Electrical Discharge Machining: Challenges with Less-Than-Conductive Material
During my time as the EDM process owner, I worked exclusively with ceramic material. In one portion of my job, I am responsible for building EDM parameter sets that will create the desired feature on the ceramic workpiece. This section gives a brief overview of some considerations one must use in order to successfully EDM a non-metal piece.
EDM 101
As a very high level description, electrical discharge machining involves energizing a circuit that is intentionally "broken" at a point. If the gap of the break is small enough, and/ or the electric potential is large enough, electricity will physically arc across the break gap.
Taking this understanding a step further, a conductive workpiece can be placed within the gap to be machined by the resulting electrical arc. EDM, as a practiced method of machining, was built on the foundation of having a workpiece that is capable of conducting electricity - metals!
Electrical discharge machining has certain advantages over other conventional practices because the EDM machine tooling never makes physical contact with the workpiece. EDM processes can machine extremely hard metals like titanium, and can create ultra-precise features without the limitations of conventional practices
Metal EDM
Resistance from Power Source to Gap - Negligable
Spark Gap - Intended Resistance
Metal Workpiece Resistance - Negligible
DC Current
By measuring the electrical potential (voltage) at a point before the workpiece and a point after the workpiece, the resistance encountered at the spark gap can be controlled. This ultimately dictates the size of the spark gap and the efficacy of the process.
What if the workpiece is not conductive? Simply put, EDM as a machining option would be off the table. That being said, if the workpiece is physically capable of conducting current, even if there is resistance, EDM may still be an option.
Research within the aerospace field has uncovered different materials, usually ceramics, with the potential to out-perform metal predecessors. The challenges with these otherwise ideal materials lie in their manufacturability.
Non-Metal EDM
Resistance from Power Source to Gap - Negligable
Spark Gap - Intended Resistance
Non-Metal Workpiece Resistance - Comparatively Greater
DC Current
Due to the resistive nature of the non-metal material, the voltage measurements will look drastically different to a metal comparison. New EDM parameters accounting for this incurred resistance will have to be discovered before the non-metal substance can be machined - if at all.