A.C. or Alternating Current: Electric current that flows back and forth in cycles, constantly changing polarity. Opposite of DC or direct current.
Arc: In Electric discharge machining (EDM), a single discharge of current perceived as an electrical spark. Hundreds of thousands of these arcs result in the machining in EDM. Commonly confused with D.C. arc which is harmful to both the work-piece and electrode. See: D.C. Arc.
Arc Duration: The length of time EDM current crosses the gap producing work. Usually referred to as on-time and is measured in microseconds.
Automatic Tool Changer: Mechanical device that can remove/replace electrodes and is used to augment CNC spark erosion machines ability to run for long periods without operator intervention. (Abbr. ATC).
Automatic Wire Feed: Mechanical device used to provide autonomy to wire EDM operations by allowing automatic re-threading of a broken wire during machining or automatic wire separation and re-threading when moving from part to part or from detail to detail. (Abbr. AWF).
Automatic Work-piece Changer: Mechanical device used to augment the unattended operation of both Spark CNC EDM and Wire CNC EDM equipment. Pre-set work-pieces are moved into the work tank for machining and are removed and exchanged for others when completed. When used in conjunction with an ATC or AWF, almost complete machine autonomy can be achieved allowing long-run production or many different parts to be machined unattended.
Barrel Effect: The term given to the condition caused by the wire vibrating or resonating within the cut thus producing size discrepancies. The greatest deviation usually occurs in the middle of the wire’s cutting length.
Bicycle Effect: The term given to the behaviour of the wire during wire EDM operations where it lags behind the EDM wire guides, cutting the corner. Therefore, the bicycle effect is the difference in the programmed path of the EDM wire guides (the front tire) and the actual path of the wire (the back tire). Also called corner wash-out. This effect is greater during faster wire EDM cuts and on taller parts.
Copper Graphite: A graphite electrode material that has been infiltrated with micro-fine particles of copper. Used for increased electrical conductivity and electrode strength.
Copper Tungsten: A sintered product composed of tungsten particles infiltrated with micro-fine copper powder. Used as an EDM electrode material for improved wear resistance.
DC Arc: In EDM, the continuous, uncontrolled flow of current between the electrode and work-piece caused by poor flushing, gap contamination, short ionisation-times, etc. Often even the smallest of dc arcs can severely damage the work-piece and electrode. Not to be confused with ‘arc’, which is actually a good controlled spark. See: Arc.
DC or Direct Current: Electric current that always flows in a single direction, maintaining a constant polarity.
De-ionisation: In wire EDM, the process of removing conductive ions from the water dielectric by passing it through a container of resin beads.
De-ionised Water: Used as the dielectric in wire EDM. Water that has had all salts and metallic ions chemically removed by the machine’s resin system. See: Resin.
Dielectric: A material that does not conduct electricity but can sustain an electric field. In EDM it is the dielectric fluid that acts as an insulator until a certain distance (spark gap) and voltage are achieved, then it ionises and supports the electric field through which machining current passes. Traditional dielectrics used in EDM are special ‘oils’ for spark EDM Machines and water for wire EDM machines although both have been used in each machine type.
Dielectric Strength: The ability of a dielectric to resist voltage break down and maintain its insulating qualities against premature electrical discharge. Its rating would be the minimum voltage required to produce an electrical spark through a dielectric medium. A high dielectric strength is a desired property of an EDM oil.
Discharge: In EDM, the controlled flow of current across the gap producing a spark.
Duty Cycle: This is the measure of efficiency of an EDM frequency. It is the percentage of on-time relative to the sum of on-time and off-time.
Edge-start: In wire EDM, the machining of any part when entering from an outside edge rather than a hole or opening.
Electrical Discharge Machining: The carefully controlled process of metal removal using a series of electrical sparks enveloped within a dielectric fluid. Material is removed in conductive work-pieces by using either a shaped electrode (spark EDM) or a wire electrode (wire EDM). (Abbr. EDM)
Electrode: The tool that allows the transfer of electrical energy to EDM a work-piece. It must be electrically conductive and can be shaped electrode as used a spark EDM or a wire as used in wire EDM.
Electrolysis: Electrolysis occurs in the presence of dissimilar metals, water, and electric current and results in the decomposition of a material by the action of an electrical field. In EDM, electrolysis contributes to rusting of ferrous materials, oxidation of non-ferrous materials, bluing of titanium, and the pitting and cobalt depletion of carbides.
Filtration: The important process of removing chips, debris, and contaminants from the dielectric fluid by passing it through a porous filter media. Typically this media can be paper or diatomaceous earth. There are also centrifuge and electrostatic filtration systems in use. Clean dielectric supports faster, safer cutting and better finishes and increased resin life.
Finish Cut: In EDM, the term used for a final pass of the wire or electrode to size and finish an EDM machined detail. This is typically done with reduced current and high frequencies.
Flash point: In EDM, the temperature at which dielectric oil, at a specific vapour/air ratio, will support momentary combustion. Simplified – the higher the flash point, the safer the dielectric oil.
Flushing: The process of physically forcing or exchanging contaminated dielectric from the spark gap with fresh clean dielectric.
Frequency: The number of on/off cycles in one second. Measured in units called hertz, equal to one cycle per second.
Fuzzy Logic: Advanced computer technology used in many high-end, decision-making control units. Fuzzy logic now augments some EDM controls allowing it to think its way through difficult or changing machining conditions.
