Spark erosion
The electric discharge machining (short EDM of Engl. Electrical discharge machining , and electrical discharge machining , EDM removal ( DIN 8580 ) or electrical discharge machining ) is a thermal, material-removing manufacturing method for conductive materials based on electrical discharges ( spark ) between the electrode as a tool and the conductive workpiece is based.
Machining process
The processing takes place in a non-conductive medium, the so-called dielectric (mostly oil or deionized water). Either the workpiece, tool and dielectric are in a basin or the dielectric is fed through hoses to the area to be eroded, where it washes around the tool and workpiece.
The electrode tool is moved to within 0.004 to 0.5 mm of the workpiece. At the right moment, the flashover of sparks is brought about by increasing the applied voltage. The sparks cause the material to melt and vaporize at points. The removal result is influenced by the intensity, frequency, duration, length, gap width and polarity of the discharges. The tool is moved with the help of a CNC control. Complex geometric shapes can be produced.
The electrode material is selected to match the material to be processed. Copper , brass , graphite , copper alloys (mostly with tungsten ) and hard metal are the most common electrode materials.
A distinction is made between spark erosion drilling ( drill erosion), spark erosion cutting ( wire erosion ), in which a wire forms the electrode, and spark erosion sinking ( sinker erosion ), in which the electrode is pushed into the workpiece as a negative shape with the help of a spark erosion machine . Disk erosion is also being used more and more, with a copper , copper-tungsten or graphite disk serving as the rotating electrode.
The tool electrode is normally connected positively and the sparks are triggered by a rapid sequence of pulses of as constant an energy as possible.
history
In 1770 the English scientist Joseph Priestley discovered the erosive effect of electrical discharges.
In 1943 the Russian scientists and spouses Boris Romanovich (came Russian Борис Романович ) and Natalia Ioasafowna Lazarenko ( Russian Наталья Иоасафовна Лазаренко ) in research with the aim of damage to electrical contacts by sparking during the switching process to reduce, to the idea of the destructive To harness the effect of electrical discharges and to develop a method for the controlled processing of metals. They called the process spark erosion because a series of sparks was created between two conductors that were immersed in liquid dielectric. The principle of the discharge generator used at that time, called the Lazarenko circle , was used for a long time in the construction of generators for electrical discharge machines . In an improved form, this type of generator is still used today in certain applications.
The first machine for machining by electrical discharge machining was presented in 1955 at the European machine tool exhibition in Milan. The first NC wire EDM machine was developed and presented in Switzerland in 1969 by the AG for industrial electronics (AGIE). This combines the advantages of numerical control in connection with spark erosion. It was also no longer necessary to produce an electrode before machining. Today, CNC erosion machines are used in machine and plant construction, because so complicated shapes u. a. can also be produced in hard materials (cutting punches, dies).
Areas of application
On the one hand, spark erosion is used to process materials that are difficult to machine , the mechanical processing of which leads to the rapid wear of conventional tools. On the other hand, a very high level of precision can be achieved in the manufacture of complex shapes in conductive materials, which in other ways required significantly more effort.
Eroding is particularly suitable for producing deep and narrow countersinks and cuts as well as very complex surface structures.
Hardened steel , hard titanium alloys , hard metal , hard materials and conductive high-strength ceramics, which v. a. in the air and space travel are used, can be well handled.
Surface structures with variable roughness as well as burr-free edges can be created. With a corresponding effort, surfaces can also be polished by EDM.
The processes can be further automated by using a coordinate measuring machine and an electrode changer for drilling and die sinking.
disadvantage
- The production of electrodes is relatively complex.
- To set up the machine, each electrode must be measured (center offset, rotation, length) and the measurement data must be taken into account in the programming.
- The amount of material removed per work cycle is low. This limits the application to the removal of limited amounts of material
- Since temperature fluctuations lead to inaccuracies, the room must be air-conditioned for precise work.
- In total, the highest production costs in tool and mold making.
- The materials to be processed must be electrically conductive.
Electrode material
Depending on the process, different materials are used for the electrodes.
- Drill erosion: copper or brass tubes in different profiles (Ø 0.1 - 6.0 mm)
- Wire erosion: brass or copper wire, partly also coated (Ø 0.02 - 0.33 mm)
- Die sinking erosion: Copper or graphite blocks , which are usually given their shape with high-speed milling processes , more rarely with ultrasonic vibratory lapping . Sometimes there are also dressing devices for profiling the electrode on the erosion machines. In the case of disk erosion, a dressing lathe tool is usually permanently mounted, with which the electrode can be freely profiled and brought back into shape if it is heavily worn.
Individual evidence
- ↑ A. Behrens, J. Ginzel, F.-L. Bruhns: Arc Detection in Electro-Discharge Machining. University of the Federal Armed Forces - Laboratory for Manufacturing Technology (LFT), Hamburg 2000, p. 7, (PDF; 147 kB).
- ↑ А.Д. Верхотуров, А.Е. Гитлевич, В.В. Михайлов: БОРИС РОМАНОВИЧ ЛАЗАРЕНКО - АВТОР ВЫДАЮЩИХСЯ ОТКРЫТИЙ В ТЕХНИКЕ XX ВЕКА. УЧЁНЫЙ И ОРГАНИЗАТОР НАУКИ (К 100-ЛЕТИЮ СО ДНЯ РОЖДЕНИЯ). (PDF; 1.6 MB) July 19, 2010, accessed June 2, 2019 (Russian).