Sinker EDM
Also: sinker EDM, ram EDM, die-sinking EDM, plunge EDM, conventional EDM, cavity EDM, electrical discharge machining, EDM drilling, hole popping
Sinker EDM burns a shaped electrode into hardened conductive metal to make blind cavities, sharp internal corners and deep ribs no rotating tool can cut, to ±0.005 mm.
353 shops tagged EDM among 5,104 CNC machining companies in the Noramark directory
A graphite or copper electrode, machined to the negative of the cavity, is lowered toward the workpiece in a bath of dielectric oil. A controlled spark jumps the gap and vaporizes a speck of metal; the electrode advances and the spark repeats tens of thousands of times a second until the cavity matches the electrode plus a spark gap. The electrode wears too, so shops use a roughing electrode and one or more finishing electrodes, or orbit a single electrode to compensate. The same principle drives EDM drilling (hole popping), where a rotating tube electrode makes small, deep holes.
Sinker EDM makes the features a mill cannot: blind cavities with sharp internal corners, deep narrow ribs and slots, hex and spline sockets to a shoulder, and anything in material too hard or too tough to cut. Mold cavities and cores, forging and die-casting dies, and stamping die pockets are its main work, plus removing broken taps and drills. Tolerances of ±0.025 mm (±0.001") are standard and ±0.005 mm (±0.0002") is achievable with finishing electrodes; mold makers specify finish on the VDI 3400 scale.
It is slow and every cavity needs an electrode that is itself machined, so it is a tool-making process rather than a way to make parts in quantity. Through-profiles are cheaper by wire EDM, and cavities a ball or bullnose mill can reach in soft or pre-hard steel are cheaper by high-speed milling. The burned surface carries a recast layer that fatigue-critical dies and mirror-finish molds must polish off.
At a glance
| Typical tolerances | Standard ±0.025 mm (±0.001") on cavity dimensions; ±0.005 mm (±0.0002") with care using finishing electrodes and orbiting; ±0.0025 mm (±0.0001") on the best machines with in-process electrode measurement. Depth to ±0.01 mm (±0.0004"). The cavity is the electrode plus an overburn of 0.01-0.1 mm (0.0004-0.004") per side, so the electrode sets the geometry and the machine sets the position. |
|---|---|
| Size limits | Common machines take workpieces up to 600 x 400 x 350 mm (24 x 16 x 14") and 500-1,000 kg; large die-sinkers reach 1,500 x 1,000 mm. Cavity depth to about 300 mm (12"), limited by flushing. EDM drilling makes holes of 0.3-3 mm (0.012-0.120") diameter up to 200-300 mm deep, at 100:1 depth to diameter. The smallest internal corner radius is the electrode corner plus the spark gap, about 0.05 mm (0.002") with care. |
| Surface finish | Rough burn 3.2-6.3 µm Ra (125-250 µin), a matte, cratered surface; finish burn 0.4-0.8 µm Ra (16-32 µin); fine finishing on small areas to 0.1-0.2 µm Ra (4-8 µin). Mold makers use the VDI 3400 scale: VDI 12 is about 0.4 µm Ra, 18 is 0.8, 24 is 1.6, 30 is 3.2 and 36 is 6.3. A recast layer of 0.005-0.05 mm (0.0002-0.002") remains and is stoned or polished off where the surface sees fatigue or must be a mirror. |
| Lead time | One-off cavities in 1-3 weeks including electrode manufacture; production tooling with several cavities and finishes in 4-8 weeks. Electrode design and machining are usually half the schedule. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Any electrically conductive material, and in practice mostly hardened tool steel: H13 and S7 for die-casting and forging dies, D2, A2 and O1 for stamping dies, M2 for cutting tools, and 420 stainless for corrosion-resistant molds. 17-4 PH, 440C, 4140 pre-hard, Inconel and titanium burn well for aerospace and medical features; tungsten carbide burns slowly with copper or copper-tungsten electrodes. Copper and aluminum workpieces are possible but rare. Plastics, ceramics and glass cannot be EDM cut.
What drives the cost
- Electrode manufacture: every cavity needs at least one machined graphite or copper electrode, often several
- Cavity volume and depth: burn time scales with the metal removed and slows in deep, narrow features
- Surface finish: going from a rough burn to VDI 12 multiplies burn time several times
- Tolerances below ±0.013 mm (±0.0005"), which need finishing electrodes and orbiting
- Material: carbide and Inconel burn slower than tool steel, and electrodes wear faster on them
- Flushing and fixturing for deep or blind features
- Quantity: one electrode set can burn several cavities, but each burn still takes its full time
When to use it
- Blind cavities with sharp internal corners: mold cores and cavities, forging dies, stamping die pockets
- Deep narrow ribs, slots, and hex or spline sockets in hardened steel
- Features in material too hard or too tough to mill: hardened tool steel, carbide, Inconel
- Small deep holes by EDM drilling: cooling holes, start holes for wire EDM, injector and nozzle holes
- Removing broken taps, drills and studs without damaging the part
When not to
- Through-profiles: wire EDM needs no electrode and is faster
- Cavities a ball or bullnose mill can reach in soft or pre-hard steel: high-speed milling is faster and cheaper
- Non-conductive materials
- Volume production of finished parts; sinker EDM makes the tool, not the parts
Design tips
- Sharp internal corners are possible but the electrode corner wears first, so allow a 0.1-0.2 mm (0.004-0.008") radius wherever you can.
- Give cavity walls 0.5-1° of draft; a dead-straight wall needs orbiting and extra finishing.
- Specify finish by VDI number or Ra on the surfaces that matter and let the rest stay a rough burn.
- Keep rib depth under about 10-15 times the rib width so the electrode flushes and does not taper.
- Say whether recast removal or polishing is required and who does it.
- Supply the cavity geometry as CAD; the shop derives the electrodes with gap compensation, so do not model the electrode yourself.
- Design cavities to be milled where a tool can reach and EDM only the corners and deep features: a hybrid routing is the norm.
Frequently asked questions
- What is the difference between sinker EDM and wire EDM?
- Sinker EDM plunges a shaped electrode into the part to make a blind cavity, so it can produce any 3D shape the electrode has. Wire EDM runs a thin wire through the part to cut a through-profile, needs no custom electrode and is faster and more accurate for anything that goes all the way through.
- What tolerances can sinker EDM hold?
- ±0.025 mm (±0.001") is standard. With finishing electrodes and orbiting, ±0.005 mm (±0.0002") is achievable, and the best machines hold ±0.0025 mm (±0.0001"). Because the cavity is the electrode plus a spark gap, electrode accuracy matters as much as the machine.
- What is a VDI finish?
- The VDI 3400 scale used by mold makers to specify EDM surface texture. Each step is a fixed roughness: VDI 12 is about 0.4 µm Ra (16 µin), VDI 18 is 0.8, VDI 24 is 1.6, VDI 30 is 3.2 and VDI 36 is 6.3. Lower numbers cost more burn time; VDI 18-24 is a common general-purpose mold finish.
- How small a hole can EDM drill?
- Standard tube electrodes run 0.3-3 mm (0.012-0.120") in diameter and drill to depths of 100 times the diameter, so a 1 mm hole 100 mm deep is routine. Specialty micro-EDM drills holes under 0.1 mm. The hole is slightly tapered and carries a recast layer.
