Sinker EDM

Sinker EDM forms precise 3D cavities and fine details in hard, conductive materials using shaped electrodes and electrical discharge in a dielectric fluid.

Overview

Sinker EDM (plunge, ram, or cavity EDM) uses a shaped graphite or copper electrode to burn complex 3D cavities into conductive materials. The process does not rely on cutting forces, so it maintains accuracy in very hard or thin-walled parts and produces sharp internal corners, deep ribs, and intricate details that are difficult or impossible to machine conventionally.

Use sinker EDM when you need blind cavities, tight internal features, or textured surfaces in tool steels, carbides, and superalloys. It excels for molds, dies, and low- to medium-volume precision parts where electrode cost can be amortized. Tradeoffs: you must design and pay for electrodes, machining is relatively slow, and part cost climbs with depth, area, and finish requirements. Tolerances and surface finishes are excellent, but not as fast or cheap as basic CNC for simple geometry.

Common Materials

  • Tool steel H13
  • Tool steel P20
  • Stainless steel 420
  • Carbide
  • Inconel 718
  • Titanium Ti-6Al-4V

Tolerances

±0.0005" to ±0.001"

Applications

  • Injection mold cavities and cores
  • Die casting and forging dies
  • Sharp internal keyways and splines
  • Deep ribs and thin-wall features in tooling
  • Medical implant tooling and fixtures
  • Turbine blade root and fir-tree forms

When to Choose Sinker EDM

Choose sinker EDM when you need blind cavities, sharp internal corners, deep ribs, or fine details in hard, conductive materials that are difficult to mill. It fits low- to medium-volume parts and tooling where you can justify custom electrodes. It is especially useful when you must avoid cutting forces, burrs, or tool deflection on delicate features.

vs Wire EDM

Pick sinker EDM when you need blind cavities, bottom details, or 3D pocketed features that a through-cut wire cannot produce. It is better for mold cavities, deep ribs, and sharp corner pockets where you need controlled depth and complex surfaces, not just 2D or tapered profiles.

vs EDM Drilling

Choose sinker EDM when you need shaped cavities, complex 3D forms, or precise pocket geometry rather than straight small-diameter holes. Use it when hole geometry, corner detail, or surface finish matters more than simply creating a fast starter or cooling hole.

vs CNC machining

Use sinker EDM when tool reach, tool deflection, or required corner radii make milling impractical or impossible. It is ideal for very hard materials, deep narrow features, or sharp internal corners where end mills would chatter, wear rapidly, or leave excessive fillets.

vs 3D printing (metal AM)

Choose sinker EDM when you already have a near-net hard metal part or tool steel block and need precise cavities with tight tolerances and fine surface finish. It is preferable for production tooling and critical sealing surfaces where dimensional accuracy and repeatable finish beat the geometric freedom of AM.

vs Grinding

Select sinker EDM when the feature is internal, blind, or highly contoured so that wheel access or form-dressing becomes difficult. It is more suitable for intricate cavities and pockets where a grinding wheel cannot reach or would require multiple complex setups.

Design Considerations

  • Group similar cavity features to reuse electrodes and reduce total electrode count and cost
  • Limit extreme depth-to-width ratios in ribs and slots; very deep, narrow burns drive time and risk instability
  • Specify realistic surface finish (Ra) grades only where necessary; finer finishes increase burn time and electrode wear
  • Provide clear datums and inspection callouts so the shop can plan electrode offsets and setups accurately
  • Avoid unnecessarily tight tolerances on non-functional surfaces; reserve ±0.0005" only for critical features
  • Allow space for flushing paths or reliefs around deep features so dielectric can circulate and maintain process stability