatic beam focused to 10-100 µm gives 10⁶-10⁹ W/cm², melting and vaporising material; assist gas (O₂ for steel - exothermic; N₂ for stainless; Ar for reactive metals) blows out the melt. Nd:YAG / fibre (1.06 µm) for metals and drilling; CO₂ (10.6 µm) for sheet cutting and non-metals (not for highly reflective copper at that wavelength); excimer for micromachining. No tool, any material, very small holes and thin kerf, but HAZ, dross, taper, limited thickness (~25 mm), eye hazard (laser safety classes; IS/IEC 60825).
• Electron beam machining (EBM): electrons accelerated to 50-200 kV focused in a vacuum chamber; power density up to 10⁹ W/cm². Micro-holes 25 µm-1 mm, aspect ratio up to 100:1, perforating filters and spinneret plates. Vacuum requirement limits size and productivity; X-ray shielding needed; work need not be conductive but is usually so.
• Plasma arc machining/cutting (PAM): an ionised gas jet at 10,000-30,000 °C cuts any conductive metal fast; large kerf, big HAZ, noise, fume. Standard for thick stainless and aluminium plate; CNC plasma tables are everywhere in Indian structural fabrication.
• Ion beam machining (IBM)/sputtering: atom-by-atom removal for polishing and thinning in semiconductor work.
• Chemical machining (CHM): a maskant (screen-printed, photoresist or cut-and-peel tape) protects areas; the etchant (FeCl₃, HNO₃, NaOH for aluminium) dissolves the rest. Chemical milling lightens aircraft skins; photochemical blanking makes burr-free thin shims, lead frames and encoder discs. Etch factor = depth of etch / undercut; etching is not directional, so an undercut always occurs.

Process selection, capability and safety

• Selection depends on: work material (conductive? brittle? reflective?), shape and feature size, thickness, tolerance and Ra, batch size, tolerance to heat damage and cost per part.
• Rough capability guide (machine- and setting-dependent, so treat as indicative):

• No cutting force in EDM, ECM, LBM, EBM, AJM, WJM - hence ideal for thin, flexible, fragile and already-hardened parts, and for fixtures that only locate (see jigs and fixtures).
• Safety and statutes: the OSH Code, 2020 (in force 21 November 2025, replacing the Factories Act, 1948) and the saved State Factory Rules cover fume extraction, machine guarding and hazardous-process notification; electrolyte and etchant effluent falls under the Water Act, 1974 and the Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016; laser safety to IS/IEC 60825; EDM dielectric is a flammable liquid needing fire precautions. Verify current amendments before quoting in practice.

Key points

• Non-traditional machining - removal by thermal, chemical, electrochemical or erosive action, not by chip shearing.
• EDM - spark erosion in a dielectric; conductive work only; hardness irrelevant; melts and vaporises.
• Spark gap / overcut - cavity ends up larger than the electrode by about the gap on each side.
• Dielectric - insulates, concentrates the discharge, flushes debris, cools; kerosene or deionised water.
• Wear ratio - tool volume lost per unit work volume removed; graphite lowest, brass highest.
• Recast layer - resolidified melt plus HAZ left by EDM/LBM; micro-cracked, must be removed for fatigue parts.
• WEDM - CNC profile cutting by a consumable 0.25 mm wire; needs a start hole or an edge; no 3-D cavity.
• ECM - anodic dissolution by Faraday's law; no tool wear, no HAZ, no residual stress, burr-free.
• ECM corner limit - internal radius cannot be smaller than the machining gap; sharp corners impossible.
• ECG - ~90% electrochemical plus abrasive action; standard for carbide tool sharpening.
• USM - 19-25 kHz, abrasive slurry, brittle fracture; the standard route for glass, ceramics and quartz holes.
• AJM vs WJM vs AWJM - dry abrasive in gas; plain high-pressure water for soft materials; abrasive water jet for anything thick with no HAZ.
• LBM - focused light, any material, micro-holes, thin kerf, but HAZ, dross and taper; CO₂ for non-metals, fibre/Nd:YAG for metals.
• EBM - needs vacuum; highest aspect ratio micro-holes (up to ~100:1).
• PAM - fastest thick-plate cutting of conductive metals; large HAZ and kerf.
• CHM - maskant plus etchant; etch factor relates depth to undercut; photochemical blanking gives burr-free thin parts.
• MRR hierarchy - PAM/AWJM (highest) > ECM > EDM > USM > LBM > AJM > EBM/IBM (lowest).
• Specific energy is far higher than in turning, so these processes are chosen for capability, not economy.

