What spark machining is

Spark machining removes metal by electrical discharge. The work sits in a tank of dielectric fluid. An electrode hovers close to it. Voltage builds across the gap, a spark jumps, and a tiny crater forms in the steel. Thousands of sparks per second carve the shape.

No cutter touches the part. No cutting force pushes on thin walls. Hardness has no effect on the cut, because the process is thermal, not mechanical. Steel at 70 HRC cuts the same as steel at 30 HRC.

That last point is the whole reason mold shops own EDM machines. A mold insert is often hardened first, then cut. Milling hardened steel is slow and wears cutters fast. EDM cuts the hardened insert directly, with no softening step and no distortion from re-hardening.

Two kinds of EDM share the name. Sinker EDM, also called ram or die-sinking EDM, uses a shaped electrode to burn a cavity. Wire EDM uses a thin wire to slice profiles. This page covers sinker EDM, the kind that makes mold cavities. Wire EDM earns its own page.

The work must conduct electricity. EDM cannot cut ceramic, glass, or plastic. A conductive mold steel is the normal case. That limit rarely bites in mold work, but it rules out EDM for any non-metal insert.

How a sinker EDM cuts

The part, usually a mold insert blank, mounts on a platen and sinks into a tank of dielectric fluid. The tank sits on the machine base. The electrode mounts in a chuck on a vertical ram. A servo motor drives the ram up and down through a pinion.

The electrode carries the reverse shape of the cavity. It is the inverse of the impression, much like a hob in cold hobbing. Both the electrode and the work connect to a power supply. The electrode turns negative, and the work turns positive.

As the ram feeds down, sparks jump the small gap between electrode and work. The gap runs about 0.001 to 0.020 inch, set by the servo. Each spark melts and vaporizes a speck of steel. The fluid flushes the debris away and cools the cut.

Electrodes are usually graphite or copper. Graphite cuts fast and wears in a steady, known way. Copper holds fine detail and a better finish on small features. The shop picks by the shape, the finish, and the number of electrodes the job can afford.

Speed is the weakness. EDM removes metal far slower than milling. A deep cavity can take hours on the machine. That time is the price of cutting hard steel in shapes no cutter can reach.

The dielectric fluid

The fluid does four jobs at once. It insulates the gap until the voltage is high enough to spark. It cools the electrode and the work. It flushes debris out of the cut. It carries heat away so the surface does not crack.

Sinker EDM machines run on hydrocarbon oil. The old text on this page called it paraffin, and that was roughly right. Paraffin and kerosene belong to the same hydrocarbon family. Modern shops use a refined hydrocarbon oil made for EDM, not lamp paraffin. The name changed. The fluid family did not.

Wire EDM runs on deionized water instead. Water suits a wire that moves through the cut. Oil suits a sinker that holds a shape in one place. Mixing the two up is a quick way to spot a shop that does not run EDM often.

Fluid care matters more than most buyers know. As oil ages or fills with debris, sparks turn unstable. The cut slows, the finish roughens, and the electrode wears faster. Shops filter the fluid, watch its condition, and change it before it turns on them. Oil past five years in service is usually due for replacement, and a refractometer gives a firmer answer than a sniff test. A ScienceDirect study of EDM dielectrics tested water-based fluids against hydrocarbon oil.

Oil also brings fire risk and fumes. A good shop keeps the tank covered, the fluid level right, and the extraction running. Any buyer touring a shop can read its EDM discipline from the state of the fluid and the tank.

Electrode wear and cost

Electrodes wear as they cut. Every spark eats a little of the electrode too. That wear is the main cost driver in sinker EDM, and the main limit on its use.

Several electrodes are often needed to make one impression. A roughing electrode hogs out the bulk shape and takes the wear. A finishing electrode follows with a fresh face and light settings for size and finish. A complex cavity may need three or more electrodes, each machined to the same shape.

The bill grows with shape complexity. A simple pocket needs one electrode and little time. A deep rib with sharp internal corners needs several electrodes and many hours. That cost is why shops quote EDM by electrode count, not by the hour alone.

