Your Guide to EDM Terminology

EDM has become a common method used across industries with a number of sub-categories including MDM.  This list of EDM terms can help you understand all of the aspects of EDM.  Many of these terms are interchangeable with MDM terms:

Ammeter (Amp Meter) – A measuring instrument for electrical current.

Amp (amperage) applied – Amperage selected on power supply controls.

Amp (amperage) average – Amperage shown on the amp meter.

Arc (Arcing) A damaging flow of electricity (pulsating yellow flash) between electrode and workpiece.
Incorrect term for EDM machining (sparking).

Average current – (see average amp)

Blind Hole (cavity) – A cavity with no flush holes.

Burning – A synonym for EDM Machining.

Capacitor – An electrical component that stores a charge.  Used occasionally to intensify the spark.  (Wear on Electrodes will result).

Carbon – The raw material used to make graphite.

Center Flushing – Flush hole through the center of electrode..

Conductor – A material which will carry electric current.

Coolant – See dielectric.

Copper Graphite – Graphite infiltrated with copper.

Copper Tungsten – A blend of copper and tungsten.

Core – A non-machined area of the cavity created by the flush hole(s) in the electrode.

Corner Wear – Wear on the corners of the electrode parallel to the direction of machining, and proportional to the distance machined.

Crater – Small cavities in the workpiece created by the individual sparks.

Cut – Synonym for machining with the EDM process.

DC Arcing – see arc.

Deionization – Bringing the sparkgap back to a nonconductive state.

Depth finder – An electronic circuit which automatically stops the electrode .0001″ before it touches the workpiece without the high voltage on.  Used during “setup” for setting an accurate depth.

Dielectric – A fluid which insulates the sparkgap between the electrode and the workpiece until a high voltage ionizes the sparkgap and makes it a conductor.


Dither – See vibrator

Discharge – see spark

Dual powersupply – Two identical power supplies in one single cabinet, used to inform the operator when the electrode makes contact with the workpiece, usually with light or an acoustic signal.

Duty Cycle -“on time” +”off time”


EDM Grinding – This procedure is accomplished by mounting a rotating electrode wheel parallel to the direction of the servo.

Electrode – The “cutting tool” in the EDM process, always made of a conducting material.

End Wear – The linear wear at the end of an electrode after EDM machining.

Flashpoint – The temperature at which a flammable material will ignite.  An important factor in selecting a dielectric. 

Filtering – Removing the solid particles created during the machining from dielectric.

Flushing – The single most important Factor in EDM machining.

Flush Hole – A hole to facilitate flushing through either the electrode or workpiece.

Flush Pressure – The necessary pressure to insure an even flow over the “area of contact” between the electrode and the workpiece.

Flush Tank – Also called flush plate or flush pot, a box-like fixture mounted on the worktable. The flush hose connects to the side of the box and the workpiece is clamped either directly or on a subplate, on top, allowing the workpiece and into the empty space of the box.

Gap (spark gap)  – The distance between the electrode and the workpiece during machining.

Gap Voltage – The average voltage in the sparkgap during machining. (meter reading).

Frequency – The number of discharges (sparks) per second.

Graphite – An easy machineable electrode material with high heat resistance and good electric current transfer.

HAZ (heat affected zone) – Also called the “recast layer”.  The depth of the heat penetration altering the parent material’s metallurgical structure due to the EDM process.  Magnitude usually from .0002″ to .008″ depending on the material and the energy per pulse.

Hunting – Excessive reciprocating movement of the servo during machining caused by shorts in the stopgap.

Injection flushing – A synonym for pressure flushing.

Ionization – A breakdown of the resistance in the dielectric flowing in the sparkgap, when a voltage of sufficient magnitude is applied between electrode and workpiece.

Ionization Channel- The path of the positive and negative ions linking the electrode and the workpiece at the moment of ionization.

Microsecond – One-millionth of a second (.000001 seconds) (usec).

Multilead–One power supply with multiple independent power leads.

Nanosecond – One billionth of a second (.000000001 seconds)

Non-directional – A finish having no specific direction to its surface pattern.

Normal Polarity – Negative polarity.  A designation no longer used, as the polarity depend on the application, and as such a polarity (positive or negative) can only be normal in relation to a given application.

No Wear – No wear as such does not exist, but generally speaking, wear less than one percent is called “no wear”  range, a deposit or plating can occur so the electrode actually grows, but as this phenomenon is unpredictable it has no practical value.

Oil through Chuck – A sealed adjustable chuck for tubular electrodes which allows a flow of dielectric fluid through the chuck to the electrode.

“On Time” – Duration of the applied amperage during one pulse cycle measured in nanoseconds.

“Off Time” – The time in microsets between the applications of open gap voltage.

Overcut – The dimension the finished cavity is larger than the electrode. (2 x sparkgap).

Redressing – Removing the worn part of an electrode, bringing it back to its original shape.

Recast Layer – See HAZ.

Reverse Polarity – see normal polarity.

Servo – The electro-mechanical mechanism which automatically maintains the proper spark gap during machining.

Spark -controlled discharge between electrode and workpiece.

Sparkgap – The distance between the electrode and workpiece during machining.

Spark Erosion – Synonym for EDM.  Mainly used outside the U.S.

Split Lead – See Multi lead.

Stepped Electrode – An Electrode constructed in two or more concentric tiers, to be used for through-hole machining,  The first smaller portion is used for the roughing, and the larger subsequent tiers are used for the medium to finishing machining.

Trepanning Electrode -An electrode constructed like a cookie cutter.  Used to machine through holes, this way increases the machining speed by minimizing the amount of workpiece material to remove.