Some features simply cannot be produced with a rotating cutter: a deep narrow pocket in hardened tool steel, a genuinely sharp internal corner, a slot a few thousandths of an inch wide in a delicate part. That is where electrical discharge machining comes in. EDM removes metal not with mechanical force but with thousands of tiny electrical sparks per second, each vaporizing a microscopic amount of material. Because there is no contact and no cutting force, material hardness is almost irrelevant — hardened steel, carbide, and exotic superalloys erode about as readily as mild steel. The tradeoffs are speed, cost, and the requirement that the workpiece conduct electricity.

How Spark Erosion Works

The workpiece and an electrode are submerged in a dielectric fluid — oil for sinker work, deionized water for wire — and connected to a pulsed power supply. As the electrode closes to within a few ten-thousandths of an inch, the voltage breaks down the fluid and a spark jumps, vaporizing a tiny crater in the metal. The dielectric flushes the debris away and cools the zone. Repeat thousands of times per second while the electrode advances, and the electrode's shape is progressively reproduced in the part.

Two consequences follow directly. First, accuracy is excellent — single-digit microns are achievable — and surface finish is controlled by spark energy, from a uniform matte texture down to a nearly polished face by running progressively lighter finishing passes. Second, material removal rate is slow compared with milling. You pay by the machine hour, so the design goal is to make the spark do as little work as possible. Because finish is a specified, purchased attribute here rather than a byproduct, it belongs on the drawing alongside GD&T and dimensional requirements.

The Three Machine Types

  • Wire EDM. A thin brass wire, commonly 0.004 to 0.012 inch in diameter, acts as a continuously fed electrode and cuts through the full thickness of the part like an extraordinarily precise bandsaw. Ideal for through-profiles: punch and die sets, gears, precision flat components, and cutting already-hardened parts. Conceptually it resembles waterjet cutting, at an accuracy level orders of magnitude finer. Wire is consumed, so it never wears out of tolerance.
  • Sinker (ram) EDM. A graphite or copper electrode, machined as the negative of the desired cavity, sinks into the workpiece and impresses its form. This is how closed 3D pockets, deep thin ribs, and recessed lettering are produced — geometry no end mill can reach. Note the hidden cost: the electrode itself must be machined, often several of them for roughing and finishing, so electrode fabrication is part of the quote.
  • Hole-popper (small-hole EDM). A fast-drilling variant that produces small deep holes in hardened material, including the start holes that wire EDM needs to thread through a closed profile, and cooling holes in tooling.

When EDM Is Genuinely the Right Answer

  • Injection molds and stamping dies. The majority of EDM hours worldwide go into tooling: deep cavities, sharp corners, thin ribs, and textured detail in hardened steel. Precision molds essentially would not exist without it, which is one of the quieter reasons behind the numbers in injection molding costs and in the choice between aluminum and steel tooling.
  • Machining after heat treatment. Hardening distorts parts. Cutting the critical dimensions by EDM after heat treat sidesteps the distortion problem entirely, because hardness does not slow the process down.
  • Sharp internal corners and narrow slots. A milling cutter always leaves a radius equal to its own radius. A fine wire or a precisely shaped electrode achieves internal corner radii no cutter can produce.
  • Very tight tolerances. When a drawing calls for a few microns, EDM is frequently the only practical route — and the cost implications of that callout are the ones described in what drives machining cost.
  • Delicate and thin parts. With no cutting force, there is nothing to deflect or chatter, so thin webs and fragile features survive.
  • Hard and exotic materials. Carbide, tool steel above 60 HRC, titanium, Inconel, and other alloys that punish cutting tools.

When Not to Use It

EDM is slow and expensive relative to conventional chip-making, so it should never be the first choice for a feature that can be milled or turned — start from the options in CNC machining for prototypes and reach for EDM only where milling stops. It works only on electrically conductive materials, which rules out plastics and ceramics. It cannot produce a truly blind internal cavity with no line of approach. For thin, shallow features in sheet material, chemical etching or laser cutting is often cheaper. And where the geometry merely requires access from several directions rather than impossible corners, moving to 5-axis machining is usually the better economic answer.

In practice, real parts almost always use a combination: mill or turn to rough shape and to all the non-critical dimensions, then EDM only the features that require it. A part designed that way can cost a fraction of the same part sent straight to EDM.

The Recast Layer Nobody Mentions

Sparking leaves a thin altered surface: a resolidified white layer over a heat-affected zone, often with microcracks and residual tensile stress. On a mold cavity that is usually acceptable, or removed by polishing. On a highly loaded or cyclically loaded component it is a genuine concern, because a cracked, tensile-stressed surface is exactly where a crack starts — the mechanism explained in material fatigue in product design. The remedies are light finishing passes with low spark energy, followed by polishing, stress relief, or shot peening on critical parts. If a part is fatigue-critical, say so on the drawing; the shop cannot infer it.

Designing to Keep EDM Hours Down

The difference between a reasonable quote and an alarming one is sometimes one internal radius on a drawing. Practical moves: allow a corner radius wherever function does not require a sharp corner; tighten tolerances only on the surfaces that actually mate; avoid unnecessarily deep narrow pockets; and specify surface finish per face rather than globally. Every one of those choices lets the shop mill more and spark less.

If you have a part or a tool with features that no cutter can reach, Projects House designs for the process and manages the shops that run it, from manufacturing process selection through first articles. Send us the drawing through our contact form and we will tell you what needs EDM and what does not.