Editing Electrical discharge machining (section)

==Types==

===Sinker EDM===
[[File:Saturn V Q2 Report - J2 Engine electrolytic erosion.ogv|thumb|Sinker EDM allowed quick production of 614 uniform injectors for the [[J-2 (rocket engine)|J-2]] rocket engine, six of which were needed for each trip to the moon.<ref>{{cite book |title=Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicle (NASA-SP4206) |author=Bilstein, Roger E. |year=1999 |publisher=DIANE Publishing |page=[https://archive.org/details/bub_gb_JnoZTbVLx0MC/page/n165 145] |isbn=9780788181863 |url=https://archive.org/details/bub_gb_JnoZTbVLx0MC}}</ref>]]
Sinker EDM, also called ram EDM, cavity type EDM or volume EDM, consists of an electrode and workpiece submerged in an insulating liquid such as, more typically,{{sfn|Jameson|2001}} oil or, less frequently, other dielectric fluids. The electrode and workpiece are connected to a suitable power supply. The power supply generates an electrical potential between the two parts. As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, forming a plasma channel,<ref name="descoeudres"/><ref name="dibitontoI"/><ref name="dibitontoII"/><ref name="dibitontoIII"/> and a small spark jumps.

These sparks usually strike one at a time,{{sfn|Jameson|2001}} because it is very unlikely that different locations in the inter-electrode space have the identical local electrical characteristics which would enable a spark to occur simultaneously in all such locations. These sparks happen in huge numbers at seemingly random locations between the electrode and the workpiece. As the base metal is eroded, and the spark gap subsequently increased, the electrode is lowered automatically by the machine so that the process can continue uninterrupted. Several hundred thousand sparks occur per second, with the actual duty cycle carefully controlled by the setup parameters. These controlling cycles are sometimes known as "on time" and "off time", which are more formally defined in the literature.<ref name="descoeudres"/><ref name="ferri"/><ref>{{cite book |first1=G. |last1=Semon |year=1975 |title=A Practical Guide to Electro-Discharge Machining, 2nd ed. |publisher=Ateliers des Charmilles, Geneva}}</ref>

The on time setting determines the length or duration of the spark. Hence, a longer on time produces a deeper cavity from each spark, creating a rougher finish on the workpiece. The reverse is true for a shorter on time. Off time is the period of time between sparks. Although not directly affecting the machining of the part, the off time allows the flushing of dielectric fluid through a nozzle to clean out the eroded debris. Insufficient debris removal can cause repeated strikes in the same location which can lead to a short circuit. Modern controllers monitor the characteristics of the arcs and can alter parameters in microseconds to compensate. The typical part geometry is a complex 3D shape,{{sfn|Jameson|2001}} often with small or odd shaped angles. Vertical, orbital, vectorial, directional, helical, conical, rotational, spin, and indexing machining cycles are also used.

===Wire EDM===<!-- [[Wire cutting]] redirects here -->
[[File:Robofil-300-WireCut.jpg|thumb|CNC Wire-cut EDM machine]]
[[File:Wire erosion.png|thumb|'''1''' Wire. '''2''' Electrical discharge erosion (Electric arc). '''3''' Electrical potential. '''4''' Workpiece]]

In ''wire electrical discharge machining'' (WEDM), also known as ''wire-cut EDM'' and ''wire cutting'',<ref name="todd">{{cite book |first1=Robert H. |last1=Todd |first2=Dell K. |last2=Allen |first3=Leo |last3=Alting |year=1994 |title=Manufacturing Processes Reference Guide |publisher=Industrial Press Inc. |isbn=0-8311-3049-0 |pages=175–179 |url=https://books.google.com/books?id=6x1smAf_PAcC}}</ref> a thin single-strand metal wire, usually [[brass]], is fed through the workpiece, submerged in a tank of dielectric fluid, typically deionized water.{{sfn|Jameson|2001}} Wire-cut EDM is typically used to cut plates as thick as {{convert|300|mm|in|abbr=on}} and to make punches, tools, and dies from hard metals that are difficult to machine with other methods.

The wire, which is constantly fed from a spool, is held between upper and lower [[diamond]] guides which is centered in a water nozzle head. The guides, usually [[CNC]]-controlled, move in the ''x''–''y'' plane. On most machines, the upper guide can also move independently in the ''z''–''u''–''v'' axis, giving rise to the ability to cut tapered and transitioning shapes (circle on the bottom, square at the top for example). The upper guide can control axis movements in the GCode standard, ''x''–''y''–''u''–''v''–''i''–''j''–''k''–''l''–. This allows the wire-cut EDM to be programmed to cut very intricate and delicate shapes.

The upper and lower diamond guides are usually accurate to {{convert|0.004|mm|mil|abbr=on}}, and can have a cutting path or ''kerf'' as small as {{convert|0.021|mm|mil|abbr=on}} using [[diameter|Ø]] {{convert|0.02|mm|mil|abbr=on}} wire, though the average cutting kerf that achieves the best economic cost and machining time is {{convert|0.335|mm|mil|abbr=on}} using Ø {{convert|0.25|mm|mil|abbr=on}} brass wire. The reason that the cutting width is greater than the width of the wire is because sparking occurs from the sides of the wire to the work piece, causing erosion.{{sfn|Jameson|2001}} This "overcut" is necessary, for many applications it is adequately predictable and therefore can be compensated for (for instance in micro-EDM this is not often the case). Spools of wire are long — an {{convert|8|kg|lb|abbr=on}} spool of {{convert|0.25|mm|mil|abbr=on}} wire is just over {{convert|19|km|mi}} in length. Wire diameter can be as small as {{convert|20|µm|mil|abbr=on}} and the geometry precision is not far from ± {{convert|1|µm|mil|abbr=on}}.

The wire-cut process uses water as its dielectric fluid, controlling its resistivity and other electrical properties with filters and [[PID controller|PID controlled]] [[ion|de-ionizer]] units. The water flushes the cut debris away from the cutting zone. Flushing is an important factor in determining the maximum feed rate for a given material thickness.

Along with tighter tolerances, multi axis EDM wire-cutting machining centers have added features such as multi heads for cutting two parts at the same time, controls for preventing wire breakage, automatic self-threading features in case of wire breakage, and programmable machining strategies to optimize the operation.

Wire-cutting EDM is commonly used when low residual stresses are desired, because it does not require high cutting forces for removal of material. If the energy per pulse is relatively low (as in finishing operations), little change in the mechanical properties of a material is expected due to these low residual stresses, although material that hasn't been stress-relieved can distort in the machining process.

The work piece may undergo a significant thermal cycle, its severity depending on the technological parameters used. Such thermal cycles may cause formation of a recast layer on the part and residual tensile stresses on the work piece. If machining takes place after heat treatment, dimensional accuracy will not be affected by heat treat distortion.<ref>[http://www.header.com/capabilities/edm.html ELECTRICAL DISCHARGE MACHINING (EDM)]. header.com</ref>

===Fast hole drilling EDM===
Fast hole drilling EDM was designed for producing fast, accurate, small, deep holes. It is conceptually akin to sinker EDM but the electrode is a rotating tube conveying a pressurized jet of dielectric fluid. It can make a hole an inch deep in about a minute and is a good way to machine holes in materials too hard for twist-drill machining. This EDM drilling type is used largely in the aerospace industry, producing cooling holes into aero blades and other components. It is also used to drill holes in industrial gas turbine blades, in molds and dies, and in bearings.