In Germany, tool and mould-making is one of the most important branches of the industry. In order to remain internationally competitive, German tool and die-makers are striving to overcome the manufactory-like individual production and achieve industrial efficiency and excellence. Successful toolmakers, therefore, strive for innovative tool concepts.
Even if several geometries can be combined on one electrode, sometimes a number of 100 ≤ n ≤ 1000 geometric-specific sink electrodes are required, which are manufactured and successively spark-eroded in order to obtain the final contour of an injection mould. This results in the disadvantage that the manufacture and replacement of the sink electrodes often account for a much larger proportion of the total production time than the actual sink erosion process. In order to reduce the electrode production time (tF) and the machine set-up time (tR) as well as realise the possibility of internal flushing during electrical discharge machining, there are research approaches to bundle tubular electrodes as individual tool electrode segments. The shaping of the outer geometry is carried out without machining processes. Previously, the overall profile was adjusted either outside the machine system or by using additionally manufactured forming tools. The fact that bundled tool electrodes are suitable for spark erosion countersinking was successfully demonstrated by comparisons with milled countersink electrodes.
On the basis of existing requirements for a tool concept that permits highly flexible shaping, the use of very hard tool electrode materials and targeted flushing of the frontal working gap (sF), an innovative tool concept for roughing machining in spark erosion countersinking has been derived. This concept is based on the bundling of partially chamfered square rods as individual tool electrode segments. By actuating each individual segment, a complex 3D form electrode can be set for spark erosion countersinking. This actuated and variable tool electrode enables spark erosion countersinking of different complex geometries with one and the same tool electrode so that the use of machining processes for shaping can be dispensed with.
The fast, actuator-based setting of the form electrode, for example, on the basis of CAD data, also enables integration into automated process chains. Geometrically simple individual tool electrode segments also offer the potential to use difficult-to-machine materials, such as carbide, as tool electrode materials. This increases imaging accuracy and process stability and reduces processing time.
The investigations were carried out on the "Genius 1000 The Cube" machine system from Zimmer & Kreim in Brensbach. A pyramid profile was prepared in three individual stages with a respective countersink depth of tSenk = 2 mm with the tool electrode surfaces AE1 = 25 mm × 25 mm for the 1st cavity, AE2 = 15 mm × 15 mm for the 2nd cavity and AE3 = 5 mm × 5 mm for the third cavity. Using the process parameters of VDI stage 44 () to achieve arithmetic mean roughness value Ra ≈ 18 μm, the profile was sunk into a stainless steel workpiece of type 1.4301 by spark erosion.
For statistical purposes, each test was carried out in triplicate. For investigations of the variable tool electrode with internal flushing, a chamfer was also attached to selected tool electrode corner segments and a flushing connection integrated. The dielectric was directed to the effective point via the chamfers, which served as rinsing channels.
It can be seen that the same removal rate of V W,i could be achieved for all three cavities using un-chamfered individual tool electrode segments without internal flushing as for the countersunk electrodes made of solid material. Using chamfered individual tool electrode segments with internal flushing, the total removal rate of V˙W,ges. is 13 % lower than for the countersunk electrodes made of solid material.
The reduced removal rate of the first cavity (V˙W,1) can be explained by the use of internal flushing with a flat countersink depth (tSenk). Process-improving effects could only be observed with the deeper third cavity. Here, the increase in the removal rate of V˙W,3 can be explained by the targeted flushing of the active site.
The lateral working gaps (sL) were lowest for chamfered individual mould electrode segments with internal flushing for all three cavities. However, the dispersion (σ) was greater than with the use of countersunk electrodes made of solid material.
The resolution can be increased by reducing the size of the individual tool electrode segments so that even complex geometries can be imaged. Rotationally symmetrical cavities can be produced by rotating the Z axis.
It was depicted that the bundling of individual tool electrode segments shows only minor deviations compared to the results with a countersunk electrode made of solid material. The advantage resulting from the saved milling times (tFräs) and set-up times (tR) outweighs the disadvantage of the process-related scattering (σ) that still exists.
The advantages of internal flushing in spark erosion countersinking can be determined by targeted technological investigations. The process-enhancing effect became apparent with deeper cavities.
In the future, publicly funded projects will include the updating and downscaling of individual tool electrode segments, technology optimization to increase process reproducibility, optimization of rinsing conditions and the use of difficult-to-machine tool materials.
30/01/2019 - Leading Spanish company Ona carried out a study to analyse a wire electro-erosion process (WEDM) as an alternative to conventional machining (broaching) of firtree profiles in discs for the aeronautical sector. The EDM company says that firtree slot WEDM could become a viable industrial option and replace broaching in future. read...
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