As we detailed in a previous article on EDM machining, the EDM It is a thermal erosion process in which metal is extracted through a series of recurrent electrical discharges between a cutting tool that acts as an electrode and a conductive part, in the presence of a dielectric fluid. This discharge occurs in a voltage gap between the electrode and the workpiece. The heat of the discharge vaporizes minute particles of the workpiece and electrode material, which are then removed from the gap by the continuously flowing dielectric.
The expansion of EDM machining in the last 45 years it has given rise to the three main types listed below, although the most used are the first two.
- Penetration EDM
- Wire EDM
- Drilling EDM (or EDM grinding)
This is the conventional type used by the first EDM machines and is based on the process that we have already described (CITE LINK TO THE ARTICLE ON ELECTRO-EROSION MACHINING). In a nutshell, the electrode is attached to the machine head which is connected to one pole – usually the positive pole – of a pulsed power supply. The workpiece is connected to the negative pole and positioned so that there is a gap between it and the electrode. Subsequently, the hole is flooded with dielectric fluid. When the power supply is connected, the hole is crossed by thousands of pulses of direct current per second, forming sparks and starting the erosion process as detailed in the article on EDM machining (CITE LINK TO THE ARTICLE ON ELECTRO-EROSION MACHINING ). As erosion continues, the electrode advances over the part while maintaining a constant gap dimension.
The finished part may exhibit several different layers. The surface layer it has small globules of metal removed from the part, as well as adhered electrode particles that are easily removed. The second layer is calledwhite cape» or «recast layer«, where the EDM has altered the metallurgical structure of the part. The third layer is the heat-affected zone or “annealed layer«, which has been heated but not melted. The distribution, extension and depth of these layers have a very important influence on the quality of the surface finish and are affected by:
- The cycles of activation and deactivation of the process.
- The duty cycle, which is the ratio of the activation cycle to the total cycle time.
- The separation distance, gap or “gap” between the work piece and the electrode.
The electrode, usually graphite or electrolytic copper, is made with the reverse or negative image of the piece to be obtained, for which it generates cavities in the room.
Among the main features of the penetration EDM we can mention:
- The dielectric fluid is mineral oil, although some machines may use water or other special liquids.
- can be obtained both intern forms What blind ways of complicated geometries.
- Steel extraction capacity: up to 2000 mm3/min.
- Minimum roughness in steels: up to less than 0.4 μm Ra.
- Applications: manufacture of deep drawing molds and dies.
The wear of the electrode used in the penetration system, added to the costs to manufacture it, drove the process of wire EDM in the early ’70s. In this process, the graphite electrode is replaced by a consumable thread, electrically charged and CNC-controlled, capable of making very fine and intricate cuts.
The principle of operation of the wire system is the same as the penetration type, since it uses a series of direct current discharges that form sparks between the wire and the workpiece, both in contact with the dielectric fluid. In some cases, the wire and part are fully immersed in the dielectric, although this can lead to electrolytic corrosion in some materials.
The fundamental difference between wire EDM and the penetration EDM is that the shape of the electrode does not directly influence the shape of the piece to be obtained, since the only thing that is intended is to make a cut in piece and not get a copy with the shape of the electrode.
The process is particularly useful for cutting fine details in pre-hardened stamping and punching dies. A continuous feed mechanism supplies fresh wire, so electrode wear is not a problem. Typical wire diameters range from 0.005 cm to 0.035 cm. These threads produce a notch slightly larger than their own diameter, eg a 0.03 cm thread leaves a 0.04 cm notch. EDM wires can run for long periods without operator attention.
Among the main features of the wire EDM we can say:
- They generate through-only geometries in the piece, depending on the path traveled by the wire, and can be performed straight cuts and taper cuts.
- The dielectric fluid is deionized water.
- Since the thread is very thin, the energy used is limited and extraction rates are low.
- Steel cutting speed: up to 500 mm2/min.
- Metal removal capacity: approx. 350 cm3/hour.
- Minimum roughness in steel: less than 0.3 μm (review cuts).
- Applications: punch-die sets, inserts for molds, electronic components, as well as for medicine and watchmaking.
Drilling EDM (or EDM grinding)
A third type of EDM is designed for the small hole drilling (between approx. 0.015 cm and 0.65 cm) but very deep, with a depth to diameter ratio of 30 to 1, or greater. For this, they are used concentric rotating electrodes up to 30 cm long that rotate at about 100 rpm and pierce the workpiece.
Basically the electrodes perform the same functions as a column drill, except that a) the extraction of the material is carried out by means of electric discharges no direct contact between electrode and workpiece, b) the hardness of the material is irrelevant and c) the accuracy of the finished hole is far superior to what any drill could produce. As the electrical discharges are generated, the rotation aids in flushing and provides even electrode wear.
At the moment, the field of application of the drilling EDM is very small and its most common use is to drill holes to start the wire EDM in already hardened materials, as well as very small and precise holes for industries such as aerospace and medical equipment.
Later in another article we will see the machines that are used for the different types of EDM machining.