CNC EDM

Electrical processing

Electrical discharge machining is a technique in which the material is removed from the processed items due to phenomena accompanying current discharges. The tool used is an electrode, and its shape is reproduced in the work piece. A spark jumps between the electrode and the work piece surrounded by a dielectric liquid (kerosene, deionized water). The discharge generates heat, the temperature rises, and the material is melted, broken and evaporated. Simply, the discharge results in erosion. The electrode is also worn.

The electrical discharge erosion can be used for electrical conductors. It is applied, in particular, for materials difficult to process, but also for other materials, when processing with other technique is impossible (e.g., due to complex shapes, or sharp inner corners).

This technique is used for processing of tools, especially injection moulds, die casting moulds and die trimms. Frequently, these applications require shapes impossible to obtain by any other technique.

Fig.1 A shape of a seat requiring electrical discharge machining (a section). A need to drill “tooth”.

Electrode

In theory, a majority of shapes can be obtained by machining. Unfortunately, certain cases are not justified from the economic point of view, e.g., because they require specialist tools. In the case of the discharge erosion, we need an approximate negative of a shape that we want to obtain. It can be easily obtained by milling a given electrode. Complex shapes can be divided between two or more electrodes.

Frequently, this technique is used for sharp corners, where excess material left by the cutter must be removed. The greater the reach required for processing, the more frequent this situation is. In this situation the system of the tool and the chuck loses its stiffness, and this is not observed in the case of the electrode, not subjected to forces occurring during machining.

Fig. 2 The electrode corresponding to the eroded shape.

Die casting moulds

Mould cavities are made of materials of high hardness and resistance to the effect of high temperatures of the alloy. In general, it can be said that after their hardening they are difficult to process. The electrical discharge machining is an ideal solution in this case.

Shapes that can be formed are mainly holes and gaps of a small diameter/transverse dimension, and great depth. The electrode can process an item either roughly or precisely. We can remove excess material left by the cutter, or apply marking, engravings, or surface texture.

Erosion proces and die cast

Electrical discharge machining (EDM) is definitely less productive in terms of the amount of material removed by it. In certain cases it appears to be indispensable, but it can be avoided when the die cast is correctly designed. Essential guidelines that help to save time and money are avoiding sharp corners and/or deep gaps in a cavity itself. Designers of die cast and moulds, as well as technologists should be aware of these issues. In most cases, it is possible to change the item design in a way improving technological options and significantly influencing the life of a mould (sharp edges – stress concentration – susceptibility to cracks).

Mould cavity surface

Electrical discharge machining by current discharges results in changes in a surface layer of the machined item. Phenomena resulting from the processing change the structure of the processed item in the layer exposed to discharges. If possible, the finishing processing should be performed with correct parameters maintained, which will ensure high quality of the surface.

Electrical discharge machining is also used to obtain a desired texture of the forming cavity. For example, when it is desirable to have a cavity with a rough surface, then an electrode shaped like the whole cavity is made and used to obtain the desired texture. This texture can, e.g., resemble a texture obtained by sandblasting.

Tab. 1 Comparison of roughness according to different labelling

Fig. 3 Comparison of surfaces after drilling according to VDI 3400. 18 on the left, 36 on the right

Other applications

In a tool shop, EDM can also have a “rescue” function, e.g., broken bits of sintered carbides that remained in the mould material can be removed by erosion.

The erosion can also be applied as a preparation for wire cutting (WEDM). When inner contours are cut, a start hole must be made within the cut shape. In some cases, that hole cannot be made before hardening or the material cannot be drilled. Then we can use the hole drilling with a so-called hole popper. A round hole is drilled with a tube-shaped electrode, with a dielectric liquid supplied through its inner duct to facilitate washing out of processed material.