EDM parameters and tolerances
Electrical discharge machining (EDM) is a non-conventional machining process where material removal occurs through controlled electrical discharges between an electrode (tool) and the workpiece, both submerged in a dielectric fluid. The process allows machining complex geometries in conductive materials regardless of their hardness, since there is no mechanical contact between tool and workpiece.
Process parameters
Section titled “Process parameters”The operating parameters determine the removal rate, dimensional accuracy, and surface finish. Below are the typical ranges for sinker EDM and wire EDM.
| Parameter | Typical range | Unit |
|---|---|---|
| Discharge voltage | 40 – 300 | V |
| Peak current | 0,1 – 500 | A |
| Pulse duration | 1 – 2000 | µs |
| Pulse frequency | 0,5 – 500 | kHz |
| Interelectrode gap (GAP) | 0,01 – 0,5 mm / 0.0004 – 0.02 in | mm / in |
| Wire diameter (wire EDM) | 0,02 – 0,33 mm / 0.0008 – 0.013 in | mm / in |
| Dielectric pressure | 0,1 – 1,5 MPa / 14.5 – 217.5 psi | MPa / psi |
| Dielectric temperature | 20 – 40 °C / 68 – 104 °F | °C / °F |
Typical tolerances
Section titled “Typical tolerances”EDM offers exceptional dimensional accuracy compared to conventional chip removal processes, provided that appropriate finishing parameters and high-quality electrodes are used.
| Machining regime | Dimensional tolerance (μm / in) | Equivalent IT grade |
|---|---|---|
| Roughing | ±10 – 50 µm / ±0.0004 – 0.002 in | IT8 – IT10 |
| Semi–finishing | ±5 – 10 µm / ±0.0002 – 0.0004 in | IT6 – IT7 |
| Fine finishing | ±1 – 5 µm / ±0.00004 – 0.0002 in | IT5 – IT6 |
| Micro-EDM or multiple passes | ±0,5 – 2 µm / ±0.00002 – 0.00008 in | IT4 – IT5 |
Applicable materials
Section titled “Applicable materials”EDM requires that the workpiece material be electrically conductive. Hardness, toughness, or brittleness do not represent limitations for the process.
| Material | Machinability by EDM | Observations |
|---|---|---|
| Carbon and alloy steels | Excellent | Includes hardened tool steels |
| Stainless steels | Excellent | Requires corrosion control by dielectric |
| Titanium and nickel-based superalloys | Very good | Moderate removal rate, risk of microcracks |
| Tungsten carbide (WC-Co) | Good | Requires graphite or copper-tungsten electrodes |
| Aluminum and its alloys | Good | Avoid chip accumulation in the gap |
| Copper and brass | Good | Often used as electrode, not as workpiece |
| Conductive ceramics (e.g., SiC, TiB₂) | Limited | Only some compositions with sufficient conductivity (>0.01 S/cm) |
| Non-conductive materials (insulating ceramics, plastics) | Not applicable | Only through conductive layer assisted EDM process |
Surface finish
Section titled “Surface finish”Surface finish in EDM depends on the discharge energy, the electrode material, and the number of finishing passes. Roughness is usually expressed as Ra (arithmetic mean roughness).
| Machining stage | Roughness Ra (µm / μin) | Observations |
|---|---|---|
| Roughing (high energy) | 3,2 – 12,5 µm / 126 – 492 μin | High removal rate, thick white layer |
| Semi–finishing | 0,8 – 3,2 µm / 31 – 126 μin | Reduction of recast layer |
| Fine finishing (graphite/copper electrode) | 0,2 – 0,8 µm / 8 – 31 μin | Multiple passes with decreasing energy |
| Mirror finish (wire EDM or sinker with micro-pulses) | 0,04 – 0,2 µm / 1.6 – 8 μin | Requires strict control of dielectric and electrode |
Advantages and limitations
Section titled “Advantages and limitations”The choice of EDM must consider the balance between unique technical capabilities and production constraints.
