Sinker EDM
Sinker EDM — the ram EDM, the plunge EDM, the die-sinking EDM — is the electrical discharge process that erodes a cavity into conductive metal with electric sparks, and it is the process that makes the cavities of moulds and dies. The EDM machine that this wiki treats carries a shaped electrode — a form of graphite or copper made to the negative of the cavity — and lowers it into the workpiece; sparks jump between the electrode and the work, and each spark melts and vaporises a tiny crater of metal, so that the electrode’s shape is reproduced in the work as it sinks in. Because the spark erodes rather than cuts, the work needs no edge on a tool: sinker EDM machines the hardened steel, the carbide and the shapes that a cutting tool cannot reach, cutting a blind cavity with square corners where no milling cutter could go. It is the process of the injection mould, the forging die, the die-cast die and the hard tool-steel cavity. This entry sets out how the sparks do their work.
The spark that cuts
Sinker EDM removes metal by controlled spark erosion, and the physics of the spark is the whole process. The electrode and the work are immersed in a dielectric fluid — a hydrocarbon oil — and separated by a tiny gap, held to a few hundredths of a millimetre by the machine’s servo. A voltage is applied across the gap, and at its closest point the dielectric breaks down: a spark leaps across, its energy concentrated on a point of the work, and the heat of the spark — hotter than the surface of the sun — melts and vaporises a tiny crater of metal from the work, and a little from the electrode too. The spark is then quenched, the dielectric flushes the eroded debris away, and another spark strikes; the machine fires thousands of sparks a second, each taking its microscopic crater, and as the servo feeds the electrode down, the craters overlap and the cavity forms. The metal removal is slow but inexorable, and it is entirely electrical — no cutting force, no tool edge, no vibration, only sparks eroding the work a crater at a time.
The electrode and its wear
The shape of the cavity is carried by the electrode, and the electrode is the heart of the process’s economics. The electrode is made to the negative of the cavity: the die cavity is sunk by an electrode shaped like the cavity’s mirror image, so an electrode of the pocket’s form is machined first — from graphite, cheap and easy to machine and able to take a fine spark finish, or from copper, tougher and longer-wearing — and then sunk into the work. The spark erodes the electrode as well as the work, so the electrode wears, fastest on its corners and its fine detail; the user allows for the wear, and makes electrodes in a set — a roughing electrode to sink the cavity fast, and finishing electrodes, fresh and detailed, to cut the final form. The gap between electrode and work is itself a dimension, so the electrode is made slightly smaller than the cavity it sinks, the allowance set by the spark gap of the finishing cut. For a complex cavity, the electrode — or a set of them — can cost more than the machining that follows.
Cutting blind and complex
Sinker EDM’s value is that it makes shapes no rotating cutter can, and the shapes are the ones that this process is chosen for. A blind cavity — the pocket of a mould or die that must be sunk to a depth with a floor — is EDM’s home: the electrode sinks straight in, cutting the bottom as well as the sides, where a milling cutter would need to enter from the side or the top. A cavity with square internal corners is another: a milling cutter leaves the radius of its own nose in every internal corner, while the electrode, shaped with sharp corners, reproduces them in the work, so the square-cornered pocket that a mill cannot finish is sunk by EDM. And the work itself may be too hard to cut: the hardened tool steel, the heat-treated die, the carbide — metal that would destroy a cutter is eroded calmly by sparks, which do not care how hard the work is, only that it conducts.
The process at work
Running a sinker EDM is a process of setting and patience, and the practice is shaped by the spark. The dielectric flushes the gap, carrying the eroded debris away, and because a deep cavity traps the debris, the process often uses flushing through the electrode or a planetary motion — the electrode orbiting slightly as it sinks, opening the gap and letting the fluid reach the cut. The cut is run in stages: a roughing setting, with large sparks and a high current, removes the bulk of the metal fast and leaves a rough, cratered surface; a finishing setting, with small sparks and low energy, cuts the final form and leaves the smooth, matte surface that sinker EDM finishes with — the surface that this wiki measures under surface finish. The fine cavities are finished, polished and re-cut as needed, and the process runs for hours, the servo feeding the electrode down as the sparks erode, until the cavity is sunk to the depth the die needs.
Sinker EDM in the shop
Sinker EDM sits beside the other electrical discharge process — the wire EDM, which cuts through work with a moving wire rather than sinking a shaped electrode — and the two share the spark and the machine family. Where wire EDM cuts through a part, sinker EDM makes cavities in it, and the two together cover the spark-eroded work of the toolroom: wire EDM cutting the through-forms, sinker EDM sinking the blind pockets and the die cavities. Sinker EDM is not a fast process — its metal removal is slow and its electrodes are tooling — so it is used where nothing else can do the work: the mould cavity in hardened steel with its square corners, the die that must hold its form, the intricate pocket that a cutter cannot reach. It erodes the cavity a spark at a time, reproducing the electrode’s form in the hardest metal to a fine accuracy and a controlled surface, making the moulds and dies that shape the parts of production.