Formulas and examples

Example 1 - ECM MRR for iron. Machining pure iron (A = 56, Z = 2, ρ = 7.8 g/cm³) at I = 1,000 A for 1 minute.

• ε = 56/2 = 28 g/mol-equivalent. m = (1,000 × 60 × 28)/96,500 = 1,680,000/96,500 = 17.41 g.
• Volume = 17.41/7.8 = 2.232 cm³ in 60 s → MRR = 2,232 mm³/min ≈ 37.2 mm³/s.

Example 2 - equilibrium gap and feed in ECM. Same iron, applied 15 V with 3 V overvoltage loss, κ = 0.02 Ω⁻¹mm⁻¹, feed 1 mm/min.

• Effective voltage = 15 − 3 = 12 V. ρ = 0.0078 g/mm³, f = 1/60 mm/s.
• h = κ(E−ΔV)·ε/(ρ·F·f) = (0.02 × 12 × 28)/(0.0078 × 96,500 × 0.016667) = 6.72/12.543 = 0.536 mm.
• Current density J = κ(E−ΔV)/h = 0.02 × 12/0.536 = 0.448 A/mm² (44.8 A/cm²). Doubling the feed halves the gap and doubles the current density - the gap self-adjusts.

Example 3 - EDM spark energy and MRR. RC circuit, C = 20 µF, charged to a breakdown voltage of 100 V, sparking at 2,000 Hz; 1.5% of the energy removes steel needing 10 J/mm³ effectively.

• Energy per spark = ½ × 20 × 10⁻⁶ × 100² = ½ × 20 × 10⁻⁶ × 10,000 = 0.1 J.
• Power = 0.1 × 2,000 = 200 W. Useful power = 0.015 × 200 = 3 W.
• MRR = 3/10 = 0.3 mm³/s = 18 mm³/min. (Doubling V quadruples spark energy - the fastest lever on MRR, at the cost of a rougher surface.)

Example 4 - EDM electrode sizing. A square pocket 20.00 mm × 20.00 mm is to be sunk with a measured side spark gap of 0.04 mm.

• Electrode side = 20.00 − 2 × 0.04 = 19.92 mm. If the roughing gap is 0.12 mm, the roughing electrode is 19.76 mm and a separate finishing electrode is needed - standard practice is one rougher plus one finisher per cavity.

Example 5 - abrasive water jet vs laser for 25 mm stainless plate.

Where it is used

• Tool, die and mould making: EDM sinking of forging dies and plastic-injection cavities, WEDM of punches, dies and extrusion profiles - Rajkot, Pune, Faridabad, Coimbatore clusters; government tool rooms (Indo-German Tool Room, CTTC Bhubaneswar) run job-work EDM and WEDM.
• Aerospace and power: HAL, GTRE, BHEL and ISRO/VSSC use EDM/STEM-type drilling for turbine-blade cooling holes, ECM for blisks and blade profiles, EBM/LBM for micro-holes and injector orifices.
• Electronics and precision parts: BEL and semiconductor/PCB suppliers use photochemical blanking for lead frames and shims, laser trimming and drilling, IBM for thinning.
• Fabrication and construction: CNC plasma and AWJM tables cutting plate for railway coaches (ICF/RCF suppliers), shipyards (CSL, MDL), Hardox wear parts and architectural metal; granite and tile processing in Rajasthan and Tamil Nadu.
• Medical, jewellery and automotive: WEDM of stents and surgical instruments, laser welding/cutting of implants, USM of sapphire and gemstones, laser marking for traceability on auto components.
• Job roles: EDM/WEDM programmer and setter, laser/plasma CNC operator, process engineer in tool room, NPD engineer specifying non-traditional routes, quality engineer assessing recast layer and edge integrity.

