Sinker EDM

Sinker EDM forms precision cavities in conductive materials by sparking a shaped electrode into the workpiece, handling hardened steels and intricate internal geometry.

Overview

Sinker EDM (plunge/ram/cavity EDM) creates 3D cavities by advancing a shaped graphite or copper electrode into a conductive workpiece while controlled electrical discharges erode material in dielectric fluid. It excels at deep pockets, sharp internal corners, fine ribs, and features in hardened tool steels that are difficult or risky to machine conventionally.

Choose sinker EDM for mold and die details, hardened parts, and geometry that needs an electrode-shaped cavity without cutting forces. Tradeoffs: slower cycle times than cutting processes, electrode design and wear add cost/lead time, and surfaces typically need finishing if you require a polished mold finish. EDM also requires start access (you can’t “grow” a cavity without an electrode path), and it won’t work on non-conductive materials.

Common Materials

  • A2 tool steel
  • D2 tool steel
  • H13 tool steel
  • 17-4 PH stainless steel
  • Inconel 718
  • Titanium Grade 5

Tolerances

±0.0005" to ±0.002"

Applications

  • Injection mold core and cavity features
  • Die casting mold inserts
  • Stamping and forming dies
  • Sharp internal corner pockets in hardened steel
  • Micro ribs and thin-wall cavity details
  • Keyway and spline cavities in hardened shafts

When to Choose Sinker EDM

Choose sinker EDM for low-to-medium quantities where a precision 3D cavity or internal detail must be produced in conductive material, especially after heat treat. It’s a strong fit for deep features, sharp internal corners, and delicate ribs where cutting forces or tool breakage are unacceptable.

vs Wire EDM

Choose sinker EDM when the feature is a closed-bottom cavity, blind pocket, or 3D form that can’t be produced as a through-cut profile. Wire EDM is constrained to through features (or requires start holes and cut paths), while sinker EDM can replicate electrode shapes into blind geometry.

vs EDM Drilling

Choose sinker EDM when you need shaped cavities, slots, or complex 3D features rather than small-diameter holes. EDM drilling is optimized for producing start holes and deep micro-holes quickly; sinker EDM is slower but can generate the final cavity geometry.

vs CNC milling

Choose sinker EDM when the part is hardened, has very deep/narrow cavities, or needs sharp internal corners and thin ribs that are impractical with endmills. Milling is usually faster for open geometry and larger radii, but struggles with extreme aspect ratios and zero-force requirements.

vs Broaching

Choose sinker EDM for short runs, hardened materials, or non-standard internal forms where broach tooling cost and lead time don’t pencil out. Sinker EDM can generate custom cavity shapes without dedicated broaches, at the expense of longer machine time.

Design Considerations

  • Define critical surfaces and tolerance zones; don’t hold tight tolerance on non-functional burn surfaces
  • Allow for electrode access and flushing; deep cavities need reliefs, flushing holes, or staged burns
  • Specify corner requirements realistically; true sharp internal corners increase time and electrode wear
  • Call out surface finish targets by area; EDM finish level drives burn strategy and secondary polishing
  • Minimize extreme depth-to-width ribs and slots; high aspect ratios increase cycle time and risk of unstable burn
  • Provide material condition (pre/post heat treat) and hardness; it affects burn rate, electrode choice, and schedule