Electrode wear also sets the accuracy limit. Sinker EDM holds about 0.0002 to 0.001 inch on most mold work, depending on wear and shape. Fine finishing passes tighten that range. A shop that quotes tighter than its electrodes can hold is selling hope.

Ask the electrode count before you accept a quote. One electrode for a complex cavity means the shop plans to dress the same electrode and accept the wear. Three electrodes means it plans to finish with a fresh face. The price differs. So does the result.

Recast layer and surface finish

Every EDM cut leaves a recast layer, also called the white layer. Melted steel that did not flush away freezes back onto the surface. Under it sits a heat-affected zone. Both are thin, and both matter.

The layer runs from under 0.0001 inch on fine finishing settings to 0.001 inch or more on roughing settings. Roughing leaves a thick, cracked skin. Finishing passes with low energy thin it down and smooth the surface.

Surface finish follows the same ladder. A finish pass can reach Ra 0.2 to 0.4 micrometers, about 8 to 16 microinch. That is smooth enough for many mold surfaces straight off the machine. A cavity that needs a mirror still goes to mold polishing after EDM.

For most mold work, the recast layer is acceptable. It sits on tool steel that will be plated or polished anyway. Electroplating over a thin, sound recast layer works fine. A thick, cracked layer under chrome will peel with the chrome.

Fatigue-critical parts are the exception. Aerospace parts, medical implants, and rotating parts cannot keep a cracked white layer. Those parts get the layer removed after EDM, by abrasive flow, electrochemical polishing, or controlled shot peening. A mold cavity rarely needs that step, but a buyer should know it exists.

One shop habit tells the story. A shop that runs a finish pass and measures the layer is doing EDM right. A shop that stops at roughing and ships the part is leaving a cracked skin on your mold.

When EDM beats milling

EDM wins where a cutter cannot go. Deep, narrow ribs with sharp internal corners are the classic case. An end mill leaves a radius in every internal corner. An EDM electrode burns the corner sharp.

Hardened inserts are the second case. A cavity in H13 at full hardness, or a D2 insert that would distort if softened and re-hardened, cuts cleanly on EDM. Milling the same insert soft, then hardening it, risks movement that no amount of polishing fixes.

Repair work is the third case. A small pocket can be burned straight into a mold bolster, and a fresh insert fitted, without softening the bolster. Softening a bolster to mill a pocket risks distortion across the whole mold. EDM leaves the rest of the tool alone.

Blind pockets and high-aspect features follow the same logic. A pocket deeper than a cutter can reach, or a feature taller than its width, belongs on EDM. So do fine textures and lettering that a cutter would blur.

EDM loses where milling is fast and easy. A shallow pocket in soft steel, a flat face, or a simple slot should never see an EDM machine. Milling cuts those in minutes. EDM would take hours and cost more for the same result.

What to ask your shop before EDM

An EDM quote should answer five questions. Electrode count, electrode material, finish pass, recast layer, and delivery time. A quote that lists only a price and a date is missing the work.

Ask how many electrodes the job needs, and what each one does. Roughing, semi-finishing, and finishing should be named, not implied. Mold steel hardness is the reason for EDM in the first place. The quote should name the steel and its hardness too.

Ask what finish the cavity will leave the machine at. A number in Ra or microinch beats a word like smooth. Ask whether a finish pass is included, or priced apart. A quote without a finish pass is a roughing quote.

Ask about the recast layer on your part. Most mold cavities keep a thin layer with no trouble. If your part runs a fatigue-critical insert, say so before the job starts, so the shop plans a removal step.

Ask about flushing and fluid. Good flushing keeps the cut stable and the finish even. A shop that changes fluid on schedule and filters it well will say so plainly. A shop that cannot answer is running old oil.

Finally, ask what happens after EDM. A cavity that needs beryllium-copper inserts for cooling, or chrome for wear, should be planned as one job. EDM, polishing, and plating in sequence beat three separate surprises. For background, the Plastics Industry Association is a useful starting point for plastics manufacturing standards.