Interview questions

• Q: Why can EDM machine hardened die steel but not an alumina ceramic? A: EDM removes material by electrical discharge, which needs an electrically conductive workpiece; hardness does not matter, but a non-conductive ceramic cannot carry the discharge current, so no spark erosion occurs.
• Q: What are the four jobs of the dielectric in EDM? A: Insulate the gap until breakdown, concentrate the discharge channel, flush away debris and cool the tool and work.
• Q: Why is there no tool wear in ECM? A: Material leaves the workpiece by anodic dissolution; at the cathode tool only hydrogen gas is liberated, so the tool is not consumed.
• Q: Why can't ECM produce a sharp internal corner? A: Dissolution occurs across the whole machining gap, so the smallest radius it can produce is of the order of that gap (0.1-0.5 mm).
• Q: How is an EDM electrode sized for a required cavity? A: Reduce each dimension by twice the spark gap (overcut) for that setting, and use separate roughing and finishing electrodes since the gap changes with current.
• Q: Which process would you choose for a 3 mm hole in 10 mm thick glass, and why? A: Ultrasonic machining - glass is hard, brittle and non-conductive, so USM's abrasive brittle-fracture mechanism works where EDM and ECM cannot.
• Q: Why is USM unsuitable for mild steel? A: Ductile materials absorb the abrasive impact plastically instead of fracturing, so the removal rate collapses and the tool wears heavily.
• Q: What is the recast layer and why does it matter? A: A thin resolidified, micro-cracked and tensile-stressed layer left by EDM; it reduces fatigue life, so it is removed by a finishing pass, polishing or etching on critical parts.
• Q: AWJM or laser for 20 mm titanium? A: AWJM - it is a cold cut with no HAZ or alpha-case, whereas a laser at that thickness gives heat damage, dross and taper.
• Q: Why does EBM need a vacuum? A: Electrons would be scattered and absorbed by air molecules, so the beam cannot be focused or delivered at full power outside a vacuum.
• Q: What limits MRR in EDM, and what is the trade-off? A: Pulse energy and frequency; raising current and on-time increases MRR but worsens surface finish, electrode wear and the heat-affected depth.
• Q: Give one hybrid non-traditional process and what each part contributes. A: ECG - the electrochemical action does about 90% of the removal with no stress or heat, while the diamond grains remove the passivating film and set the gap, giving a burr-free, crack-free finish on carbide.

Common mistakes

• Treating EDM as suitable for any material - it needs electrical conductivity, so plastics, glass and most ceramics are out.
• Reversing the electrical roles in ECM: the workpiece is the anode and the tool the cathode; swapping them would plate the work instead of removing it.
• Forgetting the spark-gap overcut when sizing an EDM electrode, giving an oversize cavity.
• Using ε = A/Z wrongly for alloys - the alloy equivalent must be combined through 1/ε = Σ(xᵢZᵢ/Aᵢ), not by averaging atomic weights.
• Unit slips in Faraday calculations: mixing g/cm³ with mm³, or minutes with seconds, is the commonest loss of marks.
• Claiming WEDM can cut 3-D closed cavities - the wire must pass through, so it needs a start hole or an open edge and produces ruled or tapered surfaces only.
• Ignoring HAZ and residual stress when recommending laser or EDM for fatigue-critical parts, where ECM, AWJM or USM leave none.

Related topics

• Theory of metal cutting · Machining and machine tools · CNC machining and CAM · Grinding and finishing processes · Metrology and quality control · Additive manufacturing (3D printing)