Electrical discharge machining Introducing EDM hole drilling – the non-contact drilling process
EDM drilling is a non-contact drilling process which makes it possible to produce the smallest of drill holes, independent of material hardness at high speed. Once mainly used for creating starter holes for subsequent wire EDM operations, it is now extensively used in mould, tool and die making.
Electrical discharge machining (EDM) drilling is different and, some may say, is more complex than typical chip-removal processes. In the EDM drilling process, energy is directed into an electrode and the workpiece is submerged in a dielectric fluid. When the two are separated by a small space – the spark gap – a pulsed electrical discharge from the electrode creates a spark that removes material from the workpiece through a process of melting and evaporation.
In typical applications, brass or copper electrodes with the correct finish-hole diameter are used to create holes in the workpiece. The electrode is charged by the EDM machine’s power supply and is fed forward to create the machining feed rate. The power usage and electrode feed rate, as well as the motion of the machine's axes, are all controlled by the machine's CNC.
A rotating spindle holds the electrode in place, and a guide ensures that the correct location is held during the drilling process. As the electrode creates a hole in the material, a high-pressure dielectric is used to flush the eroded material away - quickly and efficiently - enabling deeper holes to be made.
As holes become deeper, maintaining the high-pressure flush becomes even more important because the removed material has farther to travel up the hole to be removed. If flushing conditions are poor, or if the pressure is not high enough, material can quickly build up and will begin acting like an extension of the electrode. If this occurs, sparks will arc across the dielectric fluid and hit the workpiece in unwanted areas creating a ‘pitting’ effect on the workpiece.
Part material needs to be conductive
The only limitation to workpiece material choice in EDM operations is that it must be conductive. In mechanical (conventional) drilling applications, the tool makes physical contact with the material and is more of a friction process. The drill can "wander" or "walk" as it makes contact with the workpiece, especially if it is drilling into a concave or convex part. The problems associated with wandering can be exacerbated when drilling pre-hardened parts.
Hardened metal, material with a hardness of 70 and higher on the Rockwell hardness C scale can be drilled using EDM. EDM is a thermal process where material is eroded away rather than mechanically cut. The electrode therefore does not come in contact with the workpiece and no physical stress is put on the material.
Electrodes used for precision EDM drilling operations are tubular (not solid), with an spiral-shaped interior which when combined with high-pressure flushing ensures that machined material is not left in the hole.
The Agie-Charmilles Drill 20 is a high-precision EDM machine to drill holes with depths up to 200 mm. It can accommodate small electrodes with diameters ranging from 0.1 – 0.3 mm. According to GF Machining Solutions, The Drill 20 features an easy-to-use HMI enabling fast and real time process monitoring and optimisation, and easy job preparation and set-up. The machine is equipped with sensors that detect short circuits, temperature fluctuations and dielectric fluid levels: all vital to ensure process reliability.