| Advantages | Limitations |
|---|---|
| Machines any conductive material, regardless of hardness | Only applicable to materials with electrical conductivity (σ > 0.1 S/cm) |
| Allows complex geometries with minimum internal corner radii (0.05 – 0.1 mm / 0.002 – 0.004 in) | Low removal rate compared to conventional milling (typically 1 – 500 mm³/min) |
| Process without cutting forces: ideal for thin or fragile parts | Generates a white layer (re-solidified) and heat-affected zone (HAZ) of 2 – 50 µm |
| Repeatable dimensional tolerances of ±1 µm / ±0.00004 in | Risk of microcracks in hard materials if pulse energy is not controlled |
| Surface finishes up to Ra 0.04 µm / 1.6 μin without subsequent processes | Requires precision electrodes and electrode wear (1:1 to 100:1 depending on material) |
| High automation, unattended operation | High hourly cost (machinery, dielectric, electrodes) |
Selection guide
Section titled “Selection guide”Use the following criteria to decide whether EDM is the appropriate technology for your application:
- The material hardness exceeds 45 HRC and conventional processes show excessive tool wear → Sinker EDM.
- Complex blind cavities (injection molds, forging dies) with vertical or negative walls are required → Sinker EDM with graphite electrode.
- Contour cutting in thick plate (up to 300 mm / 11.8 in) with tight tolerances is needed → Wire EDM.
- The workpiece has thin sections (<0.5 mm / 0.02 in) that would not withstand cutting forces → EDM (both variants).
- A surface finish Ra < 0.2 µm / 8 μin is sought in materials difficult to polish mechanically → Finishing EDM with multiple passes.
- Electrical conductivity is < 0.01 S/cm → alternative processes (grinding, laser, ultrasonic).
- The removal rate must exceed 1000 mm³/min in steel → high-speed milling or turning, not EDM.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What dimensional tolerance can be achieved in wire EDM with finishing passes?
Section titled “What dimensional tolerance can be achieved in wire EDM with finishing passes?”A tolerance of ±0.001 mm / ±0.00004 in can be achieved under optimal shop conditions using at least three finishing passes (skim cuts) and thermal control of the dielectric.
What is the maximum thickness that a wire EDM machine can cut?
Section titled “What is the maximum thickness that a wire EDM machine can cut?”Standard industrial machines cut up to 300 mm / 11.8 in thickness in steel, although special high-capacity models can exceed 500 mm / 19.7 in with reduced feed rates.
What Ra roughness is typical in a fine finish sinker EDM with graphite electrode?
Section titled “What Ra roughness is typical in a fine finish sinker EDM with graphite electrode?”A typical fine finish achieves a Ra roughness of 0.4 µm / 16 μin, and with controlled micro-pulses it can drop to 0.1 µm / 4 μin.
How much white layer (recast) does a roughing EDM process generate?
Section titled “How much white layer (recast) does a roughing EDM process generate?”The white layer in roughing can reach a thickness of 30 to 50 µm / 0.0012 to 0.002 in, which must be removed by finishing passes or subsequent polishing to avoid fatigue failure.
What material removal rate (MRR) can be expected when machining tool steel by EDM?
Section titled “What material removal rate (MRR) can be expected when machining tool steel by EDM?”In roughing regime with graphite electrode and peak current of 50 A, a removal rate of up to 500 mm³/min / 0.03 in³/min is obtained; in fine finishing the MRR drops to 1–5 mm³/min.
What is the minimum taper that can be obtained in a wire EDM cut?
Section titled “What is the minimum taper that can be obtained in a wire EDM cut?”The natural taper without correction by axis control is typically 0.01 – 0.02 mm per 10 mm of height / 0.0004 – 0.0008 in per 0.4 in of thickness, although with CNC compensation it is completely neutralized.
Consulted sources
Section titled “Consulted sources”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/machine-processes-tolerance-grades-d_1367.html
- manufacturingguide.com: https://www.manufacturingguide.com/en/wire-edm
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/machine-processes-tolerance-grades-d_1367.html
- manufacturingguide.com: https://www.manufacturingguide.com/en/wire-edm