The EDM in "wire EDM" stands for Electric Discharge Machining. This means that metal is being removed from the workpiece electrically, or more specifically through millions of tiny sparks which jump between the wire (from the first part of the name) and the workpiece. The process creates extremely high temperatures which melt a little pit at both ends of the spark. This means that some material is blasted out of both the workpiece and the wire with each spark. We don't really care too much about the wire, because it is constantly moving, and renewing itself, but the workpiece is eroded away for a certain distance around the wire (the spark "gap").
As the workpiece is eroded, the table moves (X-Y) to maintain the flow of electric current. A computer in the machine interprets the user program to determine X and Y table movements, and hence, the 2-dimensional shape. Yes, it is possible to cut a limited 3-dimensional workpiece, but I don't believe that is your intention. By the way, the X-Y table movements on modern machines are in increments of 10 microinches or less, so I must question the idea that hobbing is more accurate than wire EDM. Ground gearing can, however, be more "accurate".
The surface problem with wire EDM arises from the fact that workpiece material is molten when it is blasted away from the surface. Most of it is flushed away by the dielectric fluid surrounding the cut. At the extremely high plasma temperatures of the spark, however, many things happen, including turning the dielectric fluid into a gas and sometimes blasting the molten material back onto the workpiece surface in a different location. When molten material is redeposited onto the workpiece surface, it is called "recast".
This recast consists of material which has, literally, been cast back onto the surface, so it is very hard. Underneath the recast layer (and sometimes considered a part of it) is a zone of material which has been heated and annealed, so it is softer. A hard, rough outer crust with a softer layer directly under it, does not generally make for a good working surface.
People who run wire EDM machines generally want to cut as quickly as possible, so they can get the next job into the machine to help make their monthly payements. The higher the spark settings, the faster the machine will cut. This is only limited by the fact that material is also being blasted away from the wire, so too high a spark current/time setting will cause the wire to break.
High current/time settings result in faster cutting speeds, but also in deeper pits, a rougher surface finish, and a thicker recast layer. That is why, as was previously mentioned, most modern machines do a primary, rough cut, and then several finish cuts at lower settings. These finish cuts bring the workpiece to size, but also smooth the finish and minimize the recast layer.
There are several problems associated with wire cutting a gear which is intended to operate, rather than a cavity used for molded parts. First is the problem of center location. The center hole can be wire cut before the gear, and the table-positioning accuracy, as I have mentioned, is excellent. Unfortunately, the wire must be broken, then relocated in the wire guides of the machine before cutting the actual teeth. This operation can cause runnout error which may be extreme for your application.
The second problem arises from the fact that you are cutting a spur gear and intend to use the male portion of the cut. If the workpiece is held at the edges, after the first rough cut it will either fall or be held tenuously in place at one point on the OD. Yes, it is possible to hold this more securely by leaving it attached in several places, but then we go back to the first issue... we have to break and relocate the wire even more times!
Finally, remember that steel expands and warps through the heat treating process. The wire EDM process doesn't "care" about the hardness of the material. Only the conductivity and melting point matter. If heat treating is done after the wire EDM process, the accuracy of the process itself is overshadowed by the expansion and warpage of the heat treat process.
Basically, the wire EDM process is not a really good choice for producing accurate operating gears, but an excellent choice for producing a female gear shape, such as used in injection molding cavities.
