For many decades, people have been using Electro Discharge Machining (EDM) or Spark Erosion to successfully remove material from difficult to machine location or shapes. Toolmakers come across such difficult to machine surfaces and shapes occasionally. Therefore, most good tool making workshops are usually equipped with a Spark Eroder. Spark Erosion techniques and machines are not new – this knowledge has been around for nearly two-decades and more. EDM machines are equipped with current generators, with typical currents being in the range of 75A.
Modern EDM machines are computerized and numerically controlled. With such machines, it is a very simple affair to make a single set up to cut an array of cavities. For example, with a sparker, you can drill square holes very easily. These machines can be programmed to make an undercut or cut profiles with a precision measured in microns.
Workpieces are normally metallic and must be electrically conductive. In the tool making business, aluminum or steel blocks are usual. However, a workpiece could also be a machine with a broken drill bit, stud or a broken tap that has jammed tight in a hole. Machinists also use sparkers to work on car parts.
The other important part required in the spark erosion process is the electrode. Mold makers or toolmakers use any shape such as a simple cylinder or a polygon. Other more complex operations need a CNC milled brush head, convolute or a diaphragm. If you need to remove a drill bit jammed into a car part, the machinist would normally use a cylinder such as a copper tube of small diameter. After precisely mounting the electrode in the machine head, the machinist will align its movement in the direction of the travel for the head. The alignment of the electrode and the workpiece is a precision task often requiring the help of a Digital Readout or DRO.
To start the spark erosion process, the workpiece must be immersed in a dielectric liquid. Earlier EDM machinists used paraffin as the dielectric, but now liquids that have a much higher flashpoint have superseded paraffin. Moreover, these liquids are not only safer, but also kinder to the machinists’ hands, as he has to dip them often in the liquid.
The machine is switched on and the electrode is brought closer to the workpiece. For this motion control, the mechanism may be hydraulic or electronic. As soon as a critical distance is reached, a tiny spark jumps between the electrode and the workpiece. This is an electrical discharge creating extremely hot plasma and it melts a little part of the workpiece into a tiny pool. At the same time, a small part of the dielectric also vaporizes creating a bubble around the spark.
Seen on a microscopic scale, the pool and the bubble both get larger until the control electronics stops the spark. This collapses the bubble, whereby the surrounding dielectric rushes in and flushes away the molten workpiece. The process creates a large pit on the workpiece and a smaller one on the electrode. When repeated tens of thousands of times each second, the workpiece is slowly eroded away to the required depth.
