14.9 Non-Traditional Machining: EDM, ECM, USM, AJM, LBM & EBM

Why Non-Traditional Processes Exist

Conventional machining removes metal by mechanical shear with a tool harder than the workpiece. That requirement fails in four situations, and each of them motivates a non-traditional process:

1. The workpiece is harder than any tool material — hardened die steel, tungsten carbide, ceramics.
2. The shape is impossible for a rotating or reciprocating tool — deep narrow slots, sharp internal corners, complex three-dimensional cavities.
3. The part is too fragile or thin to withstand cutting forces.
4. No heat-affected zone or residual stress is acceptable.

Classification by Energy Source

Electrical Discharge Machining

Mechanism

Tool and workpiece are separated by a small gap, typically 0.01 to 0.5 mm, flooded with a dielectric fluid — usually kerosene or a specialised EDM oil. A pulsed DC voltage builds until the dielectric breaks down and a spark jumps the smallest gap. Local temperature reaches many thousands of degrees, melting and vaporising a tiny crater of metal, which the flowing dielectric flushes away. Millions of such discharges per minute erode the shape.

Key characteristics

• The tool never touches the work, so there is no cutting force. Delicate and thin sections can be machined.
• The workpiece must be electrically conductive. This is the single most important limitation and the most frequently examined point.
• Hardness is irrelevant — hardened tool steel machines as readily as annealed.
• The tool (electrode) is usually copper, graphite or brass, and it does wear, so wear ratio matters.
• The workpiece is normally the anode (positive) and the tool the cathode, because material removal is greater at the anode with the usual polarity.
• A recast layer and a heat-affected zone are produced, and surface finish improves as spark energy is reduced, at the cost of removal rate.

The dielectric serves three purposes: it insulates until breakdown, it flushes debris from the gap, and it cools the electrode and workpiece.

Wire EDM

A continuously travelling wire, typically 0.05 to 0.30 mm brass or coated copper, replaces the shaped electrode and cuts a contour like a bandsaw. Because the wire is consumed continuously, wear does not accumulate. Wire EDM is the standard method for producing blanking dies and punches with sharp internal corners.

Electrochemical Machining

Mechanism

ECM is the reverse of electroplating. The workpiece is the anode, the shaped tool the cathode, and an electrolyte — typically sodium chloride or sodium nitrate solution — is pumped through the gap at high velocity. Applying a low voltage of 5 to 25 volts at very high current density causes anodic dissolution: metal atoms leave the workpiece as ions and are carried away as hydroxide sludge.

Material removal rate

Removal follows Faraday's laws of electrolysis:

$m = \frac{I\,t\,A_w}{Z\,F}$

where $I$ is current, $t$ time, $A_w$ atomic weight, $Z$ valency and $F$ the Faraday constant of 96,500 coulombs per mole. In volumetric terms,

$MRR = \frac{I\,A_w}{\rho\,Z\,F}$

The removal rate depends only on the current and the material's electrochemical equivalent — not on the workpiece hardness at all.

Key characteristics

• No tool wear, because the cathode is not consumed. This is ECM's decisive advantage over EDM.
• No heat-affected zone, no residual stress, no burrs — the process is athermal.
• No cutting force.
• The workpiece must be conductive.
• Sharp internal corners cannot be produced, because the field spreads. Corner radii are inherent.
• Electrolyte is corrosive and the sludge requires disposal, so the plant is expensive.

Electrochemical grinding combines a conductive abrasive wheel with ECM action, with roughly 90% of removal electrochemical and 10% abrasive. It is used for sharpening carbide tools without the heat cracking that conventional grinding causes.

Ultrasonic Machining

A tool shaped to the required cavity vibrates axially at ultrasonic frequency, typically 20 kHz with an amplitude of 10 to 50 micrometres. An abrasive slurry — boron carbide, silicon carbide or aluminium oxide in water — flows between tool and work. The vibrating tool hammers the abrasive grains against the workpiece, chipping away microscopic fragments.

Key characteristics

• Suits hard and brittle materials: glass, ceramics, quartz, germanium, hardened steel.
• Does not require the workpiece to be conductive, which distinguishes it decisively from EDM and ECM.
• Ductile materials machine poorly, because they absorb the impact plastically rather than fracturing.
• The tool is a ductile material such as soft steel or brass, and it wears.
• Removal rate is low, so it suits small precise features rather than bulk removal.

The transducer is magnetostrictive or piezoelectric, and a horn or velocity transformer amplifies the amplitude to a usable value.

Abrasive Jet and Water Jet Machining

Abrasive jet machining directs a high-velocity stream of gas, usually dry air or nitrogen at 200 to 400 m/s, carrying fine abrasive particles of 10 to 50 micrometres. Material is removed by erosion. AJM suits hard, brittle and heat-sensitive materials, and is used for cutting thin sections, deburring, cleaning and etching. Removal rates are very low.

Water jet machining uses a fine jet of water at 200 to 400 MPa, cutting soft materials such as plastics, rubber, textiles, food and composites with no heat and no dust. Abrasive water jet machining adds garnet abrasive to the stream and will cut steel, titanium and stone.

AWJM has genuine relevance in mining and heavy industry: it cuts thick plate without a heat-affected zone, and because it produces no spark it can be used in potentially explosive atmospheres where flame cutting or grinding would be prohibited.

Laser and Electron Beam Machining

Laser beam machining

A coherent monochromatic beam is focused to a spot of a few micrometres, producing power densities sufficient to melt and vaporise any material. Common industrial lasers are CO2 (10.6 micrometre wavelength, good for non-metals and thick steel) and Nd:YAG or fibre (1.06 micrometres, better absorbed by metals).

Characteristics: no tool contact, works on conductors and non-conductors alike, very small heat-affected zone, extremely fine holes and slots possible, easily automated. Limitations are high capital cost, low energy efficiency, a taper on deep holes, and reflectivity problems with copper, aluminium and gold at longer wavelengths.

Electron beam machining

A focused stream of high-velocity electrons converts kinetic energy to heat on impact. It requires a vacuum chamber, which is its principal practical limitation, but produces extremely high depth-to-diameter ratios — holes with aspect ratios of 100 to 1 are achievable. It also generates X-rays, requiring shielding.

Comparison Table

Selection Framework

The practical decision usually starts with two questions: is the workpiece conductive, and is a heat-affected zone acceptable? Those two answers alone narrow the field to two or three candidates.