In sinker EDM — also called die sinking or ram EDM — the electrode never touches the workpiece. All of the electrical discharge machining happens in a gap a few hundredths of a millimetre wide, and that gap is full of dielectric oil. The generator decides how much energy goes into each spark; the oil decides whether that energy turns into a clean, controlled discharge or into arcing, burn marks and a worn electrode. Below, the sinker EDM process step by step, and what the fluid is doing at each step.
A single discharge lasts microseconds, and the machine fires thousands of them per second. Each one goes through the same sequence:
Meanwhile the servo keeps moving the electrode to hold the gap constant, reading it through the gap voltage. Dirty oil changes how the gap behaves electrically, so the servo starts retracting and re-approaching instead of cutting. On the machine this looks like a slow, nervous burn.
Graphite is light, fast to machine and handles high currents, which makes it the usual choice for roughing and large electrodes. It also sheds fine carbon into the oil, so filtration has more to do.
Copper is the classic choice for fine finishing and fine detail, at the cost of being slower to make.
With either material the oil plays a part in electrode wear. During each spark a little of the oil cracks and leaves a thin carbon layer on the electrode, which protects it. The balance matters: clean oil and good flushing keep that layer thin and useful; contaminated oil and poor flushing turn it into deposits and unstable sparking.
The oil only helps where it actually reaches. Shops flush by pressure through the electrode or the workpiece, by suction from below the cavity, with side nozzles, and with electrode jumps — the servo lifts the electrode periodically so fresh oil is pumped into the gap.
All of these work better with a thin oil. A low-viscosity dielectric gets into narrow gaps, deep ribs and blind cavities and carries debris out of them. Thick or contaminated oil stays behind at the bottom of a deep burn, the particles build up, and secondary discharges start eating the walls and the electrode. Modern sinker EDM oils sit around 2–3 mm²/s at 40 °C for this reason.
An EDM surface is thousands of overlapping craters. Lower energy per spark means smaller craters and a finer finish, and the finishing passes run with a very small gap. On top of the surface sits a thin recast layer — metal that melted and solidified again. Debris that is not flushed out gets welded back into it, and the surface comes out rougher and uneven.
That is why fine finishing asks the most of the oil: it has to be clean, thin enough for a very narrow gap and stable in its insulation, pass after pass.
An EDM dielectric lasts as long as the filters can keep it clean. Change filters on the schedule your machine maker gives, keep the tank temperature steady, let parts drain over the tank before you take them out, and top up with the same oil. Watch for the early signs: the oil darkens, turns hazy, starts to smell, and machining slows down with more arcing. When fresh filters no longer bring the oil back to clear, it is time to change it — not because of a date on the calendar.
| Symptom | What the dielectric may be doing | What to check |
|---|---|---|
| Arcing, burn marks | Debris stays in the gap; oil does not recover between pulses | Flushing, pulse interval, filter condition, oil age |
| Slow erosion, servo keeps retracting | Contaminated oil makes the gap unstable | Filters, oil clarity, flushing into deep areas |
| High electrode wear | Unstable sparking; flushing strips the protective carbon layer | Flushing pressure, generator settings, oil condition |
| Rough or uneven finish | Debris welded back into the recast layer | Filtration, oil cleanliness, a finishing-grade oil |
| Oil darkens fast, smells | Overheating and oxidation | Tank temperature, filter load, time to change the fill |
| Level drops quickly | Drag-out and evaporation | Draining parts over the tank, tank cover, a low-volatility oil |
We keep two EDM oils in the range. EDM601 is the universal one: a low-aromatic mineral dielectric that covers roughing through finishing in the same tank, with a flash point of 102 °C. EDM101 is a synthetic, odourless oil made for oil-based wire EDM and ultra-fine sinker finishing. Both are compared side by side on the EDM oil page.
