Wire EDM Cutting

Wire EDM cutting is the process that severs a profile through conductive metal with a thin, charged wire, eroding its way through with electric sparks rather than cutting edges. The wire EDM machine that this wiki treats is the tool of the process; this entry is the process itself — how a part is cut by a wire, and how that cutting is planned and run. A wire, no thicker than a hair or a pin, is stretched taut through the work and charged; sparks leap between the wire and the metal, eroding a narrow slot — the kerf — through the work, and the machine guides the wire along a programmed path, so the wire cuts a profile out of the part as it travels. Because the wire erodes rather than cuts, and because it is guided by the machine to a fine accuracy, wire EDM is the precision way to make dies, punches and accurate shapes in hardened steel. It is the through-cutting partner of the sinker EDM, which sinks blind cavities.

Cutting with a moving wire

The wire is a continuously moving electrode, and its motion is part of how it cuts. The wire — a spool of brass, coated or plain, drawn to a diameter finer than a millimetre — is fed from the spool through the work and away, so that the short length in the cut is always fresh; the used wire is discarded, not reused, so the electrode never wears and never needs reshaping, which is one of the process’s great advantages. The work and the wire sit in a bath or a jet of dielectric — deionised water — and the power supply fires sparks across the tiny gap between the wire and the work, each spark eroding a microscopic crater from the work; the water flushes the debris away and keeps the cut cool. The machine’s servo holds the gap as the wire advances, and the wire is guided above and below the work by precise guides, so the cut it makes is straight and true through the thickness of the part. The result is a narrow, accurate slot cut by erosion, following the path the wire is guided on.

The start of the cut

Wire EDM cuts through a part, so the cut must begin somewhere, and the beginning is part of the process’s planning. Because the wire cannot start in the middle of solid metal, a hole must first be made for it: a start hole is drilled through the work where the profile is to begin — by a small hole drilling EDM, by a conventional drill, or by drilling before the work is hardened — and the wire is threaded through it. The machine then cuts from the start hole out along the profile. When a part is being cut out of a larger piece, the cut is made through the stock, and the severed slug — the piece cut free — must be supported so it does not fall and trap the wire as the cut is completed, which is why the workholding of a wire job often supports the part and the slug together. The start holes, the threading and the support are the setup that the wire process needs before the cutting begins.

The profile and the passes

The wire cuts a profile by following the part’s outline, and the accuracy of the profile is built in passes. The cutting is done in a first cut — the roughing pass, which cuts through the full thickness of the work along the profile, removing the bulk of the material and leaving a surface with the characteristic EDM finish — and then in skim cuts: further passes of the wire, re-cutting the same profile with a smaller spark and less material to remove, each skim improving the straightness, the size and the surface of the cut, until the final skim leaves the profile to its size and its surface. The first cut is fast but leaves a rougher, recast surface and a slight taper where the wire lagged; the skims correct it. And because the wire can be tilted by the machine’s guides, the process also cuts tapers — the angled sides of a die’s clearance or a mould’s draft — the wire leaning as it cuts so the profile changes through the thickness. The number of skims is set by the accuracy and the finish the part needs, trading time against quality.

What wire EDM cuts well

The process is chosen for the work that suits it, and the work is precise, through-cut and hard. Wire EDM makes the die and the punch of a stamping tool — the die plate with its shaped hole and the punch that fits it, cut from hardened tool steel to a fit that hand work could not match; it cuts the extrusion die, the blanking punch, the form tool, the parts of a progressive die. It cuts hard and exotic materials that defeat machining — the hardened steels, the carbide, the high-temperature alloys — eroding them with no regard for their hardness, and it cuts fine, accurate shapes: the narrow slot, the sharp internal corner (as sharp as the wire’s radius allows), the intricate profile that a milling cutter could not reach and that must be made in a hard part. It cuts with no cutting force, so it is also the process of the delicate, thin and precise part that a force would distort. Its place is the toolroom and the precision shop, cutting the profiles that must be exact.

Wire EDM in the making of a part

Wire EDM sits in the sequence of making a precision part, and its place is fixed by what it does best. The work is machined, heat-treated and ground to prepare its features, the start holes are made, and the wire then cuts the profile that must be true in the hard, finished part — often as the last operation but one, followed only by a light grinding or a polish of the critical surfaces. It is slower than machining and not for the blind cavity — that is the sinker’s work — but for the through-profile in hard metal, the wire is the most accurate way to cut it, eroding a narrow kerf along a programmed path with a wire that never dulls. Wire EDM cutting is thus the process of the die and the punch, the precision profile and the hard-to-machine shape: a thin charged wire guided through the work, spark by spark, cutting the accurate outline that the part needs.