EDM after heat treat — recast, stress, and the nitriding handoff

Recast thickness scales with discharge energy.

Rough sinker EDM (high amperage, long pulse on-time) leaves 20-50 µm of recast on H13. Cavity floors and corners often run higher than walls because flushing is worse there.

Semi-finish settings leave 10-20 µm.

Finish settings (low amperage, short pulse on-time, often called fine-finish or trim cuts) leave 3-10 µm if the operator runs them all the way down, and the surface lands around Ra 0.8-1.6 µm.

A specification that protects nitriding looks like this on the print. Cavity surfaces: Ra not greater than 1.6 µm, recast depth not greater than 10 µm verified on a witness coupon cross-section, no detectable microcracks in critical zones (radii, corner intersections, flash-land transitions). For high-cycle production dies aiming above 100,000 hits, tighten to Ra 0.8 µm and recast under 5 µm on radii.

Wire EDM produces less recast than sinker on equivalent settings because the wire flushes better and the pulse parameters are different, typically 5-15 µm on the final skim cuts. Wire is appropriate for through-features and trim-die windows. It cannot cut blind cavities, so sinker remains the only choice for most forging impressions.