Editing Electrical discharge machining (section)

===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.