This is an injection mold for a rubber part. On the right, the cavity has been EDM machined to a coarse finish to give the part a matte texture. The left side of the mold and most of the right side was milled with CNC.

Shown are a couple of electrodes used to EDM a mold cavity. These are machined from copper or graphite and are consumed in the EDM process. The wear on the corners of the blackened electrode on the right is normal for the process; a second or even third electrode must be made and used up to finish the part to correct geometry.Each electrode is time consuming to make and set up in the machine. The process is also intrinsically slow, these electrodes are about 2 inches long and are burned into a hardened steel block about 3/8 inch deep. Each burn took several days to complete and required 3 electrodes to get the detail and finish required in the part.

This is a 32 microinch finish; about as fine as can be produced in a reasonable amount of time.This tiny gear required about an hour on the machine; making the finish significantly better would require another electrode and many hours of additional burning. Extremely fine finishes are very time consuming and expensive to obtain.