Wire EDM
The wire erosion (including wire cutting ( English wire cutting ), Wire EDM , spark cutting or WEDM ) is a forming production process (cutting process) of high precision for electrically conductive materials which, according to the principle of the spark erosion is working: A sequence of electric voltage pulses generated sparks, the material from Workpiece ( anode ) is transferred to a thin wire ( cathode ) running through it and into the separating medium, the dielectric . The wire is then disposed of. The accuracy of the process is based on the fact that the spark always jumps over at the point where the distance between the workpiece and the wire is minimal.
Cutting process
The eroding wire is wound on a spool and from there is guided over pulleys and the brake roller to the upper wire guide. The wire is pulled through the workpiece and through the lower wire guide with a defined wire tension in the range of 5 to 25 Newtons and a speed of up to 25 m / min by two opposite drive rollers and then disposed of. The wire guides above and below the workpiece guide and support the wire and suppress vibrations. Furthermore, the wire guides serve to have a defined deflection point when cutting conically. The workpieces are cut in a liquid dielectric . This usually consists of deionized water . However, some special machines also work with erosion oil. Through permanent flushing, the dielectric removes the erosion waste from the cutting gap and cools the wire, which has to absorb a high current with a small cross section.
Basically, the wire is polarized positive and the workpiece negative. This means that the electromigration away from the workpiece , which is also relevant for the removal , takes place (metal ions are positively charged). The polarity can also be different or alternating in the recuts. This depends on the technology of the respective machine manufacturer.
If the erosion wire approaches the workpiece at a very small distance, an electric field forms at the point with the smallest distance in which positively and negatively charged ions are strongly accelerated. These ions form an ionized channel between the workpiece and the electrode that conducts electricity. Now the ions collide in the discharge channel, which leads to a visible spark. At the same time, a gas bubble forms from the evaporating dielectric and material (electrode and workpiece). The pressure in the gas bubble increases evenly and plasma is formed . The bubble enlarges until it is spatially limited by the electrode and workpiece. Now the current is interrupted by initiating the pulse pause and the bubble implodes. The implosion tears molten material from the workpiece and also from the electrode. If the pulse pause is initiated too late (pulse duration too long), an electric arc can occur, which leads to a wire break.
The generator settings for the operator are stored in technologies by the manufacturer . They contain e.g. B. the discharge duration, discharge pause, current strength and voltage (also open circuit voltage), capacitor capacity, working voltage (servo voltage), maximum feed and the basic generator circuit (mode or pulse mode).
The maximum generator output for wire EDM machines is usually specified with their roughing output in square millimeters per minute (mm² / min). There are now machines that work with up to 500 mm² / min. Since such speeds destroy more than they are profitable, the main cut or roughing cut (cut through solid material) is carried out at 150 mm² / min to 250 mm² / min. The reference height of 60 mm applies.
Often, lower generator settings are used to recut in order to achieve higher accuracies and better surfaces. In order to achieve accuracies in the range of less than 2 µm, up to eight times are cut, depending on the manufacturer.
Other erosion processes are die sinking and drill eroding .
machinery
The machines for wire erosion are mostly designed as C-frames and have five axes. The machine table performs the X and Y movements and the upper head, which is attached to the Z axis, performs the U and V movements (U parallel to X and V parallel to Y). By working together X, Y, U and V, so-called 4-axis contours are possible, e.g. B. on the bottom a square and on the top a circle.
Wire electrodes
Brass is usually used as the wire material. But copper , tungsten and steel are also increasingly being used. To increase cutting performance and accuracy, erosion wires are coated with zinc and other materials and / or thermally treated. The standard diameter is 0.25 mm in Europe and 0.2 mm in Asia. Due to the low tolerance (1 µm to 2 µm), erosion wires are available in the range from 0.02 mm to 0.33 mm. The latest developments allow the use of two different wire diameters in one processing.
Advantages and disadvantages
Wire erosion can be used to process all conductive materials regardless of their hardness. Extremely small cutting widths are possible even with a large material thickness. The machined contours are sharp-edged and also meet the highest demands in terms of dimensional accuracy and dimensional accuracy.
In contrast, there are the long processing times and the associated high costs. The use of water as a dielectric can lead to rust formation and pitting if the workpiece remains in the machine for a long time.
Even if, in principle, all electrically conductive materials can be machined using wire erosion, only materials of very high quality are suitable for a reliable and precise work process. Structural steels, for example, are very difficult to wire-cut due to their significantly lower homogeneity in their alloy and contamination compared to tool steels. Rolled or non-calmed semi-finished products can also make the eroding process more difficult or even impossible due to their tendency to deform during machining.
See also
literature
- Peter Schierbock, Dieter Weckerle: Eroding technology. EDM, EDM. Textbook and exercise book . 2nd Edition. Soester Fachbuchverlag, Soest 2008, ISBN 978-3-933867-06-3 .
Individual evidence
- ↑ Reference manual Charmilles Robofil 440
- ↑ http://www.gfac.com/fileadmin/user_upload/Germany/Produkte/Drahtwechseller/Automatischer-Drahtwechseller_AWC.swf ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
