MANUFACTURING
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Figure 1: EDM electrode. Minimum diameter is 6 µm. |
Electrodischarge machining (EDM) is well understood from an application point of view and well characterised for larger components. EDM is not so well characterised for microwork and each job setup requires a significant amount of process optimisation. This optimisation is usually an iterative process that aims to match the material of the work piece with the electrode material and the control parameters such as voltage, power, spark duration and frequency so that the best eroding conditions are obtained.
All EDM techniques use the principle of creating electrical sparks between an electrode and the work piece. A voltage difference is applied between the work piece, which has to be electrically conducting or semiconducting, and the electrode. When the voltage is high enough, an electrical discharge takes place that generates heat in the work piece or the electrode, and often both. The heat that is generated is intense, localised and of a short duration. It causes a small spot on the work piece and often the electrode to melt or evaporate. This creates a small crater or pit in the work piece and by repeated pitting, the shape of the electrode is eroded into the work piece. There are two main types of EDM: volume EDM and wire EDM.
Volume EDM
This type uses a solid electrode, which is the “counterpart” of the material that is to be removed from the part (the resulting cavity). Microvolume EDM is a similar process, but is optimised to use small electrodes to create features in the range of 20 µm to 1 mm. This is useful for drilling deep holes and slots and complex-shaped cavities of high-aspect ratio (feature depth to width ratio). Microvolume EDM processes can be used for milling by moving the part in the x y direction. A micro-volume EDM electrode for drilling holes is shown in Figure 1.
Wire EDM
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Figure 2: EDM cutting a profile in titanium alloy. . |
EDM uses electrodes made from many different materials and copper, including beryllium copper, graphite, brass, tungsten and tungsten carbide. Wire EDM employs a continuously running thin wire as the electrode and is useful for cutting profiles in plates (Figure 2). It often uses a steel wire that has been coated with brass, tungsten wire and other materials of good conductivity, high strength and high melt point. The material employed for the electrode has to be matched with the material of the work piece so that inprocess variations are controlled effectively. This is critical for achieving micron tolerances, especially when the spark gap can be as big as 30 µm. If the dielectric fluid is not flushed away properly during the drilling process, the hole size may become larger as the hole gets deeper. This is due to a change in the dielectric strength caused by contamination with metallic debris from the work piece or the electrode. This affects the spark gap distance and it is usual for the hole to become bigger as the spark gap enlarges.
Microwire EDM is particularly suited to making small parts with tight tolerances and with good-quality surface finishes. Dual microwire EDM machines use two wires of different diameters and the finest wire that can generally be used is 20 µm diameter, but 30-µm wire is more frequently used because it is a little easier to handle.
The main advantage of microwire EDM is the versatility of the process and the shapes that can be produced. It is also useful for machining and producing small and delicate parts because the process is effectively noncontact. The wire does not touch the surface, thus, there should be no lateral forces on the part. Furthermore, because it is a thermal process, microwire EDM is excellent for machining hard substances such as hardened carbon steels or tungsten carbide or difficult-to-machine metals such as titanium.
Applications
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Figure 3: (click to enlarge) Microfluidics mixer with a minimum channel width of 135 µm.. |
EDM is used for many applications, particularly for making one-off or small batches of small parts in specialist steels, which makes it ideal for medical applications. It is also useful for manufacturing microinjection mould tools, especially for high-production volume polymer parts. One medical application is shown in Figure 3. This is a microfluidics mixer used for mixing medical reagents in an analysis instrument. The microchannels have a high surface-to-volume ratio, which helps create turbulence and hence stirring of the chemicals. Many fluidics devices can be generated using microwire EDM techniques.
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Table I: (click to enlarge) Comparison of techniques for micromilling. |
Conventional micromachining and micromilling techniques use small cutters and these techniques differ from EDM by removing material using a physical cutting technique rather than a thermal noncontact technique. These techniques are good for microapplications where the aspect ratio is low, typically 1 or 2, and the minimum feature thickness required is not less than 30 or 40 µm. EDM, and particularly microwire EDM, are useful for making high-aspect ratio features. Holes of 200-µm diameter and 6-mm depth are possible and electrodes of high-aspect ratio such as 20-µm diameter and 2-mm long are possible. A comparison of the capabilities of the different machining techniques is given in Table I. The surface finish measure is shown as Ra, this is a statistical measure of the average roughness measured over a given length of surface.
Laser micromachining is advancing rapidly and small features are possible; however, these also have a limited aspect ratio especially where small holes or slots are concerned. Microvolume and micro-wire EDM have shown themselves to be useful tools for machining microfeatures and parts of down to a few microns in
size with high-aspect ratio in many different conductive and semiconductive materials.
Dr Robert Hoyle is MicroBridge Operations Manager at the Manufacturing Engineering Centre, The Parade, Newport Road, Cardiff University, Cardiff CF24 3AA, UK. tel. +44 29 2087 0018, e-mail: hoylert@cardiff.ac.uk, www.mec.cf.ac.uk.
