Lesson 6.6CutChapter 6 · Cutting and non-traditional machining

Sinker EDM

Sparks copy a shaped electrode into a blind cavity

Sinker EDM (electrical discharge machining, also called die-sinking or ram EDM) burns a shaped cavity into metal with tiny electric sparks. A graphite or copper electrode, shaped as the cavity's negative (bumps where the cavity needs hollows), sinks into the steel without ever touching it. Many molds for plastic parts, such as caps, phone cases and car interior trim, get their deep ribs and sharp inside corners this way.

Photo: CC BY 3.0, via Wikimedia Commons

Sinker EDM · cross-section of the machine and close-up of the spark gapCircled numbers match the steps below; the close-up on the right magnifies the gap under the electrode's tip.

How it works

Shape the electrode. A graphite or copper electrode is milled into the exact negative of the cavity, slightly undersize to leave room for the spark gap.
Submerge the workpiece. The steel block sits on the table in a tank of dielectric oil (an electrically insulating fluid). The oil blocks current until a spark forms, cools the metal and carries debris away.
Close the gap. A servo (a motor that constantly corrects its own position) lowers the electrode until only a tiny gap remains, typically a few hundredths of a millimeter. The two never touch.
Spark. The pulse generator fires short voltage pulses. Each one breaks through the oil, and a spark at roughly 8,000–12,000 °C melts and vaporizes a microscopic crater.
Flush and advance. Between pulses, oil sweeps the debris out of the gap. The servo holds the gap constant while the electrode sinks deeper, at thousands of sparks per second.
Finish gently. The machine switches to weaker sparks, often with a fresh finishing electrode and a small orbiting motion. Smaller sparks leave smaller craters, giving the final size and texture.

Key facts

Materials: Any electrically conductive material: hardened tool steel, tungsten carbide, titanium, nickel alloys, copper. Not plastics, glass or most ceramics.
Typical parts: Mold and die cavities, deep narrow ribs and slots, sharp inside corners, blind keyways and internal hexagons in hardened parts
Part size: From features under 1 mm up to cavities in mold blocks roughly a meter across
Precision: Typically about ±0.01 mm; a few microns (thousandths of a millimeter) with careful finishing
Surface finish: Matte and finely cratered. Roughness Ra (average height of surface bumps) runs from about 10 µm when roughing to about 0.2 µm when finishing.
Batch size it suits: One-offs and tooling (every mold is unique); small precision parts in modest series
Tooling and machine cost: Machine roughly $100,000–500,000; one custom electrode per shape, often several (roughing and finishing)
Speed: Slow: typically 2–400 mm³ of metal per minute; a mold cavity often takes hours to days

Strengths and limits

Strengths

Cuts hardened steel and carbide as easily as soft metal
No cutting force, so thin ribs and fine details survive
Sharp inside corners and deep blind cavities of almost any shape
Even, repeatable textures straight from the machine

Limits

Works only on electrically conductive materials
Slow: hours or days for a deep cavity
Needs a custom electrode for each shape, and electrodes wear
Leaves a thin recast layer (re-solidified, brittle skin) that may need polishing

Where it's used

Plastic injection moldsCavities for caps, housings, connectors and car interior parts
Die casting and forgingHardened dies for aluminum housings and forged hand tools
AerospaceShaped cooling holes in nickel-alloy turbine blades
ElectronicsMolds and stamping dies for tiny connector parts
Tool roomsRemoving broken taps and drills stuck in expensive parts

How to recognize it

A uniform matte, "frosted" surface made of tiny overlapping craters, with no tool marks running in one direction.
Sharp inside corners and deep, narrow ribs that no spinning cutter could reach.
On molded plastic parts, an even fine-grained matte texture often comes from a spark-eroded mold (graded on the VDI 3400 texture scale).
In the shop: dark grey graphite electrodes that look like precise "negatives" of the part.

Compare with

Wire EDM6.5same sparks, but a thin moving wire cuts through-profiles; sinker EDM makes blind (closed-bottom) cavities with a shaped electrode.Milling5.2much faster on soft metal, but a round cutter cannot leave sharp inside corners and struggles with hardened steel.Electrochemical machining6.7also copies a shaped tool, but dissolves the metal by electrolysis, with no heat damage and almost no tool wear.

Watch it

Practical Machinist · The basic physics of EDM, explained for beginners

temmy candra wijaya · An animation of the electrode, gap and sparks

Defiance Machine · A real machine burning a cavity under oil

Sodick Inc. · Automatic electrode changes for unattended running

See real photos

Wikipedia: Electrical discharge machining — principle diagram, sinker and wire EDM machines
Wikimedia Commons photo search

Remember

Sinker EDM copies a shaped electrode into any conductive metal, however hard, using tiny sparks in oil.
It is slow and needs custom electrodes, so it pays off for molds, dies and shapes nothing else can make.
Look for sharp inside corners and a uniform matte, cratered surface.