CNC WEDM

Cutting

In its principle, wire electrical discharge erosion/machining does not differ much from electrical discharge machining; therefore, the general information can be found in the article about electrical discharge machining (EDM) (link). Here, I would like to focus on machining where a wire rewound from one spool to another is used as an electrode. The processed item is immersed in a dielectric liquid, and dielectric is applied into an area of current discharges.

This technique is interesting, because the surface of the tool electrode works in that section where there is sufficient spark gap. The shape of the electrode (wire) is reflected in the workpiece in the form of a created gap. Both outer and inner contour machining is possible, as well as processing the workpiece surface with the electrode surface, without cutting off larger portions.

The process of electrical discharge erosion can be used for electrical conductors. Usually, the wire is used to cut contours in steel and sintered carbides, but this technique can also be easily applied to other metals and their alloys.

How do we cut?

The wire can be used for both outer and inner contour machining. In the case of outer contour machining, the wire starts to cut from the outer edge of the processed material. No additional preparation activities are necessary. In some cases, the machining process needs to be divided into two parts, so the workpiece can be mounted (as the desired shape is inside).
The inner contour machining requires drilling of start holes. In the case of hardened steel, the hole can be drilled in soft material, before hardening. However, in some applications, such holes cannot be prepared before hardening, and then the hole drilling EDM is used. A tube-shaped electrode drills a required start hole through which a wire is later fed.

Fig. 1 Model with a technological hole marked for inner contour machining.

In the case of wire cutting, the workpiece is not subjected to any high forces. The wire is rewound from one spool to another, and current discharges cause erosion of the processed piece in a machined area. Any possible stresses develop in a surface layer, subjected to direct effects of the discharges.

The cutting can either be rough or finishing. The first technique is used to obtain a specific shape with some excess material. On the second passing of the wire, a layer of the machined surface is removed, with process parameters adjusted. This way, greater precision and better surface quality, or reduced roughness, can be achieved.

Mould cutting

Moulds used in die casting are technologically sophisticated tools. Their designs must take into account the wear of crucial components. Frequently, a designer must design a cavity divided in a way ensuring easy opening and good heat transfer, and taking into account economic aspects of the production.

Fig. 2 A forming insert with an additional division, for production of an internal insert. Wire cutting is necessary to adjust the hole shape to the outer contour of the insert.

Mould cavities are formed on inserts. Frequently, they are additionally divided into smaller inserts. When a shape for insert installation is a hole that is either round or rectangular with rounded corners, the process is easy. It is more complicated when the parting line runs along a casting fin. Such contour can be milled, but wire cutting (WEDM) is a much better and more precise method for its finishing. The machining can easily be conducted in material that was already thermally processed. This way both forming cavities and inserts can be produced.

The wire electrical discharge machining (WEDM) is a very useful technique in many areas of tool construction. Of course, it is not necessary, but has its clear advantages, and in many cases it is indispensable. The costs of processing is significantly higher than in the case of machining, but selection of an appropriate technology should be optimised as far as possible. Frequently, a right balance needs to be struck between the time, the precision, and the price.