I believe that "ram" edm is what they call "sinker" edm. In this process a "carbon" male electrode is used to erode away the material in question leaving a female replica of the outline of the profile of the electrode in the material. Example cutting star shape with sharp corners into a waspaloy blank.
Wire edm is more akin to using a bandsaw. The wire acts as a continous electrode, i.e., the bandsaw blade. The work piece is moved while the wire position remains fixed. The profile of the piece is programed in X-Y coordinates and the piece moved. The wire cuts a narrow cut " the width of the wire" path around the desired profile. Usually very accurately, within 0.0005".
Ram EDM also called spark erosion or die sinking is a machined part, usually copper, graphite or some tungsten alloy that has a current passed through it that erodes its shape into the part being machined. Whilst you can do through holes its main strength is cavities as in mould tools, it is however a fairly slow process if you require any kind of surface finish and the electrodes wear fairly quickly.
Wire erosion cannot do cavities only through holes, however they can be parallel, tapered or even have a different profile top and bottom.
The big advantage with both processes is they can be done on hardened metal.
Just to add to that both systems have a “spark gap” the more current you put through the greater the gap, the greater the speed but the worse the finishing. Consider the spark gap as an offset of the wire or electrode.
Spark erosion needs a sacrificial electrode and sometimes a number of them, especially if you have steep angle faces where a large spark gap on the sides takes more cleaning up than on the bottom face as you only travel in the Z-axis.
Wire erosion is programmed so needs no electrode (except the wire itself) and by second cutting or skim cutting tolerances of 0.0002” are easily achievable however you will end up with radii in the corners of about 0.006”.
So basically spark erosion is an alternative to CNC milling cavities, especially where corners are needed at depth.
Wire erosion is more accurate, but can only do through holes or profiles, not cavities, but also consider laser, plasma, or water jet cutting.
