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How to CNC machine an EDM electrode

STL machining EDM electrodes for injection molds in the Czech Republic specializes in Radio Controlled (RC) model helicopters. Their main use for DeskProto is to CNC machine the canopy (the freeformed outside body of the helicopter), to be made using polyester layup techniques.
This gallery project shows a different application: creating injection molds by EDM machining.
An interesting project as all steps in the process of moldmaking and injection molding are shown. Perhaps we can later add a gallery project showing a helicopter model.

The parts to be made are for the JETIbox, made by It is the controller of a system for "viewing, processing and storing telemetric data", so a sort of flight data logger for any RC model aircraft. The original Jeti-box had small membrane switches, however these small switches were not easy to use: as the user is looking to the flying helicopter he needs to be able to find and use the switches without looking. So larger buttons were requested, with different shapes for tactile distinction.

Wireframe drawing of a button
The two buttons rendered
The two buttons, drawn in 3D CAD.

LaHeli have complete production facilities in-house, including injection molding machines. They also manufacture molds for injection molding, using both CNC machining and EDM machining (Electrical Discharge Machining).
Two new buttons have been designed, to be manufactured by injection molding. The 3D geometry for each button was created using Rhino, and then exported as STL file.

The STL file in DeskProto
The toolpaths in DeskProto
The geometry and the toolpaths in DeskProto.

Creating the cavity in each mold was done using EDM machining (spark erosion), where an electrode is used to remove metal from the mold by letting sparks (electrical discharges) jump from the electrode to the mold. The electrode needs to have the shape of the desired product, and in most cases an electrode will be created by CNC machining.

The electrodes for these two molds have been machined in copper, and of course LaHeli have used their DeskProto CAM software to create the toolpaths. See the illustrations above. A Cross strategy has been applied in order to get a smooth resulting surface. Note that the electrode needs to be a bit smaller than the required cavity, as some distance for the sparks is needed between electrode and mold. This spark-gap can be easily set in DeskProto as a "negative skin", in this project a spark gap of 0.1 mm has been used.

One of the electrodes before machining
One of the electrodes during machining
On the small CNC milling machine (Opti BF20 Vario CNC).

Machining has been done on LaHeli's Optimum BF20 Vario machine with CNC upgrade. A small cutter was needed (diameter 2 mm) to make the arrow shape clearly visible. Machining time was about 30 minutes for each electrode.

One of the resulting electrodes, before EDM
The EDM machine in action
The resulting electrode, and next the EDM machine in action.

EDM machining is done submerged in a "dielectric liquid" (see the reflections in the illustration above), as streaming liquid is needed to carry away the solid debris. The electrode is regularly lifted in order to create sufficient flow. And of course it will sink deeper and deeper (very slowly) during the process.

The same electrode, now blackened
The resulting cavity in the steel mold
The electrode after finishing the EDM process, and the resulting cavity.

You can see that afterwards the copper electrode has become black. In the steel cavity you can see that the surface is rough due to the sparking process. How rough can be controlled using the EDM settings.

The injection molding machine
The resulting products in plastic
The injection molding machine, and two of the produced buttons.

The two buttons have been injection molded in ABS plastic. One of the photos above shows Laheli's two small injection molding machines, clearly recognizable by the large funnel on top (dark grey) that contains the plastic granules.

The old Jeti-box with membrane switches
The new Jeti-box with distinctive buttons
The old and the new Jeti-box.

The buttons are used in the new JETIBOX Profi, shown in the photo above. The new buttons make this box easier to use than the old version, as the buttons can be found without looking, and as they provide a better defined click-action than the previously used membrane switches.