Free EDM Spark Gap & Electrode Sizing Tool

1. The Spark Gap Overcut Phenomenon in Sinker EDM

In Electrical Discharge Machining (EDM), the tool electrode never physically touches the workpiece. A dielectric fluid (hydrocarbon oil or deionized water) insulates the interface until the voltage breaks down, generating a plasma channel at $8,000^circ ext{C}$ to $12,000^circ ext{C}$. This sparks across an active physical gap called the spark gap (or overcut).

Because sparks fire laterally in all directions, the eroded cavity is always larger than the electrode by the spark gap distance on every side: $$g_{side} = k cdot I_p^{0.38} cdot t_{on}^{0.28}$$ Where $I_p$ is peak pulse current, $t_{on}$ is pulse duration, and $k$ is an empirical constant governed by the dielectric fluid and workpiece conductivity.

2. Electrode Undersize & CNC Orbiting Motion

Modern CNC sinker EDM machines rarely plunge electrodes in a static 1D stroke. They employ planetary orbiting (circular, spherical, or vector orbiting). Orbiting sweeps debris out of the gap, dramatically reduces arcing risk, and produces uniform mirror finishes.

To produce a cavity of target width $W_{cavity}$, the physical electrode must be machined to: $$W_{electrode} = W_{cavity} - 2 cdot (g_{side} + R_{orbit})$$ Toolmakers typically fabricate at least two electrodes: a roughing electrode with large undersize ($0.25 - 0.50 ext{ mm}$ per side) to hog out bulk material at high current, and a finishing electrode with minimal undersize ($0.02 - 0.05 ext{ mm}$) to achieve precise tolerances and fine surface finish.

3. Pulse Energy & Surface Integrity

Each electric discharge delivers discrete thermal energy: $$W_e = V_e cdot I_p cdot t_{on}$$ High energy pulses ($> 50 ext{ mJ}$) excavate large craters, resulting in high Material Removal Rates (MRR) but leaving a thick, brittle recast layer (white layer) that contains micro-cracks. Fine finishing pulses ($< 1 ext{ mJ}$) yield velvet or mirror surface finishes ($Ra < 0.4 ext{ }mu ext{m}$) with negligible recast depth.