Spark Erosion or Electro Discharge Machining (EDM) has been a successful means of machining difficult to machine materials or shapes for many decades. As shapes that are difficult to machine occur frequently in toolmaking, it is not unusual to find a good toolmaking workshop equipped with a Spark Eroder. At Cranford Engineering we have used sparkers for over twenty five years and have a great deal of know-how at our disposal. We currently have two “Sparkatron” spark erosion machines both with 75 amp generators and one of which is CNC (Computer Numerically Controlled). This means that it can not only cut an array of cavities with a single set up but it can also be programmed to cut undercuts and profiles with micron precision. “Drilling” square holes is pretty straightforward if you have a sparker.
How the process works.
Start with a workpiece: This can be anything electrically conductive and would normally be metallic. It might be a block of steel or aluminium in the toolmaking business. It might also be a car part or piece of a machine containing a broken stud, broken drill or broken tap jammed tight in a hole.
Next you need an electrode: In the toolmaking or mould making business this could be any shape from a simple cylinder or polygon to a highly complex, CNC milled brush head or diaphragm or convolute or whatever. In the case of a busted tap or drill jammed into a car part the electrode would normally be a cylinder or small diameter copper tube. The electrode is precisely mounted in the machine head and most commonly aligned with the direction of head travel.
When the workpiece and electrode are correctly positioned, often with the help of a Digital Readout (DRO) then the process can begin.
The workpiece would normally be submerged in a dielectric liquid. In the early years of EDM the dielectric was paraffin but this has since been superseded by dielectrics that have a much higher flashpoint. These are not only safer but much kinder to hands which inevitably get splashed or dipped into the liquid. The machine is started and clever motion control electronics (and sometimes hydraulics) move the electrode closer to the workpiece and when the critical distance is achieved, a tiny spark jumps between the electrode and the workpiece. The spark (or electrical discharge) is an extremely hot plasma that melts some of the workpiece creating a tiny pool. It also vaporises a small amount of dielectric creating a bubble around the spark . On a microscopic scale, the pool gets larger and the bubble gets larger until the spark is stopped (by the control electronics) whereby the bubble collapses and the dielectric rushes in a flushes away the molten material. Thus a tiny pit has been created on the work-piece and a smaller one on the electrode. Repeat this tens of thousands of times a second and you have a modern spark erosion machine. The workpiece and the electrode are eroded or worn away. The electrode is worn much less than the workpiece, normally in the ratio of about 1:100 and this is normally termed “electrode wear”. If there are many cavities to be spark eroded it would generally require a number of electrodes.
Other things to note are:
- Flushing the work area can also be quite critical. The head is often pulsed vertically to help flush the area and this can speed the metal removal rates.
- The finish achieved is directly related to the spark size. Bigger or longer sparks will result in bigger pits giving a rougher surface.
- Wear rates are directly related to removal rates, the slower the removal rate – the higher the wear.
- Finer finishes are obtained by using smaller sparks, lower removal rates and higher wear rates.
- Graphite or copper electrodes can be used. There are also some copper alloys that have been specifically designed for spark eroding and they can give improved machinability or wear resistance or both. Graphite is available in different grain sizes for different requirements.
- There are no cutting forces in EDM. Tricky set-ups are possible.
All in all, Spark erosion is an extremely useful process to be the master of. It allows you to remove material in a highly controlled manner. It is not as fast as conventional machining but there probably isn’t a good toolmaking business in the world without one.