A sinker EDM can cut accurate internal threads in hardened steel, carbide and other materials a tap cannot survive. It does this with a threaded electrode that either rotates in step with the Z-axis feed or orbits inside a pre-made bore. Because the spark does the work, there is no cutting force on the part, no broken tap to dig out and no risk of distorting a wall that already has hours of finish work in it.

That matters most when a hardened die turns up with a missing tapped hole, a print changes after heat treatment, or the material is simply too hard for a tap or thread mill. EDM threading is slower than conventional threading, but on a valuable hardened part it is often the only safe way to add one.

Jobs That Call for a Burned Thread

Threads added after heat treatment. A redesigned mold plate, a hole left off the original drawing or a worn thread on a forging die all start with a part that is already hard. Tapping hardened steel is slow and risky, and annealing a local area can move a finished part. The electrode never touches the wall, so nearby surfaces stay true. Threading after hardening also avoids the distortion that can shift a thread cut soft and then sent through the furnace.

Carbide parts. Cemented carbide conducts electricity, so it burns. A tap would chip it, and thread milling carbide calls for diamond tooling and a careful setup. Sinker EDM is a practical way to thread carbide spindles, punches and wear parts after sintering.

Unusual thread forms. If you can machine a thread form onto a graphite or copper tungsten blank, you can sink it. Acme, buttress, left-hand and custom pitches are all possible once the electrode exists.

When to leave it to a tap. In soft steel, aluminum or prehardened stock, a tap or thread mill finishes in seconds what EDM takes many minutes to burn. If a quality cutting tool can make the thread without real risk of breakage, use it, and save the sinker for parts where a broken tap could scrap the job. If you already have a broken tap stuck in a hardened part, our guide to EDM tapping for broken tap removal covers getting it out first.

Two Ways a Sinker Forms a Thread

Rotation synchronized with Z

The electrode turns on a C-axis while Z feeds down, advancing exactly one lead for every full turn. The electrode screws itself into the start hole while the spark gap keeps it off the wall, and it backs out along the same helix. Before rotary axes were common, this was done with lead-screw attachments or by turning the electrode by hand while advancing the ram. With true Z and C synchronization, lead holds accurately over the full depth. This method copies the electrode's form directly into the bore and suits full-form threads well.

Orbiting or helical interpolation

The second approach keeps the electrode from spinning on its own axis. A threaded electrode enters a pre-machined hole and moves in a circular path of gradually increasing diameter until it reaches full thread depth, with the orbit timed to the lead so each tooth stays in its own groove. Helical interpolation, using X, Y and Z together, achieves a similar result. An undersized electrode can rough the thread and then finish it as the orbit expands, and the motion opens space on one side of the gap so debris can escape.

Check what your control can do

Before ordering electrodes, confirm:

• C-axis: indexing or continuous rotation that can sync with Z
• Orbit routines: expanding circular orbits that can be timed to a lead
• Interpolation: coordinated X, Y and Z motion along a helix
• Retract path: reversal along the same helix without touching the teeth

Running an older sinker that cannot sync rotation with Z or time an orbit to a thread lead? Our engineers can look at your machine and thread specs and tell you what it would take. Talk to an Engineer.

Specifying the Electrode

Material

Graphite is the usual choice for threads in hardened steel. It machines easily, burns quickly and costs less. Copper-impregnated graphite adds strength for fine pitches where plain graphite crests might chip. Copper tungsten handles the hardest jobs, including carbide and small threads, because it wears slowly and holds sharp crests, though it costs more and takes longer to machine.

Pitch and undersize

The electrode must match the thread form and pitch exactly, because pitch error accumulates over depth and a tiny lead error will make the thread bind at full engagement. Size is handled differently. The burned thread comes out slightly larger than the electrode because of the spark gap, so each electrode is made undersize by the expected gap for its power setting. Roughing electrodes need more undersize than finishing electrodes because roughing settings create a wider gap.

Plan for crest wear

The crests at the major diameter wear faster than the roots, so a single electrode can lose its form partway through. Use a roughing electrode to remove most of the material, then switch to a fresh finishing electrode. Threads timed to a square or keyed shank let roughing and finishing electrodes register the same way in the holder.

Buy or make

Standard threaded electrodes in common sizes are widely available with timed shanks ready to mount. Making your own makes sense for nonstandard forms, unusual lengths or holders that need a specific shank, provided you have a lathe or mill that holds pitch and form and a way to time the thread to the shank.

Getting the Thread to Pass Inspection

Prepare a centered start hole. Start with a hole close to minor diameter, drilled or burned on center. In hardened parts, the start hole is often burned with a plain electrode or a small-hole EDM. Any location error in the start hole becomes a thread error later, so indicate it before threading.

Leave room in blind holes. The electrode's lead-in chamfer and the imperfect first threads use up depth, and debris collects at the bottom, so allow clearance below the last full thread.

Keep the gap clean and stable. Debris trapped between teeth causes secondary sparking that can oversize the thread and damage its form. Through-electrode flushing helps in blind holes, and orbiting or rotation pumps fluid in and out. Keep dielectric clean and at a steady temperature, since temperature affects gap size and wear, both of which show up in pitch diameter. Ramp energy up as more of the electrode engages and keep finishing settings low. A power supply with reliable arc protection makes a noticeable difference in a tight thread gap.

Inspect to full depth. Pitch diameter decides fit, and GO/NO-GO plug gauges are the standard check. Thread the GO plug to the full required depth, not just the opening. Where the print calls out form, check crest and root shape with a replica or optical comparator. On critical parts, look for recast and microcracks, especially in carbide. Lead errors that repeat across parts point to an axis issue rather than the electrode, which is a good reason to keep machine calibration current.

Setting Up for Repeatable Threads

Use EDM when the part is hardened, made of carbide or too valuable to risk with a tap. Choose synced rotation if your control has a C-axis, or a timed orbit if it does not. Use graphite for most hardened steel and copper tungsten for carbide and fine pitches, undersize each electrode for its gap, and finish with a fresh electrode. The same sinker EDM capabilities for hardened parts that produce cavities and ribs apply directly to threaded holes.

Threading hardened molds, forging dies or carbide parts takes a machine that holds lead over depth and a stable burn in a tight gap. EDM Zap has built EDM equipment in the USA since 1971, and our sinker EDM equipment is designed for exactly this kind of precision work. Request a Quote, or call (630) 852 1699 to talk through your thread sizes and materials.

Frequently Asked Questions

Can you EDM an internal thread in hardened steel?

Yes. Sinker EDM burns threads in hardened tool steel at any hardness because the sparks melt the metal without applying cutting force. It is commonly used to add or repair threads after heat treatment.

Do you need a C-axis to EDM threads?

No. A C-axis synced to Z is the most direct method, but a threaded electrode can also be orbited in a pattern timed to the lead.

Can EDM produce threads in blind holes?

Yes, and blind holes are a common reason to choose EDM over tapping. Leave clearance at the bottom for the lead-in and debris, and use through-electrode flushing or orbiting to keep the gap clean.

What drives the cost of EDM threading?

Burn time, the number of electrodes and the material. Carbide and fine pitches need copper tungsten electrodes and more finishing passes, while graphite electrodes in hardened steel usually cost less.