Drilling Holes for Wire EDM
Before cutting an aperture, you need a starting hole for the wire to pass through. These are often drilled by EDM itself:
EDM drilling specs:
- 0.04-inch hole through 4-inch thick steel: ~3 minutes
- Electrode: brass or copper tubing
- Practical minimum hole diameter: 0.012 inches (limited by overcut, tubing rigidity, and wear)
- Practical maximum: ~0.120 inches (too much material for larger sizes)
- Exception: EDM commonly drills large or deep holes in tungsten carbide (e.g., 0.2-inch holes through 2.9-inch thick carbide in 49 minutes)
EDM drilling tips:
- Rotation isn't required but helps flushing and reduces electrode wear
- Deionized water directed through the hollow electrode flushes debris
- Because of the extremely small cutting area, dielectric is often not filtered but replaced continuously with clean fluid
- Blind holes are difficult — often require cut-and-try methods
Wire EDM Surface Characteristics
Wire EDM produces fundamentally different metallurgy than sinker EDM:
- Carbon is extracted from the recast layer (opposite of sinker EDM)
- Copper atoms migrate into the recast layer from the wire, slightly softening the surface
- HAZ is very shallow — high amperages with very short on times
- With proper settings, HAZ depth below 1 micron (0.00004 inches)
- Arc gap maintained within 0.1 micron (0.000004 inches) of programmed position
Wire EDM Water System
- Water is deionized by an integrated deionizer
- Chemical balance is critical for good dielectric properties
- The deionizer improves the water's properties as an insulator
The Complete EDM Glossary
Understanding EDM requires mastering its specialized vocabulary. This reference covers every critical term:
| Term | Definition |
|---|---|
| Anode | Positive terminal. In EDM, incorrectly applied to the tool/electrode |
| Barrel Effect | Wire EDM condition where the cut center is wider than entry/exit points due to secondary discharge from flushing pressure |
| Capacitor | Electrical component storing charge; in some EDM supplies, capacitors discharge directly across the gap |
| Cathode | Negative terminal. In EDM, incorrectly applied to the workpiece |
| Colloidal Suspension | Particles too fine to settle; EDM debris forms this in dielectric fluid |
| Craters (Pits) | Small cavities left on the EDM surface by each spark |
| Dielectric Filter | Removes particles from 5 µm down to 1 µm from the fluid |
| Dielectric Fluid | Non-conductive fluid circulating in the gap; provides insulation, cooling, and flushing |
| Dielectric Strength | Voltage needed to ionize the dielectric fluid across the gap |
| Discharge Channel | Conductive pathway of ionized dielectric and vapor between electrode and workpiece |
| Dither | Slight up-and-down ram movement to improve cutting stability |
| Duty Cycle | On time as a percentage of total cycle time |
| EDG | Electrical discharge grinding using a rotating graphite wheel electrode |
| Electrode Growth | Plating action at low power where workpiece material builds up on the electrode |
| Electrode Wear | Material removed from the electrode; measured as end wear, corner wear, or volumetric |
| Electro-forming | Electroplating process used to make metal EDM electrodes |
| Energy | Measured in joules (volt-ampere-seconds) |
| Farad | Unit of electrical capacitance |
| Gap | Closest point between electrode and workpiece where discharge occurs |
| Gap Current | Average amperage flowing across the machining gap |
| Gap Voltage | Voltage across the gap while current flows; open gap voltage is the voltage before current flows |
| Heat-Affected Zone (HAZ) | Layer below the recast layer with altered metallurgical properties |
| Ion | Atom or group that has lost/gained electrons, carrying an electrical charge |
| Ionization | Change in dielectric fluid making it electrically conductive |
| Low-Wear | Electrode wear between 2–15% by volume |
| No-Wear | Electrode wear below ~2% by volume |
| Orbit | Programmable electrode motion: planetary, vectorial, or polygonal |
| Overcut | Clearance between one side of the electrode and the adjacent cavity wall |
| Overcut Taper | Difference between overcut at the top and bottom of the cavity |
| Plasma | Superheated, highly ionized gas in the discharge channel |
| Recast Layer | Solidified molten metal on the workpiece surface; hard, brittle, carbon-enriched |
| Secondary Discharge | Discharge occurring as conductive particles are carried along the electrode side |
| Spark In | Locating method using high-frequency, low-amperage settings with no cutting action |
| Spark Out | Orbiting technique that moves the electrode in the same path until sparking ceases |
| Square Wave | Electrical wave shape from a solid-state power supply |
| Stroke | Distance the ram travels under servo control |
| UV Axis | Mechanism for upper head movement on wire EDM to generate inclined surfaces |
| White Layer | The heat-affected surface layer; may be extremely hard martensite or an annealed layer |
| Wire Guide | Replaceable precision diamond insert sized to match the wire |
| Wire Speed | Rate of axial wire feed through the workpiece (not cutting rate) |
Putting It All Together: the practitioner's Transformation
Remember the practitioner — the shop owner staring at that impossible part drawing?
After investing in a sinker EDM machine with CNC orbiting capability, here's what his first job looked like:
The part: D2 tool steel mold cavity, hardened to Rc 62, with 0.0005-inch tolerance internal corners.
His setup:
- Electrode: Premium fine-grain graphite, positive polarity
- Roughing pass: 67% duty cycle, graphite positive, 50 amps/in² — removed 0.28 in³/hr with less than 1% corner wear
- Semi-finish pass: 50% duty cycle — removed most remaining stock and HAZ from the roughing pass
- Finish pass: 33% duty cycle, high frequency, minimal offset — brought the surface to specification
- Recast layer: Removed by vapor blasting with glass beads
- Total electrode cost: Three electrodes — rough, semi-finish, finish
The result: A cavity that matched the drawing to ±0.0003 inches, with a surface finish of 32 µin. Ra, delivered in 40% of the time it would have taken with conventional jig grinding — on material that was literally too hard to cut with carbide.
the practitioner didn't just solve one problem. He unlocked an entirely new category of work his shop could accept. Hardened die repairs. Prototype mold cavities. Exotic alloy features that sent other shops running.
That's the real power of EDM. It doesn't just solve the problem in front of you — it expands the boundaries of what your shop can manufacture.
Your Next Step
If you're evaluating EDM for your operation, start here:
Audit your current impossible jobs — every part you've quoted "no-bid" on because the material was too hard, the feature too complex, or the tolerance too tight. That's your EDM opportunity list.
Match the machine to the work — if your primary need is die profiles and through-cuts, start with wire EDM. If you need 3D cavities and mold work, sinker EDM is your first machine.
Master the duty cycle — understanding the relationship between on time, off time, frequency, and current is 80% of becoming a productive EDM operator. Use the tables in this guide as your starting baseline.
Respect the recast layer — plan your cut sequences so each pass removes the previous pass's HAZ. This single practice separates professional EDM work from amateur results.
What's the hardest part you've ever been asked to machine — and did you have to turn it down? That's the conversation where EDM begins.