Gap Voltage: Measurement of electrical voltage across the gap between the work-piece and the electrode. When the electrode or wire is too far away to allow machining this is referred to as the open gap voltage and is typically very high (up to 300V). During EDM machining it is called working gap voltage and is much lower (as low as 35V).
Generator: EDM power supply
Graphite: A carbon-based electrode material. Classified as a metalloid because it exhibits certain properties and characteristics of a metal yet it is not actually a metal. EDM graphites are made from various grain sizes of carbon compounds (amorphous carbon) and mixed with coal tar and other binder materials. It is then baked at extremely high temperatures until all volatile’s and gases have been driven off and all remaining components have been ‘graphitised’. Graphite as an electrode material provides EDM users with ease of manufacture, good metal removal rates and high resistance to wear.
Heat Affected Zone: The area immediately below the recast layer that has been influenced by heat from the EDM process. Its depth depends greatly upon the work-piece material and EDM conditions. (Abbr. HAZ).
Insulator: Any substance that prevents the flow of electric current.
Ion: The electrical condition of a previously neutral atom or molecule that has assumed a definite electrical charge (positive or negative) after gaining or losing one or more electrons. The EDM spark travels within an ionised channel or conductive path.
Ionisation: In EDM, when a path of molecules within the dielectric become polarised or aligned allowing resistivity to drop and current to flow between the electrode and work-piece.
Metal Removal Rate: The measurement of material removal by EDM usually expressed in cubic millimetres per minute (Abbr. MRR).
Off-time: The rest part of the spark cycle. Required allowing re-ionisation of the dielectric and aid in flushing. The duration of this increment significantly affects the overall speed and stability of the machining operation.
On-time: The work part of the spark cycle. Current flows and material is removed only during the on-time. The duration of this increment will significantly affect the overall machining speed, work-piece finish, and electrode wear.
Open Gap Voltage: Maximum voltage potential before ignition and flow of current. Seen on the voltmeter when the start button is pushed and the electrode or wire is approaching the work-piece.
Orbiting: The use of circular or other symmetrical movement of the electrode or worktable used to size and finish a work-piece. This orbit is usually a canned cycle around the centreline of the work-piece detail rather than a hand-programmed movement. Vectoring out from centre is another capability.
Polarity: The direction of ion alignment and current flow. In EDM, this term usually refers to the electrode’s polarity – positive or negative.
Recast/Recast Layer: The name of the condition of the EDM machined surface after having been melted than re-solidified back onto the parent material during the off-time part of the EDM cycle. The recast layer is in metallurgical terms very different than the parent material, and is also influenced by the type of electrode and dielectric used.
Resin: Term given to the beads containing the strong chemicals that condition the water dielectric used in wire EDM.
Resistivity: In wire EDM, the measurement of the resistance of the water dielectric. Opposite of conductivity.
Roughing: Term given to rapid metal removal without regard to finished dimensions. Opposite of finishing.
Secondary Discharge: Situation when conductive or semi-conductive particles suspended within the dielectric touch the sides of the electrode or work-piece and effectively reduce the spark gap causing random discharges upon the previously finished work-piece surface. This can cause pitting, irregular finishes, and tapered surfaces.
Servo-system: The driving mechanism of any controlled axis (axes).
Sinker/Die Sinker: Another name for spark EDM. Taken from the antiquated cavity-making process called hobbing, (not to be confused with gear-hobbing) where a hob is forced into the work-piece under great pressure, effectively cold-forming a cavity. This process was called sinking a die or die sinking, so the term ‘sinker’ for EDM evolved naturally because an electrode was sunk into the work-piece instead of a hardened hob.
Skim-cut/Skimming: Term given to low-power cuts or passes in wire EDM operations used for finishing and sizing a work-piece. Also called trim-cut/trimming.
Slug: The scrap material that drops out of the work-piece after wire cutting. Unless the detail is quite small, the ability to produce a slug is far more efficient than ‘pocketing’ or EDM machining the entire volume of a shape.
Spark: The electrical discharge between any charged conductor and a ground.
Spark Gap: The distance between the electrode or wire and the work-piece during machining. All information and electronic feedback to the control regarding servo advance and retract is obtained in the spark gap. This information includes parameters such as machining current, servo voltage, gap contamination flushing conditions, etc.
Start Hole: A pre-drilled opening in a work-piece providing a place for threading the EDM wire.
Stress-cuts: Special wire-cuts placed strategically around the final part configuration before it is machined to free it from possible influence or movement caused by any stresses resident within the surrounding material.
Submerged Machining: Type of machining used in wire EDM operations when the part is completely submerged by the dielectric to aid in flushing. Good candidates for submerged machining are parts with steep tapers, parts with varying thickness, and parts with intersecting openings and interrupted cuts. Edge starts are made easier when the part is submerged.
Surface Finish: Referring to the roughness or smoothness of a given surface.
Surface Integrity: The metallurgical structural quality of the EDM machined surface and sub-surfaces.
Wear: The thermal consumption of the EDM electrode during machining. There are four types of wear: end, corner, side, and volumetric.
White Layer: The term given for the thermally changed EDM machined surface. This condition can vary with different work-piece materials, electrode materials, and the type of dielectric used. The work-piece surface hardness can be affected resulting in hardening or annealing of the EDM machined surfaces. See: Recast/Recast Layer
Wire EDM: Utilising the same basic EDM theory as a spark EDM, it is a specialised machine that uses electric current passing through a wire electrode to cut precision shapes in conductive materials.
Working Voltage: The measure of voltage during the cutting process. Opposite of open gap voltage.
