Traditional machining methods like turning, milling, grinding and drilling all involve tools that are harder than the workpiece. The essence of these processes is using a harder material to cut a softer one — somewhat like using a knife to cut vegetables or peel fruit.
Electrical Discharge Machining (EDM) is different. It can use softer tools, such as copper, to cut harder parts, like tool steel or carbide. So what is the principle of EDM, and why can soft tools machine hard materials?
How EDM works
In EDM, the tool does not physically contact the workpiece — there is no cutting force as in traditional machining. The tool serves as the electrode, and there is a gap between the tool and the workpiece, called the spark gap. This gap is typically between 0.005 mm and 0.3 mm, sometimes reaching 0.5 mm or larger.
Typically, the tool acts as the cathode, connected to the negative terminal of a DC pulse power supply, while the workpiece acts as the anode, connected to the positive terminal. The gap is filled with dielectric fluid.
When the tool approaches the workpiece, sparks are generated across the gap, producing extremely high electrical heat — up to 8,000–12,000°C — which melts and vaporizes part of the material on the workpiece surface. EDM is essentially an electrothermal erosion process: the melted material is washed away by the dielectric fluid.
How are sparks generated?
As the electrode approaches the workpiece, the electric field strength in the spark gap region exceeds the dielectric strength of the fluid at a certain point, causing the fluid to break down and become conductive. Current then flows from the electrode to the workpiece in the form of sparks.
Dielectric strength is the maximum voltage an insulating material can withstand before it breaks down and becomes conductive. Once the threshold is exceeded, the insulator breaks down and conducts electricity. When the pulse stops, the dielectric fluid returns to its insulating state.
Why is high-frequency current needed?
High-frequency pulses provide brief on-times, which ensures the heat generated by each spark is insufficient to transfer deep into the tool or workpiece, protecting them. During the off-time, the dielectric fluid helps flush away the eroded material and replenish the spark gap. This happens very rapidly — thousands or even tens of thousands of sparks occur every second.
Common types of EDM
- Wire EDM — the fine wire (brass, zinc-plated brass or tungsten, typically 0.18 mm or 0.25 mm in diameter) acts as the electrode, with deionized water as the dielectric fluid.
- EDM drilling — a narrow nozzle near the workpiece creates intermittent sparks that melt and vaporize material; the dielectric fluid delivered through the nozzle cools the process and flushes away material.
- EDM sinking (molding) — electrodes made from copper, brass, graphite, tungsten, copper-tungsten or carbide are used, suitable for creating complex parts with high precision.
Materials and accuracy
EDM can theoretically machine any conductive material. Since its material removal rate is slower than traditional methods, EDM is best suited to hardened tool steels, titanium, carbide, tungsten carbide, chrome-nickel alloys and other materials that are difficult to machine conventionally.
EDM offers high precision: wire EDM can achieve ±2.5 µm with surface roughness as fine as 0.15–0.2 µRa, while EDM sinking can reach accuracies of ±0.05 mm down to ±0.004 mm.
Advantages and disadvantages
Advantages:
- Wide material compatibility — any conductive material, regardless of toughness, hardness or microstructure
- No deformation — no physical contact means no cutting force
- Capable of machining complex contours via molding EDM
- High precision with excellent surface finish and tolerance control
- No burrs or tool marks on the workpiece surface
Disadvantages:
- Not suitable for non-conductive materials
- Slower material removal rate than conventional processes, which can increase time and cost
- Higher power consumption, making it more expensive than conventional milling or turning
Since EDM uses electrical and thermal energy to remove metal, it is largely independent of the strength and hardness of the material being machined — effectively achieving “soft tools cutting hard materials.”