We had a mold project come in that needed EDM work on some deep, narrow rib cavities that simply couldn’t be reached with a cutter. The mold was a four-cavity H13 tool steel block, hardened to 50 HRC, with ribs 0.060″ wide and 0.80″ deep. Aspect ratio like that is where sinker EDM earns its keep.
We machined copper graphite electrodes on our CNC mill — one electrode per rib, with enough taper built in to account for the EDM gap. Set up on our Charmilles sinker EDM with orbital motion to help flush the dielectric through those narrow slots. We used a fine finishing setting for the last pass to get the surface to around 32 Ra, which is what the mold polisher needed to take it to a Class B finish.
The whole EDM phase took about 22 hours across all four cavities. When you’re putting electrodes into hardened steel that deep, there’s no other realistic option. The mold came out clean and the customer was in production within two weeks of delivery. Sinker EDM doesn’t always get the glamour that wire does, but it solves problems nothing else can.